Ordinary radiator- move water inlet to top tapping?

[DerekG]

Ive got a large, heavy & expensive but otherwise ordinary radiator to
which I want to fit a Honeywell "Rondostat" digital TRV.

Sadly it has an LED display that can't be read with the 'stat in the
traditional location at the bottom.

The question is if I re- arrange the hot water inlet to use the top
tapping will the bottom half of the radiator still get hot ?

DerekG

[harry]

On Feb 14, 1:10 am, DerekG <asparagus.wee....@miniac.demon.co.uk>
wrote:

Diagonal connections will give you the maximum heat output. The
manufacturers heat ratings are all for diagonal connections because it
makes them look more efficient.

Diagonal connections are the traditional way radiators are connected.
Using the two bottom connections is just done to make the pipework
look neater. And save a bit of pipe.

[GB]

Does it make any difference whether the water goes in at the top and out
the bottom, or vice versa?

[Alan Braggins]

In article <511ca82a$0$23478$5b6a...@news.zen.co.uk>, GB wrote:
>On 14/02/2013 08:52, harry wrote:
>> On Feb 14, 1:10 am, DerekG <asparagus.wee....@miniac.demon.co.uk>
>> wrote:
>>> Ive got a large, heavy & expensive but otherwise ordinary radiator to
>>> which I want to fit a Honeywell "Rondostat" digital TRV.
>>>
>>> Sadly it has an LED display that can't be read with the 'stat in the
>>> traditional location at the bottom.
>>>
>>> The question is if I re- arrange the hot water inlet to use the top
>>> tapping will the bottom half of the radiator still get hot ?
>>

>> Diagonal connections will give you the maximum heat output. The
>> manufacturers heat ratings are all for diagonal connections because it
>> makes them look more efficient.
>>
>> Diagonal connections are the traditional way radiators are connected.
>> Using the two bottom connections is just done to make the pipework
>> look neater. And save a bit of pipe.
>>
>Does it make any difference whether the water goes in at the top and out
>the bottom, or vice versa?

Google suggests inlet at the top is traditional; with inlet at the bottom
you can get a cooler lower corner at the other end as the hot water rises
as it comes in.

And it turns out that top and bottom at the same end is actually even
better than diagonal (and is used for quoted ratings). But the effects
are small in modern systems.

http://www.diybanter.com/uk-diy/326601-radiator-inlet-outlet-positions.html
http://www.navitron.org.uk/forum/index.php?topic=18116.0
http://www.plumbingpages.com/featurepages/CorrectionFactors.cfm

[Andy Champ]

On 14/02/2013 09:02, GB wrote:
> Does it make any difference whether the water goes in at the top and out
> the bottom, or vice versa?

ICBW but it seems to me it'll be better with hot fed in at the top. That
way the top of the radiator will be hottest, and the outlet at the
bottom coldest (which you want for boiler efficiency) and you'll also
have the advantage of a counter current flow between the air and the
water: the air will rise and be coolest where the rad is coolest, and
hottest where the rad is hottest.

Andy

[Andrew Gabriel]

In article <511ca82a$0$23478$5b6a...@news.zen.co.uk>,

No difference at all.
The outlet has to be at the bottom, but the inlet can be anywhere.
If you feel a radiator with inlet at the bottom, you will see the
hot water rises by convection to the top in the first vertical
channel. It makes no difference if this happens inside the radiator
or if you pipe it to the top outside the radiator.

The only time the inlet must be at the top is for a gravity fed
system (no pump), to generate the dynamic pressure difference
in the height of the radiator which generates the gravity flow.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

[fred]

In article <511ca82a$0$23478$5b6a...@news.zen.co.uk>, GB
<NOTso...@microsoft.com> writes

When you are using top and bottom connections the (hot) flow must be at
the top and the return must at the bottom. The logic behind this is that
hot water entering at the top loses heat progressively and falls to the
bottom of the rad where it is pumped out on the return.

If you reverse this then hot water entering at the bottom rises
immediately (in the first few vertical channels) to the top of rad where
it is pumped out (still hot) on the return. The result is a cooler than
normal surface temp on the lower part of the rad resulting in reduced
output.

This applies to both Top Bottom Opposite End (TBOE) and Top Bottom Same
End (TBSE) connections.
--
fred
it's a ba-na-na . . . .

[Fredxx]

This needs to be clarified as it doesn't fully answer GB's question. I
once got a TIBO the wrong way round and I got a hot bottom entry area
and hot at the top and everywhere else cold. I cannot stress that if
choosing a top connection, it must be on the flow side and the exit must
always be a bottom connection.

[RobertL]

On Thursday, February 14, 2013 9:02:29 AM UTC, GB wrote:


> Does it make any difference whether the water goes in at the top and out
> the bottom, or vice versa?

yes. hot water must go in at the top and out at the bottom. Also the connections should ideally be diagonally opposite each other (e.e "top left and bottom right", or "top right and bottom left").

Robert

[Man at B&Q]

On Feb 14, 12:22 pm, Fredxx <fre...@nospam.com> wrote:

>
>  I
> once got a TIBO the wrong way round and I got a hot bottom entry area

Use more lube next time.

MBQ

[Onetap]

On Thursday, February 14, 2013 8:52:25 AM UTC, harry wrote:

> Diagonal connections will give you the maximum heat output. The
>
> manufacturers heat ratings are all for diagonal connections because it
>
> makes them look more efficient.
>
>
>
> Diagonal connections are the traditional way radiators are connected.
>
> Using the two bottom connections is just done to make the pipework
>
> look neater. And save a bit of pipe.

More bollocks.

TBOE was traditional, but only whilst gravity circulation was in use.
With any pumped system there is very little difference in the flow rates between TBOE and BBOE.
The manufacturers' radiator ratings will be done with the pipe arrangement specified. There should be correction factors for other temperatures.

[RobertL]

On Thursday, February 14, 2013 4:43:47 PM UTC, Onetap wrote:
> On Thursday, February 14, 2013 8:52:25 AM UTC, harry wrote:

> > Diagonal connections will give you the maximum heat output. The
> > manufacturers heat ratings are all for diagonal connections because it
> > makes them look more efficient.
>
> > Diagonal connections are the traditional way radiators are connected.
> > Using the two bottom connections is just done to make the pipework
> > look neater. And save a bit of pipe.

> TBOE was traditional, but only whilst gravity circulation was in use.
> With any pumped system there is very little difference in the flow
> rates between TBOE and BBOE.

Not according to Faber & Kell's Heating and Air Conditioning of Buildings:

TBSE = 1.00
TBOE= 0.97 to 0.95
BBOE = 0.78 to 0.84

I am interested to find that TBSE is better than TBOE.

Robert

[Onetap]

I think the book is dated and recent measurement won't support that. I saw figures (in the CIBSE magazine ISTR) circa 1984 that said BBOE > TBOE, but think I've seen contradicting numbers since. I don't think it's a precise science and the differences are marginal.

[Onetap]

On Thursday, February 14, 2013 12:22:17 PM UTC, Fredxx wrote:

> This needs to be clarified as it doesn't fully answer GB's question. I
>
> once got a TIBO the wrong way round and I got a hot bottom entry area
>
> and hot at the top and everywhere else cold. I cannot stress that if
>
> choosing a top connection, it must be on the flow side and the exit must
>
> always be a bottom connection.

Why? How did the water pass from the hot bottom to the hot top, without warming the area of the radiator between them?

The head provided by a pump would be many times the head generated by gravity circulation, so the water will still flow through the rad if connected bottom in top out. I can't imagine why anyone would intentionally do that, but it should still get hot and emit heat.

[Andrew Gabriel]

In article <1261bf7b-ad94-40a5...@googlegroups.com>,

The hot water will flow in the bottom, straight up the first channel,
and out of the return (passing along the top if it's on the opposite
side). The water in the rest of the radiator stays cold, and doesn't
circulate, and most of the radiator remains ineffective.

[Andy Champ]

On 14/02/2013 20:07, Onetap wrote:
> Why? How did the water pass from the hot bottom to the hot top, without warming the area of the radiator between them?

It warms a little bit of the vertical, not all of it. As FredXX found.

Andy

[David.WE.Roberts]

Thank you for expanding the TLAs since I couldn't quite work out where the
Billion Barrels of Oil Equivalent (BBOE) fitted into the topic ;-)

Cheers

Dave R

[polygonum]

On 15/02/2013 13:06, David.WE.Roberts wrote:
<>
>
> Thank you for expanding the TLAs since I couldn't quite work out where the
> Billion Barrels of Oil Equivalent (BBOE) fitted into the topic ;-)
>
> Cheers
>
> Dave R
>

TLAs? FLAs!! Surely?

--
Rod

[Onetap]

So FredXX says, but I am asking the company to apply their wisdom and say why that should be; since I don't think it would happen unless there are some other factors involved.
I have spent a lot of time designing, commssioning and balancing fluid systems. I can't see it happening, unless there's something else happening.

It is plausible with TBSE connections,

It is unlikey with TBOE connections. If you look at a TBOE radiator, there are equal flow distances between the inlet and outlet connections, regardless of which vertical channel the water passes through.
You would expect there to be equal flow rates through all of the radiator channels, regardless of which connection was top or bottom.

The things that might cause it to happen, IMHO, are;
radiator with some channels obstructed with rust or corrosion products (they often are);
second-hand radiator with internal baffles/blanking;
radiator airlocked;
TBSE connections, especially with a CI sectional radiator (big waterways);
CI radiators generally (negligible hydraulic resistance).

[The Natural Philosopher]

Cold spots will be indicative of restricted flow.

If water has to speed up to bass a bottleneck it will in general dump
less energy on the way.

The whole argument is rather academic though, since the total rated
power of most radiators is usually far in excess of the boilers ability
to supply that power. I.e. the designs are such that most radiators will
be throttled back by the balancing valves or TRV's.

So 'radiator efficiency' is only relevant when trying to get the most
out of a small area. You aren't wasting fuel by having low 'efficiency'
radiators.


--
Ineptocracy

(in-ep-toc’-ra-cy) – a system of government where the least capable to
lead are elected by the least capable of producing, and where the
members of society least likely to sustain themselves or succeed, are
rewarded with goods and services paid for by the confiscated wealth of a
diminishing number of producers.

[RobertL]

On Thursday, February 14, 2013 5:21:51 PM UTC, Onetap wrote:

> I think the book is dated and recent measurement won't support that. I saw figures (in the CIBSE magazine ISTR) circa 1984 that said BBOE > TBOE, but think I've seen contradicting numbers since. I don't think it's a precise science and the differences are marginal.

I imagine Manufacturers will have been modifying the designs to be good for BBOE as that is what people like to use.

I connected all mine TBOE to I want that to be the best!

Robert

[harry]

On Feb 14, 4:58 pm, RobertL <robertml...@yahoo.com> wrote:
> On Thursday, February 14, 2013 4:43:47 PM UTC, Onetap wrote:
> > On Thursday, February 14, 2013 8:52:25 AM UTC, harry wrote:
> > > Diagonal connections will give you the maximum heat output.  The
> > > manufacturers heat ratings are all for diagonal connections because it
> > > makes them look more efficient.
>
> > > Diagonal connections are the traditional way radiators are connected.
> > > Using the two bottom connections is just done to make the pipework
> > > look neater. And save a bit of pipe.
> >  TBOE was traditional, but only whilst gravity circulation was in use.
> >  With any pumped system there is very little difference in the flow
> >  rates between TBOE and BBOE.

The difference arises not because of flow rates but the distribution
pattern of the water.
With diagonal connections, the water is more uniformly distributed and
there are fewer "dead spots".
Pumped circulation or gravity, makes little difference.
Assuming a standard radiator with "manifolds" top and bottom.

[Colin Stamp]

On 15/02/2013 13:39, Onetap wrote:
> On Thursday, February 14, 2013 10:59:59 PM UTC, Andy Champ wrote:
>
>> It warms a little bit of the vertical, not all of it. As FredXX found.
>
> So FredXX says, but I am asking the company to apply their wisdom and
> say why that should be; since I don't think it would happen unless there
> are some other factors involved.
> I have spent a lot of time designing, commssioning and balancing fluid systems.
> I can't see it happening, unless there's something else happening.

Surely the something else is convection isn't it? When the hot, light
water goes in at the bottom, it'll float up to the top of the rad,
flooding through the top "tank". Flow rates inside the rad due to the
pump are low, so convection can dominate and the hot water floats up
through the first few channels. If it didn't, BBOE would never work.
The cooler water already in the rad will pool at the bottom, where the
exit ought to be. If the exit is at the top, then it'll be the hottest
water in the rad that gets pushed out by the incoming water - obviously
not what you want. A notional BTOE (as opposed to TBOE) rad will only
get hot up one end and along the top.

>
> It is plausible with TBSE connections,
>
> It is unlikey with TBOE connections. If you look at a TBOE radiator,
> there are equal flow distances between the inlet and outlet connections,
> regardless of which vertical channel the water passes through.
> You would expect there to be equal flow rates through all of the
> radiator channels, regardless of which connection was top or bottom.

No. I would expect convection to be the driving force in deciding which
channels the water flows through. If hot water finds itself at the
bottom of a cooler channel, it'll flow strongly up it. With the exit at
the top, a large pool of cold water will accumulate at the bottom of the
rad.

Cheers,

Colin.

[fred]

In article <ao6qmc...@mid.individual.net>, polygonum
<rmoud...@vrod.co.uk> writes

tetra-, tetr-. (Greek: four; a number used as a prefix) B-)

Although QLA probably falls off the tongue better.

[polygonum]

Let's not mix languages :-)

QLCSA
quattuor littera compendium seu abbreviatio

(Gratias Google Vertere)

--
Rod

[fred]

In article <b-KdnbUxP8Tu4YPM...@brightview.co.uk>, Colin
Stamp <dus...@stamp.plus.com> writes

>
>No. I would expect convection to be the driving force in deciding which
>channels the water flows through. If hot water finds itself at the
>bottom of a cooler channel, it'll flow strongly up it. With the exit at
>the top, a large pool of cold water will accumulate at the bottom of the
>rad.
>

I've seen this very effect on a system I was balancing for a pal. All
rads TBOE but one wouldn't balance correctly and room temp was down.
Large pool of cool water from the bottom up was exactly how is
manifested and swapping the flow solved it immediately.

Also, whacking more flow through makes little difference, you get a
little more mixing but half the rad is still cold so I wouldn't expect
more than half output from it and that would be geometry dependent.

[harry]

On Feb 15, 8:56 pm, fred <n...@for.mail> wrote:
> In article <b-KdnbUxP8Tu4YPMnZ2dnUVZ8k2dn...@brightview.co.uk>, Colin
> Stamp <dust...@stamp.plus.com> writes

The factors governing heat exchange are.

Time (spent in the heat exchanger),
Turbulence,
Temperature (difference)

The more you get of any of them the greater amount of the available
amount of energy is transferred per unit of water.


So in fact if the water spends longer in the radiator, it will come
out cooler.

However the total heat emitted will be greater as the flow increases.
(The temperature difference will be less between inlet and outlet so
the average surface temperature will be greater.)

[Colin Stamp]

On 16/02/2013 07:16, harry wrote:

> The factors governing heat exchange are.
>
> Time (spent in the heat exchanger),
> Turbulence,
> Temperature (difference)

You're forgetting probably the most important one - the effective
surface area of the heat exchanger. If the rad has a big cold patch,
then its effective surface area for heat exchange is obviously much
smaller than it should be.

> The more you get of any of them the greater amount of the available
> amount of energy is transferred per unit of water.
>
>
> So in fact if the water spends longer in the radiator, it will come
> out cooler.
>
> However the total heat emitted will be greater as the flow increases.

If the outlet is at the top, then increasing the flow will increase the
output power slightly, but then you'll end up with a badly unbalanced
system. The rad in question will be a huge flow-hog and the boiler won't
be able to achieve its rated output because of all the still-hot water
coming back on the return.

> (The temperature difference will be less between inlet and outlet so
> the average surface temperature will be greater.)

The average surface temperature isn't the average of the inlet and
outlet temperatures. With the outlet at the top, it'll be well below
both of them.

Cheers,

Colin.

[Onetap]

On Friday, February 15, 2013 6:20:36 PM UTC, Colin Stamp wrote:
>
> Surely the something else is convection isn't it? When the hot, light
>
> water goes in at the bottom, it'll float up to the top of the rad,
>
> flooding through the top "tank". Flow rates inside the rad due to the
>
> pump are low, so convection can dominate and the hot water floats up
>
> through the first few channels. If it didn't, BBOE would never work.
>
> The cooler water already in the rad will pool at the bottom, where the
>
> exit ought to be. If the exit is at the top, then it'll be the hottest
>
> water in the rad that gets pushed out by the incoming water - obviously
>
> not what you want. A notional BTOE (as opposed to TBOE) rad will only
>
> get hot up one end and along the top.
>
>
>
> >
>
> > It is plausible with TBSE connections,
>
> >
>
> > It is unlikey with TBOE connections. If you look at a TBOE radiator,
>
> > there are equal flow distances between the inlet and outlet connections,
>
> > regardless of which vertical channel the water passes through.
>
> > You would expect there to be equal flow rates through all of the
>
> > radiator channels, regardless of which connection was top or bottom.
>
>
>
> No. I would expect convection to be the driving force in deciding which
>
> channels the water flows through. If hot water finds itself at the
>
> bottom of a cooler channel, it'll flow strongly up it. With the exit at
>
> the top, a large pool of cold water will accumulate at the bottom of the
>
> rad.

I still don't agree with that. Please note that I'm not saying it won't happen, I'm saying it wouldn't anticipate it happening.

The convection just generates a pressure difference due to the different masses of the H&C water columns.

With BBOE and no convection (air, rad, water at the same temperature) water will still flow up the first channels and down the last, the flow rate being such that the frictional pressure loss equals the pressure difference across the inlet & outlet provided by the pump.

With BBOE plus convection, it works as above, but pump and convection forces work together.

With TBOE (wrongly connected, flow in bottom & return out opposite top) the pump and convection forces are opposing one another. The pump will IMHO overwhelm the convection forces and ther will be little difference. It will be a parallel flow heat exchanger and the air movement will be reduced.

Domestic hot water cylinders are connected F in bottom, R out the top, but no-one has ever claimed it stops them working.

[Andrew Gabriel]

In article <213f8646-eb71-491c...@googlegroups.com>,

Onetap <one...@talk21.com> writes:
> I still don't agree with that. Please note that I'm not saying it won't happen, I'm saying it wouldn't anticipate it happening.
> The convection just generates a pressure difference due to the different masses of the H&C water columns.
> With BBOE and no convection (air, rad, water at the same temperature) water will still flow up the first channels and down the last, the flow rate being such that the frictional pressure loss equals the pressure difference across the inlet & outlet provided by the pump.
> With BBOE plus convection, it works as above, but pump and convection forces work together.

Convection forces are very important. The hot water rises instantly in
the first channel, and the rest of the radiator behaves as TBOE, with
the hot water descending uniformly down all the other channels, as it
cools, and remains stratified almost equally across them all. See
http://www.paceplumbing.co.uk/images/thermal-image-radiator-after-flushing.png

> With TBOE (wrongly connected, flow in bottom & return out opposite top) the pump and convection forces are opposing one another. The pump will IMHO overwhelm the convection forces and ther will be little difference. It will be a parallel flow heat exchanger and the air movement will be reduced.
> Domestic hot water cylinders are connected F in bottom, R out the top, but no-one has ever claimed it stops them working.

The heating coil isn't.
But neither are they much of an analogy to a radiator.
If you are refering to the outer cylinder, then that's the same as
the air convecting up a radiator, with the cold air going in the
bottom, and the hot air coming out the top, again by convection.

[Colin Stamp]

On 16/02/2013 15:28, Onetap wrote:

> I still don't agree with that. Please note that I'm not saying it won't happen, I'm saying it wouldn't anticipate it happening.
>
> The convection just generates a pressure difference due to the different masses of the H&C water columns.
>
> With BBOE and no convection (air, rad, water at the same
> temperature) water will still flow up the first channels and
> down the last, the flow rate being such that the frictional pressure
> loss equals the pressure difference across the inlet & outlet
> provided by the pump.

I wouldn't expect that. The vast majority of the water will go whistling
straight through the bottom tank and out of the other end. To get to the
top of the rad, it would have to fight the restriction of one of the
channels, both up and then back down. in BBOE, it needs convection to
have a reason to do that.

>
> With BBOE plus convection, it works as above, but pump and
> convection forces work together.

Actually, the pump would work against convection with BBOE, but the
effect is very small because the velocity of water in the rad due to the
pump is so low. Convection easily overwhelms it.

>
> With TBOE (wrongly connected, flow in bottom & return out opposite
> top) the pump and convection forces are opposing one another. The pump
> will IMHO overwhelm the convection forces and ther will be little
> difference. It will be a parallel flow heat exchanger and the air
> movement will be reduced.

I think you're over-estimating the velocity of the water inside the rad
due to the pump. For example, a 1kW rad might need about 1.2 litres per
minute at "normal" temperatures. The top and bottom "tanks" might be
similar in size to 28mm pipe, so that's just 3ish cm/sec even where
*all* of the water has to flow through one of the tanks (near the inlet
or outlet). Once it splits up and goes through the numerous channels,
it's just barely drifting along. Convection will have no trouble
reversing any flow pattern the pump might tend to cause inside the rad.

>
> Domestic hot water cylinders are connected F in bottom, R out the
> top, but no-one has ever claimed it stops them working.

The coil in mine has flow into the top. The flow velocity due to the
pump is much higher than in a radiator in any case.

Cheers,

Colin.

[Onetap]

On Saturday, February 16, 2013 6:18:16 PM UTC, Colin Stamp wrote:
> On 16/02/2013 15:28, Onetap wrote:
>
>
>
> > I still don't agree with that. Please note that I'm not saying it won't happen, I'm saying it wouldn't anticipate it happening.
>
> >
>
> > The convection just generates a pressure difference due to the different masses of the H&C water columns.
>
> >
>
> > With BBOE and no convection (air, rad, water at the same
>
> > temperature) water will still flow up the first channels and
>
> > down the last, the flow rate being such that the frictional pressure
>
> > loss equals the pressure difference across the inlet & outlet
>
> > provided by the pump.
>
>
>
> I wouldn't expect that. The vast majority of the water will go whistling
>
> straight through the bottom tank and out of the other end. To get to the
>
> top of the rad, it would have to fight the restriction of one of the
>
> channels, both up and then back down. in BBOE, it needs convection to
>
> have a reason to do that.

It definitely does do that. It's the electrical resistance analogy with multiple flow paths. You could work it out from the same principles, i.e., flow in = flow out, differential across all the flow paths is the same, flow along any one path is determined by the resistance and the differential.

>
> > With BBOE plus convection, it works as above, but pump and
>
> > convection forces work together.
>
>
>
> Actually, the pump would work against convection with BBOE, but the
>
> effect is very small because the velocity of water in the rad due to the
>
> pump is so low. Convection easily overwhelms it.

Convection assists the pump, driving more of the flow up the alternative flow paths, along the top header, rather than straight through the bottom header.

[harry]

If you have ever had a system run purelyb y convection you will
realise what a puny force it is.
In a pumped system, it is neither here nor there.

[Onetap]

On Saturday, February 16, 2013 7:00:30 PM UTC, harry wrote:

> If you have ever had a system run purelyb y convection you will
>
> realise what a puny force it is.
>
> In a pumped system, it is neither here nor there.

I have.

It can be a significant factor, but then I could work it out if I felt inclined.

The differential pressure, provided by the pump, across the radiator is throttled down in the course of balancing until the flow is adequate.

Most radiators can work adequately on a one pipe system with no diverter tees (Harry will have to google for that), with convection alone driving nearly all of the flow through the radiator. That suggests to me that the differential pressures across the rad, whether provided by pump or convection, may be of a similar order of magnitude.

It's a hypothetical question since no-one would knowingly connect a radiator TBOE with the flow in the bottom. I don't see why it wouldn't work, is all.

It was a useful exercise, since my pondering of this gave me a bloody good idea about something else.

[Andy Champ]

On 16/02/2013 19:58, Onetap wrote:
> It's a hypothetical question since no-one would knowingly connect a radiator TBOE with the flow in the bottom. I don't see why it wouldn't work, is all.

We've already seen lots of people saying that with BBOE the water comes
in at one end and convects to the top in the first channel or two. The
rest of the rad is then full of cooling water, which eventually makes
its way to the bottom tank and then out.

In BTOE it'll come in at the bottom, convect straight up to the top tank
in the first channel or two - and then have no reason to ever go back
down to the rest of the rad, as it can leave from the opposite top.

Andy

[Colin Stamp]

On 16/02/2013 18:56, Onetap wrote:
> On Saturday, February 16, 2013 6:18:16 PM UTC, Colin Stamp wrote:
>> On 16/02/2013 15:28, Onetap wrote:
>>
>>
>>
>>> I still don't agree with that. Please note that I'm not saying it won't happen, I'm saying it wouldn't anticipate it happening.
>>
>>>
>>
>>> The convection just generates a pressure difference due to the different masses of the H&C water columns.
>>
>>>
>>
>>> With BBOE and no convection (air, rad, water at the same
>>
>>> temperature) water will still flow up the first channels and
>>
>>> down the last, the flow rate being such that the frictional pressure
>>
>>> loss equals the pressure difference across the inlet & outlet
>>
>>> provided by the pump.
>>
>>
>>
>> I wouldn't expect that. The vast majority of the water will go whistling
>>
>> straight through the bottom tank and out of the other end. To get to the
>>
>> top of the rad, it would have to fight the restriction of one of the
>>
>> channels, both up and then back down. in BBOE, it needs convection to
>>
>> have a reason to do that.
>
> It definitely does do that. It's the electrical resistance analogy with
> multiple flow paths. You could work it out from the same principles, i.e.,
> flow in = flow out, differential across all the flow paths is the same,
> flow along any one path is determined by the resistance and the
differential.

That's why I said the vast majority would flow through the bottom tank.
Not *all* of it. The top and bottom tanks generally have a much wider
bore than the vertical channels. Any path in BBOE that includes a bit of
top tank, also has two vertical channels in it, along with the same
amount of tank as the straight through path and four right-angle turns
for good measure. The overall resistance to flow of a top-tank path will
be a lot higher than the straight-through path, so the flow will be much
lower.

Let's be charitable and assume that, say, 30% of the water does manage
to go through the top tank. The 70% that completely bypasses the core of
the rad would be absolutely disastrous in terms of system balancing.
Luckily, with the boiler turned on, convection sorts BBOE out.

>>
>>> With BBOE plus convection, it works as above, but pump and
>>
>>> convection forces work together.
>>
>>
>>
>> Actually, the pump would work against convection with BBOE, but the
>>
>> effect is very small because the velocity of water in the rad due to the
>>
>> pump is so low. Convection easily overwhelms it.
>
> Convection assists the pump, driving more of the flow up the alternative
> flow paths, along the top header, rather than straight through the bottom
> header.

The pump-only flow pattern has most of the flow going straight through
the bottom tank. Convection virtually completely wipes out that flow
pattern and replaces it with one where virtually all of the water goes
via the top tank. Hence convection fights the pump - and wins
comprehensively.

Note that this only applies *inside* the radiator, not in the pipes.

Cheers,

Colin.

[Onetap]

On Saturday, February 16, 2013 8:40:20 PM UTC, Andy Champ wrote:

> In BTOE it'll come in at the bottom, convect straight up to the top tank
>
> in the first channel or two - and then have no reason to ever go back
>
> down to the rest of the rad, as it can leave from the opposite top.

See above, electrical resistance analogy.

If all the water tries to pile up the first few channels, the resistance of that flow path increases and other channels become a path of less resistance.

It balances out so that all possible flow paths have the amount of water flow that makes their resistance equal. You could get all the flow going up the first few channels if they were very wide.

[Colin Stamp]

On 16/02/2013 19:00, harry wrote:
>
> If you have ever had a system run purelyb y convection you will
> realise what a puny force it is.
> In a pumped system, it is neither here nor there.
>

It is puny *outside* the radiator. The pump can easily overwhelm the few
mm of head it generates. *Inside* the radiator, it's a completely
different story. The entire radiator has a very low flow resistance and
it only requires a small flow, so the pressure drop across it is very
small. Water drifts very gently through the channels and convection can
easily hold sway. I'm glad it can or my all BBOE system wouldn't work.

Cheers,

Colin.

[Colin Stamp]

On 16/02/2013 19:58, Onetap wrote:

> On Saturday, February 16, 2013 7:00:30 PM UTC, harry wrote:
>
>> If you have ever had a system run purelyb y convection you will
>>
>> realise what a puny force it is.
>>
>> In a pumped system, it is neither here nor there.
>
> I have.
>
> It can be a significant factor, but then I could work it out if
> I felt inclined.
>
> The differential pressure, provided by the pump, across the
> radiator is throttled down in the course of balancing until the flow
is adequate.

Yep. That's the crux of it. In a properly balanced system, the flow
through each rad is pretty small. Coupled to low end-to-end flow
resistance of the rad, that makes for a very low pressure differential
across the rad itself (not the tails) due to the pump. So it turns out
that the effect of the pump inside the rad is very feeble, just gently
sweeping the water from the inlet to the outlet.

>
> Most radiators can work adequately on a one pipe system with no diverter
> tees (Harry will have to google for that), with convection alone driving nearly
> all of the flow through the radiator. That suggests to me that the
differential
> pressures across the rad, whether provided by pump or convection, may
be of a
> similar order of magnitude.

I wouldn't be surprised.

>
> It's a hypothetical question since no-one would knowingly connect a radiator
> TBOE with the flow in the bottom. I don't see why it wouldn't work, is all.

Well, a couple where that has accidentally happened have come to light
on this thread, and they've both shown the same symptoms. Of course, you
could argue that there my be thousands of others where it hasn't
mattered, but I know where I'd put my money...

Cheers,

Colin.

[Colin Stamp]

If you want to pursue the electrical resistance analogy, then you'll
have to extend it to include convection.

You could do that by putting a voltage source in each channel, pushing
flow up that channel. The voltage of the sources will depend on the
temperature difference across their channel. In practice, the first
channel will have its bottom end much hotter than its top end, so its
voltage source will be large and most of the current will be driven up
that channel. Any current that sneaks along the bottom tank will raise
the temperature at the bottom of the second channel, increasing the
voltage of the source in that channel and causing it to suck up any
remaining current that the pump, strangled by the lockshield valve, can
provide.

Cheers,

Colin.

[harry]

I am very familiar with with one pipe heating systems having spent
forty years in the NHS where they are universal in older hospitals
thanks.
And without diverter tees (which can be sweep tees, or cup tees),
radiators work very poorly indeed.
This should be obvious or it would not be neccessary to make such
tees.
The main reason for a top connection is to give the diagonal
connection (and hence a better water distribution pattern) and to put
the control valve at a convenient height.

[harry]

Radiators do exist where the bottom manifold is split with a divider
halfway along. This forces all the water up the vertical channels,
into the top manifold and down the remaining channels and back into
the other side of the bottom manifold. No top connections are provided
in such radiators.

[Onetap]

On Saturday, February 16, 2013 4:01:07 PM UTC, Andrew Gabriel wrote:
>
> Convection forces are very important. The hot water rises instantly in
>
> the first channel, and the rest of the radiator behaves as TBOE, with
>
> the hot water descending uniformly down all the other channels, as it
>
> cools, and remains stratified almost equally across them all. See
>
> http://www.paceplumbing.co.uk/images/thermal-image-radiator-after-flushing.png

I'm starting to change my mind on this. That image does seem to show all the hot water (white area) going straight up the first channel. Presumably the waterways are oversized (relative to a pipe) for the flow rate that is going through them.

I'm not wholly convinced since the 'before' picture on that site does not show what I'd expect from a sludged-up radiator. There are numerous thermal images of radiators on the internet, but none as 'black & white' as this one; they've probably chosen an image that best suited their sales patter.

TBOE connections would seem to be a waste of time unless you want the valves high up. The extra heat is probably due to the extra pipe.

[halln...@gmail.com]

I have read this whole thread with interest thanks. It strikes me that it would be just as good to connect the f and r at the top on opposite sides of the radiator. Any thoughts>

[The Other John]

On Fri, 16 Mar 2018 11:06:23 -0700, hallnorm99 wrote:

> Any thoughts

Yes, it seems you can't read dates! This is a 5 year old thread!

--
TOJ.

[Fredxx]

At least he had the decency to quote the post he was replying to.

[Vir Campestris]

On 16/03/2018 18:06, halln...@gmail.com wrote:
> I have read this whole thread with interest thanks. It strikes me that it would be just as good to connect the f and r at the top on opposite sides of the radiator. Any thoughts>

Return must be at the bottom. Otherwise the cold water at the bottom
never moves, and the hot runs across the top.

Andy