Central heating bypass valve

[Gordy]

As all my radiators have TRVs fitted I think my central heating (Open Vent)
needs a bypass valve.

Looking at the diagrams it should be fitted after the pump, before the
diverter valve and then to return, but why can't it be fitted between flow
and return on just the central heating side of the plumbing??

This would certainly be a lot easier for me access wise.

Any thoughts??

[harry]

The bypass valve is just a means of maintaining constant pressure on
the radiators as the TV valves close down.

As they all close down, pressure rises in the system which tends to
defeat their operation.

[Gordy]

So Just putting it between the central heating flow and return somewhere
would achieve the desired operation??

[A.Lee]

Gordy <joeb...@home.com> wrote:

> So Just putting it between the central heating flow and return somewhere
> would achieve the desired operation??

Or, the easiest, and cheapest option, is to either leave one rad on
full, or take off the adjustable head, so the flow will always go
through that rad, even if the others are at full heat, with their TRVs
shutting off the flow through them.
In reality, is your house really that warm that every radiator TRV
shuts at the same time? Because that is the only time when the bypass
valve will come into play.

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[Fredxx]

Interesting, two replies but no answer.

I assume this is a system with a diverter motorised valve, where the hot
water circuit is guaranteed to provide sufficient flow, but where you're
worried that the central heating circuit may be closed down if all the
TRVs are shut. In this case I can see no problem with placing a bypass
valve on just the central heating side.

The other posters are correct in suggesting that it is normal to have
one radiator without a TRV, and this radiator is normally in the
vicinity of a thermostat.

I would therefore suggest that instead, you don't fit a bypass but
remove the TRV body from ideally your hallway radiator and add a
thermostat there, if you don't already have one.

[Roger Mills]

On 20/01/2013 17:50, Gordy wrote:

I'm not sure that you *do* necessarily need a by-pass valve.

The usual reason for fitting one would be if the boiler requires pump
over-run but there is no guaranteed flow path. This is often the case
with S-Plan systems which, with all zone valves shut, leave nowhere for
the over-run flow to go unless there's a by-pass.

But if you have W-Plan system with a diverter valve (or is it a Y-Plan
system with a mid-position valve?) it can never close completely -
either the HW or CH ports (or possibly both) will be open. If just the
HW port is open, there will be a flow path through the HW cylinder's
indirect coil. If just the CH port is open, there *should* be a flow
path through one or more radiators.

When you say that all your rads have TRVs, do you mean *literally* all?
Is there no room stat? If there is, it is usual for the rad nearest to
that *not* to have a TRV.

The generic fitting instructions for by-pass valves are likely to cover
the general case (including S-Plan) where they *do* need to be fitted
after the pump and before any zone valves, but in your case - if you
decide that you *do* need one - I can see no reason why it can't be
where you suggest, just by-passing the CH circuit because, except in
CH-only mode, there will always be a flow path through the HW circuit.
--
Cheers,
Roger
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[Onetap]

It should be an automatic diverter valve, it's like a pressure relief valve and the valve opens,
directing water straight into the return, as the pump pressure creeps up/flow rate decreases/TRVs close.
Honeywell and others make them. I've one in my shed from when I upgraded to something better; see below.

A simple by-pass will reduce the boiler/system efficiency all year round.

A more elegant, though much more expensive, solution is a modulating boiler and weather compensation.
Most recent boilers have the optional add-ons to achieve this.

In mild weather, the boiler flow temperature is reduced, so the TRVs don't have to throttle the flow so much to control room temperatures.
Flow through the boiler remains relatively high.

[harry]

On Jan 20, 7:12 pm, "Gordy" <joeblo...@home.com> wrote:

Normally the tee in the pipework is between the pump and the heating
system.
The pump is usually in the flow these days, so just "downstream" of
the pump.

Unlike a safety valve it is designed to open progressively.
It's important to correctly adjust it so that as the TVs close, it
opens, so keeping the pressure constant.
So when all the TVs are open, it will be closed.
And when all the TVs are closed, it will be open.

It also maintains a constant flow of water through the boiler so
making it more efficient and reducing local hot spots in the heat
exchanger. Also boiler thermostat works better.

Diverter valves were an earlier idea.
Instead of just shutting the flow off, water is diverted straight back
to the return. This overcomes the problem of maintaining constant
water flow through the boiler but doesn't solve the problem of varying
pressures in the radiators/heating system.



In practice, if pressure falls away in the heating system, the more
remote radiators go cold and the nearer ones stay hot. ie, the flow
pattern in the heating system changes.

So what is needed is constant pressure on the heating system and
constant volume going through the boiler.

The bypass valve does both.

[Scion]

A.Lee spake thus:

The bypass valve is also used when the zone valve(s) close if the boiler
over-runs in order to prevent the water overheating.

[Gordy]

Thanks for all the replies, I should have been a bit more specific, the
system is a 'Y Plan' but with no room stat, All the Rads have TRVs, but I
suspect in reality the Bathroom, which is quite large with only a heated
towel rail never gets close to shutting down.

When I mentioned the diverter valve I was referring to the Motorised valve.

I only started thinking about a bypass valve as I have just had a new (oil)
condensing boiler and I believe they are a bit more fragile than my old
1980s Worcester 20/25

I agree that whilst the hot water and heating is on then as long as hot
water is required then there will always be a return path, that's why I had
come to the conclusion that if the bypass was in the central heating circuit
then it would achieve the aim of never 'dead ending' the pump

Cheers

"Gordy" <joeb...@home.com> wrote in message
news:1Iqdnc0FHKXDs2HN...@bt.com...

[Andrew Gabriel]

In article <7eudnXcoTrkzWGDN...@bt.com>,

"Gordy" <joeb...@home.com> writes:
> Thanks for all the replies, I should have been a bit more specific, the
> system is a 'Y Plan' but with no room stat, All the Rads have TRVs, but I
> suspect in reality the Bathroom, which is quite large with only a heated
> towel rail never gets close to shutting down.
>
> When I mentioned the diverter valve I was referring to the Motorised valve.
>
> I only started thinking about a bypass valve as I have just had a new (oil)
> condensing boiler and I believe they are a bit more fragile than my old
> 1980s Worcester 20/25
>
> I agree that whilst the hot water and heating is on then as long as hot
> water is required then there will always be a return path, that's why I had
> come to the conclusion that if the bypass was in the central heating circuit
> then it would achieve the aim of never 'dead ending' the pump

That's right.
Some boilers have a minimum length of pipework which must exist
through the bypass loop circuit (I presume to provide sufficient
volume to allow time for the boiler to respond to the increasing
loop temperature without overheating).

OTOH, some have a built-in bypass loop.

Check the installation instructions for yours.

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

[Onetap]

On Monday, January 21, 2013 10:47:55 PM UTC, Gordy wrote:
> Thanks for all the replies, I should have been a bit more specific, the

> I only started thinking about a bypass valve as I have just had a new (oil)
>
> condensing boiler and I believe they are a bit more fragile than my old
>
> 1980s Worcester 20/25

Condensing boilers and by-passes are not a good mix, in that the latter often stops the former condensing.
Read the manual & see what the maker's suggest.

You should have an indoor thermostat, or indoor air temperature sensor. I'd get that fitted before anything else.