Cleaned domestic how water heat exchanger, what next?
dog-man
firing up and down etc.
Central heating is fine.
I have a Baxi Genesis 80 combi.
I have thoroughly cleaned the DHW heat exchanger using Fernox ds-3 and
using a jet washer.
AS far as I can tell the heat exchanger is clean.
What do I look for next and how do I test it for failiure before
replacing any parts?
I currently have to run the bath hot water tap very slowly to get a
nice hot bath.
Steve................
Colonel Edmund J. Burke
news:bd4b25ac-8144-4287...@f21g2000prn.googlegroups.com...
> My hot water gets hot then cold then hot then cold etc with the boiler
> firing up and down etc.
>
Obviously another ill made English product. Next time buy American.
dog-man
better, but in the mean time, it would be nice to run the tap normally
and get plenty of hot water at a reasonable flow rate.
Steve..........
Colonel Edmund J. Burke
IS IT STEVE OR DOG MAN? MAKE UP YER FUCKING MIND!
newshound
"dog-man" <st...@dog-man.com> wrote in message
news:d8ccbf9d-6cd1-4c17...@k14g2000pre.googlegroups.com...
Not much comfort I know, but that's what I did after endless similar
problems with a Halstead. I replaced one HX and put in a magnetic filter at
the same time but the performance deteriorated quite quickly. There were
other problems too, but in the end I tried cleaning the HX with strong
hydrochloric acic and that didn't sort the performance. I suppose I ought to
chop the old ones up and look inside, but havn't quite got round to it. I
suspect that if you do have magnetite in your system, the plate heat
exchangers in combis do block up quite readily.
In fact I went back to a system boiler with a conventional indirect
cylinder. And with a Stuart Turner pump on the DHW I now have lots of lovely
hot water wherever I want it.
John Rumm
> My hot water gets hot then cold then hot then cold etc with the boiler
> firing up and down etc.
The boiler cycling on DHW production is normally a sign of only drawing
a very low flow rate of hot water on a working boiler. However a plate
HE that is not working correctly could also cause this.
> Central heating is fine.
>
> I have a Baxi Genesis 80 combi.
>
> I have thoroughly cleaned the DHW heat exchanger using Fernox ds-3 and
> using a jet washer.
> AS far as I can tell the heat exchanger is clean.
Which side of the PHE did you clean? They can block with limescale on
the secondary (i.e. fresh water) side, and with other "crud" on the
input side.
Are you getting corrosion elsewhere in the system? i.e. lots of rads
filling with gas and needing bleeding etc?
--
Cheers,
John.
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| John Rumm - john(at)internode(dot)co(dot)uk |
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A.Lee
> My hot water gets hot then cold then hot then cold etc with the boiler
> firing up and down etc.
>
> Central heating is fine.
>
> I have a Baxi Genesis 80 combi.
>
> I have thoroughly cleaned the DHW heat exchanger using Fernox ds-3 and
> using a jet washer.
>
> AS far as I can tell the heat exchanger is clean.
>
> What do I look for next and how do I test it for failiure before
> replacing any parts?
I had exactly the same fault a couple of years ago. I changed the DHW
heat exchanger (after previously cleaning the old one).
Changed the diverter valve (this can send the hot water to the radiators
rather than keeping it for the hot water - you have checked this? )
Easy way to check, all rads cool, run the hot water for 5 minutes or so,
see if the first radiator is getting warm - if so, then the divertor
valve could be faulty.)
If the above have been checked then there are really only 2 probably
causes - pump failing, or blocked primary heat exchanger.
How old is the boiler?
When it was fitted, was it connected up to old pipework with giving that
a good clean out?
Has it had inhibitor in it from new?
Mine was a classic one fitted by a cowboy - fitted to old pipework, not
cleaned out, no inhibitor. The HE was cast iron, so easily corroded. I
attempted to get the HE out one day, but every fitting was seized into
place, so I gave up, and bought a new boiler.
Alan.
--
To reply by e-mail, change the ' + ' to 'plus'.
dog-man
I immersed it for around 2.5 hours in Fernox ds-3, keeping it hot and
flushing it with a jet washer a few times.
Radiator water seems quite clear. There was some black flakes in the
HE but not a substantial amount.
I am wondering if perhaps the diverter valve needs replacing because
it's not fully switching to the hot water setting but can switch fully
to the central heating setting.
I will swap the flow switches tomorrow to see if it makes any
difference.
Steve..............
newshound
> I had exactly the same fault a couple of years ago. I changed the DHW
> heat exchanger (after previously cleaning the old one).
> Changed the diverter valve (this can send the hot water to the radiators
> rather than keeping it for the hot water - you have checked this? )
> Easy way to check, all rads cool, run the hot water for 5 minutes or so,
> see if the first radiator is getting warm - if so, then the divertor
> valve could be faulty.)
>
> If the above have been checked then there are really only 2 probably
> causes - pump failing, or blocked primary heat exchanger.
>
I'd call the DHW one the secondary heat exchanger, the primary being the one
above the gas burner
Andrew Gabriel
dog-man <st...@dog-man.com> writes:
> I cleaned both sides of the heat exchanger.
>
> I immersed it for around 2.5 hours in Fernox ds-3, keeping it hot and
> flushing it with a jet washer a few times.
The symptoms exactly match it still being scaled up, and I am
doubtful that what you did will clean it.
I did a friend's Baxi 105 (IIRC). Put the exchanger in a large
plastic washing up bowl. Mixed up a litre of DS-3 in a jug, and
poured it through slowly. Beware of the acidic spray/vapour.
Then transfer the collected solution back to the jug, and pour
it through again. Each time the DS-2 turns from yellow to green,
it's used up, and you need to mix up a fresh load. Keep doing
this until you can't make it go green anymore. I think it took
3-4 litres. A heat exchanger might not seem particularly large,
but it has a very large surface area which needs descaling.
--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]
A.Lee
Thats what I meant - if the secondary/DHW HE has been cleaned, and it
still doesnt work correctly, it is likely to be the primary HE that is
scaled up with rust and limescale.
dog-man
Steve..........
dog-man
understand how that could affect the hot water side of things and not
the heating side.
Steve...........
John Rumm
> I cleaned both sides of the heat exchanger.
>
> I immersed it for around 2.5 hours in Fernox ds-3, keeping it hot and
> flushing it with a jet washer a few times.
>
>
> Radiator water seems quite clear. There was some black flakes in the
> HE but not a substantial amount.
>
> I am wondering if perhaps the diverter valve needs replacing because
> it's not fully switching to the hot water setting but can switch fully
> to the central heating setting.
That would not cause the symptom you are seeing. Basically when the
primary water is diverted through the PHE its not shedding heat fast
enough, and hence the primary gets hot enough for the boiler to cycle
off on its internal stat. If you were bleeding water into the CH, that
would consume more heat from the primary water rather than less.
> I will swap the flow switches tomorrow to see if it makes any
> difference.
I would try circulating descaler through the DHW side of the PHE rather
than just soaking it.
dog-man
Steve.....
A.Lee
> I suppose I could clean the primary one as well, but I don't
> understand how that could affect the hot water side of things and not
> the heating side.
It does affect the heating side, but you cannot tell, unless you have
measured the temperature of the rads every few months at the same
settings.
The heating side is rarely put to full output, whearas the water output
is nearly always at full output when on.
Also, the radiators are usually on for a long time, so have longer to
get up to a reasonable temperature. If this is 10 degrees below the
maximum that was once available, you wouldnt notice, as the boiler
performance has been gradually declining over the years.
Again, it may be that, or may be a faulty divertor, or possibly pump.
dog-man
No evidence of any dirt etc coming out of it.
I also cleaned the primary HE and that was filthy. Loads of black
flakes and what appeared to be rust came out of that. I have a
generous amount of inhibitor in the system so how the hell could it
get that dirty?
Gave it a good clean with Fernox ds-s and then used a jet washer on
it.
All back together and running but so far it doesn't seem to have made
any difference to my hot water then cold water and so on problem.
I am tempted to try a new 3 way valve diverter.
Steve..........
dog-man
the central heating side has become even hotter after cleaning the
primary heat exchanger.
I ran the hot tap with all radiators cold and there was no heat
getting to the central heating flow pipe so I guess that rules out the
3 way diverter valve as being at fault?
So what next apart from taking a sledge hammer to it and replacing it
with a new boiler?
Steve...
John Rumm
I am not familiar with that boiler, but does it have a DHW flow
temperature sensor? Some modern combis have a probe that senses the
outgoing water temperature and use it to control modulation etc. If
yours has something similar then it could be its giving a false reading
and telling the boiler PCB that the DHW is hotter than it really is.
Also is there a DHW temperature adjustment? Is that set high enough? and
again, is it working correctly?
dog-man
The temperature knob is set at full. I have tried it at several
positions and it always makes a difference so is working.
There is a DHW and Heating thermistor. Perhaps I should try swapping
them over as they are identical.
Steve..........
John Rumm
> On 6 Dec, 12:16, John Rumm<see.my.signat...@nowhere.null> wrote:
>> On 06/12/2010 11:36, dog-man wrote:
>>> So what next apart from taking a sledge hammer to it and replacing it
>>> with a new boiler?
>>
>> I am not familiar with that boiler, but does it have a DHW flow
>> temperature sensor? Some modern combis have a probe that senses the
>> outgoing water temperature and use it to control modulation etc. If
>> yours has something similar then it could be its giving a false reading
>> and telling the boiler PCB that the DHW is hotter than it really is.
>>
>> Also is there a DHW temperature adjustment? Is that set high enough? and
>> again, is it working correctly?
> The temperature knob is set at full. I have tried it at several
> positions and it always makes a difference so is working.
> There is a DHW and Heating thermistor. Perhaps I should try swapping
> them over as they are identical.
Does the service manual give a range of expected resistances? A quick
check with a multimeter would probably tell you want you need to know.
Still if easy to swap, then that sounds like a worthwhile test.
(I had a combi once that developed a fault on the DHW sensor. This was
one where it "tempered" a couple of litres of water which it held
internally to give "instant" how water. Alas the heating of this on
switch on took priority over other boiler functions. So the failure of
the probe also caused the software to lockout the whole boiler at
startup, because it could not sense its way though the initial heating
of the water. I had to fool it with a fixed resistor while a new probe
was on order!
dog-man
I unplugged the connector from the DHW thermistor and I was surprised
to see it had no effect whatsoever.
I was expecting the boiler to refuse to supply hot water, but it did
and it behaved as it usually does.
water gets hot then cold then hot and so on.
Steve...........
John Rumm
If its doing the same with it not even in circuit, it might indicate
that its gone open circuit. Could you measure the resistance across the
unplugged end?
dog-man
one and it's looking like that was the fault.
Hot water seems much better at the moment. Will know for sure when I
run a bath later.
Thanks to all for suggestions.
Steve...........