Hot water at half temperature. What's wrong?

[stakhanov]

I have a Chaffoteaux et Maury Britony Combi 80 and the CH part works fine. Over the years the DHW has got cooler and cooler (i have noticed this because i have had to gradually turn the thermostatic mixer shower up to near maximum over time.) I imagine this has something to do with the secondary heat exchanger. Last week, however, my hot water went down to about half its normal heat.

I took the secondary heat exchanger off and descaled and flushed. I put it back on and i think it helped a little but it's still not as it should be. Do you think i need to replace the secondary heat exchanger altogether or is there some other part that might cause this sort of problem?

As i understand it, if there was a problem with the DHW thermistor or the control PCB, then the reduction in heat would be as a result of a reduction in heat in the primary circuit. But this seems to be fine. The flow coming out of the DHW heat exchanger is also fine so i don't think it's blocked or anything. This only leaves the secondary (DHW) heat exchanger unless someone can suggest something else to try before i waste £100 on an unneccesary part.

 

[chrishutt]

Several possibilities, but you need to work more methodically through the possibilities, using a manual (preferably the manual for the Baxi Genesis!). If you're not up to this, you need to find a competent boiler engineer with experience of this make.

 

[stakhanov]

I have the manual for this boiler. I have followed the flow chart, but my problem doesn't lie clearly within any of the faults in the flow chart.

I have thought it through methodically. There is nothing wrong with the primary heating circuit as the CH works fine. There is nothing wrong with the flow so there is no blockage or 3-way valve sticking problem. The water is warm so the secondary heat exchanger is working at least partially. The boiler is pretty simple so the only other things other than the secondary heat exchanger that have anything to do with the DHW supply are the DHW thermistor and the control PCB. This is where my lack of knowledge with boilers causes me problems. From what i have read, the thermistor regulates the DHW temperature by reducing/increasing the gas supply to the burner (through the control PCB), thereby reducing/increasing the temperature of the outgoing water. As i said though, the water in the primary circuit is plenty hot enough so it can't be the thermistor telling the PCB to tell the burner to burn less. I can't see how the PCB could affect the temperature of the DHW without reducing the heat in the primary circuit.

That leaves the secondary heat exchanger. I was hoping that someone might either be able to confirm this, or suggest some other faults that could cause it (with an explanation of how it would cause it.)

You say that there are several things that could cause it. Perhaps you could suggest some.

 

[chrishutt]

Perhaps you could suggest some.

Oh ****!

Well, first off the cold spell started a week or so back, so mains water temperatures dropped, so combi hot water temperatures would drop too. that might at least partly explain the "problem", so we need to know what the inlet and outlet temperatures are on DHW and the flow rate in litres/minute, plus whether the burner gas rate is right.

That should shut you up.

 

[stakhanov]

Firstly, you're not really in Cuba are you (or how would you know about our cold weather? BBC world service?)

I had thought about the colder mains water and yes, i think that may well be a part of it. I will be borrowing my brother's temperature probe tomorrow so i will post the relevant info. As for the burner gas rate, i have no way to measure it. But, i would have thought that it's ok as the CH still works ok.

More to come. For now, consider me 'shut up'.

 

[chrishutt]

Well you're not really Stakhanov, are you, or you wouldn't be whingeing about having to have cold showers.

i would have thought that it's ok as the CH still works ok.

Not so. The central heating will run quite happily on a lower gas rate than is required for DHW. So you might find that one of the gas valve solenoids is faulty, which you would only notice on DHW. When you run HW very slowly, can you see the flame changing between high and low? If not, this could be the problem.

 

[stakhanov]

Right then. I have checked the temperatures while i was drawing DHW at about 6 litres a minute, and they are as follows:

(NB. SHE is secondary heat echanger, not my wife)

Output from primary heat exchanger (i.e. before going into SHE): 60 deg
Input to primary heat exchanger (i.e. after leaving SHE): 57 deg
Input to secondary heat exchanger (i.e. cold water feed): 18 deg
Output from secondary heat exchanger (i.e. DHW): 56 deg

Does the output from the secondary heat exhanger seem too low? It feels hot (can't hold pipe for more than a quarter of a second).

I did turn the DHW flow right down and the flame on the burner did alternate between high and low (and off sometimes). Itused to be that when i turned down the flow like this, the water would come out very hot but now it is just cool.

So, any ideas?

 

[scatmanjohn]

Hi...

1,Why 6 litres per min
2,What did you use to measure the temps because they don't add up.

You have a 3 degrees temp drop on the primary side across the "SHE"
but have a 38 degrees temp rise on the dhw side of the "SHE".

18 degrees inlet temp with these weather conditions seems high

The performance for this boiler should be a 35 degrees temp rise 9.5 l/min

You need a flow cup and ideally a good quality electronic contact thermometer

 

[chrishutt]

the temps ........ don't add up.

No reason why they should. The temp drop of 3C on primary side would relate to a much higher flow rate than the 38C rise on secondary side.

In fact it would have to be a flow rate of 38/3 x 6 litres/min = 76 litres/min which sounds a bit high (anyone know what sort of flow rate one should expect on primary side?), so maybe the temperatures aren't that accurate.

Basically a temp rise of 38C at 6 litres/min is equivalent to 24C at 9.54 litres/min, which is barely 2/3 of the 35C rise claimed by manufacturer at that flow rate. So if your temp measurement are accurate, you're losing 1/3 of the heat that you should be getting into your hot water.

Later edit.. You can't check the burner pressure, but you can check the gas rate on the meter. When running flat out on DHW your boiler should have a heat input of 28.7kW which requires a cubic foot of gas in 38 seconds. If your gas meter measures in cubic feet, count how many seconds for dial to make complete revolution and post result.

 

[Agile]

You dont want hot water at over 50*C.

I dont know for sure about your model but most modern boilers have a maximum temperature of about 56*C

try running 9 litres per minute and see id the boiler increases the temperature by 35*C.

Tony Glazier

 

[chrishutt]

Good point Tony, I hadn't thought of that.

 

[scatmanjohn]

scatman wrote:
the temps ........ don't add up.

chrishutt wrote:

No reason why they should....so maybe the temperatures aren't that accurate.

chrishutt... Not quite sure what you were saying..was i correct or just talking from my a*s* hole

still think 18*C cold inlet is high unless he is located on the med

 

[meldrew's_mate]

Stakhanov, your primary water temperature seems a little low at 60 C. What temp does it enter the PHE?

Are you sure the electrically driven diverter valve is travelling fully over on DHW duty? You can check this by measuring the CH flow temp. whilst drawing HW, it should not heat up (easier if you have the heating cold before you start)

If my memory serves me correctly there are only two gas rates on this model, low and high. Generally you should have high rate on DHW duty at 9 l/min and you can hear the solenoids "clunking" in and out on lower loads.

 

[chrishutt]

Scatmanjohn, you're right about 18C being high for inlet temp of cold mains in this weather, but of course there might be a longish run of internal pipework to warm it up.

You're also right about the temps being suspect. It's difficult to get stable temperatures at 4 locations for long enough to take all measurements, especially if flow rate is as low as 6 l/m (yes, you were right about that too) which could lead to DHW temp limit cutting gas rate.

However I thought you were implying that the primary temp drop (3C) and the secondary temp rise (38C) should be equal, which is not so. Apologies if that's not what you meant.

 

[scatmanjohn]

Chrishutt...looking at what i wrote, i agree it was not very clear.

Thought i had lost the plot for a minute then