Worcester 240 DHW more warm than hot

[ajph]

I have an ageing Worcester 240 RSF boiler that has given good service for the three years that I have had it. I'm keen to hang on to it as general opinion seems to be that they are very good boilers, but it doesn't seem to be capable of heating the same flow rate of water now as it did when we moved in to the house. The central heating doesn't seem to be affected.

The loss of DHW heating capacity seemed to come about after I drained the CH circuit to be able to remove a radiator. This was over the summer, and the circuit stayed drained for four days. Initially I tried to run some hot water in this state as I wrongly assumed that there was an independent heat exchanger for the DHW circuit. This resulted in the boiler making some strange noises (I think this was probably the pump running dry) and I realised my mistake. I have since read the boiler manual, as well as posts on this list and feel a lot better informed!

After refilling the system the boiler seemed to be OK, the pump didn't seem to have been too badly affected by my actions as the CH worked (and continues to work) well. It quickly became obvious though that to achieve the same temperature of hot water a much lower flow rate was required than previously. This has remained the case despite trying a number of remedies, some from this list, and some from a neighbour who is a gas engineer.

I have had the diverter valve replaced, cleaned the CH circuit using DS40, and removed and cleaned the DHW heat exchanger. Both cleaning attempts produced satisfying quantities of mucky water, but none of these things have noticeably improved the hot water output. There is a good flow of water through the hot water taps, and when there is DHW demand the boiler burns continuously, ie it's not overheating in the primary loop.

In my reasoning, this all seems to suggest that the boiler isn't producing the same level of heat output as it used to. I know that for DHW the burner is supposed to automatically adjust its output to achieve a preset water temperature. I am wondering if this isn't happening now for some reason. Am I barking up the wrong tree completely? Is this simply a symptom of an old boiler? I would appreciate any advice or suggestions as to what to do next.

Thanks very much,

Andy

 

[Agile]

A nicely witten description of what you have done.

Unfortunately nothing you have said gives any clue to your problem!

You need to test the actual hot water output by measuring the in and out temperature when the flow is set to 9 litres per minute when the temperature rise should be 35 degrees.

If it really is low then a gas reg engineer needs to check the combustion settings and that should identify the problem.

Measuring the flow and return temperatures on DHW delivery will also give useful information in case the pump is weak.

All the temperature measurements need a contact electronic test meter. IR meters are not much use.

Tony

 

[ajph]

Thanks for that Tony, I'll have a word with the neighbour to see whether he has a contact test meter and see what I can find out.

 

[ianmcd]

First thing to check is the actual flow rate of the DHW through your boiler this is easy to test and you dont actually need any specialist equipment although they do exist, find a container that you know the volume of and open a hot tap fully and time how long it takes to fill eg if the container holds 10L and it takes 1 min to fill it with tap full on then flow rate is 10L/min

 

[ajph]

Thank again for the sensible suggestions. I've not been around much for the past week, but I've just now had a look at the DHW temperature:

Max flow rate is somewhere between 10 & 11 l/min

When running the DHW @ 9 l/min I measured the following:

Cold water supply to boiler: 10°C
Hot water return from boiler: 32°C

So the temperature rise is 22°C @ 9 l/min. This was measured with a cheap contact thermometer, but even taking assumed inaccuracies due to that into account, the temp rise is a lot less than the 35°C that you quoted Tony.

I guess then it's worth getting a professional gas engineer in to have a look at it?

Thanks, Andy

 

[Gasguru]

Open the hot taps fully.

Gas rate the boiler see the FAQs at the top.

Note if the flow pipe to the rads get hot.

Post back the result.

 

[Agile]

As long as the meter is using a thermocouple they are surprisingly accurate!

Most inaccuracies are cancelled out for these type of measurement because if you measure in/out water of 10 C and 32C and they are actually say 10.5 C and 32.5 C then the rise is still correctly measured.

You now have something which can measure the flow and return temps during DHW delivery. They may give us some clues. As will the gas rate measurement.

Tony

 

[ajph]

Ok, I measured the gas consumed by the boiler whilst running DHW with the tap fully open by photographing the meter twice with my phone over a few minutes. The result:

Over 220 sec - 0.087m3 of gas

This equates to 15.3kW if I've read the gas rating post correctly.

The outbound central heating pipe from the boiler warms up slightly when the DHW is left on, but only to lukewarm close to the boiler, by the time it reaches the floor (about 50cm) the pipe temp drops off considerably.
I have noticed this before and thought that it was probably conduction rather than a faulty diverter valve, although slightly puzzlingly the return CH pipe is always much cooler, which it shouldn't be if the same amount of heat was being conducted down it?

Anyway, is it fair to say that the boiler rating is considerably less than the listed max power of 24kW which would be expected when the DHW demand was as high as possible? What is the likely cause of this?

Thanks, Andy

 

[ajph]

Almost a year has passed, and I've finally got to the bottom of the problem. I asked my gas engineer neighbour to test the burner pressure when there was DHW demand and it stayed at the lowest value regardless of the water flow rate. It transpired that the Modureg valve had become stuck and wasn't opening up. Now that it's freed again, all's back to normal