Boiler output for CH vs HW

[jackthom]

Thanks for all the feedback and input.

My base problem is that if the HW is being heated, the boiler (Glow worm 24 hxi) overshoots its temperature setpoint and goes into a repeating cycling mode. Fires up at full power and overshoots before it has a chance to go into modulating mode. It doesn't do this if ONLY CH is on but does still do it if both HW and CH are in play. HW tank is about 4 years old and is located very close to the boiler. This has being going on for years but with incoming gas price rises, I'm looking to get it running as efficiently as possible. I've downrated the boiler down to 15kw but that hasn't made any perceivable difference to the problem.

I suspect the issue is that the modulation capabilities of the boiler is the issue. It's insistence at running at full power for the first minute or so of firing up rather than modulating sooner means it gets into this repeated cycling mode.

Try to find out exactly which cylinder you have. Post a few pictures if possible.

A modern rapid recovery cylinder would almost certainly solve this problem.

 

[fixitflav]

My base problem is that if the HW is being heated, the boiler (Glow worm 24 hxi) overshoots its temperature setpoint and goes into a repeating cycling mode.

Why is that a problem?

Fires up at full power and overshoots before it has a chance to go into modulating mode. It doesn't do this if ONLY CH is on but does still do it if both HW and CH are in play.

That's interesting, as when CH and HW are both calling the demand is higher than CH only. Are you sure they are both calling? If you have weather compensation it's either/or, usually with HW preference, as the reduced boiler flow temperature could be too low to heat the cylinder.

 

[DP]

I don't know where you're coming from here. I was just trying to explain to the OP how the boiler operates if the demand is much lower than the boiler rated output.

Clearly you are advising from a platform that is lacking on knowledge. What makes you think the boiler output is going to be greater than what the cylinder will consume. Look at what the poster is asking. It no way mirrors what you have

Bypass is there to allow pump to circulate water when the MV have shut down or someone is foolish enough to put TRVs on every radiator. When water is passing through the ABV, the burner will not be on

 

[fixitflav]

What makes you think the boiler output is going to be greater than what the cylinder will consume.

I'd have thought it was obvious. The boiler wouldn't be cycling otherwise.

Look at what the poster is asking. It no way mirrors what you have

I have, and I've tried to give helpful comments.

Bypass is there to allow pump to circulate water when the MV have shut down or someone is foolish enough to put TRVs on every radiator

Yes, I know, what has that to do with the discussion?

BTW it's kWh, not kw/h.

 

[DP]

Thanks for all the feedback and input.

My base problem is that if the HW is being heated, the boiler (Glow worm 24 hxi) overshoots its temperature setpoint and goes into a repeating cycling mode. Fires up at full power and overshoots before it has a chance to go into modulating mode. It doesn't do this if ONLY CH is on but does still do it if both HW and CH are in play. HW tank is about 4 years old and is located very close to the boiler. This has being going on for years but with incoming gas price rises, I'm looking to get it running as efficiently as possible. I've downrated the boiler down to 15kw but that hasn't made any perceivable difference to the problem.

I suspect the issue is that the modulation capabilities of the boiler is the issue. It's insistence at running at full power for the first minute or so of firing up rather than modulating sooner means it gets into this repeated cycling mode.

I suspect the issue is elsewhere.
Your boiler Will I am sure have a modulating burner
More information is required to locate the cause
May well be a motorised valve failure.

 

[DP]

d have thought it was obvious. The boiler wouldn't be cycling otherwise.

Yes it would if the demand is there but the valves are closed

I have, and I've tried to give helpful comments.

You fail to understand what is happening

 

[DP]

My base problem is that if the HW is being heated, the boiler (Glow worm 24 hxi) overshoots its temperature setpoint and goes into a repeating cycling mode. Fires up at full power and overshoots before it has a chance to go into modulating mode

Check the motorised valve is open for cylinder zone
If the valve is closed as cylinder has reached temperature but the demand is still in place from the valve, boiler will fire at 15Kw, heat output not going anywhere other than through the ABW, rapid temperature rise and cycling takes place

 

[BlueLoo]

Clearly you are advising from a platform that is lacking on knowledge. What makes you think the boiler output is going to be greater than what the cylinder will consume. Look at what the poster is asking. It no way mirrors what you have

Bypass is there to allow pump to circulate water when the MV have shut down or someone is foolish enough to put TRVs on every radiator. When water is passing through the ABV, the burner will not be on

Why is putting trvs on every rad foolish?

If you are going to be the judge of competency and knowledge, you at least have to have those qualities yourself.

 

[BlueLoo]

Thanks for all the feedback and input.

My base problem is that if the HW is being heated, the boiler (Glow worm 24 hxi) overshoots its temperature setpoint and goes into a repeating cycling mode. Fires up at full power and overshoots before it has a chance to go into modulating mode. It doesn't do this if ONLY CH is on but does still do it if both HW and CH are in play. HW tank is about 4 years old and is located very close to the boiler. This has being going on for years but with incoming gas price rises, I'm looking to get it running as efficiently as possible. I've downrated the boiler down to 15kw but that hasn't made any perceivable difference to the problem.

I suspect the issue is that the modulation capabilities of the boiler is the issue. It's insistence at running at full power for the first minute or so of firing up rather than modulating sooner means it gets into this repeated cycling mode.

Glancing at the IOM,
Glow worm repeatedly state that the boiler controls are basic and any HW control is only possible via a upgraded control module.

Now, i wonder if your boiler is set up like a std non condensing boiler with the CH out set at 75+c and the return 11c less? The HW following suit because it has no option to do otherwise?

Whereas the CH should be set for a return temp in the condensing zone and the HW set to +60c?

I think this needs either boiler controls added and/or external controls such as a X-plan layout.

One of the better plumbers in here noted that he sees boilers installed in a non-condensing format all the time.

Might be worth looking at. If your Boiler is never in a condensing mode because a clueless plumber is clueless then you will be wasting gas.

 

[DP]

Why is putting trvs on every rad foolish?

If you are going to be the judge of competency and knowledge, you at least have to have those qualities yourself.

TRVs is the item I mentioned, not smart TRVs
Imagine all the TRVs close, boiler demand still there from a programmer
Boiler fires, heat goes no where, rapid temperature rise, boiler cycles until the timer goes off.

One radiator is NOT TRVed, boiler fires, boiler Ramps up to max power, but as there is flow through a load, output temperature is monitored and corrected by closed loop circuit which needs a slight lag so boiler output will soon be at minimum and if that is too much, burner shuts down and boiler cycles.

If you are going to be the judge of competency and knowledge, you at least have to have those qualities yourself.

I am no judge of competency but base my knowledge on working on domestic systems everyday. Does That make me a voice to be heard? Certainly not. But I feel a poster asking for guidance needs good advice that a diyer may or may not have ( evidenced by nonsense that has been written about indirect coils rated at 3kw and furring taking place- a symptom I do not see in Glasgow). Clearly point I was making when I said not to TRV all the radiators a direct comparison to an S plan with faulty valve that used closed but demand exists a thing I find common in the day to day task

If you knew why all radiators should not be TRVed, you would not be asking. This is not my utterance but industry standard.

 

[BlueLoo]

TRVs is the item I mentioned, not smart TRVs
Imagine all the TRVs close, boiler demand still there from a programmer
Boiler fires, heat goes no where, rapid temperature rise, boiler cycles until the timer goes off.

One radiator is NOT TRVed, boiler fires, boiler Ramps up to max power, but as there is flow through a load, output temperature is monitored and corrected by closed loop circuit which needs a slight lag so boiler output will soon be at minimum and if that is too much, burner shuts down and boiler cycles.



I am no judge of competency but base my knowledge on working on domestic systems everyday. Does That make me a voice to be heard? Certainly not. But I feel a poster asking for guidance needs good advice that a diyer may or may not have ( evidenced by nonsense that has been written about indirect coils rated at 3kw and furring taking place- a symptom I do not see in Glasgow). Clearly point I was making when I said not to TRV all the radiators a direct comparison to an S plan with faulty valve that used closed but demand exists a thing I find common in the day to day task

If you knew why all radiators should not be TRVed, you would not be asking. This is not my utterance but industry standard.

My point is simple and has been said to others.
Don't be in such a rush to criticize posters. Sometimes the topics are complex and obtuse and criticism may be mislaid.

 

[fixitflav]

Yes it would if the demand is there but the valves are closed

It's possible the HW valve has closed but the power the boiler is still there, due to a faulty microswitch, but we're a long way from concluding that. We don't know what controls he has, hence what valves. And if that were the case, the boiler would be on (though cycling) 24/7, which it would be worth the OP mentioning.

 

[DP]

Sometimes the topics are complex

Granted. Us breakdown engineers look at all manner of boiler malfunctions. As such often the information given by a client ( posters on the forum) is gleaned for salient information plus experience of previous encounters and then advice given.

Have you noticed some regulars have stopped offering advice?

 

[DP]

We don't know what controls he has, hence what valves.

Really? On an unvented cylinder? What valve would you expect to find or not find on a system fitted with an UV Cylinder

Would you even admit you could be wrong?

 

[Bargain Bucket]

Just to clarify, I have a Y plan system controlled by Hive. Room thermostat for CH and tank thermostat for HW. Separate time programmes for CH and HW. No outside compensation. Boiler is set at 65 degrees. HW tank set for 60 degrees.