Boiler output for CH vs HW

[DP]

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.

Is your cylinder unvented? Are you sure?
What is the make and model
An unvented cylinder would in almost every case have a 2 port motorised valve to stop flow to the cylinder when desired temperature has been reached.
A midposition valve can under certain conditions make a false demand on the boiler
A midposition valve would not result in almost steep temperature rise unless the valve is open to radiator port and all radiators are closed.

Easy enough to find out why you are having issues by carrying out on site temperature distribution check

Your boiler is set to 65 and cylinder to 60. Try raiding the boiler temperature to 70 or 75. My money is on the cylinder not reaching desired thermostat set temperature so boiler keep firing, very little heat lost to load and boiler shutting down.

 

[Bargain Bucket]

Is your cylinder unvented? Are you sure?
What is the make and model
An unvented cylinder would in almost every case have a 2 port motorised valve to stop flow to the cylinder when desired temperature has been reached.
A midposition valve can under certain conditions make a false demand on the boiler
A midposition valve would not result in almost steep temperature rise unless the valve is open to radiator port and all radiators are closed.

Easy enough to find out why you are having issues by carrying out on site temperature distribution check

Your boiler is set to 65 and cylinder to 60. Try raiding the boiler temperature to 70 or 75. My money is on the cylinder not reaching desired thermostat set temperature so boiler keep firing, very little heat lost to load and boiler shutting down.

To confirm: it is a VENTED cylinder (RM 86775). I mistyped in my initial post. I have tried with the boiler at 80. The problem still happens, albeit at a higher temperature. I think you are spot on with your thoughts on the cylinder not reaching the desired cylinder stat temp and little heat lost being lost to the load. I just wonder how to deal with it. If only this stupid boiler wouldn't fire at full power initially and would try to modulate up (or would start to modulate down sooner rather than run flat out for a minute or 2 before modulating down). Do newer boilers have a better modulation algorithm or do they all fire on full power for a minute or 2?

 

[DP]

To confirm: it is a VENTED cylinder (RM 86775). I mistyped in my initial post. I have tried with the boiler at 80. The problem still happens, albeit at a higher temperature. I think you are spot on with your thoughts on the cylinder not reaching the desired cylinder stat temp and little heat lost being lost to the load. I just wonder how to deal with it. If only this stupid boiler wouldn't fire at full power initially and would try to modulate up (or would start to modulate down sooner rather than run flat out for a minute or 2 before modulating down). Do newer boilers have a better modulation algorithm or do they all fire on full power for a minute or 2?

What is the boiler make and model?

 

[Bargain Bucket]

Its a Glow worm Ultrcom 24 hxi

 

[DP]

Its a Glow worm Ultrcom 24 hxi

When the cylinder is cold, is your boiler cycling then?

Visualise the output water that will have cooled down sufficiently post burner shutdown to say 58 degrees ( you pick a number if you want). Remember the circulating water will be at the temperature of the water in te cylinder.
Boiler fires up. Very little power will be needed to raise boiler temperature from 58 degrees to 65.

On your boiler the fan runs at low speed, ignition takes place, fan speed maxes then spools down to maintain stable output. Know of one boiler that fires at 75% output and then will go to full power 15 minutes later if load not satisfied.

Try raising the boiler temperature to 75 degrees or cylinder temperature dropped to 55 degrees. 55 degrees will be where the thermostat is but hotter higher up the cylinder

 

[fixitflav]

OP - I'm still not clear why you think there's a problem. Is the boiler noisy or showing other signs of distress? What's the history - has the system worked differently (better) in the past, or have you just moved in, or changed something?

I think you are spot on with your thoughts on the cylinder not reaching the desired cylinder stat temp and little heat lost being lost to the load.

DP has argued all along that the heat input to the cylinder is much higher than your (or my) suggested 3 kW. IMO it's irrelevant, the boiler should still cycle without causing a problem (unless you identify the problem) and in any case the higher the input to the cylinder, the lower the need for the boiler to go into cycling mode.

 

[fixitflav]

What valve would you expect to find or not find on a system fitted with an UV Cylinder

Well, what did you expect? We know now it's on Y-plan, so it has a 3-port mid-position valve, but we didn't know that when I posted. It could have been two 2-port valves, or possibly something else.

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

On the contrary, from what we know now (vented cylinder, 3-port valve), it's quite similar to what I have. Apart from sizes, which clearly won't be the same, the only difference is mine is W-plan.

 

[DP]

and in any case the higher the input to the cylinder, the lower the need for the boiler to go into cycling mode.

This one takes the biscuit. Heard it all now

 

[fixitflav]

This one takes the biscuit. Heard it all now

You may have heard, you clearly haven't understood

 

[DP]

Coming from a dabbler who changes his tune like the weather that is rich indeed. Carry on confusing the posters who seek help from these that can provide it as you clearly are lacking in that field.

You clearly have lost your marbles trying to compare W and Y Plans as similar with little difference. Just because both are three port valves means squat.

 

[fixitflav]

Coming from a dabbler who changes his tune like the weather that is rich indeed.

Can you point out where I have done that?

You clearly have lost your marbles

When you resort to insults it's a clear sign you have no confidence in what you're saying.

You clearly have lost your marbles trying to compare W and Y Plans as similar with little difference. Just because both are three port valves means squat.

You're clutching at straws there. The only difference between W- and Y-plan is that Y has a midposition valve, so can have HW and CH on at the same time, whereas W-plan is either/or, with HW preference. That's a small difference, and almost certainly has no bearing on the OP's concern, which is what happens when the heat demand is much lower than boiler output, either when cylinder demand is low (as he suggested) or when TRVs have closed and there is some bypass (which he didn't). Either of those scenarios is possible with W- or Y-plan.

and in any case the higher the input to the cylinder, the lower the need for the boiler to go into cycling mode.

To go back to this - if the cylinder demand is high, say similar to CH demand, in HW only mode the boiler is less likely to stop on control-stat in HW only (on/off type) or to modulate (that type). Surely that is obvious.

Anyway, the OP has not answered any of my questions, which could help diagnose the problem (if any), so unless he does I'm out of here.

 

[Bargain Bucket]

OP - I'm still not clear why you think there's a problem. Is the boiler noisy or showing other signs of distress? What's the history - has the system worked differently (better) in the past, or have you just moved in, or changed something?


DP has argued all along that the heat input to the cylinder is much higher than your (or my) suggested 3 kW. IMO it's irrelevant, the boiler should still cycle without causing a problem (unless you identify the problem) and in any case the higher the input to the cylinder, the lower the need for the boiler to go into cycling mode.

Sorry - missed your questions. The boiler is not noisy (ticks a little bit but no bangs, kettling, gurgling, etc). Up until 3 years ago, I'd never paid it any attention so I'm unsure if the problem has been here for all 7 years we've lived at this address or if it's something that has started more recently (but at least 3 years ago when I first became aware of this problem). 3.5 years ago, we had the cylinder relocated from an upstairs airing cupboard in the bathroom, to an area under the stairs. I became aware of this problem the following winter. As I said, the problem may have been happening before the cylinder relocation. And to re-iterate what I perceive to be a problem, with HW or HW+CH (but never with CH only), the boiler will cycle constantly once the water in the cylinder starts to get fairly warm (but not warm enough for the cylinder stat to be satisfied and remove the demand signal). The boiler will happily run at 65 (or whatever else I set it to e.g. 80) but at some point will overshoot by 2 degrees. Once it's done that, it will shut down and run the external pump for a few minutes, fire up again, overshoot the boiler setpoint by 2 degrees within 30 seconds, shut down and keep repeating this cycle until the end of the timed period as controlled by the timer. In other words, it heats the water in a series of small bursts. This cannot be efficient and cannot be good for the boiler. I would have hoped that the boiler would either modulate down much sooner after fire-up or would start low and modulate up.

 

[fixitflav]

with HW or HW+CH (but never with CH only), the boiler will cycle constantly

I still think that's odd. It sounds like when calling for HW+CH, it actually goes into HW-only mode. Perhaps a problem with the valve, or maybe the Hive control system. Issues of wiring with Hive and similar come up regularly on here. Fixing it wouldn't solve the main problem, but at least it would happen less often, ie just when HW-only is calling.

the boiler will happily run at 65 (or whatever else I set it to e.g. 80) but at some point will overshoot by 2 degrees. Once it's done that, it will shut down and run the external pump for a few minutes,

Curious! It sounds like a problem with the boiler modulation, since if modulating properly it should maintain flow temperature, and only rise to shutdown point at very low demand, and even then only re-fire after a reasonable differential (like my on/off boiler). According to the manual, the 24-hxi should modulate to minimum output 5.25kW, which seems pretty good. Maybe worth getting Glowworm to look at it. Could be a problem with the electronics.
Are you sure the pump stops after a few minutes? It shouldn't, it should continue to run while the cylinder stat is calling. That might extend the time it takes to re-fire, by holding the temperature up a bit longer.

I would have hoped that the boiler would either modulate down much sooner after fire-up or would start low and modulate up.

So would I.

I don't know whether that's any use, but maybe food for thought.

 

[DP]

Sorry - missed your questions. The boiler is not noisy (ticks a little bit but no bangs, kettling, gurgling, etc). Up until 3 years ago, I'd never paid it any attention so I'm unsure if the problem has been here for all 7 years we've lived at this address or if it's something that has started more recently (but at least 3 years ago when I first became aware of this problem). 3.5 years ago, we had the cylinder relocated from an upstairs airing cupboard in the bathroom, to an area under the stairs. I became aware of this problem the following winter. As I said, the problem may have been happening before the cylinder relocation. And to re-iterate what I perceive to be a problem, with HW or HW+CH (but never with CH only), the boiler will cycle constantly once the water in the cylinder starts to get fairly warm (but not warm enough for the cylinder stat to be satisfied and remove the demand signal). The boiler will happily run at 65 (or whatever else I set it to e.g. 80) but at some point will overshoot by 2 degrees. Once it's done that, it will shut down and run the external pump for a few minutes, fire up again, overshoot the boiler setpoint by 2 degrees within 30 seconds, shut down and keep repeating this cycle until the end of the timed period as controlled by the timer. In other words, it heats the water in a series of small bursts. This cannot be efficient and cannot be good for the boiler. I would have hoped that the boiler would either modulate down much sooner after fire-up or would start low and modulate up.

Two pipes that go to your cylinder, touch the pipes when HW or HW+CH is on. Is there temperature differential across these two pipes.

Come back with your finding. Rather than take a shot in the dark, Will approach your system problem with proper diagnoses.

Another check you could carry out is to check temperature differential across two 22 mm pipes and ascertain if the differential is the same across cylinder connection to the indirect coil.

If you have clamp thermometers, even better, take temperature readings.

 

[fixitflav]

Sorry - missed your questions. The boiler is not noisy (ticks a little bit but no bangs, kettling, gurgling, etc). Up until 3 years ago, I'd never paid it any attention so I'm unsure if the problem has been here for all 7 years we've lived at this address or if it's something that has started more recently (but at least 3 years ago when I first became aware of this problem). 3.5 years ago, we had the cylinder relocated from an upstairs airing cupboard in the bathroom, to an area under the stairs. I became aware of this problem the following winter. As I said, the problem may have been happening before the cylinder relocation. And to re-iterate what I perceive to be a problem, with HW or HW+CH (but never with CH only), the boiler will cycle constantly once the water in the cylinder starts to get fairly warm (but not warm enough for the cylinder stat to be satisfied and remove the demand signal). The boiler will happily run at 65 (or whatever else I set it to e.g. 80) but at some point will overshoot by 2 degrees. Once it's done that, it will shut down and run the external pump for a few minutes, fire up again, overshoot the boiler setpoint by 2 degrees within 30 seconds, shut down and keep repeating this cycle until the end of the timed period as controlled by the timer. In other words, it heats the water in a series of small bursts. This cannot be efficient and cannot be good for the boiler. I would have hoped that the boiler would either modulate down much sooner after fire-up or would start low and modulate up.

Don't know whether you've made any progress on this, but from the symptoms I wonder if it might be one of the thermistors faulty. Glowworm might be able to comment. I tried to attach the installation manual, showing the thermistors. It keeps failing, but you can download it.