Boiler output for CH vs HW

[Bargain Bucket]

For a standard boiler providing heat for an unpressurised 2 pipe radiator system and for an unvented indirect hot water cylinder : if you need 18kw for the heating and 3 kW for the hot water, how on earth is the boiler supposed to deal with such a massive difference in required heat output? In theory I guess it should modulate down for the HW but my boiler seems to overshoot before it has a chance to modulate down.

 

[flameport]

A modern unvented cylinder isn't 3kW of input from the boiler. It would take several hours to heat up if it was.
More likely to be 10-20kW depending on size.

Electric immersion heaters are normally 3kW, but that's a limitation of the electrical supply, and they do take a few hours to heat a cylinder.

 

[Bargain Bucket]

Sorry. My bad on the cylinder. I meant a vented indirect cylinder.

Anyway, the cylinder requires far less in terms of kW than the CH system does. So how does a boiler rated at, say, 18kw deal with a load that requires far less than the 18kw output? I guess by modulation. So what modulation ratio do I need to look for in a new boiler? My current boiler cycles constantly when heating the HW cylinder.

Thanks.

 

[fixitflav]

Sorry. My bad on the cylinder. I meant a vented indirect cylinder.

Anyway, the cylinder requires far less in terms of kW than the CH system does. So how does a boiler rated at, say, 18kw deal with a load that requires far less than the 18kw output? I guess by modulation. So what modulation ratio do I need to look for in a new boiler? My current boiler cycles constantly when heating the HW cylinder.

Thanks.

Modulation may or may not be an advantage, but it makes no difference to its ability to cope with low load, as in HW mode. If the boiler is not modulating it goes on/off on its control-stat.
That's just how mine works, non-modulating 15kW boiler, estimated demand on HW close to your 3kW, I expect the coil is well furred up. It cycles on/off 3-4 times before the cylinder stat is satisfied. In CH mode it never reaches control-stat setting.
Edit - just noticed, I should have added that you said it in the last sentence of your post.

 

[DP]

Sorry. My bad on the cylinder. I meant a vented indirect cylinder.

Even these require high input to be able to reach workable hot water delivery in a very short period. Just because it is not a sealed cylinder does not mean the indirect coil is limited to 3kw

 

[fixitflav]

Even these require high input to be able to reach workable hot water delivery in a very short period. Just because it is not a sealed cylinder does not mean the indirect coil is limited to 3kw

Whatever the figure is, the OP just wants to know how the boiler will handle it (and I've told him what I think). In any case 3 kW is the typical immersion heater rating, which is enough for normal HW usage.

 

[DP]

Whatever the figure is, the OP just wants to know how the boiler will handle it (and I've told him what I think). In any case 3 kW is the typical immersion heater rating, which is enough for normal HW usage.

What has 3kw immersed got to do with indirect coil in a hot water cylinder
And pray explain how you know your 15Kw boiler only outputs 3kw to the cylinder. How did you come up with that nugget when your boiler is non modulating boiler

 

[fixitflav]

What has 3kw immersed got to do with indirect coil in a hot water cylinder
And pray explain how you know your 15Kw boiler only outputs 3kw to the cylinder. How did you come up with that nugget when your boiler is non modulating boiler

3 kW immersion indicates that 3kW transfer from the coil is likely to be OK.
See my #04 where I explain how average 3 kW is transferred, it's quite simple.

 

[DP]

3 kW immersion indicates that 3kW transfer from the coil is likely to be OK.
See my #04 where I explain how average 3 kW is transferred, it's quite simple.

If that was the case why may cylinder manufacture say water in cylinder will be hot in say x minutes but immersion heater is only 3 kw.

Just because your cylinder is not making proper use of boiler output does not mean that is the norm, neither is furring of the coil across length and breadth of the country.


Look at the coil spec in the picture below

 

[DP]

117 litre cylinder heated to raise the temperature by 50 degrees

=6.8 kw/h

just because your boiler is cycling does not mean the coil is rated at same input as the immersion heater
Boiler switching off for three to four minutes means boiler is letting the cylinder catch up.

BTW, 6.8 kw/h is the energy required to heat 117 litres of water by 50 degrees C. 6.8 kW consumed for 1 hour.

The load, ie the indirect coil, could well be say rated at 18 kw. Boiler initially will run at full power until the secondary water temperature starts to rise. Same scenario where a big pot of water will take all the energy from a burner, even a BIG one, but to maintain temperature at target point, burner can be at minimum flame.

 

[JohnD]

Sorry. My bad on the cylinder. I meant a vented indirect cylinder.

What colour is your cylinder?

 

[JohnD]

my boiler seems to overshoot before it has a chance to modulate down.

What model of boiler have you got?

Is it fully pumped?

 

[DP]

For a standard boiler providing heat for an unpressurised 2 pipe radiator system and for an unvented indirect hot water cylinder : if you need 18kw for the heating and 3 kW for the hot water, how on earth is the boiler supposed to deal with such a massive difference in required heat output? In theory I guess it should modulate down for the HW but my boiler seems to overshoot before it has a chance to modulate down.

Forget the 3kw figure. That is for the immersion heater
Then forget about the vented boiler and unvented cylinder ‘compatibility’ as unvented cylinder is unvented on hot water side and not the coil that is connected to the boiler.
If your cylinder coil is rated say 20 kw, run the boiler to heat radiators and cylinder at different times so boiler is not like a candle in a big room

 

[Bargain Bucket]

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.

 

[fixitflav]

If that was the case why may cylinder manufacture say water in cylinder will be hot in say x minutes but immersion heater is only 3 kw.

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. The actual figure for cylinder input is irrelevant. There could be a situation where all the water goes through a bypass valve, so the heat loss is close to zero, and the boiler has no problem, it still cycles on/off (even with a modulating boiler, as demand likely to be below minimum turndown).
I estimated 3 kW for my cylinder from the boiler cycle times, but I mentioned a 3 kW immersion heater to point out that this gives enough HW for most households.

Just because your cylinder is not making proper use of boiler output does not mean that is the norm, neither is furring of the coil across length and breadth of the country.

I quite agree, and if it caused me a problem I would do something about it, but it doesn't. No doubt mine had higher input when the cylinder was new, probably the figures on your chart will drop off when they're a few years old.