System boiler modulation with low loss header LLH

[HRP123]

It would be impossible to circulate 8.5LPM through say a 90M loop, it would require a pump with a 17.5M head, you could circulate it through 20 or 25M with a manifold pump running with a 4M head, you will circulate ~ 4LPM through 90M of 16mm (12mm ID) with that 4M head which makes some bit of sense and means ~ 10kw UFH demand but your gas usage shows twice this.
You can "prove" this 20kw usage another way, just keep reducing the boiler max output in say 2kw increments until the boiler flowtemp start falling, this will verify the consumption numbers, or not.

So the open loop had pipe length of approx 220m as opposed to 90m in your calculation above.

It would be impossible to circulate 8.5LPM through say a 90M loop, it would require a pump with a 17.5M head, you could circulate it through 20 or 25M with a manifold pump running with a 4M head, you will circulate ~ 4LPM through 90M of 16mm (12mm ID) with that 4M head which makes some bit of sense and means ~ 10kw UFH demand but your gas usage shows twice this.
You can "prove" this 20kw usage another way, just keep reducing the boiler max output in say 2kw increments until the boiler flowtemp start falling, this will verify the consumption numbers, or not.

I'm trying to experiment with what level of range rating will cause the boiler flow temp when the underfloor circuit is on to start dropping.

I had a look at the existing Grundfos UPS2 25-40/60 130 pump details online - it says a 6m head.

Thanks for all your time and help.

 

[HRP123]

Yes, regarding the UFH heating requirements and boiler output, both will (have to) be the same. With your system if you want to run at a manifold flowtemp of 40C then you would have to reduce the boiler flowtemp to 59C to give the original 27C boiler dT, normally, you would have a separate UFH manifold recirc pump on each UFH system, then the boiler dT can be controlled to whatever you require irrespective of boiler or manifold temperatures but if your system is working OK then why worry as it certainly wont require more energy and is actually more efficient with those very low return temperatures.
See below schematic of a system with a LLH and UFH manifold recirc pump but with a boiler flowtemp of 65C and UFH flowtemp of 40C and (if) a required boiler dT of 20C, the boiler return will now be 45C.

View attachment 82194

Hi John

Just going through this calculation again - can I ask, you mentioned "a required dT at the boiler of 20c" - what determines this required temperature? Is it 20 to promote condensing efficiency? For example in these calculations, if I change it to a dT of 10, it increases the LPM flow rate from the boiler. In my boiler the default setting is 10 dT in the installer menu.

 

[ShaunCorbs]

Best efficiency for a condensing boiler they will run at dt10,15 etc but you loose a bit

 

[HRP123]

Best efficiency for a condensing boiler they will run at dt10,15 etc but you loose a bit

OK - thats what I thought - so if the WB 40cdi is set to a default 10 dT and I change it to 20dT - would this basically mean for example if I was heating just the UFH it would cycle more often. However given the UFH is blended down to a lower flow temp, this can actually improve overall gas consumption? Thanks

 

[John.g]

The amount of cycling in the above case (UFH only) is determind as all ways on the UFH heat demand, if its below the minimum boiler output then the boiler will cycle, the further below the boiler minimum output, the more the cycling.
Don't know how the 20C boiler dT is automatically accomplished, boiler speed control probably.
If your UFH has a 6kw demand with flow/return temps of say 40C/32C then the UFH flowrate is, 6*860/60/8, 10.75LPM and the boiler flowrate (at 20C dT) will be (8/20)*10.75, 4.3LPM, the boiler flowtemp must then be set to, 20C+32C, 52C. However a boiler flowrate of 4.3LPM is unacceptably low and a flow of 8.6LPM (0.53m3/hr) is probably the lowest acceptable then you can't run the boiler with a dT of 20C, it must be kept to 10C but that still gives a very good boiler efficiency with a boiler return of 42C (vs 32C), so boiler flowtemp setting is the important factor here IMO.

 

[HRP123]

The amount of cycling in the above case (UFH only) is determind as all ways on the UFH heat demand, if its below the minimum boiler output then the boiler will cycle, the further below the boiler minimum output, the more the cycling.
Don't know how the 20C boiler dT is automatically accomplished, boiler speed control probably.
If your UFH has a 6kw demand with flow/return temps of say 40C/32C then the UFH flowrate is, 6*860/60/8, 10.75LPM and the boiler flowrate (at 20C dT) will be (8/20)*10.75, 4.3LPM, the boiler flowtemp must then be set to, 20C+32C, 52C. However a boiler flowrate of 4.3LPM is unacceptably low and a flow of 8.6LPM (0.53m3/hr) is probably the lowest acceptable then you can't run the boiler with a dT of 20C, it must be kept to 10C but that still gives a very good boiler efficiency with a boiler return of 42C (vs 32C), so boiler flowtemp setting is the important factor here IMO.

Thanks for this explanation and the gems of heating calculation wisdom as always.

I guess the LLH helps to balance out the issues with the flow rates as well.

I need to set the system up for catering to hot water and heating from the same one boiler flow temp. So I am aiming for a 65 degree flow to comfortably achieve the 55 degrees setpoint for the 2 x 200 litre tanks, I cannot really adjust this lower.

So the boiler is set to a 65 degree flow temp from the boiler - with just the underfloor on and 40/32 flow/return dT8 - lets say the underfloor demand is 6 loops at 1.4kw (I think my loops are longer than average and a mix between 150-200cc). This should be around 9.65LPM from the boiler and should only be demanding around 10kw from the boiler. The problem is my observations so far suggest when the UF first fires up, it takes 20kw. The boiler isn't cycling during this time.

As soon as a second heating circuit comes on (rads or towel rails), the 6-7kw additional demand causes the flow temp to fall. So even though the boiler should be 100% nominal 19-20kw output, at something under that, I already see the flow temp falling to around 55-57 degrees instead of 65 degrees.

 

[John.g]

From a cold start up and assuming a UFH system contents of say 80L then it will take a 20kw boiler ~ 6 minutes to heat this from 20C to 40C, there will be some heat emitted so wouldnt expect the boiler to reach its target temp of 65C in less than 8 minutes or so, it should/will then modulate down to maintain 65C and a output of 8.4kw, the UFH demand.
If the flow temperature is falling below 65C then it can only mean that the remaining heat demand is greater than 20-8.4, 11.6kw?.
Why not run the other circuits without the UFH and monitor the energy demand once system water is up to temperature?.

 

[HRP123]

From a cold start up and assuming a UFH system contents of say 80L then it will take a 20kw boiler ~ 6 minutes to heat this from 20C to 40C, there will be some heat emitted so wouldnt expect the boiler to reach its target temp of 65C in less than 8 minutes or so, it should/will then modulate down to maintain 65C and a output of 8.4kw, the UFH demand.
If the flow temperature is falling below 65C then it can only mean that the remaining heat demand is greater than 20-8.4, 11.6kw?.
Why not run the other circuits without the UFH and monitor the energy demand once system water is up to temperature?.

Thanks will try this out, appreciate the help