System boiler modulation with low loss header LLH

[HRP123]

First rule of LLW design. “Thou shall not use multi tapping headers” Sorry but they are a PIN. And if systems arent perfectly matched and flow rates set correctly will leave you wide open to parasitic flow across the ports.

First thing I’d be checking is flow rate of your primary pump, Vs flow rate of your secondaries.

Something is pulling that flow water across the header rather than it circulating around it, so boiler then is ramping up as it’s running on its thermistor readings to control the gas rate. If the heat is going around the system rather than round the header it’s going to take an awfully long time before the temps are coming back to the boiler hot enough to start the modulation process

Guide to low loss headers - CIBSE Journal

Achieving hydraulic separation through well-designed low loss headers is key for efficient heating and cooling systems. David Palmer and Baxi Heating’s Ryan Kirkwood provide detailed design guidance, including a recommended system configuration

www.cibsejournal.com

Thanks for this. I was thinking in my novice mind that this pump flow issue could be an issue too.

The heating pumps are Grundfos Alpha 2s. The boiler pump is currently set to max constant pressure -

The UFH has been set to speed 1 of 3
CH to low constant pressure (I tried proportionate pressure but then it takes too long to heat the rads)
Towel rails to low constant pressure
HW to max constant pressure

I'm not sure how to check the flow rates of the pumps or circuits.

For example this weekend I noticed that for a couple of hours when just the CH circuit was calling for heat, the boiler was getting too hot and cycling. The same thing happens if only the UFH is calling for heat.

I have been trying to time the heating circuits to call for heat at the same time, which in combination with the low range rating (40% of 40kw) means the boiler only achieves a flow temperature of around 50 rather than 65/70. This lower flow temp isn't conceptually a bad thing I think (?) because the UFH mixes down to a lower flow temp anyway, and the rads can manage with 50 degrees too.

The problem is if the CH or Towel Rails also kick in, then there can be too much demand of on 16kw heat output currently from the boiler. Then the HW stays on for a really long time because it cannot heat the tanks to 50 degrees with a 50 degree flow temp.

 

[Phill K]

Is there a pump from the header sending water to the underfloor pump? Technically there should be as the underfloor pump is just that. Only for circulating round the underfloor
Might the underfloor is your source of leeching.

Valves are opening up, pumps shunting water and the boiler pump is then forcing the flow across the header rather than circulate it around it as there is no separation of systems occurring

 

[HRP123]

Thanks - I don't quite understand what you mean - but there is a pump from the boiler to the LLH and a pump from each flow outlet on the LLH to the circuit. So one pump for UFH, one for CH, one for Towel rails and one for HW. There are no additional pumps on the UFH manifolds inside the house.

 

[HRP123]

Is there a pump from the header sending water to the underfloor pump? Technically there should be as the underfloor pump is just that. Only for circulating round the underfloor
Might the underfloor is your source of leeching.

Valves are opening up, pumps shunting water and the boiler pump is then forcing the flow across the header rather than circulate it around it as there is no separation of systems occurring

I had the UF heating only come on last night for an hour. It consumed the full power that the boiler is rated up to currently (approx 20kw) for the hour that it came on.

Can anyone help suggest what is the best back of the envelope way to calculate what the UF circuit usage should in theory be? I have an idea of the metres of pipework and pipe centres + floor area overall. Thanks!

 

[HRP123]

I've also noticed that whenever the hot water cylinders come on, they also consume the full power of the boiler - even though in theory I think they should require less.

 

[John.g]

I had the UF heating only come on last night for an hour. It consumed the full power that the boiler is rated up to currently (approx 20kw) for the hour that it came on.

Can anyone help suggest what is the best back of the envelope way to calculate what the UF circuit usage should in theory be? I have an idea of the metres of pipework and pipe centres + floor area overall. Thanks!

Post number of loops, length of each loop, the flow/return flow of each loop, the manifold flow/return temps or any info you have, for example if you have 9 loops with a flowrate of 2.0LPM/loop and with manifold flow/return temps of 45C/37C, dT of 8C then the UFH requires 9X2x60X8/860, = 10.04kw say 10kw and the boiler output will be exactly the same, I thought we looked at this previously?

 

[HRP123]

Post number of loops, length of each loop, the flow/return flow of each loop, the manifold flow/return temps or any info you have, for example if you have 9 loops with a flowrate of 2.0LPM/loop and with manifold flow/return temps of 45C/37C, dT of 8C then the UFH requires 9X2x60X8/860, = 10.04kw say 10kw and the boiler output will be exactly the same, I thought we looked at this previously?

Yes thanks we did look at this together - but what I am trying to work out is what you mentioned in your last line - if you calculate the heat demand is 10kw, this should in theory translate to the boiler needing to burn around 10kw (lets say a little more because its not 100% efficient).

But it doesn't make sense if your theoretical UF heat demand is 10kw, for the boiler to burn 20kw right? So I'm trying to work out where this heat is going and why the boiler doesn't seem to be able to modulate down to the required level. I'm not sure for example if its because of the LLH, or the impact of the amount of cold return flow from the UFH loop when it first starts up which distorts the boiler return temp.

 

[John.g]

The boiler output will always match the UFH heat demand, if the UFH heat demand is lower than the minimum boiler output then the boiler burner will cut out at the boiler flowtemp setpoint+5C and will cycle on/off to match the heat demand, simple as that, the fuel input is the boiler output/its efficiency.
Can you post a few details of the UFH as requested above?

 

[HRP123]

The boiler output will always match the UFH heat demand, if the UFH heat demand is lower than the minimum boiler output then the boiler burner will cut out at the boiler flowtemp setpoint+5C and will cycle on/off to match the heat demand, simple as that, the fuel input is the boiler output/its efficiency.
Can you post a few details of the UFH as requested above?

Hi John, this is a schematic of what I have and which loops are typically open. I've estimated a couple of loops in orange which were added when we did an extension. Manifold 2 is one of the original ones and does not have flow meters on the heads. Manifold 1 is a new one and seems to have flow meters and I will try and do some tests today. Thanks!

 

[HRP123]

The boiler output will always match the UFH heat demand, if the UFH heat demand is lower than the minimum boiler output then the boiler burner will cut out at the boiler flowtemp setpoint+5C and will cycle on/off to match the heat demand, simple as that, the fuel input is the boiler output/its efficiency.
Can you post a few details of the UFH as requested above?

Sorry I made a mistake above and am having problems editing the post. This is the schematic attached.

So as an example I just tested the family room loops for half an hour. The gas consumption according to the meter was 10.69kwh (nothing else was open). The boiler was firing at 70 on the screen and the underfloor loop flow return at the manifold was 53.8 / 43.6 (the white JG blending valve near the underfloor pump set to around 45 degrees).

The boiler is about 6 metres away from the manifold. There is no pump on the manifold and the flow meters didn't seem to move (not sure if these are flow meters?).

 

[John.g]

Gas consumption 10.69kw/30mins???, so gas consumption 21.38kwh per hour??.
What is the boiler flow temperature setpoint and what is the actual boiler flow temperature, also can you see the boiler return temperature??.

Also how many loops are open?, I am calling a loop as like one of the 8 above.

 

[HRP123]

Gas consumption 10.69kw/30mins???, so gas consumption 21.38kwh per hour??.
What is the boiler flow temperature setpoint and what is the actual boiler flow temperature, also can you see the boiler return temperature??.

Also how many loops are open?, I am calling a loop as like one of the 8 above.

Yes, 10kwh! It is the zone called Family Room which I tested - so 3 loops. The boiler flow was 70 degrees, I didn't measure the actual boiler flow / return in this instance but generally from the flow tends to be measured at 5 degrees less than the boiler temp and the LLH return tends to be 10 degrees less - i.e. at 70 on screen it would be 65/55.

 

[John.g]

Still not clear if its 10.69kwh/30mins or 10.69kwh/hour.
If 10.69kwh/hour then loops emittance 10.69X85%, say 9kw, which means each loop emitting 3kw which equals a flowrate of 4.2LPM at a dT of 10.2C (53.8-43.6), I'm a bi surprised that a dT of 10.2C at that flowrate but thats what your info shows.
If the heat emitted is 18kw , 6kw/loop, then the flowrate is 8.4LPM/loop, very unlikely IMO. I normally read of loop dTs of ~ 8C with a flowrate of 2.5LPM so a loop would typically emit 1.4kw.

 

[HRP123]

Yes, 10kwh! It is the zone called Family Room which I tested - so 3 loops. The boiler flow was 70 degrees, I didn't measure the boiler return in this instance but generally from the LLH the return tends to be 10 degrees less usually.

Still not clear if its 10.69kwh/30mins or 10.69kwh/hour.
If 10.69kwh/hour then loops emittance 10.69X85%, say 9kw, which means each loop emitting 3kw which equals a flowrate of 4.2LPM at a dT of 10.2C (53.8-43.6), I'm a bi surprised that a dT of 10.2C at that flowrate but thats what your info shows.
If the heat emitted is 18kw , 6kw/loop, then the flowrate is 8.4LPM/loop, very unlikely IMO. I normally read of loop dTs of ~ 8C with a flowrate of 2.5LPM so a loop would typically emit 1.4kw.

It was 10kw over the half hour, so 20kw per hour.

So working on your calculation the flow rate would be 8.4 LPM. This is why I am wondering what could be taking the heat if its not going to the UFH.

I'm guessing there is some heat loss for heating up the LLH and a little bit of each closed heating circuit (the rads, towel rails and hot water) until their zone valves. Also since the two UFH manifolds are connected off one circuit, some heat would circulate to the other manifold in the house but divert through since the zone actuators are closed.

I'm not sure what else could cause the high gas usage aside from a very inefficient boiler.

 

[John.g]

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.

 

[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