System boiler modulation with low loss header LLH

[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.