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Do You Need a Buffer Tank or Low Loss Header for Underfloor Heating in a Large Property?

 
Thermostats, Volumisers, Buffer Tanks, TRVs, Pumps, Piping, Valves, Radiators & Controls
Last Post by GrahamF 14 hours ago
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GrahamF
 GrahamF
(@grahamf)
Estimable Member Member
Joined: 1 year ago
Posts: 134
Topic starter 14/10/2026 8:57 pm   [#3194]

I have come across someone on another forum who lives in a 300m2 house with UFH throughout.  He has a low loss header and 3 pumps on 3 zoned UFH circuits.  

My reaction was that the LLH and all those pumps could be causing his problems - e.g. distortion in the LLH, the parallel pumps on the secondary circuits could be fighting each other, especially as they are not hydraulic separated from each other...  

However, I am wondering whether I could be mistaken.  I have done some searches with Google AI and it came back recommending an LLH based design.  The rationale was that the UFH could be several kilometres long.  The water pump in the air source heat pump would not have enough power to push the water around the circuits.  Therefore extra pumps are required and the LLH is needed for hydraulic separation.

What is the recommended approach for such a large UFH installation?


Grant Aerona 290 15.5kW, Grant Smart Controller, 2 x 200l cylinders, hot water plate heat exchanger, Single zone open loop system with TRVs for bedrooms & one sunny living room, Weather compensation with set back by room thermostat based load compensation


   
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JamesPa
 JamesPa
(@jamespa)
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Posts: 5541
14/10/2026 11:44 pm  

Posted by: @grahamf
↑

 The rationale was that the UFH could be several kilometres long.

Well thats a very silly argument typical of AI, it will (or should be) lots of parallel loops joined at one or more manifolds, not one single string several km long!  Also it depends on the house.  If its modern, very low loss, then less flow is needed than if its a leaky stone pile.

Posted by: @grahamf
↑

What is the recommended approach for such a large UFH installation?

I cant be certain about that, but the rationale given by the AI is wrong so far as I can see.  I guess the answer is 'do the math' for the layout present and loss just like you would do for radiators if there were doubt.  Someone used to UFH may well have a feel for the likely answer. 

Posted by: @grahamf
↑

My reaction was that the LLH and all those pumps could be causing his problems - e.g. distortion in the LLH, the parallel pumps on the secondary circuits could be fighting each other, especially as they are not hydraulic separated from each other...  

Agreed, and the 'call for heat' signal, if there is one, could also be getting messed up unless someone has actually designed the overall control system (which quite likely they have not!).  That problem has occurred several times on this forum with mixed radiator/UFH systems where nobody bothered to think through how the separate control systems would mess with each other.

 


This post was modified 1 day ago 4 times by JamesPa

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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GrahamF
 GrahamF
(@grahamf)
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Topic starter 15/10/2026 7:30 am  

@jamespa thanks for your insights on this.  On reflection, assuming the same flow temperature which is not necessarily true, the amount of water flow must be about the same, whether you are using radiators or UFH.  

Another point is that I may have misled the AI in saying there were 3 circuits.  The system has UFH manifolds, so water is flowing in parallel around many loops for each circuit.  This must reduce the flow resistance and so make it easier for the water pump(s).

Your point about the control systems is well made and I had already raised that.

 


Grant Aerona 290 15.5kW, Grant Smart Controller, 2 x 200l cylinders, hot water plate heat exchanger, Single zone open loop system with TRVs for bedrooms & one sunny living room, Weather compensation with set back by room thermostat based load compensation


   
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JamesPa
 JamesPa
(@jamespa)
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Posts: 5541
15/10/2026 8:14 am  

Posted by: @grahamf
↑

On reflection, assuming the same flow temperature which is not necessarily true the amount of water flow must be about the same, whether you are using radiators or UFH.  

Strictly same deltaT is the requirement for same flow rate.  I think ufh systems are typically designed for DT5 or same as rads, but can't be certain.


4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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 Singlespeed
(@singlespeed)
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Joined: 7 months ago
Posts: 41
15/10/2026 8:18 am  

The AI answer is one of the cases where if you'd pointed out the error of it assuming one long run, it would reply.

You're quite right in correcting me. Let me explain why. In reality ......



   
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JamesPa
 JamesPa
(@jamespa)
Illustrious Member Moderator
Joined: 4 years ago
Posts: 5541
15/10/2026 8:20 am  

Posted by: @singlespeed
↑

The AI answer is one of the cases where if you'd pointed out the error of it assuming one long run, it would reply.

You're quite right in correcting me. Let me explain why. In reality ......

Indeed.  AI, at least the models we have access to, doesn't yet pass the Turing test!

 


4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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GrahamF
 GrahamF
(@grahamf)
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Posts: 134
Topic starter 15/10/2026 10:33 am  

@jamespa in some cases, failing the Turing test might be seen as an advantage. 😆


Grant Aerona 290 15.5kW, Grant Smart Controller, 2 x 200l cylinders, hot water plate heat exchanger, Single zone open loop system with TRVs for bedrooms & one sunny living room, Weather compensation with set back by room thermostat based load compensation


   
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