How can I roughly calculate UFH output at given flow temps.

[jimseng]

Hello all

Following on from my other thread about buffer tanks and secondary pumps, I have now fully engaged with the design of my ASHP and UFH design. I am at the stage where the GF UFH is down and screeded. The UFH heating company (Mec-Serv) in Bristol has gone bust and so I can no longer ask questions. However really all I want to know is how to calculate the heat output from each loop for each room. Be gentle because maths makes my brain panic but I do have a functioning brain, it just takes me a little while.

I would like to try and calculate this based on what I have, namely 16mm pipe at 150mm spacing with a given total length of loop. I guess we are going to have to assume the performance of the pipe, but how different can plastic pipe be? so presumably I have to come up with a w/m2, based on a flow rate and flow temp.

I would like to plug this into a spread sheet so I can start to estimate a suitable flow temp for the given heat loss of the house. 

Just to note, rightly or wrongly I took the design of the UFH away from the ASHP supplier and I'm not desperate to change suppliers at this stage as I have already signed the contract and I think it would be simpler to go from here. Me, my builder and plumber and I are most likely going to do the FF UFH ourselves. I am trying to get my head around all the aspects of the maths so I know where I am. Like all things in life, it seems if I want it to be done right I have to do it myself.

 

 

[JohnMo]

Simple answer you ASHP company should do this for you. They need to do a room by heat loss calculation anyway. They will know the UFH design, because you will pass it over to them. They will then be able to state at design temp the flow temp.

 

Flow temp required will also vary with a screed depth and floor make up insulation etc and floor covering. So a little fine tuning may be needed.

Edited 3 hours ago by JohnMo

[jimseng]

Can I have the complicated answer? I have the heat loss calculations. I'm really interested in understanding as much of it as I can and this bit is where I need a nudge.

[SimonD]

If maths makes your brain hurt, the best thing you can do is just buy a copy of the CIPHE Low-temperature heating and hot water guide as it has a load of nomographs (see examples here) so all you need is a ruler and this gives you a good start.

 

If you want to do the calcs yourself you can use this calculation here:

 

Q (W/m²) = 8.92(Average Floor Surface Temperature - Room Temperature)^1.1   

 

But this is really just the start because to then calculate the Mean Water Temperature for that average floor temperature, you need to know the correction factors for both the screed and the floor coverings. Why is this important? Because if you have say a carpet, you might need a flow temp of 45C to give you 27C floor temp, but with another surface like LVT you might only need a flow temp of 37.

 

This is really why you need the design documentation from the UFH company, but I know from experience that a lot of companies out there provides pants documentation, if any at all. One customer of mine used a company from Bristol when they were in a hurry and I was away and their company gave them diddly squat. I then repiped their system with hydraulic separation but left it there as I dind't want to take responsbility for a system I knew nothing about.

 

Really, your plumber should also be able to take you through this, together with any references being used if you're going to tackle this yourself. But I have to say that nowadays, if I'm contacted by someone who has problems with their UFH and they've done it through the builder/plumber I usually walk away. Get yourself a decent UFH company to do a proper design for you.

[jimseng]

Quote

This is really why you need the design documentation from the UFH company

That's the problem, they have gone bust shortly after putting the GF pipework in.

I know there are lots of factors such as floor covering, and given the current state of build and budget that might change. I was trying to get a rough guide. But those examples are certainly a good place to start.

[Great_scot_selfbuild]

@jimseng I’m loving your posts - it’s like you’re asking my questions for me! (I feel your frustrations/pain).

[JohnMo]

Here are couple of documents to get you started

UFH-System-Design.pdf UFH output calculation.pdf