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 8 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

[jimseng]

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I feel your frustrations/pain

I am taking full ownership of the design of my heating system as there are too many parties involved with not enough skin in the game. I came to this in ignorance: Never built a house from scratch, never installed a heating system from scratch, never owned an ASHP before. Therefore I made the mistake of not understanding how important the calculations are for a well designed ASHP heating system are. The ignorance led to a lack of attention paid at the beginning but I am trying to catch up and at the very least get to a position on paper where I can see a calculated flow temp for my ASHP/house that will produce the heat output the maths says I need. This will be my starting point and from there I will be able to set the system up for best efficiency/comfort. On my side I have as much solar as I can fit on the roof and a decent amount of battery. I have good insulation, MVHR and I am trying my best to get the most efficient building I can afford. The heat loss calculations I have seem correct. I already have the GF UFH down and it is should output more than enough heat but until I see the actual figure I won't be satisfied, hence me banging on about it on here!

[Nickfromwales]

37 minutes ago, jimseng said:

I am taking full ownership of the design of my heating system as there are too many parties involved with not enough skin in the game. I came to this in ignorance: Never built a house from scratch, never installed a heating system from scratch, never owned an ASHP before. Therefore I made the mistake of not understanding how important the calculations are for a well designed ASHP heating system are. The ignorance led to a lack of attention paid at the beginning but I am trying to catch up and at the very least get to a position on paper where I can see a calculated flow temp for my ASHP/house that will produce the heat output the maths says I need. This will be my starting point and from there I will be able to set the system up for best efficiency/comfort. On my side I have as much solar as I can fit on the roof and a decent amount of battery. I have good insulation, MVHR and I am trying my best to get the most efficient building I can afford. The heat loss calculations I have seem correct. I already have the GF UFH down and it is should output more than enough heat but until I see the actual figure I won't be satisfied, hence me banging on about it on here!

If you have both GF and FF UFH, then I assume you’ve allowed for a TMV on each manifold, and a buffer for hydraulic separation? 
 

You’ll need the flow temp from the ASHP to be at or slightly above the one that needs the highest output, hence you’ll need to be able to manage the temps per floor for the differing emitters and coverings.

[jimseng]

Quote

and a buffer for hydraulic separation

Everything I have seen states that there is rarely a need for hydraulic separation. These comments confuse me! I will check regarding the TMV. 

[Nickfromwales]

14 minutes ago, jimseng said:

Everything I have seen states that there is rarely a need for hydraulic separation. These comments confuse me! I will check regarding the TMV. 

You’ll be sending 2 different temps into 2 different manifolds? 
 

What is the methodology for FF UFH?

[JohnMo]

Or let the ASHP controller do the work, two zones, so two WC curves, controller can use an electronic mixer, such a an ESBE, the ASHP controls flow temp to coolest flow temp, no buffer needed, no temperature distortion, no additional pumps. Just run as fully open system with two flow temps, one unmixed and one mixed.

[SimonD]

44 minutes ago, jimseng said:

Everything I have seen states that there is rarely a need for hydraulic separation.

 

You don't need separation. As @JohnMo says just a heatpump and controller that can deal with 2 zones using an electronic mixer. 

[Nickfromwales]

18 minutes ago, SimonD said:

 

You don't need separation. As @JohnMo says just a heatpump and controller that can deal with 2 zones using an electronic mixer. 

That’s correct, but how many of these can do this out of the box, and what cost and complexity will ensue?

 

Mixers vs mixer? Eg qty 2 if UFH over 2 floors?
 

This plus the fact that this will defo need a volumiser for when the house is at / near to target temps and the heat demand reduces. 

[JohnMo]

24 minutes ago, Nickfromwales said:

That’s correct, but how many of these can do this out of the box, and what cost and complexity will ensue?

 

Mixers vs mixer? Eg qty 2 if UFH over 2 floors?
 

This plus the fact that this will defo need a volumiser for when the house is at / near to target temps and the heat demand reduces. 

Almost all currently available heat pumps do this out the box - you only need one mixer for the lowest flow temp zone.

 

Shouldn't need a volumiser either, unless you over sized the heat pump. Basically work on the basis of 20L per kW minimum output, system volume, if you achieve this no volumiser is needed.

 

Complexity - a temp probe down stream of mixer, connected to heat pump control/wiring centre. 3 wires to mixer from same control/wiring centre. Set controller to two zones, set the two WC curves, job done. 

Edited 2 hours ago by JohnMo

[jimseng]

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Shouldn't need a volumiser either, unless you over sized the heat pump. Basically work on the basis of 20L per kW minimum output

My UFH pipework volume total for both floors (based on 12mm ID and total design length) is 105L. I have accepted that it is a good idea to install a 30L volumiser which, as I understand it, is for defrost cycles. So 130L total volume. This is well above the 64l based on 20l/KW

Quote

you only need one mixer for the lowest flow temp zone.

So presumably this would be for the FF manifold as the requirement for the first floor (bedrooms) would be lower?
Is the mixer a proportional valve or a simple on off? 
isn't the whole point to balance the system with the flow meters so the whole house is treated as one giant emitter?  

[SimonD]

6 minutes ago, jimseng said:

So presumably this would be for the FF manifold as the requirement for the first floor (bedrooms) would be lower?
Is the mixer a proportional valve or a simple on off? 
isn't the whole point to balance the system with the flow meters so the whole house is treated as one giant emitter?  

 Not necessarily. If you're using spreader plates and then have carpet, the flow temp may actually be higher even with less heat load. It's all answered by the initial calculation and designs. 

 

The mixers aren't proportional or relay valves as such. They mix flow/return fluid to reduce the flow temperature of the system.

 

So the strategy is not about controlling flow, it's about controlling system temperatures. Your system is then still one giant emitter.

[jimseng]

 

Quote

The mixers aren't proportional or relay valves as such. They mix flow/return fluid to reduce the flow temperature of the system

So the reason I asked this is to find out what the heat pump is actually controlling with a mixer valve if it is set to two zones. If it isn't an on off actuator with a switched 240v feed what is it? A humanoid robot that rotates a knob? A 0-10v analogue signal? Hence the term proportional. I get how a thermostatic valve could be set but @JohnMo mentions an electronic mixer. This suggests something more sophisticated than bypassing the manifold once a temp threshold has been reached.
 

[SimonD]

46 minutes ago, jimseng said:

 

So the reason I asked this is to find out what the heat pump is actually controlling with a mixer valve if it is set to two zones. If it isn't an on off actuator with a switched 240v feed what is it? A humanoid robot that rotates a knob? A 0-10v analogue signal? Hence the term proportional. I get how a thermostatic valve could be set but @JohnMo mentions an electronic mixer. This suggests something more sophisticated than bypassing the manifold once a temp threshold has been reached.
 

 

Just like I said, the heat pump controller is controlling the flow temperature of the 2nd zone with the mixer. It doesn't bypass the manifold, it regulates the mixing of flow and return water to provide the right heat input to the zone in question. 

Edited 1 minute ago by SimonD

[JohnMo]

42 minutes ago, jimseng said:

 

So the reason I asked this is to find out what the heat pump is actually controlling with a mixer valve if it is set to two zones. If it isn't an on off actuator with a switched 240v feed what is it? A humanoid robot that rotates a knob? A 0-10v analogue signal? Hence the term proportional. I get how a thermostatic valve could be set but @JohnMo mentions an electronic mixer. This suggests something more sophisticated than bypassing the manifold once a temp threshold has been reached.
 

Something like an ESBE VRG valve and ARA600 actuator. Not an on off actuator. Your flow temp difference between GF and FF is only going to b e a few degrees. Lets say 5 degs.  GF wanting say 25 and FF 30 degs.  ASHP pushes out 30 degs, the electronic mixer just mixes down to 25 degs for GF. This flow temp changes based on outdoor temperature.

 

No robot needed, no 1-10V signal needed.