Designing a simple UFH system

[mattgibbs]

Hello all,

 

I’ve read through previous posts on this topic and like the idea of having an UFH system that is weather compensated by the heat pump with no individual control or zones. A simple system.

 

Ive contacted a recommended company and they have said this is a good idea but they would need to know the heat loss per sqm for individual rooms in order to make sure smaller rooms don’t overheat and larger rooms aren’t under heated.
They also said actuators and floor probes stop the slab from overheating so the manifold should still have actuators presumably all connected to a wiring centre which all goes against the idea of a simple system.

 

Is there anyone that has a simple UFH system that came across these issues? I’m not sure of a way to calculate heat loss on an individual room basis.

 

project details

newbuild

wet UFH in 70mm screed

ashp and mvhr

[Conor]

You need to know the heat loss from each room, as they have pointed out. LoopCAD will do this for you. This will tell you the optimum pipe spacing and flow rates. I designed ours this way and seems to have worked well.

 

You don't need floor probes or actuators. You set the flow rates as prescribed by LoopCAD and then fine tune as you get to know the realities.

 

 

[mattgibbs]

thanks for the response. I didn't realise LoopCAD could also calculate heat loss for each room,  I would give it a try but the plan that includes heat loss is $1,150 can you do it all with the free trial?

 

Can I ask if you have any pumps or just rely on the ashp flow? our floor area is near 300sqm so I'm wondering if each manifold would likely need its own pump also

Edited August 6 by mattgibbs

[JohnMo]

Loopcad and free trial will do what you need.

 

Whatever company or person you are speaking to has limited or no experience with a low heat demand house. UFH flow temps are unlikely to exceed 30 to 32. The 70mm screed will bleed down the flow temp to a floor surface temp closer to low 20s. So probes, actuators etc are not needed. Just your manifold and an isolation valve at inlet and outlet points, so 2x valves.

 

Number pumps depends on your circulation pump capacity. So really depends on how close you do the UFH loops spacing and this will define the number loops. Play with Loopcad it will help define things. dT for a heat pump is generally around 4 to 5 degrees (difference between flow and return) but 

[mattgibbs]

@JohnMo

thanks that’s great to know. And will anything I’m doing with UFH cause an issues with wanting to put radiators in upstairs bathrooms or is that just a job for the plumber to work out when I get to that stage?

[JohnMo]

You need to concider the whole design up front. Ideal is to design system to run at all the same temperature and from the ASHP circulation pump.

 

So you have 300m² of UFH downstairs plus upstairs? 

[mattgibbs]

@JohnMo

yes it’s 300sqm downstairs and almost the same again upstairs.

This is my first design in loopCAD. All spacing is at 300mm centres which based on a water temp of 34 degrees keeps most the house the right temp. There is one room overheating which I can just reduce the amount of pipe work in to compensate 

 

is 34 degrees a good temp for the ASHP or should Increase the pipework to 150mm centres and run an even lower temperature? I’ve seen on your other posts you’ve run into trouble with short cycling by running the ASHP lower then 29 degrees.

 

total heating load is coming out at 12,000w but I don’t think it shows anywhere how much the UFH is providing out of that total

 

 

 

[JohnMo]

Just looking at you Loopcad the dT looks to 10 degs in the most part, you are going to be nearer 4 to 6. So that will drop your flow temp a couple of degrees for the same mean temperature. The design temp flow temperature is only a very short window of the year most the time it nowhere that cold outside.

 

Even for 600m² you heat loss of 12kW sounds very high. Especially for Hampshire. 

[Conor]

I found LoopCAD over estimated heat loss. Says ours was 9kW, reality closer to 5.5kW. maybe I got some setting wrong.

 

I'd reduce spacing in larger areas to 150mm, 200mm in other areas. Flow temp of 32c. Should work well.

 

LoopCAD assumes the UFH is the only heat source, and will give warnings if supplimentary heating is required.,

[mattgibbs]

@Conor It does assume its the only heat souce but it doesnt design the system to heat the upstairs also (unless you have upstairs ufh). see the attached below, the current UFH is providing the 6kw of heat the downstairs requires but the highlighted box shows there is also a further requirment of 6kw for the upstairs. Based on this i would have a 6kw UFH system in a 12kw demand house.

 

Do I double the size of the system to have the full 12kw of heat in the downstairs UFH which would massively overheat the downstairs in loopCAD or just get some electric radiators in the upstairs bathrooms to cover the heat requirments upstairs.

 

@JohnMo Thanks, I increased the flow rate for a total of 16L/m which brought the DT to around 5 and lowered the flow temp to 31 degrees.

 

Jeremys spreadsheet estimated 11,200w heat loss with a 25 degree defecit so its not far off. There is around 200m2 of glazing which is our main heat loss.

 

 

[mattgibbs]

Also does anybody know how i can make LoopCAD count the supply and return pipes towards the room heat. Our plant rooms and hallways show as needing supplemental heating when in reality they are probably going to be hotbox's? 

 

Ignore the pipe layout in the triangular room as i still need to space it out. but there is a large amount of pipework in the plant room (lowest room pictured) which is showing as being to cold and same for the hallway but there is no way thats actually the reality.

 

 

[Nickfromwales]

On 06/08/2025 at 13:11, mattgibbs said:

the idea of a simple system

If it was for one room then you could have your simple system, but when the sun goes from E > S > W the whole house changes throughout the day. 
 

Based on simple terms, you will likely need the stats and actuators for the reason stated, primarily for prevention of any spaces overheating.

 

Depends on how much of your life you have spare, to play with this, fine tune it, and what compromise you’ll be willing to accept with the unavoidable temp changes that will happen from room to room.

 

Yea cannae change the laws of physics, Jim. 

[Conor]

Ouch. You'll want to work on that glazing and reduce it. It's going to cost you a lot to run and potentially very uncomfortable in the summer. Has your architect done a full heat gain / loss model?

[mattgibbs]

@Conor Glass is already all in to be fair. its also mostly south facing but fortunatly our overhangs keep to sun off the glass in the summer almost entirly. But no we didnt do a heat gain/loss model 

 

did you have any idea on my comment above about Loopcad only accounting for the downstairs heating requirements?

Edited Saturday at 15:46 by mattgibbs

[Conor]

I never had that issue, I put all three floors in to the one model and it gave me total heat demand at the top of the report.

 

Make sure you get an ASHP that can easily do cooling, you're going to need it! You'll fing likes of May and September will be the difficult months and the sun is lower and will bypass your overhangs.

Edited Saturday at 15:47 by Conor

[mattgibbs]

@Conor thats lucky, ive not found a way to merge the upstairs zone with the downstiars one.

 

I know right! ive been getting lost in the world of the fan coil units vs air con debate today but i think i will just go with cooling the downstairs slab and air con for cooling

Edited Saturday at 15:50 by mattgibbs

[JohnMo]

8 minutes ago, Nickfromwales said:

Based on simple terms, you will likely need the stats and actuators for the reason stated, primarily for prevention of any spaces overheating.

Doubt it. Your floor output just drops to zero as the it gets solar gain. On a 7 Deg day the floor surface is only going to because degree or so warmer than the room. Switching a loop off is a waste of time. The time frame to have any affect is way longer than than the solar gain lasts. Just run it all open loop on WC and leave the system to look after it's self. 

 

Now on year three with proportionally more glass. In late winter I was actually running the heating at a higher flow temp of excess PV, even though the house was warm. Then as it dropped to -3 overnight (back at WC flow temps) the heat pump actually switched itself off.

2 hours ago, mattgibbs said:

how i can make LoopCAD count the supply and return pipes towards the room heat.

Spread the pipes out further instead of all bunched up.

 

 

[Nickfromwales]

41 minutes ago, JohnMo said:

Doubt it.

And it is exactly that, the uncertainty of not being able to get this wrong.
 

It is that statement which makes me confident that I should keep doing these installs the way I do, as I cannot afford to gamble with this even 1%.

 

Lots of self builders on here have built off design, theory on paper extensively researched, and still had got it wrong. 
 

Some have had to retrospectively install air conditioning, and more….

 

Doubt cannot reside in any successful design, if you’re to go and spend money on it, even more so when it’s someone else’s money eg when we advise others or work for them.

 

All of the designs I propose are fully underwritten too, I just get the needs / wants / wishes from my clients and then transfer that into a draft spec, then get system designers (usually the people who are going to supply the kit) to underwrite and commission at the end.

 

Works wonderfully.

[JohnMo]

5 minutes ago, Nickfromwales said:

And it is exactly that, the uncertainty of not being able to get this wrong.
 

It is that statement which makes me confident that I should keep doing these installs the way I do, as I cannot afford to gamble with this even 1%.

 

Lots of self builders on here have built off design, theory on paper extensively researched, and still had got it wrong. 
 

Some have had to retrospectively install air conditioning, and more….

 

Doubt cannot reside in any successful design, if you’re to go and spend money on it, even more so when it’s someone else’s money eg when we advise others or work for them.

 

All of the designs I propose are fully underwritten too, I just get the needs / wants / wishes from my clients and then transfer that into a draft spec, then get system designers (usually the people who are going to supply the kit) to underwrite and commission at the end.

 

Works wonderfully.

That's the difference you get paid for it, so you better get it right first time.  😁 

 

I used all the wisdom at the time 2020, when we installed our heating system, thermostats etc, f'ing waste of time and money.  Caused more issues than they fixed (boiler short cycling, under and over swing of room temps), worked my way down to one thermostat, now have none, one switch from cool to heat, now we are on a heat pump. Plus a home assistant relay, which is used to soak up excess PV and charge floor at a great CoP - plus zero cost heating 30% of the time in the shoulder season.

 

Sorry against what you think, thermostats are tat, used for high temp heating systems. Just aren't needed running weather compensation and low flow temps.

[Nickfromwales]

Just now, JohnMo said:

Sorry against what you think, thermostats are tat, used for high temp heating systems. Just aren't needed running weather compensation and low flow temps.

It’s perfectly fine, I’d much rather stay up and fight than go to sleep on this lol.

 

One flow temp is not sympathetic to say one example job I did, a very long split level bungalow, lots of south facing glass.

 

On the room stat one end of the open plan area I could see 22°C, and in the WNW glazed master bed it was 18.5°C which the client didn’t like.

 

The only way to cure this was to have enough stats to shut enough actuators and allow the rooms to heat (more importantly at the SE end) to NOT heat.

 

I like your solution for a dwelling that sympathises with it, but I don’t think it is wise to suggest it to people who’s homes may not.

 

It’s why every single new clients project of mine ALWAYS starts with a blank bit of paper, with only their surname at the top.

[Susie]

19 hours ago, mattgibbs said:

Also does anybody know how i can make LoopCAD count the supply and return pipes towards the room heat. Our plant rooms and hallways show as needing supplemental heating when in reality they are probably going to be hotbox's? 

 

You can change the target temperatures for the plant and hall or tick that they have supplemental heat. 
 

sorry my trial has expired months ago now.

 

tip print every thing you can off and save to pdf all the reports so you have a copy of all the summaries and reports loopcad can produce.  

 

[JohnMo]

14 hours ago, Nickfromwales said:

On the room stat one end of the open plan area I could see 22°C, and in the WNW glazed master bed it was 18.5°C which the client didn’t like.

 

The only way to cure this was to have enough stats to shut enough actuators and allow the rooms to heat (more importantly at the SE end) to NOT heat.

Sounds similar to ours, but without the split level. I just put in way less UFH pipes in bedrooms, as the design temp of the room was lower, which what we wanted, converse could be true also if you wanted bedrooms hotter, for strange reason. Just less heat added to room at same flow temperature; pretty much the same as you do for radiators.

[SimonD]

15 hours ago, JohnMo said:

Sorry against what you think, thermostats are tat, used for high temp heating systems. Just aren't needed running weather compensation and low flow temps.

 

I know this is a debate between you and @Nickfromwales but I feel compelled to jump in on this because I think you are mistaken. System control is the biggest headache in heating systems. It's something I have struggled with in my place because of surprising and significant solar gains, even with triple glazing. It's the main reason I opted for radiators rather than ufh.

 

I know you have settled for a system that runs low flow temperatures and you have balanced your system to take advantage of the self-regulating effect of those low temperatures, but there are so many other factors that play into whether a heating system actually delivers the heat to the right place and the right time in the house to prevent both over heating and under heating.

 

I have always been a fan of using room influence together with weather compensation because it adds additional and necessary feedback into the system and this is especially important when you have highly insulated buildings because basic activities such as cooking can have such a dramatic effect on internal temperatures. Not to mention the issue of solar gain already mentioned.

 

Thankfully manufacturers are awake to this with newer systems providing the options in both design and operation to commission the systems with pure weather compensation and/or room influence to allow better tuning together with better tunability in defining the weather comp curves.

 

In my designs, and especially after completing all the heat geek training etc. I always design systems with the capacity to have room influence, especially within bedrooms and areas subject to heat input.

 

Now, the problem with many room influence solutions on the market is the tendency for this to be relay rather than modulating, especially in underfloor heating systems. I believe that the way to go is through moderating the flow rates so you still, at least theoretically, maintain decent open volume and more subtle temperature control. But this is a difficult balance to find.

 

Like @Nickfromwales, I'm also doing this commercially, so I have to cover my backside, especially because I'm taking design responsibility. It is very different if you have both the time, knowledge and resources to spend countless hours tinkering.

[Nickfromwales]

2 hours ago, JohnMo said:

pretty much the same as you do for radiators.

Not really? 
 

Radiators have thermostats in each room / space, in the form of thermostatic radiator valves, meaning each space can be controlled individually as per the temp set on ‘the stat’, so that’s actually converse to your favoured methodology? 

[JohnMo]

52 minutes ago, SimonD said:

providing the options in both design and operation to commission the systems with pure weather compensation and/or room influence to allow better tuning together with better tunability in defining the weather comp curves.

Been there done that with UFH biggest disaster of all the options I tried and tested (Atag boiler). Floor timescale for change is just too long, great for radiators, utter rubbish with UFH (unless very screed and high flow temps used).

 

55 minutes ago, SimonD said:

self-regulating effect of those low temperatures

The main reason I chose UFH, as opposed to radiators, where high flow temp means they do not self regulate. As this topic is about simple UFH, not sure radiators are relevant or room compensation.

 

If your room is 23 (solar gains etc) and floor temp is 22, there is zero output to the room. A thermostat does nothing except close of flow routes available to the heat pump, generally not good. The floor stays charged up ready to release heat as room temp drops. 

 

There are obviously two views, but if I knew in 2020, what I know now, I wouldn't have a cupboard full of needless tatt, that I have removed from the heating system. 

 

Subject is simple UFH.

Simple is no mixers, no more pumps than absolutely required. Open loop, no thermostats. Run a simple weather compensation curve. Have switch that allows cooling to be active if needed. ASHP will run only as required, set the logic so the circulation pumps are off during compressor off periods to get a great CoP. Plus a suitable system water capacity, to ensure decent run times. So about 30L to 40L of system water always engaged per min output kW of the heat source.

 

The heat pump will modulate it's self, as gets closer to target flow temp, so you don't need outside influence to do that.

 

If you don't do UFH, maybe you should explore further. Set it running it on weather compensation it looks after it's self come rain or shine. You don't need external control.