ASHP - choices, choices. Feel like I’m back at square one

[-rick-]

2 hours ago, Great_scot_selfbuild said:

I’m not doing the loop design myself (I’m drawing the line at buying a windows laptop just to use LoopCAD), and although the suppliers don’t share their designs before we commit to using them, we’ve had some say their design has 7 circuits and others say they have designed for 11 circuits. Are you advising that we make sure we ask for more than one circuit per room for the redundancy (just to make sure they factor it into their design)?

 

Gus and I have debated this before. My position is you should design the loops to output the heat you need at very low temps. Within reason more shorter loops vs fewer longer ones makes sense. Less than 100m but close to it is about the ideal loop length IIRC. But I wouldn't deliberately add extra loops if it throws off your heat output calcs, etc. In the event of a loop failure, firstly you want to find it and fix it before the concrete gets really hard and in the event that isn't possible then because you have designed the system for very low temperatures, you should be able to compensate for the loss by running temps a bit higher and adjusting flow rates.

Edited Wednesday at 10:18 by -rick-

[Iceverge]

Don't forget physics. 

 

Meaningful energy transfer will only occur when there's enough  temperature difference. A slab at 24⁰ will impart very little heat into a room at 22⁰ but a lot into a room at 15⁰. 

 

Given identical pipe spacing, on a single zone, the room with big windows or a leaky external door will extract far more energy from the UFH all by itself than the small internal room. 

 

Its not the case that a pantry will end up at 30⁰ and the hallway at 15⁰. 

 

The pantry will probably get to 22⁰ and the hallway to 20.5⁰. 

 

In a passive class house it'll all end up at 21⁰. Regardless of pipe spacing or omitting rooms or forgetting UFH and ASHP and just plugging in a cheap oil filled rad. 

[Nickfromwales]

23 hours ago, Iceverge said:

Don't forget physics. 

 

Meaningful energy transfer will only occur when there's enough  temperature difference. A slab at 24⁰ will impart very little heat into a room at 22⁰ but a lot into a room at 15⁰. 

 

Given identical pipe spacing, on a single zone, the room with big windows or a leaky external door will extract far more energy from the UFH all by itself than the small internal room. 

 

Its not the case that a pantry will end up at 30⁰ and the hallway at 15⁰. 

 

The pantry will probably get to 22⁰ and the hallway to 20.5⁰. 

 

In a passive class house it'll all end up at 21⁰. Regardless of pipe spacing or omitting rooms or forgetting UFH and ASHP and just plugging in a cheap oil filled rad. 

Amen. 🙏 

[Iceverge]

Ok.....

 

Having a brainwave here .......it's hurting......there's blood coming out my ears..............

 

Take a well insulated house with an ASHP. Lay UFH at 200mm centres  Don't bother with room by room patterns. Just slap it down in a big up and down pattern or whatever.

 

Pour 150mm of concrete over the top and then ensure no one drills more than 75mm into the concrete. And after than go to town with room layout etc. 

 

"Trim" heat the bathrooms with direct electric to make warmer if needed.

 

Any reason this wouldn't work? 

 

 

 

 

 

 

 

 

 

Edited Thursday at 18:38 by Iceverge

[JohnMo]

1 minute ago, Iceverge said:

Ok.....

 

Having a brainwave here .......it's hurting......there's blood coming out my ears..............

 

Take a well insulated house with an ASHP. Lay UFH at 200mm centres  Don't bother with room by room patterns. Just slap it down in a big up and down pattern or whatever.

 

Pour 150mm of concrete over the top and then ensure no one drills more than 75mm into the concrete. And after than go to town with room layout etc. 

 

"Trim" heat the bathrooms with direct electric to make warmer if needed.

 

Any reason this wouldn't work? 

I did similar but at 300mm centres, just didn't pipe under internal walls, kitchen units or beds and only did 100mm concrete.

[Iceverge]

I'm thinking idiot resistant ( not idiot proof, that's impossible).

 

Simple pattern to lay. Single zone. Run at a low flow temp.

 

Would it actually make any difference to the feel of the house avoiding walls beds, kitchen cabinets etc etc. 

 

 

[-rick-]

4 minutes ago, Iceverge said:

Would it actually make any difference to the feel of the house avoiding walls beds, kitchen cabinets etc etc. 

 

In a low temp system I'd guess not. My planning around this is to aim to not worry about kitchen cabinets etc. Seems like extra effort to try and route around things and in the event the layout is changed in future might you end up with an area of floor that is a touch cooler than others (I can't see it affecting room temp but might be noticable with feet on the floor).

[BadgerBodger]

I know I’ll probably be told this is wrong but I pneumatic tested mine and left pressurised while pouring. 
 

And then beyond. 
 

I forgot and knicked the pipe which was flush up to the edge of the concrete in a rebate I left so I could screed the shower to falls and boy did I get a surprise. 
 

 

[-rick-]

55 minutes ago, BadgerBodger said:

I know I’ll probably be told this is wrong but I pneumatic tested mine and left pressurised while pouring. 

 

Thats the right thing to do.

 

55 minutes ago, BadgerBodger said:

I forgot and knicked the pipe which was flush up to the edge of the concrete in a rebate o left so I could screed the shower to falls and boy did I get a surprise. 

 

Worked as intended then! Good to know of the problem straight away. Though if it really made an impression on you maybe you had the pressure a bit too high

[Nickfromwales]

5 hours ago, Iceverge said:

I'm thinking idiot resistant ( not idiot proof, that's impossible).

 

Simple pattern to lay. Single zone. Run at a low flow temp.

 

Would it actually make any difference to the feel of the house avoiding walls beds, kitchen cabinets etc etc. 

 

 

Nope.

[Nickfromwales]

5 hours ago, -rick- said:

 

In a low temp system I'd guess not. My planning around this is to aim to not worry about kitchen cabinets etc. Seems like extra effort to try and route around things and in the event the layout is changed in future might you end up with an area of floor that is a touch cooler than others (I can't see it affecting room temp but might be noticable with feet on the floor).

For the obvious "kitchen / utility units will always at least be there forever" areas, yes, reduce the UFH according to basics. Most medium sized islands I will always pipe under, but more significant sized rooms don't need the 'help' so you can avoid if you choose to. 

[BadgerBodger]

8 hours ago, -rick- said:

 

Thats the right thing to do.

 

 

Worked as intended then! Good to know of the problem straight away. Though if it really made an impression on you maybe you had the pressure a bit too high

3.5 bar… 

[Dillsue]

16 hours ago, Iceverge said:

I'm thinking idiot resistant ( not idiot proof, that's impossible).

 

Simple pattern to lay. Single zone. Run at a low flow temp.

 

Would it actually make any difference to the feel of the house avoiding walls beds, kitchen cabinets etc etc. 

 

 

Yes, Yes and No!

 

We did a large kitchen/diner with 100mm+ slab and pipe at 150mm centres, I think....it was 20+ years ago. We did the whole floor wall to wall so heated under cabinets and a partition wall. It's been great to have warm tiles:)

 

If there was concern over loss of heat output from the areas under cabinets you could put a fan in the plinth to draw out the warmed air??

[Nickfromwales]

40 minutes ago, Dillsue said:

If there was concern over loss of heat output from the areas under cabinets you could put a fan in the plinth to draw out the warmed air??

….and that’s where we bring this to a sober conclusion lol. 
 

No fecking fans under plinths!!  

[Gus Potter]

On 04/02/2026 at 10:13, SimonD said:

In terms of redundancy, what exactly are you referring to? Is it to run extra loops in parallel - for example, if I need 300mm spacings, I add additional loops so that I essentially have 150mm spacings - or is it something else?

Good question as an introduction.

 

On 04/02/2026 at 10:13, SimonD said:

I'd like to know more Gus as I'm keen to understand your perspective and reasoning for this. What are the thermal characteristics that are better and how is the system output controlled once it's in?

 

I'm from the school of calculating heat load requirements in the room and designing emitters to satisfy this at a low temperature as possible and avoiding external controls as much as possible although I recognise and use room influence where needed. This approach does use the method of designing both radiators and ufh to the demand of each space.

 

I'd like to understand more from your experience.

And this is even better! 

 

@Rick "Gus and I have debated this before. My position is you should design the loops to output the heat you need at very low temps. "

 

"But I wouldn't deliberately add extra loops if it throws off your heat output calcs, etc. "

 

I can see that my approach requires some reasoning in response to the very good points made by many (not just Simon and Rick above in response to my "philistine approach". I'm going to have a go at setting out my stall. I say from time to time that I did my first UFH a long time ago, long before BH and when is was almost for the "whacky inventor in the UK" 

 

To start as a bit of background.

 

About 30 years ago I built a house out in the country. There was a guy (Clive) who was building a house about 1/4 of a mile away that had come over from Scandinavia with the UFH idea. At the time it was innovative in the UK. I looked at the complicated controls, could not be sourced in the UK at the time, even the pipe diameter and thought.. I'll simplify it, build my own manifolds out of soldered Yorkshire joints, got some pumps and a blending valve off the shelf and installed all of that into timber suspended floors, ground and first, all coupled up to an oil fired boiler.. basically a big blow torch in a twin walled steel box that had water in it.  I could not afford "high end" but knew and believed in the principle of UFH. 

 

Now it worked ok but when it got really cold, which was quite often -5 deg over a week, often dropping to minus 15 deg overnight, I ended up having to crank the flow temperature though the UFH pipes up to 60 C. The carpets did not self destruct and the tiled floors hung in there! I learnt that floor finishes if slowly conditioned and installed properly are often more forgiving than you think. Incedentally my pal (Clive) down the road died and it was only about 5 years back that I stopped look after his widows system (500 m square house). It has twin LPG boilers, a low loss header and so on. 

 

Once I moved into my own self build.. could walk about with my socks off I became absolutely hooked on UFH. Make no mistake I love it and am a big fan. I'm not such a big fan of some of the stuff on BH.. it's too complicated!

 

Since then (Clive)  I've been involved in UFH, done some for myself and for Clients. The technology has improved a lot, especially boiler innovation, controls weather compensation and so on.. I've experimented on my own house.. tested ideas on my own houses,  made mistakes. I've also learnt a lot about FE analysis, the maths behind it and it's limitations... and know it's to be taken as a modelling tool... although the folk that have a financial interest in selling say UFH .. it's designed by FE so must be right! That is bollocks.. it's a tool, a good one at times, but it must be sense checked. 

 

In my day job now I'm an SE and designer, still learning every day! Much of my work is domestic. I'm exposed to how domestic Clients work with builders. In this context take the money and Client / Builder expectation. These differer. Expectation of quality control on site differs, drawing and contract interpretation. On BH there are good comments about, testing, site supervision..and if you do this then you can have a leaner UFH design. But the reality is, from having done this for a long time, it is that what you think you are going to get as a self builder often does not get realised on site. The self builder is 95% often not to blame.. the industry probably is. 

 

1/ My general approach:

 

To digest this I'm going to split the UFH into two different elements and generalize a bit for sake of arguement / presentation.

 

1/ What is in say a screed / concrete slab or timber floor. If something goes wrong with that then the cost of access and repair is high and disruptive to finishes not least. As a correlation. Say I have my SE hat on, I design a timber frame and the windows have timber lintels on cripple studs. But the detailing at the cill causes hidden water to leak back into the kit, rots it and things start to move. That is often going to cost a lot to fix.

 

If you then think.. well it's fair to ask the UFH pipes to perform for the same length of time as the structure.. 50 -60 years. To add more weight to this. What about your drain pipes in the house.. is it not fair to ask that they will also hang in there for 50 years? Or do you want your UFH pipe to last for say a guarentee period of 7 -10 years like double glazing units.. what about young kids that may later buy your house, is that for them to sort out, or inherit? 

 

Now know that in my first house.. 30 years ago the UFH pipes are still fine, not breaking down, getting brittle.. and that is with 30 year old pipe technology! 

 

Point.. is UFH like double glazing or an important "built in" part of the structure? Boilers, pumps etc above the floor can be easily changed. We may want to change them as more efficeint and simpler controls come on the market?

 

2/ What happens on site:

 

I can see on BH that some are saying.. if you control the works, supervise, test, make sure you are draconian with the folk installing the UFH pipes, checking all their bend radius ( see UFH pipe datail), you have a fighting chance. Some on BH are asking about running UFH pipes over concrete slab movement / sawn  joints, some say the UFH pipes can stretch a bit.

 

I can tell you as someone who designs this slab / screed stuff that this is false prophecy as you are eroding the factor of "safety" and each time you do that you risk UFH  failure. Your UFH pipes are not designed to be "stretched  by concrete / screed movement or over bent! 

 

But the reality is for most self builders is that you are going to have to trust the folk on site.. and accept that some pipes may be bent a bit tight, stood on, overlapped and get crushed at the over lap. When laying screed or concrete the pipes should be under air pressure, say 6.0 bar, not just to make sure someone does not make a hole in them but also they expand by microns and this give them a bit of play at the return bends, prevents further stretching as a pipe is already stretched on the outside of the bend when it gets bent. The pipe manufacturer's declare their product performance.. but do not include your concrete / screed.. why should they.. it's your job to do this not theirs.. they need to sell pipe and fittings and are not liable for you concrete / screed design.

 

Point.. your ordinary self builder needs to design something that can be drawn so the folk on site can understand what they have to do. Areas that are important need to be highlighted on the drawings. Pipe centres need to be the same. If the UFH drawings are presented in this way then Contractors will say.. that looks easy.. keps the cost down. You the Client will be able to identify any anomolies. Thus by all means use loop cad.. but think about buildability, the harder it is the more you will pay and maybe not get get what you think you are paying for. 

 

Now as an SE / UFH philistine and just having done stuff like this for a while. If you put in extra loops it covers you in part.. for shit that happens or a bit of dodgy pipe. If you look at the extra cost of using smaller loops its not that much. Say 15- 20% on the pipe install tops, clipping them and the extra length getting back to the manifold. It also means you don't have loops crossing concrete joints and so on.

 

3/ What happens when the UFH pipes come above floor level into the plant room:

 

(a) More loops means a larger manifold, that is one of the down sides to my appraoch. Or you can say.. if in one room if the loops are roughly the same length connect them all together before the manifold as the flows will be roughly the same. You don't have to use the redundancy you have built in, it's just there if you need it. 

 

(b) Now since I started 30 years a go I can see massive innovation in control technology.. but it's very complex. But the big eplephant is the room is ongoing replacement part technology and now often the software that you need to support this. This can come at a scary cost as replacement part costs escalate.. it's the law of supply and demand.

 

I have a Client that has bought a house recently that is filled with a fortunes worth of home automation.. he is ripping it all out as there is no manual on how to work it and the software is not updatable. 

 

I need to digress and reference @JohnMo John is an exponent of the open system that is compatible with modern systems.. This is incredibly elegant, clever, few parts needed other than an intelligent boiler. I almost think John has come full circle and back to how I started.. but in a more informed, evidenced based way and even more cost effective! 

 

(c) If we design at the moment on the least loops (using the least pipe and sod all other considerations) basis that are built into the structure then we are closing off our options for the decades ahead. Yes we will still have them but @SimonD that is partly what I mean by redundancy. The UFH design needs to be homogenous.

 

@SimonD" I'm from the school of calculating heat load requirements in the room and designing emitters to satisfy this at a low temperature as possible and avoiding external controls as much as possible although I recognise and use room influence where needed. This approach does use the method of designing both radiators and ufh to the demand of each space."

 

Simon.. that is OK. I can do that also.. but there is much more to the equation and that is why I'm trying to encourage folk to look at the whole life cycle. 

 

@-rick-

On 04/02/2026 at 10:09, -rick- said:

Having said that, I believe that modern Pex-Al-Pex pipe is significantly more robust than pipes used in previous decades so as long as everything is tested in good condition before the pour then the pour shouldn't be an issue.

I think and agree with you but you are only half way there in terms of what you do on a self build and how you can realistically control what happens on site. 

 

For all.. I've seen some commercial sites with UFH.. millions spent.. and they work even less better than many self builds!

 

4/ Use of software:

 

Now I know that many on BH rely on software.. but there is nothing like getting a bit of basic knowledge to enable you to do a sense check. I'll maybe write another time about how you do that. As an SE I do some fag packet sums.. but before that I just look at the drawings.. you do the same with UFH.. look where the big areas of glazing are, volumes and where the building is more or less insulated. 

 

5/ What are you setting out to achieve:

 

Well you can be the theorist, look to save every penny on the gas / electric bill and treat it as a hobby. That's ok by me. But my experience is that that hobby will start to cost you more and more as parts become worn or software is redundant. 

 

My personal view is that UFH is still something that yes needs to function to heat the house.. but it's also a luxery. Once you have it you realise that the furniture is always warm.. if you have a leather sofa it's not cold, house plants thrive, the air currents in the house are less noticeable, your linen in the drawer is dry.. but not too dry, I could go on. 

 

But if you have a family member that is feeling unwell or you just want to have a hot zone then more loops make sense. You may change say in 20 years time the wal layout.. you want to make that easy. This is redundancy. You may be a doggy person.. always leave a cold spot! DSome folk want a traditional panty.. leave the heating out in there. 

 

You may want to sell the house in the future and the valuer turns up and says.. does you heating actually comply with the regs? 

 

You may just be wanting a UFH that works ok for you now.. look fancy and don't really care about any future owner? 

 

6/ To conclude:

 

I'm fully supportive of UFH. I'm not so supportive of folk that "design by calculation" alone.

 

I can produce calculations that may show an element is ok, safe.. but that is not best design. Best design is about marrying elements into the rest of the design.. called holistic design. Here good holistic design is about getting the best you can out of the money you have. Self building is just that.. we get to have something you can't buy off the shelf. 

 

Some on BH are driven by calculation, trying to get a home that is most energy efficient on paper..the day after completion.  I push back at this and say.. in real life say ten years down the road it is unlikely to work out the way you think at the moment.