@davidnolan22 Inspired by someone on buildhub I asked ChatGPT and Duck.ai (which uses chatgpt and other LLMs) the following question:

"Suppose I have underfloor heating with pipes at 150mm centres buried 15mm deep into concrete that is 100mm thick, sitting directly on damp soil assumed temp at sufficient depth 10C. Suppose that on top of the concrete is say 5mm of luxury vinyl tile and the room above is at 20C. How much heat goes down and how much goes into the room? You may assume, for simplicity, that the heating continues for several months, that the room is infinite is size and that the pipes are supplied with water at 35C"

They used slightly different methods (which they gave) and both had to make simplifying assumptions (unsurprisingly).  ChatGPT assumed a room of infinite area, Duck.AI asked me to specify the room size so I gave it 7m square and 10m square.  They gave their methods which look reasonable but I haven't verified completely (and Im not sure if I will).  In summary ChatGPT says 80% of the heat will go down, Duck.ai says 50% will go down.    This is at variance with what UrbanPlumber says here, but his somewhat handwaving argument, for which he admits he has no evidence, makes an assumption which I think is incorrect.  However your installer may have based his statement on this, its not the first time I have heard it said.

Frankly I don't know, but I would say that, based on what we have so far, there is at least 50% probability that at least half the heat goes down in an uninsulated UFH arrangement such as yours.  If this is correct (and I think you would need to do a finite element simulation to prove it) then it could easily account for your problem.  

If you want me to DM you the full conversation with Duck.ai and ChatGPT Im happy to do so.  It might be food to ask the exact same question of another LLM that we know doesn't use ChatGPT at all

@briansprankle, good luck. Keep us posted. Glad they’re trying to help after seeing your post here.

@jamespa 

hmmm..... one to ponder. 

I've no idea how dry or wet the soil is. the old part of the house is from 18th century.

But, I've put a 20K floor, 20k kitchen down since then.... we are not digging any floors up..... 

Posted by: @davidnolan22

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@jamespa 

hmmm..... one to ponder. 

I've no idea how dry or wet the soil is. the old part of the house is from 18th century.

But, I've put a 20K floor, 20k kitchen down since then.... we are not digging any floors up..... 

 

It may be clutching at straws but heat has to go somewhere, is it possible to insulate the external edge of the solid floors so that the whole floor becomes a giant low temperature heat sink?  Not ideal but long term would reduce heat loss possibly to a managable level.  Soil directly under the floor would gradually warm and reduce heat loss.  Maths for this is well above my pay grade but it just might help without digging up the floor.

Posted by: @wanttobegreen

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It may be clutching at straws but heat has to go somewhere, is it possible to insulate the external edge of the solid floors so that the whole floor becomes a giant low temperature heat sink?  Not ideal but long term would reduce heat loss possibly to a managable level.  Soil directly under the floor would gradually warm and reduce heat loss.  Maths for this is well above my pay grade but it just might help without digging up the floor.

You need to insulate the edges and beneath - 'heat rises' applies only in fluids not solids, in solids the heat spreads isotopically.    If you do this the slab does indeed become a large storage radiator.  If you dont you are right that soil underneath will gradually warm, but its likely too large a storage radiator to be useful.  I cant do the maths either (I dont think its soluble analytically) but I did ask ChatGPT a question and it estimated (with plausible methodology given) that the time constant for the warming of the soil was order weeks or months.  Thus the problem might get better as the season progresses - but isnt particularly practical as a 'solution' unless you heat throughout the year.

Posted by: @davidnolan22

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@jamespa 

hmmm..... one to ponder. 

I've no idea how dry or wet the soil is. the old part of the house is from 18th century.

But, I've put a 20K floor, 20k kitchen down since then.... we are not digging any floors up..... 

 

Understood.  Solutions I can see if you cans somehow squeeze enough out of the heat pump are

• Supplementary heating
• Replace UFH with rads (because then the slab is in contact with air at 20 not pipes at 35)

Sorry to be the bringer of bad news, but UFH on an uninsulated slab is a massive gamble so far as I can see and, as you say, its not coming up so now the options are limited.

Posted by: @davidnolan22

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@jamespa 

hmmm..... one to ponder. 

I've no idea how dry or wet the soil is. the old part of the house is from 18th century.

But, I've put a 20K floor, 20k kitchen down since then.... we are not digging any floors up..... 

 

Understood.  Solutions I can see if you cant somehow squeeze enough out of the heat pump are

• Supplementary heating
• Replace UFH with rads (because then the slab is in contact with air at 20 not pipes at 35)

Sorry to be the bringer of bad news, but UFH on an uninsulated slab is a massive gamble so far as I can see and, as you say, its not coming up so now the options are limited.

If this were my nice house in Sheffield (sorry I'm still envious of the location) I would bite the bullet, go for the first and get a suitable tariff to run it on, or a nice log burner if its permitted where you live.

@jamespa 

We have a wood burner, and use it.  But one day they might be banned.

All this is pretty depressing to be honest. During the build we had one window company go bankrupt on us, other issues building control who we paid 4000 to go bankrupt..... This house has killed me

Posted by: @davidnolan22

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@jamespa 

We have a wood burner, and use it.  But one day they might be banned.

All this is pretty depressing to be honest. During the build we had one window company go bankrupt on us, other issues building control who we paid 4000 to go bankrupt..... This house has killed me

I feel for you.  I have never done a major 'build'  personally but I have done several professionally, and I'm sorry to say that incompetent tradesmen and contractors going bust midway is par for the course.  Of course there are good people in the construction industry, but I would estimate it's at the 50-70% level max.  The rest are either incompetent, scammers, lazy & couldn't care less, or several of the above.  Despite never doing a major build I personally lost £5k on a scammy roofer a few years ago.

From what you say your installer guaranteed performance,  so you likely have the law on your side.  Depending on your appetite for fighting you may be able to use that to your advantage.  There are degrees to which you can take this, another individual here has steadfastly refused to let go for a year or more and is, I think, eventually heading for success albeit ar the cost of mych frustration.  That's one approach.  Another is to keep that approach in reserve and aim eg for getting some sort of fix, not the perfect one, free.  Alternatively just move on and fix it in whatever way is now possible, which is what (in some ways to my shame) i did with the roof.  You will need to think about what sits best with you.

Keep posting if you want more suggestions,  people here are willing to help!

Posted by: @wanttobegreen

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It may be clutching at straws but heat has to go somewhere, is it possible to insulate the external edge of the solid floors so that the whole floor becomes a giant low temperature heat sink?  Not ideal but long term would reduce heat loss possibly to a managable level.  Soil directly under the floor would gradually warm and reduce heat loss.  Maths for this is well above my pay grade but it just might help without digging up the floor.

this has been modelled for the passive house planning package and as a result there are simplified values come out. Heat loss with no insulation under the concrete slab uses a UValue of 2.0 (several variations based on water content level of soil/water table/ whether on a hill & the warmer water runs away etc). When the house has external insulation attached insulating footings too, and dug down to ~0.7m (practical limit) the U value of this ‘heat island’ is 0.7. In either case a lot of heat goes down into the ground not up into the house.

So if you want to use ufh you need to~10-15cm of insulation the minimum is building regs.

@editor I was surprised to read your praise for Mitsubishi for remote connectivity and telephone support for diagnosing problems. Here's my experience:

I have had a Mitsubishi Ecodan R290 10 kW heat pump since early September. The unit does not come close to meeting Mitsubishi’s published performance specifications.

Actual performance vs Mitsubishi specification:

• COP at 35°C flow, 7°C external: 1.4 (spec 5.1)
• COP at 35°C flow, 2°C external: 1.2 (spec 3.24)
• Noise level at 1 m: 68 dB (spec 47 dB)

My energy bills have doubled and my home is cold. It has had a real impact on our comfort and our life.

What happened when I asked Mitsubishi for help?

Despite reporting the issue on 22 October, Mitsubishi repeatedly delayed, passed responsibility around, and failed to take ownership. Their communication has been slow, contradictory and at times obstructive:

• Technical support acknowledged a site visit was needed 10 November, yet Mitsubishi failed to schedule one, then required a “warranty claim number” that their own staff failed to issue.
• I repeatedly had to chase, re-explain the issue, and even agreed to pay for the visit upfront just to get progress.
• Forms were “lost”.
• After weeks, the request was marked “urgent”, but still no visit date.
• When the unit finally failed with an error code (P8), Mitsubishi still offered no practical assistance or timeframe.

I also submitted a formal complaint referencing their own ISO9001 obligations. No response.

• The heat pump supplied is defective and far outside its published specification.
• Mitsubishi’s after-sales support has been unresponsive, disorganised and extremely slow.
• We are left with a cold home and extremely high electricity bills.

I expected far better from a company of Mitsubishi’s reputation. Based on my experience, and seeing over 140 other one-star reviews on TrustPilot with similar themes it is hard to see why anyone would select Mitsubishi.