My SE is recommending I do NOT embed the UFH in the insulated slab...

[zzPaulzz]

Hi all. I just had this message from my SE: 

"You have previously indicated that you would like to place the underfloor heating pipes within the structural slab. As I see it this could create several issues. Firstly, there will be a great deal of stress in the slab, usually we would cut joints in the slab. These joints would allow the slab to crack along predetermined lines. These joints would be saw cut but I feel this would be very risky to cut the slab with the underfloor heating pipes within it. Also the stress within the slab may cause stress in the pipes themselves possibly leading to a burst.

Further to this if the underfloor heating pipes are embedded in the structural slab if there are any issues with the system, a pipe bursts ect, then the slab will need to be broken out to fix this. However, breaking out part of a raft will be difficult to repair, you will also have to take out the screed ect to be able to do this."

From discussions here and other forums I've don't think I'm asking for anything unusual but can  I point him to industry best practice that would help ease his concerns? 

[Canski]

maybe lower the finished height of the structural slab and put your heating pipes and screed on top ?

[IanR]

Can you clarify whether you are having a slab or a raft?

 

Typically a slab is poured inside of strip foundations and isn't load bearing, where as for a raft there are no seperate strip foundations and your load bearing walls sit on the raft, passing the loads into it.

 

My answers below assume you have an insualted raft foundation.

  

43 minutes ago, zzPaulzz said:

Firstly, there will be a great deal of stress in the slab, usually we would cut joints in the slab.

 

What flow temp is he considering? Has yours been calculated? If you are at PH levels of energy loss then flow temps are likely to be <35°C. Typically the raft temp will only be 1°C or 2°C above your target air temp, ie. 21°C or 22°C. I see greater increase in raft temp from solar gain than I do from UFH.

 

My raft is 465m² and I have no cut joints.

 

33 minutes ago, zzPaulzz said:

Further to this if the underfloor heating pipes are embedded in the structural slab if there are any issues with the system, a pipe bursts ect, then the slab will need to be broken out to fix this

 

Agreed with regards the repair, but not that high stresses may cause a burst. I did have to repair an UFH pipe, as I cut into it. My pipes were on top of the steel mesh and it was relatively easy to expose and repair.
 

 

39 minutes ago, zzPaulzz said:

you will also have to take out the screed ect to be able to do this."

 

Why are you having a screed?  Perhaps you should list your floor build up. Typically an Insulated raft only has floor finishes above it.

[zzPaulzz]

Yes, load bearing insulated raft foundation for a passive house.  I do not want to use a screed - only floor finishes (SE proposed a screed). My design flow temp is 34ºC.

 

Edited yesterday at 15:45 by zzPaulzz

[IanR]

58 minutes ago, zzPaulzz said:

Yes, load bearing insulated raft foundation for a passive house.  I do not want to use a screed - only floor finishes (SE proposed a screed). My design flow temp is 34ºC.

 

 

Is this an early "conceptual" section, or the finished design?

 

I'm surprised your FFL is not at least 150mm above the outside ground level. It's also an untypical sub-floor build up, there's no perimeter water management and a lack of insulation "overlap" where the raft steps from floor thickness to the beam thickness. What insulation and thickness is being proposed?

 

My raft from AFT is doing similar to what yours is trying to do, but looks more like (but not identical to):

Ref. https://www.advancedfoundationtechnologylimited.co.uk/our-products/passive-building-foundations/

Edited yesterday at 16:43 by IanR

[zzPaulzz]

Conceptual, yes. I’m using the MBC passive frame which is a twin wall structure with cellulose insulation in between. The internal frame is load bearing on the slab. External frame is on the EPS. Then there’s another small concrete base for the low brick wall ‘skirt’ (timber clad above). 

[Bramco]

1 hour ago, zzPaulzz said:

I’m using the MBC passive frame which is a twin wall structure with cellulose insulation in between.

 

If you are using MBC, why aren't you using their insulated slab?   They know what they are doing.

 

Ours has the UFH pipes installed as part of the slab before the concrete pour. Never heard anything from an SE about having to cut expansion slots or whatever. Not sure what our ground floor is but it's got to be 175m2.  Our flow temperature is probably just above 25C. Given you are using the MBC PH build, you must be aiming for PH-ish energy requirements. Why would you run flow at 35C

 

Anyway, the great thing about these slabs are that they are like having a thousand of the old storage heaters which you can batch charge on cheap electricity - probably your plan - but it makes for a v comfortable house. I'd sack off your SE and get MBC to do the slab.

 

 

[zzPaulzz]

MBC’s quote was high, and assumed benign ground conditions.  They also said I’d probably get it done cheaper locally. I’ve got clay and trees on the site. I do agree though that if my SE can’t get his head around the UFH in-slab then I’ll go elsewhere. 
Re the flow temp, I’ve used LoopCad to estimate it based on a 3kw peak heat output over 100m2 ground floor and a 300mm spacing. If I go to 200mm then I’d probably be where you are at. 

[Onoff]

A142 mesh, fibres with removable timber boards at the designated crack lines. Remove and fill later?

 

Unless your wanting a polished concrete floor? 

[zzPaulzz]

That’s sounds like one solution though might be hard to execute. No polished concrete. I’m assuming the joints to stop cracking aren’t full depth as he mentioned cutting them in after the pour. 

[Nick Laslett]

That is a real shame. I was determined to have an insulated raft with embedded UFH.
 

I could not be happier with the outcome, even though the engineered timber flooring is covered with underlay and correx the ASHP is running on weather compensation with a max flow temp of 27°. The temperatures of the house is 19-20°, the floor feels cold to touch and it doesn’t feel like there is any heating. It is a bit like magic. MVHR still to be commissioned, so windows are open to provide some ventilation. So far very happy. I think the house might actually be exceeding the heat loss from Jeremy’s spreadsheet. 

[zzPaulzz]

@Nick Laslett me too, so not giving up yet!  However, I am finding it hard to source any ‘professional guide’/standards for UFH in-slab to pass to my SE. All the suppliers seem keen to push their proprietary UFH tracks and screed solutions, perhaps because they can sell more ‘stuff’?

[Nick Laslett]

@zzPaulzz I went down a rabbit hole on the different types of UFH pipe, because I wanted the pipe embedded in the concrete, but it was hard to find details from suppliers if they endorsed this use case for their product. 
 

Optimum underfloor were one of the few at the time that had a technical drawing showing their pipe embedded in concrete slab. 
 

https://www.optimumunderfloor.co.uk/s/Optimum-Installation-Guide.pdf

 

@garrymartin provided details of the Warmup specification for UFH pipe embedded in concrete in this post. 
 

I have the generic Kore Technical Drawings from my foundation design that show the UFH in the raft. In the NSAI certificate for their system it states tha UFH can be in the raft. They have a recent blog post that elaborates; “When fitting underfloor heating pipes, the pipes can be clipped to the steel mesh or to the KORE Floor Insulation.”

Edited 23 hours ago by Nick Laslett

[Gus Potter]

7 hours ago, zzPaulzz said:

Hi all. I just had this message from my SE: 

"You have previously indicated that you would like to place the underfloor heating pipes within the structural slab. As I see it this could create several issues. Firstly, there will be a great deal of stress in the slab, usually we would cut joints in the slab. These joints would allow the slab to crack along predetermined lines. These joints would be saw cut but I feel this would be very risky to cut the slab with the underfloor heating pipes within it. Also the stress within the slab may cause stress in the pipes themselves possibly leading to a burst.

Further to this if the underfloor heating pipes are embedded in the structural slab if there are any issues with the system, a pipe bursts ect, then the slab will need to be broken out to fix this. However, breaking out part of a raft will be difficult to repair, you will also have to take out the screed ect to be able to do this."

From discussions here and other forums I've don't think I'm asking for anything unusual but can  I point him to industry best practice that would help ease his concerns? 

Hope this helps a bit. The following is general but this is the way I do it... the following is simplistic and I mess about later on technically..to value Engineer where I can.. but roughly..

 

If you want to cast your UF into the structural slab (think of a structural slab like a wide reinforced concrete beam.. you don't put movement joints in a beam). By definition this would be a continuous slab with no crack /movement joints. To design a structural slab for strength, shear and deflection etc I would take the effective structural depth as being to the underside of the UF pipes. The bit above that is neglected... just like a screed. But in this case you just cast the slab to the full depth.

 

If you have say a ground bearing slab with movement joints then each bay gets it's own UF loop that does not cross the joints.

 

UFH design is much based on common sense and past experience.

 

To my mind there are too many folk that are trying to make wet UF seem like an exact science. This I can tell you from forty years experience.. it is NOT!.. well it may be for the first year or on a calculation sheet (if you get really lucky) but after that when the controls start playing up and no one has a clue about the design philosophy / how to operate and maintain it. Then you actually need to look at how the stuff gets installed on site which can blow any theory out the water any way! In the heat of battle on site I would challenge most folk to stick to drawings!

 

Cut yourself some slack. There is no point in paying for a UF design you can't deliver practically on site. I see this all the time... folk just waste their money.

 

I'm a massive fan of UFH and have put these systems into my own self builds and renovations. But I always go for the simple stupid.. like they do often in Nordic Countries.

 

Now simple stupid does not mean the system is uncontrollable. But it needs the home owner to be aware of how their house warms and cools,that is you weather compensation!

 

@zzPaulzz Go back and have another chat with your SE. It seems like you are working at cross purposes.

 

 

 

[Nickfromwales]

1 hour ago, zzPaulzz said:

@Nick Laslett me too, so not giving up yet!  However, I am finding it hard to source any ‘professional guide’/standards for UFH in-slab to pass to my SE. All the suppliers seem keen to push their proprietary UFH tracks and screed solutions, perhaps because they can sell more ‘stuff’?

@Nick Laslett HAS got this, and it's doing great, is what he's saying. Not sure if you're confused there, sorry.

 

FYI @zzPaulzz, I have done a number of MBC PH TF M&E installations, (I got recommended directly by MBC or otherwise got awarded the project after MBC engaged), and there has been UFH in a 100mm constructional slab on EVERY SINGLE ONE . 

 

I have worked with many SE's to date, and found Hilliard Tanners to be the most approachable and pragmatic by far (including myself designing foundations for ICF builds with insulated rafts + UFH etc) and they accommodated my methodology; only once they had suitably scrutinised everything I had sent them and they were then 100% satisfied to produce a design that we could then use on site to instruct the groundworkers to build the foundation from.

 

I've NEVER fitted expansion gaps on a 'passsive raft', NEVER cut expansion gaps in anything other than much thinner and non-constructional screeds, and most were tiled over. Zero cracks / other evidence of cracking etc whatsoever. I always leave it a full 6-9 months to allow the slab to cure before applying the tiles, other floor coverings can go down much sooner, but the builds normally progress sympathetically to that timeframe being acceptable / possible.

 

The running temps of the UFH in the raft are just so low the damn things don't move much at all, if anything!

 

The MBC PH TF I have just moved clients into has a flow temp into the raft of 26^oC, and I'll bet that gets dropped back 1-2^oC by next winter, once they've got the house bone dry and properly acclimatised. 140m2 L-shape slab.

[Nickfromwales]

14 minutes ago, Nickfromwales said:

and there has been UFH in a 100mm constructional slab on EVERY SINGLE ONE

To add:

 

100mm, and with only 1x layer of reinforcing steel mesh too  

 

Thicker at intermediate junctions and ring beams etc, obvs.

[IanR]

1 hour ago, zzPaulzz said:

 I am finding it hard to source any ‘professional guide’/standards for UFH in-slab to pass to my SE. 

 

Insulated Rafts remain pretty niche in the UK, so you'll not find a standard way of incorporating UFH. They are however quite standard in Sweden, which is where Advanced Foundation Technology originate from. AFT Engineered and supplied my insulated raft including UFH from ThermoTech (Sweden) - Olof even dropped by to help install the UFH. The Swedes do their UFH a little differently to the UK but they take their plumbing very seriously.

 

 

 

 

[Nickfromwales]

3 minutes ago, IanR said:

 

Insulated Rafts remain pretty niche in the UK, so you'll not find a standard way of incorporating UFH. They are however quite standard in Sweden, which is where Advanced Foundation Technology originate from. AFT Engineered and supplied my insulated raft including UFH from ThermoTech (Sweden) - Olof even dropped by to help install the UFH. The Swedes do their UFH a little differently to the UK but they take their plumbing very seriously.

 

 

 

 

Nice job.

 

I very much like the idea (simplicity) of installing the mesh atop the UFH pipes, as the UFH goes down very quickly and simply, just it pains me to trust the groundworkers to not damage the pipes during the steel installation.... If one company is doing the lot then it's managed and there should be some quality control and robustness of their procedures, but if left to Tom > Dick > Harry then I'll always prefer leaving the meat get off site and then I install over the top before the concrete goes down.

[Gus Potter]

37 minutes ago, Nickfromwales said:

I have worked with many SE's to date, and found Hilliard Tanners to be the most approachable and pragmatic by far (including myself designing foundations for ICF builds with insulated rafts + UFH etc) and they accommodated my methodology; only once they had suitably scrutinised everything I had sent them and they were then 100% satisfied to produce a design that we could then use on site to instruct the groundworkers to build the foundation from.

Yes they are a good personable team. In fact I'm working with them on a project. I also have found Hilliard and his team very approachable and willing to engage to date.  My outline SE brief from the Client is to coordinate a Hilliard basement design with a superstructure. In some ways my brief is to act as the overall supervising Engineer for the project. I'll spot check say 10% of Hilliard's calcs and make sure their stuff makes sense in terms of overall stability and performance.. everyone has a boss / someone watching over.. even Tanners!

 

44 minutes ago, Nickfromwales said:

I've NEVER fitted expansion gaps on a 'passsive raft',

For all .. Nick mentions a " passive raft" this tends to be a structrual concrete slab and by my previous post you would expect that to have no movement joints.

 

But you can also do passive house design with a direct ground bearing slab on insulation and this would often have movement joints. We make these thin as we can with minimal anticrack reinforcement or use concrete with fibres to control the cracking.. @saveasteading has implemented this approach.

 

@Nickfromwales I think we are both singing from the same hymn sheet here.. practical and buildable design!

[IanR]

16 minutes ago, Nickfromwales said:

just it pains me to trust the groundworkers to not damage the pipes during the steel installation.... 

 

The Thermotech pipe helped here, it's very robust and doesn't kink. It's Ø17 polyethylene which won't damage from the mesh being trodden on squeezing the pipe.

[Nickfromwales]

10 hours ago, IanR said:

 

The Thermotech pipe helped here, it's very robust and doesn't kink. It's Ø17 polyethylene which won't damage from the mesh being trodden on squeezing the pipe.

Yes, I noted that, and I use Pex - Al with the aluminium liner as part of the MLC construction, hard as nails, you really have to pretty much maliciously damage it tbh, but still stops one from sleeping well in the hotel bed if done 'that way'. Done a couple, and it went well, but I insisted on being there and also having the pressure up to around 4bar for those ones. 

 

Conversely, on others, I rarely pressure test as the pipe is just too good. MBC don't test iirc, and they're putting this stuff in by the multiples of km's per month. I've heard of only one failure since 2016!! That client isolated one loop, and then all was fine (ish), and then said they couldn't really notice it wasn't heated for that part of the slab  The joy of a passive raft and a well insulated, airtight house with MVHR!! 

[Gus Potter]

1 hour ago, IanR said:

The Thermotech pipe helped here, it's very robust and doesn't kink. It's Ø17 polyethylene which won't damage from the mesh being trodden on squeezing the pipe.

Hey Nick you know I'm a bit of a philistine, old skooool. Below is part of my temporary UF manifold I knock up 4 -5  years ago or say (yes compression fittings and so on)! Still working. Also I designed my first UFH system 30 years ago and have done a few other designs since then.

 

 

I look after a big house (500 m sq) that has 16 dia pipe. It makes maintenance harder and more expensive.

 

Folks. Stick to British standard 15 dia pipe. Yes you'll get less flow in the pipe.. but see in a few years time when you come to look at the maintenance costs.. you may think.. that philistine Gus had a point!

 

17 mm dia.. I just smell shite.. sell it cheep and bump up the part replacement costs.

 

Folks.. a good plumber will cost you £250 to £300 a day maybe plus vat.  Now if they have to get in the van to find 17mm dia replacement parts they are going to charge you for their time in addition.

 

Just to lay this on a bit thick.. for the unwary. My wife likes to put a rug on the floor which insulates it. We also have some big furniture that traps the heat from the UFH. UFH needs to be designed with a broad brush approach and pragmatically to compensate for how we may change our life style.

 

For the unwary.. say a valuer comes round to your house and asks.. does your heating perform if we have big furniture and want to lay down some rugs? That could be a bit of a bummer? They may also ask if you have a maintenance contract and if so the quarterly cost.

 

I can tell you from experience that you can get UF to work with a 15 dia BS compatible pipe. In fact now that the insulation regs have got more stringent the challenge is less than it was before. As an aside.. a larger dia pipe delivers the heat in a more concentrated way.. now look at the acceptable temp range of say an engineered wood flooring! You actually have a better chance of meeting the flooring guarentee requirements with a 15 dia pipe at closer centres than a larger dia pipe.

 

For all UFH comes with challenges! From time to time you just can't meets all the manufacturer's requirements and you just have to take in informed view.

[IanR]

22 minutes ago, Gus Potter said:

17 mm dia.. I just smell shite.. sell it cheep and bump up the part replacement costs.

 

??   All available over the counter at my local plumbing outlet. Pretty standard size across the Continent.

[Nickfromwales]

14 minutes ago, Gus Potter said:

Folks. Stick to British standard 15 dia pipe

 

14 minutes ago, Gus Potter said:

I can tell you from experience that you can get UF to work with a 15 dia BS compatible pipe.

You can certainly get the stuff but it's pert not pex, and is a bit less 'hardy' IMO.

 

18 minutes ago, Gus Potter said:

Now if they have to get in the van to find 17mm dia replacement parts they are going to charge you for their time in addition.

Ok, less gin and more tonic needed here   "Testing, testing, 1,2,3. Can you hear me at the back?" lol.

 

The UFH pipes only emerge from the floor, for less than 1000mm in most instances, and then get made off to the manifold. There's nothing to change / fix / visit the van for here! Maintenance plumbers won't ever be changing that pipework??

 

For completeness, yes, don't use the same non-standard pipe for all the domestic internal plumbing of the hot & cold feeds, 15mm all the way, but NOT in the MLC as that's just overkill; and a PITA to pull in single-handed, ask me how I know.

 

23 minutes ago, Gus Potter said:

17 mm dia..

That was from a Scandinavian (IIRC) company, imported on a design / supply / install basis. I've done 16mm / 17mm / 19mm / 20mm and so on, Vogel & Noot were 17mm IIRC, so it's no matter tbf as it's going under the floor to never be seen again  

 

"Thank you, and good night" 🤗

 

[saveasteading]

1 hour ago, Gus Potter said:

@saveasteading has implemented this approach.

Everybody is right here....my pennyworth tomorrow.