Slab cooling/heating - to do or not...

[MarkH]

Hello.

We're soon to get cracking on our build again now the tourists have gone and I'm no longer employed to drive them in circles on a boat. We have arrived at the point where we must decide whether or not to install the pipes for slab heating/cooling and I'm right on the fence. On the one hand I think such a system might bring unnecessary complexity into a very simple building (and we like simple a lot). On the other hand maybe we're going to need to shift heat from the building on certain days and slab-cooling might be the best way to do this, we might also want to add heat...

 

I'm thinking more straightforward cooling methods exist for cooling: opening a window, summer bypass on mvhr (which we'll have anyway). And for heating an inline air heater on the mvhr would be a simple addition.

 

I'd appreciate any input from you all.

Thanks!

 

Mark

[jack]

If the question is just whether to put pipes into the (presumably insulated) slab, then I think you'd be crazy not to, mainly because you simply can't realistically go back and do it later if it turns out you need it.  If you don't need it, well at least you can tell a prospective buyer that they're there if you ever sell, because no-one will believe you when you tell them there's no UFH or radiators!

 

We're more or less Passivhaus standard, and I think our house would have been downright cold in the middle of winter without the slab being heated to around 21-22 degrees.  We have the additional problem of concrete floors throughout our ground floor - this makes the floor, and hence the house, feel cooler than if it was wood or carpet.

 

[jamiehamy]

With no science behind it, I can share our experience thus far. We have 12m2 of glazing on the lower SW elevation. My worry was that it would heat up too much on sunny days. We have no UFH, but 80mm of concrete slag on top of the Tetris. We built the overhang to be 500mm, which means in the height of summer, almost no direct sun hits inside in the middle of the day bar the sill area. In addition, we specified Solar Tint glazing to reduce the heat gain (on that elevation only). We have only been wind and water tight for a month, but we have an inside and outside temperature sensor installed to track patterns. 

 

In short, the internal temperature downstairs is fairly contant and only increases of decreases very slowly and steadily. When you track against the outside air temperature, even when it's 22degrees outside, it might only reach 16.8 inside before sundown (there is no heating yet). But during the night when it might drop to 11degrees outside, the internal temperature will not go below 15degrees before dawn. Our overall U-value is targetting 0.15. 

 

So, hopefully the theory has paid off and we will not suffer overheating in the summer. If required, we can open windows and put ion by-pass mode, but slab cooling was never on the cards - too complex. Only time will tell tho!

[jack]

9 minutes ago, jamiehamy said:

So, hopefully the theory has paid off and we will not suffer overheating in the summer. If required, we can open windows and put ion by-pass mode, but slab cooling was never on the cards - too complex. Only time will tell tho!

 

I suspect we're coming into the most dangerous time for potential overheating.  The sun is lower in the sky so penetrates further into the house and under overhangs, plus we still have some warm periods.  Certainly our house has at times been a little stuffier lately than I've wanted but it's generally been fixed by opening windows.  

 

We haven't yet bothered to enable cooling on the ASHP and I'm not sure we ever will.  In part this is because downstairs is always comfortable - it's the bedrooms that get warm, and they'd get hardly any cooling effect from reducing the slab temp.

[Mikey_1980]

Definitely put the pipes in .

 

We have a large double story glazed gable, the Temperature has reached 28 in one of the bedrooms on the hotest days this year but cools quicky with the windows open and just circulating the water in the slab, not cooling it evens up the house temperature and helps cool it in the evening.  we haven't got curtains up on yet but I expect these will help on the hotest days if drawn as they will be thermally lined.

 

The Kitchen stays at around 21/22 , I exptect becuase of the large vaulted ceiling and the 2 MVHR extracts in their.

 

 

[jamiehamy]

13 minutes ago, jack said:

 

I suspect we're coming into the most dangerous time for potential overheating.  The sun is lower in the sky so penetrates further into the house and under overhangs, plus we still have some warm periods.  

 

 

That's my hope! Bear in mind we're always a bit cooler up here, when the sun is low, hopefully it'll get to the back where it otherwise would not get. I suppose it depends how much the Solar Tint reflex - iirc the G-value was around 0.35 - would that be right? Partner still wants two 2.4x900 rooflights over the kitchen and dining area at the front but I'm pushing back until I know what the heating cycle looks like. You can see the tint in this pic - when you're below the level of the window it reflects the sky, but from above no difference. 

[Declan52]

Just put the pipes in and put caps on them. If you need to use the system then it's there if not all you are out a roll out two of pipe.

[Stones]

Another voice -  put them in.  You tehn have the option to use it simply as a regulating mechanism - pumping water around to keep house at stable temp, or adding heat.  This could be done simply and cheaply (in terms of capital outlay) using an inline immersion heater like this:

 

http://www.ebay.co.uk/itm/like/201418279478?lpid=122&chn=ps&adgroupid=28306099746&rlsatarget=pla-184492182546&adtype=pla&poi=&googleloc=9046984&device=c&campaignid=628859225&crdt=0

 

 

[TerryE]

I've got so much on my plate at the moment that I considering deferring ASHP selection and installation until a second phase starting 12-15 months after we have moved in and settled down. 

 

We (actually MBC) have put three UFH circuits in our slab as well as the ducting and electricity feeds.  So it will just be a case of retrofitting the ASHP.  This also gives a clean break with any Planning / BReg sign-offs.

 

We will need supplemental heating in the winter from day 1, so I am looking at installing an inline heater to drive the UFH as a single zone as @Stones suggests.  We will have a far better handle on the heating / cooling requirements after instrumenting the house through a full year of occupation, and we can then correctly size any ASHP or alternative system.  

 

PS.  I really can't understand all this fascination with glass. @jamiehamy, you must have what 15m² of glass there.  It's U-value is at least 10× worse than wall and this must generate up to ~8kW solar gain in direct sunlight which must all play havoc with your internal temperature control.   

Edited September 9, 2016 by TerryE

[jamiehamy]

13 minutes ago, TerryE said:

 

PS.  I really can't understand all this fascination with glass. @jamiehamy, you must have what 15m² of glass there.  It's U-value is at least 10× worse than wall and this must generate up to ~8kW solar gain in direct sunlight which must all play havoc with your internal temperature control.   

The fascination is about the view  

[TerryE]

It is a fantastic view,  but you only use two of these to appreciate it:

 

 

Seriously, I can understand why you have chosen all the window area -- as long as you are happy with the heating and cooling consequences of that choice, and have addressed them in your design. 

 

In our case we currently live in the centre of a village in a farmhouse built as part of a ribbon development from 1680-1850s. As with other farms, the farmland that was part of the farm attached to the house was split off and developed as a housing estate in the early 70s.  The LPA have insisted that we build our new house which sits in a plot divided off from our garden in a style that is sympathetic to the adjacent traditional stone cottages --  but to our rear we overlook some rather tired and poorly maintained 1970s houses, so no nice view  

[Stones]

9 hours ago, TerryE said:

I've got so much on my plate at the moment that I considering deferring ASHP selection and installation until a second phase starting 12-15 months after we have moved in and settled down. 

 

We (actually MBC) have put three UFH circuits in our slab as well as the ducting and electricity feeds.  So it will just be a case of retrofitting the ASHP.  This also gives a clean break with any Planning / BReg sign-offs.

 

We will need supplemental heating in the winter from day 1, so I am looking at installing an inline heater to drive the UFH as a single zone as @Stones suggests.  We will have a far better handle on the heating / cooling requirements after instrumenting the house through a full year of occupation, and we can then correctly size any ASHP or alternative system.  

 

I like the idea of doing this, and had my projected heating requirement been low enough that direct electric was a contender, then I think I would have done the same.  I suppose the only issue might be self discipline.  You put money aside to buy a heat pump a year down the line, and in the intervening period, you find a more pressing purchase requirement.

 

 

[TerryE]

4 hours ago, Stones said:

I suppose the only issue might be self discipline.  You put money aside to buy a heat pump a year down the line, and in the intervening period, you find a more pressing purchase requirement.

 

We have a slightly different financial dynamic:  we did the initial work from our savings, but have funded the bulk by taking out a new mortgage on our existing farmhouse, which we will sell as soon as we can after we are nearing completion, so we are cash constrained before this sale but not after. 

 

For us the issue might be exhaustion: if the direct electric solution is cheap enough and good enough, then we'd need a compelling argument to rework the system, for example a 10-year payback or some desirable functional improvements.   As I was discussion with Jan at least we would be in the situation we we could say with confidence that we need N kWhr/day heating if the ambient is 10°C, M kWhr/day heating if 0°C, etc.. We do / don't need slab cooling, ... so we could properly size and spec our system.

[MarkH]

Thanks everyone.

 

It seems like putting the pipes in anyway might be sensible. Considering our 85m2 of floor area (mostly one open plan room) is a single circuit ok? I'll be attaching the pipes to the reinforcement mesh. I'm getting different answers to the question of whether a single loop is ok or multiple loops leading to a manifold are the way to go.

 

Incidentally, we're also lumbering ourselves with loads of glass and the problems that entails. Besides having a nice view and being very lucky to have no-one else nearby we also like a house with a loose barrier between inside and out and all that (inefficient and expensive) glazing does give that. We're mitigating the problems with shading and overhangs and aren't worried all that much by achieving a constant 21 degrees so for us it's worth it. Probably. Hopefully.

 

 

[PeterW]

85sqm @150mm centres is around  550m of pipe give or take so it would be 5 circuits. 

 

They can can all be linked together as one back at where you want the manifold if it's just for shoving heat around but they should be reasonably similar in length. 

[TerryE]

As Peter says, you need to differentiate between heating zones and heating circuits.  You don't want joins in pipes and the standard pipe length is 100m so each circuit wants to be at most 100m and they all need to be as close as possible to the same length.  If you balance them and heat them together and in parallel then this is a single zone.  We have roughly 75m² slab so we were goign to have 4 circuits but dead areas under units, etc. dropped this so we decided to drop to 3, but in our case we are using the UFH system to pump at most a couple of kW into the roof environment so 3 vs 4 (and moving to 175mm centres) isn't an issue for us.  Given that you have a lot glass and the hot spots that this brings, then staying with the standard 150mm might be better in your case.

Edited September 12, 2016 by TerryE

[MarkH]

Thanks a lot. Good info.

[MarkH]

One other thing...

 

Is there any reason why I can't run the pipes up to the loft space above the bathroom (ours is a single storey building) and site the manifold etc there? I don't think that should be a problem...

 

Hard to find info on simple slab cooling or circulation systems, I've contacted a few companies with no useful response:

 

 

Quote

 

"Hello Mark,

We do not do any cooling.

Underfloor heating works well, Underfloor cooling is not recommend.

Kind regards,

"

 

 

 

Thanks!

[jack]

Underfloor cooling is usually cautioned against due to fears of condensation.  If you don't cool it down so much that there's a condensation risk, you'll be fine.

[Nickfromwales]

14 minutes ago, MarkH said:

One other thing...

 

Is there any reason why I can't run the pipes up to the loft space above the bathroom (ours is a single storey building) and site the manifold etc there? I don't think that should be a problem...

No problem whatsoever. 

As long as the pipe runs are kept ~ 100m ( I've run 130m without issue in less than ideal jobs and they've still been ok  ) then you'll be fine. 

Make sure there is an automatic air vent on each manifold rail ( one on flow and another on return ) and job done. 

No different to the ones I've done in 2-storey houses where I've put the manifold upstairs. . 

You need to Uber insulate the pipes between the slab and the manifold ( anywhere where it's not in screed basically ) so if it's ever heated the heat only goes where you want, and vice versa if you ever cool to stop condensation risk ). 

[MarkH]

Thanks! 

[jamiehamy]

On 9/9/2016 at 16:47, MarkH said:

On the one hand I think such a system might bring unnecessary complexity into a very simple building (and we like simple a lot).

 

I think the thread has proven this nicely  

[MarkH]

3 hours ago, jamiehamy said:

 

I think the thread has proven this nicely  

 

Ha! Maybe... Well we're going to install the pipes anyway - the cost isn't significant - blank them off and time will tell whether we ever progress from there. 

[Nickfromwales]

I can just see the spin-off thread of "how to retrofit Ufh pipes into cured concrete?"

Couldn't resist, sorry. 

[Onoff]

6 hours ago, Nickfromwales said:

I can just see the spin-off thread of "how to retrofit Ufh pipes into cured concrete?"

Couldn't resist, sorry. 

 

You can get a special tool to do it: