I am reviewing the floor make-up of my build and would appreciate some thoughts. Question is - is a thicker slab better and how to control its temperature?

The house overall is approaching Passive standards in insulation and air tightness. The floor is going to be concrete slab with insulation (150mm PIR) and a screed. In the ground floor 'bedroom' part of the house, I am intending the build-up to be Compacted base, Concrete, insulation, 65mm screed. This area will have minimal heating. In the main open plan area (about 36m2) I intend to have UFH from the Solar Thermal and the build-up would be compacted base, insulation, concrete then screed (with UFH pipes). The idea would be to have a bigger thermal mass to try and offset low solar in winter and a bit of a heat dump in summer.

I realise the greater thermal mass slows down the response rate and my concern is it will be too slow. I am guessing that the slab needs to be at some minimum temperature even in the summer without it being so hot it actually heats the house. This is a building near Edinburgh so long periods of high temperature are usually not an issue. In winter, keeping a steady state should not be too hard, but in summer what sort of temp should the slab be at? Will running 18C water through it maintain it at a good temperature?

Alternatively, should I use the same as the rest of the house and just UFH a thinner screed? That seems the wrong solution.

Posted By: TimberWhat is the response time of the rest of the building? If timber frame with low thermal mass, a slow response time from the UFH system can cause issues with winter overheating, unless you have an intelligent control system.

Low Thermal mass building (looking like light steel frame). In winter I anticipate keeping a fairly steady state temperature. Part of the reason for the high thermal mass. ST will probably be insufficient for DHW but should be able to do a lot of the space heating at lower temps.

If you're really near passiv then keep the temperature rise of the slab controlled. Don't use thermostats in the air, but bury them in the slab. Use them to control the heat flow and keep the slab more or less at your desired ambient temperature. That way, if the room heats up then no heat is transferred from the slab since there's no temperature differential.

A normal UFH install provides around 10W/m2.K, which in PassivHaus terms means only a 1C rise to supply all the necessary heat even at peak load.

It's an elegant, if expensive, solution which also allows potential to use low temperature solar capture heat for space heating.

Posted By: MarkBennettIf you're really near passiv then keep the temperature rise of the slab controlled. Don't use thermostats in the air, but bury them in the slab. Use them to control the heat flow and keep the slab more or less at your desired ambient temperature. That way, if the room heats up then no heat is transferred from the slab since there's no temperature differential.

That was my thinking exactly. What sort of thermostats? I had considered running the water at a lower temperature so it simply reaches equilibrium (water and slab at same temperature).

Posted By: MarkBennettA normal UFH install provides around 10W/m2.K, which in PassivHaus terms means only a 1C rise to supply all the necessary heat even at peak load.

That is what I am hoping.

Posted By: MarkBennettIt's an elegant, if expensive, solution which also allows potential to use low temperature solar capture heat for space heating.

I don't think it will be too expensive as the tank of water will be there anyway, intend to do the pipework and controls etc myself and all I have done is reverse the intended floor build-up.

Posted By: tonyis a heating system even needed?

A question of risk. I would rather risk I do not need it than risk I do need it and don't have it.

Hi borpin

I'm building at a similar latitude (55.3N) albeit that I am on the very south coast of Sweden.

I have also considered solar heating so I may be able to help (or confuse) you.

First off have you or will you do a PHPP or similar analysis of how your building will perform? As the saying goes "if you're not assessing you're guessing". I found it very helpful to have good figures to model solutions for.

My house will also perform just above certified passive house levels, probably at 22kW/m2/a depending on my final choice of MHRV unit.

The house is around 200m2 and the expected heating demand in December, January & February is around 1000kWhr each month. I estimate that I would need about 60m2 of evacuated tubes to cover most of that demand, (my roof is 45deg pitch facing South East) but I haven't verified that with a tool like T*Sol or Polysun which would be advisable if you were going to go this route.

One of the problems of course with having such a large array is having to dump large amounts of heat during the summer, I think the best solution for this is to place a coil below your insulated slab (and have vertical insulation to a depth of maybe 1m at the edge of the slab) and dump excess heat to that so that you have a pool of heat in the ground below your slab. You may get some release of heat upwards into the house in the winter, but the main benefit will probably be to reduce your heat loss from the house to the ground. (My slab is already poured and I was too late thinking about solar heating to do this).

As regards the thickness of the slab I think that if you are going with solar heating it would be best to have a thick slab as it will help to smooth out the delivery of heat to the house over a 24hr period.

The danger of course is that if you have an uncontrolled fast reacting heat source such as a stove or solar gain from large south facing windows you could have periods of over heating.

But the bigger danger with solar heating (and ultimately why I decided to not go with it) is that if you have an extended period of gloomy weather with no solar input the temperature of the house will start to drop, so you need a carefully controlled back up heating solution to maintain heat in the house. In such circumstances heating your tank on Economy 7 could provide you with a relatively low cost solution for this, but unfortunately for me I don't have such a tariff available.

Hopefully that gives you a little food for thought.

Despite not going for solar heating myself I do believe that it is possible to do it even at our latitudes but it needs to be carefully designed and controlled to be effective.

Viking House seems to have the most experience on here with solar heating and what he does is to circulate the water from the solar panels direct to the UFH in the winter and thus avoid temperature drops across heat exchangers and the tank.

Of course to do that he needs to operate the solar panels via a drain back system. It also makes controlling the flow temperature in the slab impossible bar setting a minimum flow temp.

Thanks to your blog post borpin I took a look at your house design this morning. I'm no expert on this but relating this to my own calculations I think you will struggle to install enough solar panels on you South West facing roof to get a high solar fraction of your heating demand.

Your roof is at a lower pitch than mine (is it around 37-40degrees?) and the optimum at our latitude is I think around 60 degrees to maximise winter solar. And you also have a lot of rooflights which will disrupt how you can install panels.

You could go for an integrated solar roof but personally I am unconvinced of their efficiency and have not seen any independently tested & verified outputs for them.

You also have a projection which will shade significant portions of the roof at various times during the day.

I think before you go any further on this you need to do some calculations of how much solar energy you can harvest in the winter months and what the heating demand of your house will be.

Posted By: Chris P BaconOf course to do that he needs to operate the solar panels via a drain back system. It also makes controlling the flow temperature in the slab impossible bar setting a minimum flow temp.

I can't see any reason why you couldn't use a 4 port mixing valve or a thermostatic head with separate solar & underfloor heating circulation pumps mixing at a low loss header.

David

Posted By: tonyIn general it is not possible to catch enough heat via solar panels to heat a house in winter, it can help. It is probably better to spend on insulation during construction while it is easy

I have UFH pipes installed but have never filled them or used them (air tested) insulation and air tightness work for me.

Absolutely true Tony, even with 60m2 of good quality evacuated tubes some electrical back up would be required but I calculated it at slightly less than the electrical consumption of an ASHP to provide the same amount of space heating/DHW.

In my own case building fabric is at the limit of what is practical for most elements, 0.1 for the insulated slab 0.08 for the sloping roof and 0.095 for the walls, the windows could be improved but it is not economic for me to do so. Air tightness is planned to be below levels required for certification.

Beyond that I am limited by local planning laws in what I can build and cannot orientate the house to the optimum position.

The company supplying my frame are saying that they expect the house to perform significantly better than it does on paper and they have data from a number of previous builds to back that up, I am not counting on it but it would be a nice bonus if it happens.

Posted By: Chris P BaconThe company supplying my frame are saying that they expect the house to perform significantly better than it does on paper and they have data from a number of previous builds to back that up, I am not counting on it but it would be a nice bonus if it happens.

Despite all the PHPP etc, buildings do not seem to always perform as predicted!

Posted By: borpinI intend to have UFH from the Solar Thermal.

Ahh you see how one could have arrived at the notion that you were looking at pure solar heating.

How many m2 of solar panels are you going to fit? What is there predicted output during the heating season?

the big problem with trying to use solar for winter heating is that it provides the minimum when you need the maximum. I try to save some of the summer maximum til the winter.

I optimised solar hw for late autumn, early spring and am already saving excess heat for next winter

under my house

Well a while since i bin here, but regardless of Passive Haus design or not, I bes a confirmed beliver in Thermal Mass, since what we got works.
I have not had the underfloor heating on in three weeks, but the central located wall theromometer reads 17 deg C , and if? perhap?s I light the logburner in the front room later, it will................,
inside of 15/25 minutes force us to open the door to the stairwell, to cool down.
Our kitchen (in which I currently sit, at 19:30 Hrs) and the living room lie fully to the West so we get the full benefit of the evening sun when we are in residence.
Which said evening Sun is at the moment proper glorious as we sit with the kitchen patio doors open.
Wor bedroom obviousley lies in the East Wing, so we get the morning Sun there.
During this summer period we heat the domestic hot water with 28 sec oil.
I leave it running for about 1 hr max each morning and with a 210 litre tank that does until the next morning.
This period also heats a couple of radiators in our bedroom and the bathroom, probably only runs about 20 or 30 mins out of the 1 hour on-time.
I somewhat disbelivengily checked the oil tank this evening after three weeks of running for hot water and could not discern any difference from about three years ago, seriousley!
It,s a plastic tank so condensation is not keeping the surface level up!!!!
PS
The Guiness was good
the Bushmills is even better
An I'm on a Promise
As I said life is GOOD

PPS
Sometimes, just sometimes
Less Design n More Living
just WORKS

Figures, what figures?
All I know is that a week or a few days ago I figgered we might need to fire up the log burning boiler to dump a lok o heat into the underfloor,as the thermometer was reading 15 deg C , but didnay be bothered, simply lit the log burner in the front room, a couple o wermer days and hey presto we is back up to 17 deg..
All the log burner done was keep ussens warm o an evening in the front room.
Solar gain is great stuff hi Sur!

IMHO a mistake to put solar thermal direct to UFH. Times you may not need it, but it 'has' to be on. And times it can't supply it when you DO need it. The times when you might need it but solar can't supply it - unless of course you have garnered it when it's there, and used it when it's not! Go via TS instead, so UFH input is multi-faceted. Also can't agree with others on no air stats I'm afraid.

Orangemannot - whatever it is you're on, I want some!