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Heating and cooling: supplementing heating with air with UFH?

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  1.  
    • CommentAuthorjamesheath
    • CommentTimeNov 29th 2016
     
    Hi all,

    I'm in the throws of building a non-certified, but (hopefully) compliant, passive house, and am struggling with conflicting advice over the heating.

    The house is big - 450m2 TFA, and three storeys.
    We have an insulated slab, and then a timber frame sitting on top of it.

    The main heating is by a combination of towel rads in all bathrooms, and wet batteries on the MVHR air supply.
    These are supplied from a 1500L thermal store, which itself is fed by a fairly large solar thermal array, a gas boiler, and a wood burning stove (I know I know - crazy in a passive house, but we wanted one!).

    This may well all work wonderfully, but there are two concerns...

    1/ It is possible that more heat will be required on really cold days - PHPP says we can't heat with hot air alone (we're about 1kw short) that may be met by the towel rads, but the heat from them is surely mostly heating the air anyway? We could just light the wood burner, but I may not be around to do that, and it then wouldn't happen.

    2/ It seems possible that if the house is warm enough on average, there may be stratification of the temperature over 3 floors. The lowest floor has a one bed flat for my Dad - and the living room there needs to be warm. The next floor up is our living area - where we want it warmest. the top floor has the bedrooms, where if anything, we'd be happy for it to be a degree or so cooler. (and yes, on typing this out, I realise we probably ought to have designed it the other way up).

    Our ideal plan would be to build the house, spending just a little extra on allowing for retrofitting of stuff to fix these issues If they arise, spending more to combat specific issues after living in it for a winter, rather than over spending now.

    Luckily, one step in that direction is relatively easy to do - there's a wet heating circuit in for the towel rails, so we can fairly cheaply leave some points of that behind plasterboard to allow retrofitting of small rads. It just so happens that nearly all of these can actually go on the other side of a wall from the towel rads, or where the heating pipes were running anyway.

    The other option in addition to that, that I'm wrestling with now, is whether to put UFH circuits in to the insulated slab. For the most part the lowest floor will have the slab polished as the floor finish.
    To minimise the spend on something that may never be used, I would likely lay the pipes and temp sensors only (I would pressure test though). This would probably cost 300 - 400 quid, and a couple of days of my time wrestling with PEX pipe.
    I'd then add the manifold, mixing valve, controls etc if needed.

    If needed this would dribble some background heat in to the bottom of the house, and the hot air would then do the rest and get the room temps right.
    I'd likely control the temp of the slab to a few degrees above the desired ambient temp, and let the hot air do the rest.

    Is the UFH:
    a) a bad idea
    b) a waste of cash
    c) riddled with difficulties

    -if- people out there think the UFH has legs (it would be nice to have a slab just slightly warmer than the room I think?), what kit is out there to control the slab to a given temperature? I was going to just put some DS18B20 one wire temp sensors in the slab & roll my own control, but that might be dumb if what I need is out there.
    (I'm comfortable with code & soldering irons!)

    I'm also really keen to hear from others living in passive houses with unheated insulated slabs - do they feel cold? and anyone in a large house heated predominantly with air - does the top get other than the bottom?

    Sorry that was so long winded!!

    Thanks,

    James
    Hove
    • CommentAuthorgyrogear
    • CommentTimeNov 29th 2016 edited
     
    <blockquote><cite>Posted By: jamesheath</cite>whether to put UFH circuits in to the insulated slab</blockquote>

    Well, you will only get one chance to install UFH tubing in a slab, so IMO, definitely go for it ! PEX or whatever has peanut-level cost up-front, and like you say, add the extra gear later if required.

    Perhaps also consider radiant walls ?

    You will likely get some stack effect as you say, so you might also consider a duct solution to drag overly warm upstairs air back d/s where it would be of more use (or at least leave reservations / penetrations somewhere, to allow installing a stack later...).

    gg
  3.  
    At the points where you are making provision to add radiators I would ensure that you also have an electrical supply so that you can use fan coils rather than rads. They will give you a greater output for a given size and so can be reasonably discreet.

    Because of the higher surface area for heat loss your top floor will tend to be a degree or two cooler than your middle floor.

    We have a low energy house which we heat with UFH on the ground floor and electrical heating battery in the MVHR for the first floor. At 900W the heater is too small to heat the entire house via the MVHR.

    If you are not already doing so I would design the MVHR in three separate branches, one for each floor, with a duct heater in each branch so that you have a modicum of control of the temperature on each floor given their varying heat requirements.
    • CommentAuthorcjard
    • CommentTimeNov 30th 2016
     
    Concern 1: use a fan heater for the coldest days. The cost of purchase (£20 from Argos) plus the lifetime energy use at a COP of 0.9 will trounce the cost of a more involved heating system. And it's portable.

    Concern 2: it might end up defying your notion of physics, as warm air is pretty lazy and a poor way to get heat around he place. I have a fairly large (280sqm over 3 floors) not-passiv-but-well-insulated-and-reasonably-airtight house with a huge open stairwell that is the full height of the building. The ground floor and first floors are heated, the top floor has some token amount of ufh pipe fed off the manifold from mid floor, but gets most of its heat from the sun, as there is about 40 sqm of roof glazing. Right now the heating system runs, but there are no doors as it's still being built. The ground floor is noticeably warm, with the stat set to 15 (feels like 19), the mid floor somewhat cooler feeling(never measured the temp, feels like 17) and the top floor varies wildly due to the sun
    Main point is the warm air downstairs, though it does seem to circulate because I can feel a slightly cool draught when standing next to the full height hall wall, doesn't heat the upstairs to any significant degree, nor does the air elsewhere but the hall move about much. Some things may change when the MVHR is activated, but air is generally a crappy way to transport heat energy so I'm not expecting a sea change in the general behaviour of the house. All in, the temps are about what I want, with the living area slightly warmer but maybe stuffing the ceilings with acoustic wool has limited the heat transfer through the floor.. This is further compounded by her wanting 2.2 tog carpet upstairs so a few tweaks may be required in future. My caution to you is, really don't expect the "hot air to do the rest" even with an MVHR pushing it around.. Passivhauses can get away with having just duct heaters in the MVHR for heating, but that doesn't necessarily mean they can be effectively heated by warming one area and then trying to get that air moved to a colder part of the house

    If you're laying pipe now, and it's a sensible idea, use the aluminium stuff (pert Al pert or pex Al pex) as it's nicer to work with, stays bent. See if the place that you buy it from will loan you a staple gun. Two person job, I've a video somewhere of the lads laying mine- took four of them about 4 hours to do 180 sqm including connecting up 2 manifolds so it shouldn't take you long, but I'd advise getting a helper to staple as you lay. Don't forget it can be used to cool a house too, which may come in useful.
  5.  
    Thinking back, the first winter we were in the house 3 years ago we relied primarily on the heater in the MVHR as I hadn't installed any controls for the UFH at the time.

    Our PHPP estimated that we required 2,6kW at peak load. However we found that the 900W heater was sufficient to maintain the heat in the house almost all of the time.

    The main problems occurred if we were away for more than 3 or 4 days at a time in mid-winter. Without the passive gains from occupancy, cooking etc. the indoor temperature would gradually decline and on return would need a big boost from a number of electric fan heaters to get it back to normal.

    Another point is to play around with the ventilation rates in PHPP. By increasing the ventilation rate you should find a point at which your duct heater can provide sufficient heat to maintain the heat in the house at peak demand. Then look to see if your MVHR unit can provide that ventilation rate, if so then just boosting the ventilation rate during the coldest times will be sufficient to maintain the indoor temperature at the required level.
    • CommentAuthorjamesheath
    • CommentTimeNov 30th 2016
     
    Thanks all for your really helpful input.

    Gyrogear - I'd agree that it's not huge effort or expense to get them in now...

    Crispy - good point about power to rad locations and split duct heaters - noted. ditto the option to over-ventilate - the MVHR units are oversized. on your GF do you have the slab exposed anywhere - if so, in your first year in the house, prior to having the UFH on, was the slab noticeably cold?

    cjard - yep - my wife suggested the fan heater. TBH it really does need to be considered. I get that I can't rely on the MVHR to move the heat around.

    All - If I bite the bullet and put the pipes in, what spacing do I put them at? assuming 15mm pipe, standard for a new build seems to be 200mm, but that comes out with crazy amounts of heat. Given I'm not expecting fast response times (like I'm expecting a week for it to warm up, and then maybe a day or two if I want to change the temp).
    I did some fag packet calcs and came up with anywhere between 400mm and 1m !!

    Thanks,

    James
  7.  
    Surely an electric heater has a COP of 1.0 not 0.9?
    •  
      CommentAuthordjh
    • CommentTimeNov 30th 2016
     
    Posted By: jamesheathIt seems possible that if the house is warm enough on average, there may be stratification of the temperature over 3 floors. The lowest floor has a one bed flat for my Dad - and the living room there needs to be warm. The next floor up is our living area - where we want it warmest. the top floor has the bedrooms, where if anything, we'd be happy for it to be a degree or so cooler. (and yes, on typing this out, I realise we probably ought to have designed it the other way up).

    As others have said, do not ignore the fact that the top floor has a much larger exposed surface area - the roof, so it is not likely to be any warmer than downstairs and may be colder.

    How well insulated are your MVHR supply ducts? They need to be insulated if you expect any heat to make it from the heat battery all the way to the rooms. I have an electric duct heater, which is slightly oversized from the PHPP design value but the only room where warm air comes out of the supply valve is the bedroom closest to the MVHR unit. You can tell the difference between unheated and heated air at the other terminals but only just. It was unexpected, but I don't really mind because the heat is going into the house anyway - just into the first floor structure instead.

    The duct heater is limited by thermostats and flow sensor, so as already said increasing the ventilation boosts the heating capacity.

    My approach to heating was to suck it and see. I decided against gas or a wet system, so I just laid some extra power radials. So far though I've just been plugging things in to the ring circuits to establish what works. Last winter I installed the duct heater and running that overnight was sufficient last year. This month I've also used a 400 W quartz heater I run overnight in the hallway to spread a bit more heat around because there's been very little sun. I don't yet know how much power the duct heater actually draws though. It's rated at 1.8 kW but it has two thermostats that might well be reducing the effective power.

    Our slab is unheated and doesn't feel cold but perhaps not a useful comparison. It's got a battened timber floor on top and we normally wear slippers anyway. If you're already committed to a wet system and UFH pipe is really so cheap then I think I would stick it in.
    •  
      CommentAuthordjh
    • CommentTimeNov 30th 2016
     
    Posted By: jamesheathAll - If I bite the bullet and put the pipes in, what spacing do I put them at? assuming 15mm pipe, standard for a new build seems to be 200mm, but that comes out with crazy amounts of heat.

    I'm no expert but doesn't the heat output depend on the flow temperature and isn't the secret of automatic slab temperature regulation in a well-insulated house to use an extremely low flow temperature, just a few degrees above the air temperature?
    •  
      CommentAuthorjoe90
    • CommentTimeNov 30th 2016
     
    Our new build ( to near passive levels) had the UFH pipes at 150mm, I thought this was too close together but was told it would aid uniformity of heat levels and heat up times would be better.
    • CommentAuthorringi
    • CommentTimeNov 30th 2016
     
    Given:
    a) Heat pumps work best with a low flow temperature
    b) You may want to put a heat pump in one day
    c) Closer spacing of the UFH pipes allows a lower temperature of water in them
    I would be putting the pipes in as close as I could without getting issues on bending the pipes etc, therefore 150mm seems about right.
    • CommentAuthormw116
    • CommentTimeNov 30th 2016
     
    I wouldn't spread the pipes too far apart - you end up with warm and cold lines on the top of the slab. I think it depends on how deep they are, but 150-200 sounds about right. And think carefully about how you layout the pipe - the floor at the hot end of the pipe gets significantly warmer than the floor at the cold end...
    •  
      CommentAuthorjoe90
    • CommentTimeNov 30th 2016
     
    My suppliers guide says best to put "hot" end of the loop next to patio doors or windows as these are the places that cool the most/quickest.
  14.  
    Posted By: jamesheath on your GF do you have the slab exposed anywhere - if so, in your first year in the house, prior to having the UFH on, was the slab noticeably cold?
    No exposed slab, but various areas of ceramic tile, hall, utility room & bathroom.

    Even now with UFH these areas are cold in barefeet for most of the day. The UFH only runs for about an hour a day and the flow temp is around 28-30C, when it is running and for a while afterwards the floor feels warm to touch but don't expect to be walking around with toasty feet all day if you install UFH.

    Our UFH pipe is 16mm dia. and installed at the bottom of a 100mm slab, it's laid at approx. 450mm centres in four loops all run as one zone on the ground floor.
    • CommentAuthorjamesheath
    • CommentTimeNov 30th 2016
     
    Thanks again all.

    Joe 90 - good point about running out past sliding doors etc!

    Chris P - what's your flow temp and your return temp? (I'm not sure if the 28-30 you state is flow & return, or flow etc) Your pipes are spread a good bit wider than most peoples, but your setup seems to be much closer to my plan than most. It sounds like the temp of your floor varies quite a bit through the day - any idea how much?
    My slab is a good bit thicker at 175mm, so everything will be slower to react.

    Thanks,

    James
  16.  
    Posted By: jamesheath
    Chris P - what's your flow temp and your return temp? (I'm not sure if the 28-30 you state is flow & return, or flow etc) Your pipes are spread a good bit wider than most peoples, but your setup seems to be much closer to my plan than most. It sounds like the temp of your floor varies quite a bit through the day - any idea how much?
    My slab is a good bit thicker at 175mm, so everything will be slower to react.
    It's been quite a while since I last measured the flow and return temps but from memory the flow temp is around 28-30C and the return normally around 2 degrees lower.

    I can't remember having measured the surface temp. of the floor when the UFH is running but just now it is sitting at the same temp. as the rest of the ground floor ~21.5C

    You won't need to worry about reaction times, your nice thick slab will give you a great buffer if you do use UFH. One of the main aims of Passiv hus is to have a nice stable indoor environment, so no large fluctuations in temperature and no draughts.
    • CommentAuthorringi
    • CommentTimeNov 30th 2016
     
    Posted By: Chris P BaconThe UFH only runs for about an hour a day and the flow temp is around 28-30C


    Have you tried running it for longer but at a lower flow temp?
    • CommentAuthorringi
    • CommentTimeNov 30th 2016
     
    Posted By: jamesheathMy slab is a good bit thicker at 175mm, so everything will be slower to react.


    You will NEED some heating that is a lot faster to respond, otherwise when the system has been off for some time, you will overshoot. Electric fan heaters will do the job.
  19.  
    Posted By: ringi
    Posted By: Chris P BaconThe UFH only runs for about an hour a day and the flow temp is around 28-30C


    Have you tried running it for longer but at a lower flow temp?

    It had been my plan to run the system with a flow temp of around 23C but never managed to get the gammy mixer valve to control the flow at that temp.

    Posted By: ringi
    Posted By: jamesheathMy slab is a good bit thicker at 175mm, so everything will be slower to react.


    You will NEED some heating that is a lot faster to respond, otherwise when the system has been off for some time, you will overshoot. Electric fan heaters will do the job.

    Unless you are planning on spending half the winter in the Caribbean when would the system be ever off for some time?

    I don't have any time clock on our system it's just set to a minimum room temp. of 20.5C and it runs as needs be.
    • CommentAuthorringi
    • CommentTimeNov 30th 2016
     
    Posted By: Chris P BaconUnless you are planning on spending half the winter in the Caribbean when would the system be ever off for some time?


    It only takes someone going away for 2 weeks and setting the room temp to the frost setting (5c).
  21.  
    Posted By: ringi
    Posted By: Chris P BaconUnless you are planning on spending half the winter in the Caribbean when would the system be ever off for some time?


    It only takes someone going away for 2 weeks and setting the room temp to the frost setting (5c).

    Why on earth would you do that?
    • CommentAuthorgyrogear
    • CommentTimeNov 30th 2016 edited
     
    Posted By: joe90My suppliers guide says best to put "hot" end of the loop next to patio doors or windows as these are the places that cool the most/quickest.


    My understanding is somewhat opposite: our electric radiant UFH (1983 vintage...) stops 2 meters shy of our south-facing sun windows.

    The thinking is dual: on the one hand, this stops hard-earned cash being injected straight out via the window; and on the other hand, the "bare area" maximizes solar gain from off of the window (since it is the sun that heats up that part of the slab...) (because the slab is cooler because it has no emitters in it). So this means an economy as regards the cost of constructing the heated slab: in our case, the slab is 12 x 5 = 60 m2, and the "empty" zones are (2 x 2.8) x 2 windows = 11.6 sq.m "missing radiant" = basically 20% less.

    The second part of the installation comprises a second slab (north rooms), for another 40 m2.
    So out of a total of 100 m2 of slab, basically 12% is unheated.
    System draws 6.5 kW therefore over 88 sq.m that gives 73 w/m2.

    On a somewhat different note, we have actually retired these floors - i.e. they are not used. Despite the nice side of UFH, it turns out pretty expensive nonetheless, a kWh being a kWh. As we've gradually improved the house, we found we did not need them, to be honest...

    (however, I am currently installing (thanks to inputs from members of this site...) an electric UFH mat in the concrete stairs, as this bit was obviously neglected in the original house plan, whereas to me now, given our new "savings" context, it is a no-brainer... ).

    gg
    •  
      CommentAuthorjoe90
    • CommentTimeDec 1st 2016
     
    But, I am using the UFH to harvest the excess solar gain from the patio doors by operating the UFH pump with no heat input to balance the heat into non solar heated area,s of floor. Usually there is no solar gain when the heating is required ( evenings) .
    • CommentAuthorgyrogear
    • CommentTimeDec 1st 2016
     
    OK, cheers, I get you ! Obviously with electric, one cannot do that :sad:

    gg

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