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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorjamesheath
    • CommentTimeMar 10th 2017

    The (quite large) Passive House I'm having built for me is going to be heated by a combination of (mostly) air heaters on the MVHR, and towel rads in the bathrooms and cloakroom.

    The M&E designer says I don't need TRVs on the towel rads - by tweaking the mixer valve on their heating circuit and adjusting the Towel rad valves, we'll be able to get it right.

    TRVs are not hugely expensive (though I would need a few), and would seem to me would give me a set temperature.

    Does anyone have any direct experience of living with such a set up?


    • CommentAuthorJohn Walsh
    • CommentTimeMar 10th 2017
    Could you say anything about the heat source for the rads? Is it the case that the system is small (relative to a regular house heating system),that if you had TRV's on all the rads there's a potential problem with the 'boiler' (if that's what you are using) effectively having nowhere to put the hot water (if that's what you're using) and ends up cycling on/off (in a way that's not good for the system).
    Surely you want towel rads as hot as possible not controlled by air temp?
    • CommentAuthorjamesheath
    • CommentTimeMar 10th 2017
    John - good point - 1500l thermal store, solar thermal panels, wood stove, backup gas boiler, so boiler cycling not an issue I think....

    Victorianeco - maybe - won't that end up with very hot bathrooms? this is PH airtight & instulated.....
    If it's very airtight and insulated then surely they won't 'open' as they wont need to call for heat?
    • CommentAuthorringi
    • CommentTimeMar 10th 2017
    You say "Passive House" therefore little need for heat....
    Then say "1500l thermal store, solar thermal panels, wood stove, backup gas boiler" so a VERY expensive heating system....

    Personally I would forget the wood stove and 1500l thermal store and just put in a normal DHW tank with solar PV (not thermal) along with the gas boiler assuming you are on main gas, but if not on mains get just use electric heating.
    • CommentAuthorJohn Walsh
    • CommentTimeMar 10th 2017
    Getting back to the OP, the question was about in-use experience (not about the design, to which it sounds like a lot of effort has already gone in - apologies if my post caused a diversion).

    I don't have direct experience and on such a bespoke system there's likely to be a certain amount of trial and error. Is it possible to buy valves with interchangeable heads, so you'd retain flexibility?, e.g. fit regular valve heads to suit your M&E person and retro-fit thermostatic heads as and when?
    • CommentAuthorgravelld
    • CommentTimeMar 10th 2017
    What's your current estimate for how often these rads will be on? Hopefully it won't be many days? Is it worth just going with the simplest option for now and retrofitting with a TRV and bypass loop if needed?
    • CommentTimeMar 11th 2017
    Posted By: jamesheathThe (quite large) Passive House I'm having built for me is going to be heated by a combination of (mostly) air heaters on the MVHR, and towel rads in the bathrooms and cloakroom.

    Do you think that will work? The towel radiators will certainly heat the bathrooms, perhaps overheat them unless you are careful, but I don't think they will contribute much to the overall heating. The heat they put into the air in the bathrooms will mostly be immediately extracted and passed to the MVHR where it will heat the incoming fresh air. The post heater will then reduce its output so that its output temperature stays the same as it was without the towel radiators.

    But having said that, it's just a theory; it's not like I've tested it. I use radiant heaters to supplement my post heater when required, to try to warm the fabric of the building directly. I think a large proportion of the heat from my post heater is dissipated into the fabric via the ducts rather than emitted into the rooms too, since the air temperature at the supply vents is only mildly warm instead of hot.
    • CommentAuthorjamesheath
    • CommentTimeMar 11th 2017
    Hi DJH,

    TBH I'm not sure! I didn't design the heating system - I'm the client (albeit a fairly engineering savvy one).
    I have the same concerns - there's only so much heat you can get in to air - it doesn't have a very high specific heat capacity.
    Do you think it would be wise to insulate the ductwork to get the heat to the rooms? I'm not clear whether that has been planned.
    As you say, I think there's a risk the bathrooms will just get too hot, hence the TRVs - at least I can then set the temperature.
    • CommentAuthorowlman
    • CommentTimeMar 11th 2017
    This isn't like your home but:-
    I have TRVs on my rads but only because they've been there for some time. The truth is I don't need them.
    Some years when I installed my biomass system I fitted an automatic mixer valve between the CH flow and return near to the large accumulator. This valve has a control sensor/thermostat situated in the hallway.
    If I had a fully automated zoned system I'd have need one of these mixer valves and thermostats on each zone.
    House temp is regulated by the temperature of the water in the system, not by any form of restrictor valve on the rads. My CH goes on in the autumn and stays on 24/7 until Spring.
    It works fine.
    • CommentTimeMar 11th 2017
    Who did design it - are they passivhaus systems designers? I confess I just busked it and made it up as I went along.

    The main concern is simply getting the requisite amount of energy into the building, but it does help if it is distributed. Our house has an east-west spine wall with the main rooms to the south of the spine and they tend to be a little warmer because of solar gain, plus incidental gains from cooking and the TV etc. There's one bedroom to the north of the spine but it's the only room where the temperature at the vent is noticeably warm, because it happens to have a short duct from the distribution box, so it balances out.

    The shower rooms are unheated and whilst they're not places we linger in winter, they're not uncomfortable to have motivated me yet to connect up some sort of heater to the wiring that I put in place in case. If I did, I'd either use a 'heat lamp' or a radiant bar bathroom heater. Towels dry just fine because of the MVHR.

    My post heater is deliberately slightly oversized, so I expect its output is usually limited by the duct temperature sensor. It would easily have coped with keeping the house up to design temperature for the past two winters except we limit it to drawing E7 power, so sometimes we add a bit more power at night with radiant heaters. It's an all-electric house.

    I didn't really think about heat loss from the ducts; I knew we'd lose some heat through the double-wall plastic but I lazily expected it to take a while to get the duct warm and then the air from the vents would be warm, a bit like running hot water through pipes. But it doesn't seem to work that way; I can tell whether the heating is on by putting my hand to the supply vents, but the only one that feels properly warm is the northern bedroom. Luckily all my ducts run through the first floor so the heat is going into heating the floor structure and I can detect where the ducts run using my IR thermometer. So it doesn't matter in my house whether the ducts are insulated or not; you'll need to think about your situation to assess whether the same is true.
    • CommentAuthorjamesheath
    • CommentTimeMar 11th 2017
    Thanks - useful info - there don't seem to be that many projects in the UK using hot air to heat.

    It is a passive house designer, and they are confident about their system. I realise that the design is likely over the top (the wood burner for e.g. whilst having 80% delivery to HW, is still a "lifestyle" choice, not an economic one). The duct bit is interesting though - our ducts are spiral galvo steel, so will be a LOT more conductive, though like you they are mostly in the floors between rooms.
    • CommentTimeMar 11th 2017
    I think best practice with metal ducts is to insulate them, although I'm not sure. What's the main heat source?

    There's a school of thought that a gas boiler with a few radiators is still a good way to go, partly for cost effectiveness and partly because it will cope if the project turns out to be out of spec and additional heat is required. I didn't want the service cost of gas nor the complexity of a wet system, so my backup plan was just more electric heating. If you're having your house built, is meeting passive house spec part of the contract?
    Posted By: djhThere's a school of thought that a gas boiler with a few radiators is still a good way to go

    Especially if you ever want to sell it !!
    • CommentTimeMar 11th 2017
    Posted By: Peter_in_Hungary
    Posted By: djhThere's a school of thought that a gas boiler with a few radiators is still a good way to go

    Especially if you ever want to sell it !!

    Very true, but personally I didn't go through all the hassle of building my house in order to sell it.:bigsmile:
    • CommentAuthorjamesheath
    • CommentTimeMar 12th 2017
    Main heat source is solar thermal, followed by wood burner, and then gas boiler when I can't be bothered to light the wood burner. (yes yes - crazy over-design & not financially sensible I now realise).
    I'm fitting a single radiator in the most "at risk" location, and am putting tees in to the towel rad heating circuit to allow retro fitting of extra rads if they prove necessary, though I don't think they will.
    I'm not selling this house - not after the effort it's taken just to get this far & it's not even built!!
    • CommentAuthoralec
    • CommentTimeMar 12th 2017
    put crudely TRVs need you to overheat before they start closing...much better to use compensation controllers...although the issue then becomes where do you site it?
    • CommentAuthorringi
    • CommentTimeMar 13th 2017
    TRVs have three use cases I can think of.

    1) If you don’t wish to heat a room as much because it is not in use, however unless you have VERY close fitting doors, in a well insulated home all the rooms will tend to the same temperature.

    2) If one room heats up faster than other due to solar gain and you wish to stop it overheating while the rest of the rooms still need more heat.

    3) If the radiators are not sized “correctly” or not balanced so one room heats up faster than other rooms.

    As far as I can see all of these use cases are of limited benefit in a very well insulated home.

    TRV also give you more EPC points, if you care about the EPC rating.....
    It seems an expensive and complex heating system for a 'Passive' house. Have you done a cost/benefit for alternative heating solutions? Our neighbour eventually just used a small number of wall mounted electric radiators - very low capital cost. If you're properly passive you should be able to work out your expected annual heating cost (in kWh) and can look at the relative cost of that using electricity/gas etc. Add together purchase, installation, annual servicing, repairs for your heating system and estimate what the lifetime cost per year is going to be.

    In our house temperature difference between rooms seems minimal irrespective of whether UFH heating is on in a particular room. Why not electric underfloor in the bathrooms set to come on for short periods at the times you expect the rooms to be used?

    <blockquote><cite>Posted By: djh</cite>Towels dry just fine because of the MVHR</blockquote>

    Absolutely - one of the nicest things about it. Long, simple unheated, towel rails so that the towels aren't hanging in too many layers. You can shower morning and evening in our house and large towels are always dry.

    Towel rails covered in towels are a very ineffective form of heating.
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