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).

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.....

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.

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?

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.

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.

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: 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.

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?

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.