heating a passive house

[smart51]

I've been getting online quotes for windows and doors and notice that it is not much more expensive to go for windows with a centre pane U value of 0.6.  For larger units, that yields a whole window U value of about 0.11 which is impressively good.  200mm carbon filled EPS gives walls with a U value of 0.15, plus floors and ceilings need to be 0.13 anyway.  This takes us into passiv haus territory.

 

With new build airtightness and MHRV set to give 0.3 ACH, we're looking at a total heating demand of 1.5kW to maintain 20°C above outside temperature or 2.2kW to maintain 30°C above outside temperature.  Clearly we need some heating but not much.  Under floor heating seems a bit overkill.  What is a sensible way to provide this? 

[jayc89]

How will you be heating water? I'd still be tempted to go for some form of UFH if only for the cooling function in the warmer months. 

[ProDave]

Almost exactly the same heat input requirement as us, just over 2kW when delta T is 30 degrees.

 

A 5Kw ASHP is doing the job nicely, heating the house and the DHW (in a 300L UVC)

 

Are you doing much of the build DIY?  If so when you shop around, the cost of UFH parts is not a great deal so not much extra cost if you were having the ASHP anyway for water heating?

 

We also have a 5kW wood burning stove that is also more than capable of heating the whole house.

 

One thing is pretty certain from our own experience and others is you won't need heating upstairs, so that simplifies the heating to a few loops downstairs.

[smart51]

6 minutes ago, jayc89 said:

How will you be heating water? I'd still be tempted to go for some form of UFH if only for the cooling function in the warmer months. 

Air source heat pump, which of course could do heating.  Heating demand would be 10W/m2 with a 30°C temperature difference. I suppose a very sparsely laid out UFH would save a little bit of cost.

 

Since typing the question, I realise I'd forgotten to calculate solar gain.  The southern wall is entirely glass.  On a sunny winter solstice, that could be worth 3.6kW averaged over 24 hours.  Even a cloudy day could be enough, which is probably the point of passiv haus! 

[Kelvin]

I’m glad to read confirmation on my view of no need to heat upstairs if you get the insulation and air tightness right as my plan is no rads upstairs. 

[JohnMo]

We have a 192m2 floor area, UFH pipes on 300mm centres.  But with a smaller area you have watch flow rates as they can easily be way to low for a heat pump.  Ours is about 12-13l/m so ok for a 3to 5kW heat pump.  If the floor area was smaller you need to looking at 100-150 spacing to ensure you have a suitable flow rate to keep the HP happy.

 

Don't base any calc on having solar gain, as it may not be there.  Couple of overcast days with no gain can make a big difference.

[Nick Laslett]

25 minutes ago, smart51 said:

 

Since typing the question, I realise I'd forgotten to calculate solar gain.  The southern wall is entirely glass.  On a sunny winter solstice, that could be worth 3.6kW averaged over 24 hours.  Even a cloudy day could be enough, which is probably the point of passiv haus! 

It looks like cooling will be more of an issue for you. Does the glass in the southern wall cover both floors? Once the heat is inside a well insulated house, it is very difficult to remove without A/C. 
 

My biggest takeaway from the forum when I first started my research was all these well insulated houses were overheating. A certified Passive House tries to balance the solar gain with window overhangs. 
 

I put in upstairs UFH purely as part of a summer cooling solution. I also went with solar cooling window coatings, because despite the reduction in solar gain, it is easier to add heat in winter, than cool in summer unless you have A/C. 

[jack]

1 hour ago, smart51 said:

For larger units, that yields a whole window U value of about 0.11 which is impressively good. 

 

I assume you mean 1.1? 0.11 would indeed be impressive for U_w!

 

14 minutes ago, smart51 said:

Since typing the question, I realise I'd forgotten to calculate solar gain.  The southern wall is entirely glass.  On a sunny winter solstice, that could be worth 3.6kW averaged over 24 hours.  Even a cloudy day could be enough, which is probably the point of passiv haus! 

 

And on a cloudy summer solstice, it'll be worth little to nothing in terms of solar gain.

 

Also, even highly insulating windows are generally a net negative in terms of heating. Yes, they allow for solar gain (when the sun is out - obviously not always the case in a British winter!), but they leak heat 24/7. In your case, that's at a rate of something like 5-10 times greater than the rest of your insulating elements, and almost certainly more than any solar gain they enable.

 

With large areas of south-facing glass, I'd be way more concerned about overheating in summer. Unless you have external shading, I'll bet dollars to donuts that you're going to cook without active cooling.

[ProDave]

43 minutes ago, Kelvin said:

I’m glad to read confirmation on my view of no need to heat upstairs if you get the insulation and air tightness right as my plan is no rads upstairs. 

Like many, I fitted electrical points in each bedroom for a panel heater, because I simply did not trust that here in the Highlands you could survive without heating in the bedrooms, and like most that have done this, those electrical points were simply a waste of cable and have never been connected.

[smart51]

11 minutes ago, Nick Laslett said:

It looks like cooling will be more of an issue for you. Does the glass in the southern wall cover both floors? Once the heat is inside a well insulated house, it is very difficult to remove without A/C. 
 

My biggest takeaway from the forum when I first started my research was all these well insulated houses were overheating. A certified Passive House tries to balance the solar gain with window overhangs. 
 

I put in upstairs UFH purely as part of a summer cooling solution. I also went with solar cooling window coatings, because despite the reduction in solar gain, it is easier to add heat in winter, than cool in summer unless you have A/C. 

The house is single story and the roof will overhang the south facing windows by 2.5m .  In the summer months, no direct sunlight will get in through the windows, save for early in the morning.

[SteamyTea]

1 hour ago, smart51 said:

I realise I'd forgotten to calculate solar gain.  The southern wall is entirely glass.  On a sunny winter solstice, that could be worth 3.6kW averaged over 24 hours

Do you mean 3.6 kWh over 24 hours, which is 150 W.

Or a peak power of 3.6 kW, which may only last seconds.

Or a mean of 3.6 kW for 24 hours, which would be 86.4 kWh.

 

 

[Iceverge]

Been there have the T.

 

Block built passivehouse, cavity wall, EPS beads. Plug in rad for heating. 

 

Words of caution.

 

0.3 ACH is difficult. You'll probably need to do it yourself. I was pathological to get there.  

 

A true window install of 1.1w/m2k without overwapping external insulation is almost impossible. Windows are routinely 5 times worse than a wall for heat loss. Also EPS beads real life I suspect are closer to a k value of 0.04W/M/K.For a 200mm cavity work on a U value of 0.2.

 

Passivhaus 20deg is not warm enough for everyone, in every room. 

Our house lives closer to 21/22 which will have an effect on your bills.

 

With no central heating you rely on air-circulation to even up temps. If doors are closed ,ground floor rooms will cool down if the never get any sunlight. More so if you have windows. Equally, overheating, west windows are the worst as they're a pig to shade. A small room with a closed internal will get hot at any time of the year. 

 

Finally and most importantly you may or may not have to co-occupy with other humans. 

 

Passivhaus goes out the proverbial triple glazed window when you have a gale howling through the hallway for an hour a day as the front door is permanently open. Likewise if you open ever window for 24hrs a day in a heatwave you will have overheating. 

 

If I'm home alone the resistance rad on a timer with open internal doors works well.

 

Kids+partner add so many variables in real life however......... you may want to spend £500 on UFH pipes just in case. 

 

 

TLDR. 

 

If you plan on having windows and/or non scientifically fundamentalist lodgers an active heating/cooling system might be prudent. 

Edited September 9, 2022 by Iceverge

[Gone West]

2 hours ago, smart51 said:

What is a sensible way to provide this? 

IIRC we calculated a heating requirement of 1.14kW for a 30C temperature difference in our PH. We designed the house so we wouldn't need a conventional central heating system with UFH and ASHP. We heated the house with three electric towel rails in the bathrooms and a 585W EASHP providing warm air heating through the MVHR system when required. We heated the whole house to 23C. In order to compare what level of comfort you would like it is necessary to know the temperatures others have in their properties.

[Conor]

Do you think you'll ever sell the house? Do you think a house with no central heating will be a turn off for potential buyers? That's why we stuck in UFH and an ASHP. May not be used, but no way somebody is going to pay market rate for a 300m2 house with no heating. £10k well spent.

 

When we first moved in, three 1.5kW fan heaters on for 12hrs at a time kept the house at 18-20c when it was 8c outside and the house still drying out. So I'm certain we could live comfortably without the ASHP and UFH... But it's in and will be used.

[Gone West]

Just now, Conor said:

Do you think you'll ever sell the house? Do you think a house with no central heating will be a turn off for potential buyers?

It wasn't a turn off when we sold our house last year. We had three people bidding against each other at significantly higher than the asking price. We had an EPC of A95 without renewables which is what people seemed more interested in.

[Conor]

5 minutes ago, Gone West said:

It wasn't a turn off when we sold our house last year. We had three people bidding against each other at significantly higher than the asking price. We had an EPC of A95 without renewables which is what people seemed more interested in.

That's great to hear. I think with the current energy ripoff that should be more and more common. Our architect was the one that insisted we put in the UFH as has had instances of lower valuations and reselling issues with some of his PH clients. The local estate agent we're friendly with said the same thing that a house with no CH would be an issue for our location. 

[Kelvin]

The recent energy crisis will help change people’s attitudes towards heating with gay abandon because it used to cost buttons to better understanding that reducing the need to heat your house in the first place makes more sense. 

Edited September 9, 2022 by Kelvin

[JohnMo]

1 hour ago, smart51 said:

The house is single story and the roof will overhang the south facing windows by 2.5m .  In the summer months, no direct sunlight will get in through the windows, save for early in the morning.

 

I would check your solar gains at this time of the year.  As said once the heat is in its difficult to get it out with out Aircon.  Have you thought about a split Aircon system, high does heating and cold very efficiently.

[smart51]

3 hours ago, SteamyTea said:

Do you mean 3.6 kWh over 24 hours, which is 150 W.

Or a peak power of 3.6 kW, which may only last seconds.

Or a mean of 3.6 kW for 24 hours, which would be 86.4 kWh.

 

At noon on the winter solstice, there will be over 9kW of sunlight coming through the windows.  In thinking through this answer I realise I've taken care of solar azimuth and daylight hours but not the angular change from east to west.  Even so, the volume of sunlight through the day would be the equivalent of 900W for 24 hours.  On those days, the thermostat won't switch the heating on.

On balance I think it wise to fit a heating system and use the ASHP to heat it.  Underfloor heating it is, but with pipes fitted at generous intervals.

[Iceverge]

Have a read about @TerryE system. Some searching will be needed on your behalf but it's well worth it. 

 

It's a very measured look at this exact topic. 

 

 

 

[Russdl]

9 hours ago, smart51 said:

This takes us into passiv haus territory.

If you’re heading that way then I’d urge you to look into the PHPP. Very DIY’able if you have the time and patience or you can chuck a chunk of cash at a consultant - but check their credentials. Carefully. 
 

As mentioned, it sounds like overheating will be your biggest problem. Our issue comes from east facing windows, but we’ve got a workaround for those now. West facing windows can also be an issue, south facing - not so much especially with the large overhang’s you’re planning. External shading will sort all those potential problems. 
 

As for heating a passive house, ours has a Willis heater for the ground floor UFH (which wasn’t working properly last winter due to a valve on one of the circulation pumps not being fully open) and that sufficed. I’m sure it will be more than sufficient this winter. 
 

Electric UFH in the two bathrooms plus electric towel rails and that’s it. 
 

You basically don’t need much at all, and it doesn’t need to be complex. Courtesy of @TerryE