Heating in a well insulated house

chichikleine

Hi,

I would be very grateful for any info re heating systems.
We have just started building a 5 bedroom (210 m2), well insulated house, on an exposed site. It is an "upside-down" house, with 4 bedrooms downstairs and living area/master bedroom upstairs. We will have a woodburning stove upstairs and also plan to have HRMV.
What we are not sure about, is what kind of additional heating-system to install. Air-source heat pump with (wet) underfloor heating only downstairs? Or (dry) electrical underfloor heating only downstairs. We envisage the house only needs to be heated properly in the evenings/holidays. We have quite a bit of solar gain and also plan to have PV-panels.

Many thanks in advance!

Chichikleine

Robwiz

When you say how well, what are your target U values for walls, floor, roof and windows? Has your architect given you heat loss figures? In addition to insulation, it's essential that your builder pays close attention to air tightness – to get the best from your mvhr and to minimise ventilation losses.

It's quite possible that the wood burning stove can emit sufficient heat to keep the whole house warm – your challenge will be distributing it to the bedrooms downstairs as warm air rises!

I don't believe that underfloor heating is right choice for a well-insulated house because it is not very responsive. You probably won't need to heat the bedrooms and bathrooms for long periods of time – only for bathing and dressing. Therefore, electric radiant would probably be ideal because the capital cost is low and assuming your demand is also low, the running costs would not be significant. You could either have it controlled by timers, PIR sensors or room occupants switch it on when needed.

Alternatively you could install an air to air heat pump and use it to maintain a background temperature downstairs of say 16º C in winter – a small unit will have a high coefficient of performance (COP) – at least four – so you could bring more heat into the house per kWh of electricity used.

chichikleine

Hi Robwiz,

Thanks very much for your reply.
U-values are pretty good (floor 0.11, ext wall 0.19, roof 0.11, windows (triple glazing) 0.8) and builder is very much aware about the air-tightness.
I am not sure what an air-to-air heat pump is. Is this the same as a air-source heat pump?. Could this be sensibly used with UFH, to maintain a background temp of 16 C?

Ectophile

Depending on how much extra heat you're expecting to need, you might find it cheaper to just install a few bog-standard convector heaters.

They will cost more to run, but cost almost nothing to install.

Of course, I wouldn't recommend that unless the house is very well insulated, or the electricity bills could be frightening.

chichikleine

Hi Ectophile,

Thanks very much for that.Electrical UFH seemed attractive, because the bedrooms downstairs are quite small. However, I appreciate that the reaction time of a convector heater will be faster than an UFH system...

matelodave

Do a few calculations if you are considering u/f heating in the bedrooms & bathroom. I have it installed, running from an Air Source Heat Pump and the disadvantage that I've come across is the amount & type of furniture in the room
Assume a bedroom of 4m x4m = 16sq.m. Then stick a double bed, wardrobe/cupboards , chest of drawers and a couple of bed side cabinets in there and you could easily cover up between a third and half of the floor area, thus reducing the output of the heating in those rooms by 30-50%.

If the furniture is all on legs and the air can circulate then it's probably not a big problem but stuff that sits on the floor (like a big divan bed with drawers or wardrobes than sit directly on the floor) will block any heat.
Likewise you can't have heating under the bath, shower, furniture or toilet so you need to uprate the output in those rooms to compensate.
Our system struggles a bit in the bedrooms when it's very cold because of the furniture but is OK all over the rest of the bungalow. The towel rail in the bathroom is connected to the heatpump as well so helps compensate for the reduced floor heating area.

If the house is really well insulated then I'd consider either a convector or panel heater for the short period that you are likely to heat those rooms or even radiators running on a heatpump if you are using one in the rest of the house. - you only need it warm when you get up and possibly just before you go to bed.

chichikleine

Hi Matelodave,

Thank you very much for your reply. It is really good to get feedback from someone who has experience with the different set-ups!

Robwiz

Hi chichikleine

Your U values are excellent and your new build must be pretty close to passivhaus standards. Given that level of insulation, I wouldn't bother with wet central heating.

An air to air heat pump is an air source heat pump but it doesn't transfer heat to water. You have a fan coil unit installed indoors and a compressor unit outside. The indoor unit can be programmed like a central heating unit with set back temperatures and on/off times. The advantage of air to air is that because the difference between the warm and cold air temperatures never exceeds 30º the efficiency (technically known as coefficient of performance or COP for short) is high – the best models achieve a COP of around 5 – which means that for every kWh used to run the heat pump it will move 5 kWh of heat from outside the house to inside. In contrast, air to water heat pumps rarely exceed a COP of 3 – they are required to heat water up to 55º C and the laws of thermodynamics dictate that this can only be done at the expense of efficiency (search on Carnot's Law if your want the detailed theory).

A 2 kW rated heat pump can be bought and installed for around £1000 to £1200 which is good value compared to a conventional central heating system or even wood burning stoves.

Johnandabby

Robwiz wrote: »

Hi chichikleine

Your U values are excellent and your new build must be pretty close to passivhaus standards. Given that level of insulation, I wouldn't bother with wet central heating.

An air to air heat pump is an air source heat pump but it doesn't transfer heat to water. You have a fan coil unit installed indoors and a compressor unit outside. The indoor unit can be programmed like a central heating unit with set back temperatures and on/off times. The advantage of air to air is that because the difference between the warm and cold air temperatures never exceeds 30º the efficiency (technically known as coefficient of performance or COP for short) is high – the best models achieve a COP of around 5 – which means that for every kWh used to run the heat pump it will move 5 kWh of heat from outside the house to inside. In contrast, air to water heat pumps rarely exceed a COP of 3 – they are required to heat water up to 55º C and the laws of thermodynamics dictate that this can only be done at the expense of efficiency (search on Carnot's Law if your want the detailed theory).

A 2 kW rated heat pump can be bought and installed for around £1000 to £1200 which is good value compared to a conventional central heating system or even wood burning stoves.

This would be a split or multi split system, rather than an air source heat pump system. More typical for offices, shops, restaurants etc thsn houses, but a highly responsive system and able to provide cooling as well as heating - you may need that if you make the house too air tight.

One issue is that the indoor fan coil units look hideous - there are concealed units but they are more expensive, and need space to install.

Another is thst you need a indoor fan coil unit for each room, with the minimum size probably much too big for your rooms. You also have to have an outdoor condenser for probably every 3 indoor units.

Third is that the system uses refrigerant, which is great at carrying thermal energy but not so good if there's a pipe leak in your bedroom when you're sleeping - may not be an issue with small systems but hotels have to have safety systems to detect refrigerant leaks.

It will be a real shame to get this decision wrong - I would recommend that you speak to passivhaus consultant about the options, or at least a services engineer. Really need to see the drawings and details.

passivhaus schemes generally use a heating element in the mvhr to provide heated air, which could be fed by direct electric or gas boiler or ashp depending on size and frequency of use.

Johnandabby

chichikleine wrote: »

Hi,

I would be very grateful for any info re heating systems.
We have just started building a 5 bedroom (210 m2), well insulated house, on an exposed site. It is an "upside-down" house, with 4 bedrooms downstairs and living area/master bedroom upstairs. We will have a woodburning stove upstairs and also plan to have HRMV.
What we are not sure about, is what kind of additional heating-system to install. Air-source heat pump with (wet) underfloor heating only downstairs? Or (dry) electrical underfloor heating only downstairs. We envisage the house only needs to be heated properly in the evenings/holidays. We have quite a bit of solar gain and also plan to have PV-panels.

Many thanks in advance!

Chichikleine

my first question would be how are you heating hot water for taps etc? Doing this by direct electric would be expensive. An ImmerSUN or similar (I have no affiliation!) product would help you make full use of the pv by heating some of the water, but what about the rest?

If it's an exposed site you may find with increased wind pressures that the air leakage rate is higher. I agree that your heating requirements may be small but you would still need heating in the bedrooms in the mornings as well, when there is no solar gain.

chichikleine

Hi JohnandAbby,

Thanks very much for taking the time to send me your thoughts on this. We have no mains gas, so would need another system for hot water to taps and showers. We are a family of 5, 3 teenage sons, so quite a lot of showers;). I have read about the immerSUN system, but don't really know how efficient this would be.
We have roof-space for 10 PV-panels.
Is there any way to calculate what would be the most cost-efficient way to provide us with hot water?

Many thanks!