Life after Heat Pump-Cold

bernithebiker

Cardew wrote: »

Well everything I have read states it is just as bad to oversize a heat pump as undersize.

This from a Government publication:

Fair enough, but I guess it comes down to how 'protected' you want to be from extra cold weather.

For example a house may be specced with a 10kW HP, which will be fine down to -10. But to be good for -15, an 'over-specced' 12kW may be required.

Probably the best way to do it is go with the 'correct' spec 10kW and have a backup wood stove or similar for the coldest days.

Cardew

bernithebiker wrote: »

Fair enough, but I guess it comes down to how 'protected' you want to be from extra cold weather.

For example a house may be specced with a 10kW HP, which will be fine down to -10. But to be good for -15, an 'over-specced' 12kW may be required.

Probably the best way to do it is go with the 'correct' spec 10kW and have a backup wood stove or similar for the coldest days.

Personally I think people should cater for the '95th percentile' of weather conditions and get a couple of £10 3kW fan heaters for those occasions when the pump can't cope. It isn't a disaster if you spend a couple of pounds to cope with extreme conditions - certainly cheaper than spending £thousands on a stove - and many properties have no chimney anyway.

[Deleted User]

Off the top of my head our heating requirement was 11 or 12kw.

The closest Ecodans were 8.5kw or 14kw, quite a big gap... And did not want any other make.

Mitsubishi recommended we went with the 14kw unit being an inverter which seems to be about right when compared to other properties in the vacinity.

Not sure if this would be referred to as oversized???

Cheers

paceinternet

people should cater for the '95th percentile' of weather conditions

I agree with this concept and we are using a system with this type of capacity. But:
The heat pump alone will not enable a property to "warm up" in the way a 30 or 40 kw gas boiler will. If you want to have it come on from cold a short time before you expect to be in a warm room it needs significantly more heating capacity, or an additional bivalent type heating system to support it. Or you accept that you run it for longer hours.
Example. Run it at a lower temperature overnight, maybe 30 deg return water or programmable room stat at 10, 12, 15 whatever, then at the required temperature 30 mins before it is actually needed.
I don't see the problem with installing a 14kw inverter pump in a building with a 10kw heat loss design if it helps the users lifestyle and the installation cost is acceptable. Or even a fixed speed buffer tanked system.
But you do need an answer for -15 degC ambient and lower in some areas of the country if the customer is to be happy.

lovesgshp

jeepjunkie wrote: »

Off the top of my head our heating requirement was 11 or 12kw.

The closest Ecodans were 8.5kw or 14kw, quite a big gap... And did not want any other make.

Mitsubishi recommended we went with the 14kw unit being an inverter which seems to be about right when compared to other properties in the vacinity.

Not sure if this would be referred to as oversized???

Cheers

You had no real option JJ. The 8.5 would have been too small for a 10% undersize and as no 11Kw unit, you have the best alternative in my opinion.

albyota

Oversizing is certanly an issue with the fixed speed 'direct on line' equipment, but with inverter driven kit, oversizing (within reason) is not really an issue, however if a building only needs 6kW then you would certainly not specify a 10 or 12kW system but you should check to see what the output is likely to be at low ambient temps, e.g. you still want 6kW output at minus 10, so a 8 or 9 kW unit should be chosen. JJ's system is correctly spec'd.

lovesgshp

paceinternet wrote: »

I agree with this concept and we are using a system with this type of capacity. But:
The heat pump alone will not enable a property to "warm up" in the way a 30 or 40 kw gas boiler will. If you want to have it come on from cold a short time before you expect to be in a warm room it needs significantly more heating capacity, or an additional bivalent type heating system to support it. Or you accept that you run it for longer hours.
Example. Run it at a lower temperature overnight, maybe 30 deg return water or programmable room stat at 10, 12, 15 whatever, then at the required temperature 30 mins before it is actually needed.
I don't see the problem with installing a 14kw inverter pump in a building with a 10kw heat loss design if it helps the users lifestyle and the installation cost is acceptable. Or even a fixed speed buffer tanked system.
But you do need an answer for -15 degC ambient and lower in some areas of the country if the customer is to be happy.

A heat pump is meant to be run as much as possible. After all, you would not turn your refrigerator off to save energy. It is recommended not to decrease return flow temp by more than 4C for rads, or 1.5C for UFH. 2 zone systems on room stats can be used, but normaly with a buffer tank. e.g. ground floor 20C, bedrooms 17C.
In this type of setup, the heat curves to the buffer tank would normaly be increased over the temperature ranges.

[Deleted User]

Cheers all,

Out of interest to anybody I had a look at the Ecodan and it said the flow temp to the rads [fan coils/ufh] was 37c. Does this sound about right for the current cold snap when snow had landed?

Thanks

lovesgshp

jeepjunkie wrote: »

Cheers all,

Out of interest to anybody I had a look at the Ecodan and it said the flow temp to the rads [fan coils/ufh] was 37c. Does this sound about right for the current cold snap when snow had landed?

Thanks

Could be ok. You have to look at the heat curve temp against external temp. Obviously you could "tweak" the values if needed. What external temps have you?

[Deleted User]

Geotherm wrote: »

Could be ok. You have to look at the heat curve temp against external temp. Obviously you could "tweak" the values if needed. What external temps have you?

When I looked the temp was probably about say -1c.

Thanks