Pro's and cons of a heat pump

alanblaby said:

so a radiator roughly twice the size will be required.

The extra thing to notice/add is that most gas boilers are timed (big heat pulses), while Heat Pumps are 24/7 (smooth & stead) with a small temperature offset overnight. This can allow similar sized radiators (if designed properly!) or reuse of existing radiators. 

Hardest bit is to stop expecting the 'hot bum' effect from leaning on radiators!. 35C feels cool.

I've turned down our LPG boiler's flow temp well below the condensing point (that's 55C mentioned) to a "3" (~45C) and it keeps the house steady & comfy (except when my wife turns all the radiators off because there's no one in the room!). An we are on micro bore piping. 

Philip Oakley said:

The extra thing to notice/add is that most gas boilers are timed (big heat pulses), while Heat Pumps are 24/7 (smooth & stead) with a small temperature offset overnight. This can allow similar sized radiators (if designed properly!) or reuse of existing radiators. 

Unfortunately, that will not work. There is a formula for working out the rad size, and there is no way a same size radiator will work properly at two wildly different flow temperatures.

Does it not depend upon what you are aiming for?

With a traditional setup, the radiators have to be powerful enough to heat the room fairly quickly. That is not just the air in the room, but the walls and furniture.

Once the room is up to temperature, all you have to do is to balance the losses, so if you run your heat pump continuously, it need not be so powerful.

Unfortunately it doesnt work like that. If the rads were sized for the lowest flow temperature, it would work, but not the other way round.

Say a room has a 1kW heat loss at the design temperature (-3), that means it will need a 1kW heat source on all the time to keep the room at a steady temperature (20 deg. C). The flow (and return) temperature will determine the size of the radiator.

A radiator sized for a gas boiler (55 flow, 35 return) will, typically, have an output 50% higher than if the same rad was fed by a heat pump (35 flow, 30 return). In that case , if sized for the gas boiler, the room would only reach half of the required temperature, as the radiator cannot put out enough heat to raise the room temperature. It doesnt matter how long the heat is on, it is impossible for that radiator to heat the room sufficiently, as it can only output 500 Watts, when the room heat loss is 1000 Watts.

A radiator sized for a gas boiler (55 flow, 35 return) will, typically, have an output 50% higher than if the same rad was fed by a heat pump (35 flow, 30 return).

It's probably worse than that - the heat transfer into the room is proportional to the temperature difference between the rad and the room - in the gas case proportional to 25 degrees (average of 55 and 35 = 45, less 20) whereas for the HP case it's 32.5 less 20 = 12.5 degrees - so the gas case is double (100% higher).

Traditional calculations (for gas) are indeed done for steady-state heat loss (at worst case outdoor temperatures) - but then pipes and radiators (like for electrics) are chosen from standard size so tend to be oversized by a reasonable margin - plus of course the outdoor temperature for most of the year considerably above worst case - so that's where the quicker warm-up times come from. Try heating a house from stone cold in the middle of winter and even by gas it'll normally take a considerable time.

   - Andy.

A radiator sized for a gas boiler (55 flow, 35 return) will, typically, have an output 50% higher than if the same rad was fed by a heat pump (35 flow, 30 return).

It's probably worse than that - the heat transfer into the room is proportional to the temperature difference between the rad and the room - in the gas case proportional to 25 degrees (average of 55 and 35 = 45, less 20) whereas for the HP case it's 32.5 less 20 = 12.5 degrees - so the gas case is double (100% higher).

Traditional calculations (for gas) are indeed done for steady-state heat loss (at worst case outdoor temperatures) - but then pipes and radiators (like for electrics) are chosen from standard size so tend to be oversized by a reasonable margin - plus of course the outdoor temperature for most of the year considerably above worst case - so that's where the quicker warm-up times come from. Try heating a house from stone cold in the middle of winter and even by gas it'll normally take a considerable time.

   - Andy.