Pro's and cons of a heat pump

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.

Few points, in no particular order:

1. If you'd like to calculate the room by room heat loss, to understand the required heat outputs from radiators, heat pump size etc, then 'Heat Punk' is a free online modelling tool you can use to enter house and room details and do these calculations. I mention this as it then gives you some design details for a reference to compare against proposals from heat pump installers, plumbers etc quoting for the job.

2. If you do opt for a gas boiler, then I would ensure any pipework is future-proof for an A2W heat pump in the future, so no micro-bore pipe, minimum of 15mm to each radiator and 22mm or 28mm primary (depending on the size of the property) - the extra cost will be minimal compared to huge inconvenience and higher running costs in the future when you do eventually move to a heat pump.

3. Consider lifestyle and how the house will be used, air-to-water heat pump systems are typically operated continually, always-on type heating, as they take many hours to get a house up to temperature, does that fit with lifestyle / household usage?

4. Depending on size of house, property details etc, have you also considered air-to-air type heat pumps, these are like the air-conditioning units you find in commercial settings, they are like a highly efficient fan heater so can heat rooms up much quicker and are more responsive while providing efficiency like air-to-water heat pumps, CoPs of 3 or more.. They also provide cooling, so can be useful if the property gets hot in the summer. Consider thing like 'multi-split' units with one outdoor unit, connected to multiple indoor units covering multiple rooms.

Seal Tight, but Ventilate Right ??

Indeed - one option that again works well for me (in a renovated 1910s build stone house with lots of insulation) is heat recovery ventilation. If you're doing a major refurb it would be a lot easier to run the air ducts - but sometimes they can be incorporated into existing more easily - if the layout is helpful. Saves a significant amount of ventilation heat loss in winter, makes sure everywhere is fresh (even if you want to close the windows for security when out) and can even help a little with summer time cooling if you go for a model with a summer by-pass. Mine has a in-built humidistat so draws reduced power most of the time but then automatically boosts when humidity rises (i.e. someone takes a bath or shower, or cooking).

   - Andy.

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.

Problem is COP needs to be at least 3 in practice as with on peak rates per KWH the outside air temperature needs to be above 10C to get even 20C inside economically. 

A1 quote " COP follows a linear or near-linear curve. As outdoor temperatures drop, the COP decreases because the temperature lift (the difference between the outside air and the water circulating in your radiators) increases" 

Problem is COP needs to be at least 3 in practice as with on peak rates per KWH the outside air temperature needs to be above 10C to get even 20C inside economically. 

Modern HPs do reasonably well - mine quotes a COP of 4.8 for 7 degrees outdoor temp and 35 degrees flow (and 3.6 for 45 degree flow). There are times when you need a higher flow temperature though (or immersion backup) - e.g. for domestic hot (tap) water heating. For mine the COP drops to 2.8 for 55 degrees (or even lower if outdoor temperatures are lower) - but heat pump tariffs give you a much lower kWh price so that compensates things financially provided you can program the system to take advantage of the right periods (my cheap rates are 0500-0900 and 1300-1600 so is quite handy for both morning and evening hot water demand). 

    - Andy.

The hidden confusions are with respect to the hidden operating conditions, and the 'safety' margins (just like the electrical cable specs have lots of margin in the majority of cases) with the selection of radiators from tables.

It's not clear how these putative 'marginally designed' radiators are ever going to get up to temperature if their max output is minimum needed to sustain operation, never mind heat the room and its thermal masses in the first place (especially for the typical/historic burst mode operation).

Even then there are hidden assumptions about how long the cold spells last (or margins added so they can assume week long perfect '-3C' or whatever). 

If heat pumps are operated in burst mode, rather than continuous (low & slow), then like you say, it's not going to work.

The corollary is that often existing systems (especially with better insulation & draught control) will work, when they would not have if the other improvements hadn't happened.

alanblaby said:

Unfortunately it doesnt work like that.

If you are going to run your heat pump 24/7, I agree, but that is not what I was describing.

Let's say that CH comes on at 06:00 and goes off at 08:00. You go out to work. CH comes back on at 16:00. Whilst you were out at work, there were 8 hours of heat loss. Now you need the CH to put back that heat within a reasonable time, say 2 hours.

It's similar to cooking your spuds. You put the water in the pan and turn the burner full on. The water boils so you put the potatoes in the pan. The temperature drops a bit so you leave the burner full on. Then when the water is boiling, you turn the burner down.

Alternatively, you could leave the pan on the stove all day just simmering for when you need it.

Philip Oakley said:

Gas is a 'heat the room' solution. Heat pumps are 'heat the house'. 

That depends on the technology, for air-to-water heat-pumps, using radiators or underfloor heating then yes, but for the air-to-air units with their individual room units, they can heat specific rooms separate from the rest of the house.