Air pump, electric boiler or both!

Reed_Richards

tacpot12 said:

Your 1850's terrace house has a lot of thermal mass. Also, if you are mid-terrace, you have two other heatings systems on either side keeping your house warm, so you will not have to run an ASHP 24x7 in the winter. You should be able to have it off between say 9pm and 3am. In this time, the temperature in the house will fall, but not buy much, and the period beween 3am and when you get up should warm the house back up again. 

You won't get a lot of solar gain during the winter days, due to small windows, but you will get a cost saving running the ASHP from the solar panels and due to the warmer outdoor temperature (as more heat is available to collect from the environment).

You need to have the radiators AND radiator pipe sizes checked if you go down the ASHP route. Most people don't need changes because of how the rules of thumb used by plumbers in the 1970s, 80s and 90s tended to oversize radiators, and because microbore pipes were not used that commonly, but you need these to be checked if you are to avoid a big shock with the running costs.

The risk of the installation not being efficient is one that the ASHP industry hasn't solved yet. A good installer will be able to install an ASHP and be able to (more or less) guarantee the performance.

Other things to consider are:

• have you got enough insulation in the loft (270-300mm is ideal) 
• have you got good double-glazing 
• have you eliminated all the draughts

Really, the size of the ASHP indoor unit (assuming it includes a water tank) is the biggest problem you have to solve. If you can't fit the ASHP indoor unit into the garage, then an oil combi-boiler would be the best solution. 

I think there is a mixture of good information and misinformation in the above comments.  My house, built in 1980, needed a complete set of new radiators.  It's much safer to assume that you will need new radiators rather than assume that you won't.

Only certain types of ASHP need an "indoor unit".  If you get a "monobloc" type (as I have) then the entire unit is outdoors.  You will need a hot water tank because there is not such thing as a "combi heat pump" but your house must have had a hot water cylinder in the past, even if it doesn't now.  

thrope

Would again strongly recommend advice from a different local oil installer. Ideally look for one who doesn't recommend an oil combi but would put in heat only boiler with existing tank (or replace if necessary). Installers like combis because they are quick and easy to install but they can be less reliable and parts are expensive. Especially with oil there is a compromise compared to gas becausr the oil burner cannot modulate, it is either on or off, so having a high power required for hot water means it can be oversized for the heating. Having an installed modular system (external pumps etc) means all the parts are commodities and can be eailsily replaced as required.

So I think you should look at a non combi oil option.

Another point about ashp is that the flow temperature is much lower. This is why you might need larger radiators, but also means it takes much longer to heat up. If anyone is in or out all the time, eg smokers or using a BBQ on a cooler evening, the limited capacity of the ashp will struggle to keep warm. While oil would be able to respond.

Of course not good for efficiency to not be insulated, but I like having that option as life sometimes requires it.

Reed_Richards

thrope said:

Another point about ashp is that the flow temperature is much lower. This is why you might need larger radiators, but also means it takes much longer to heat up. I

No, that thing about heat-up time is a fallacy.  If a radiator gives out X kW of heat when it reaches its specified operating temperature then that is what you get and your room will heat up as fast as that X kW of heat causes it to.  So the only difference will be in the time it takes the radiator to reach the specified operating temperature.  That's a race between an ASHP heating the water to maybe 50 C compared to an oil boiler heating the water to maybe 70 C.  That race could go either way, the difference is only likely to be a matter of minutes.

If the oil boiler does heat up faster it's because the radiators were over-sized for the actual heat requirement.    

thrope

Right, but I think regular heating systems do tend to be oversized. And I don't think it is a matter of minutes, my experience with ASHP was that they do not heat up the water quickly like a normal boiler, but heat it very slowly. So not minutes but hours or even days.

I stayed in a house with ASHP underfloor heating and when we arrived it took 2 days to get up to temperature (and then was extremely hot because the people staying there before us had shifted up the curve). Was not possible to change the temperature responsively as we are used to with radiators (IE 18 during the day, turning up to 20 for the evening, because it takes best part of half a day to increase a few degrees.

I think ASHP are super efficient at maintaining a steady state temperature with a fixed heat load. But if that heat load is not fixed, for example have a log burner or gas fire, or like to air the house out in the winter because of pets, dust, asthma sufferer, smokers, children playing in and out, having a bbq on a chilly evening and going in and out all the time to check on it, then they cannot adapt as quickly.

Reed_Richards

The same fallacy.  People often use heat pumps in conjunction with underfloor heating and they confuse the behaviour of the underfloor heating with that of the heat pump.  Underfloor heating can be very slow to respond, particularly when embedded in a big slab of concrete with insulation underneath the slab (so you turn your floor into a giant storage heater).

I live in a house with an ASHP and radiators.  It's not slow to respond.      

shinytop

Reed_Richards said:

The same fallacy.  People often use heat pumps in conjunction with underfloor heating and they confuse the behaviour of the underfloor heating with that of the heat pump.  Underfloor heating can be very slow to respond, particularly when embedded in a big slab of concrete with insulation underneath the slab (so you turn your floor into a giant storage heater).

I live in a house with an ASHP and radiators.  It's not slow to respond.      

I'll add to that by saying that although boiler rads are normally 15-20 degrees hotter than ASHP ones, the same doesn't apply to ufh; they should run at about the same flow temp.  

Mstty

I wonder if it's worth mentioning that if you get an ASHP get a comfy chair to sit in front of the control panel. You will be there are lot in the first year.