@springswood

We have two Daikin single split A2A systems, 3.5kW in the living room and 2.5kW in the main bedroom. This is in a 4-bed detached, 1970s house, with as much retrofit insulation as we could practically do. The whole house heat loss is around 6-6.5kW. We also have gas central heating.

The Daikin units (FTXM25A/R and FTXM35A/R) both modulate down to 800W output and around 125W minimum input. They have a SCOP of 5.2 on heating, which was achieved over the last heating season.

All through the heating season, we used the 3.5kW unit in the living room to put a steady supply of heat into the house on cheap rate overnight electric. This was very effective and cost less than 2p per kWh of useful heat. Compared to gas prices from Oct 1st at say 90% boiler efficiency works out at 8.8p per kWh, so the A2A costs less than 25% the price of gas for overnight heating. In autumn and spring, we were often able to cover all our heating requirements by also running both A2A units at a light load during the day (off solar/battery) without needing the gas CH to come on at all. The units are easily powerful enough, the issue is one of spreading the heat around sufficiently evenly. With less heat loss from the house in autumn and spring, it seemed there was more time for the warmth to spread around and just using the A2A was fine. However, once we got into cold winter days, the kitchen ended up not being warm enough as did the bathroom and one of the bedrooms, and we let the gas CH take over during the day to even the temperatures out. Year to date, our gas consumption, also used for hot water and cooking, is down from 14400 to 8400 units, due to using the A2A.

In summer, the A2A was fantastic at dealing with the heat waves. With 34C outside temperatures for hours on end, our whole house was easily maintained at 23-25C with the occasional help of an extra fan to push the cool air along the upstairs corridor towards the other bedrooms.

One suggestion would be to get A2A units in only the places where they are most needed, perhaps just two indoor locations as we have done. You'll need a multi-split (one outdoor unit) to avoid the need for planning permission (2 outdoor units are permitted development on detached properties, 1 otherwise). If you keep your gas CH, then you have the best of both worlds, cheap heating via A2A whenever you can and gas when you need extra and for the hot water. That way you have cooling in summer and redundancy in your heating system. If the boiler packs up at some point, you can still heat the house well enough.

A more comprehensive solution would be to look at something like a Daikin Multi+ system. This is a single outdoor unit, 6.8kW or 8.6kW on heating, that can be connected to 3 or 4 indoor units, and also provides hot water. If you went down that route and got rid of your gas boiler at the same time, then you would be eligible for the £2500 BUS grant for replacing a gas boiler with an A2A system that can cover both he space heating and the hot water requirements of your house. You could also do away with your gas supply altogether and save on the annual standing charge.

Note, with multi-splits, you don't need the outdoor unit to match the total output of the indoor units, for example a 6.8kW outdoor unit would be perfectly fine with 3x 2.5kW indoor units.

I've mentioned Daikin systems, only because I have Daikin A2A in my house and I am very happy with it.

Thanks for that @rob-of-york. The two way split I described is what you suggest - putting the units where they're most needed. For years now I've used the loop energy monitor app and found that when heating demand is low I can save a lot by only heating the living room. Probably because of the overheads involved in warming all that water to circulate in radiators. Cooling is not the main objective. Still as the living room and first floor bedroom are west facing and suffer solar gain in the summer this plan also puts them in the right place.

I'm working on passive ways of reducing indoor temperatures. I recently bit the bullet and replaced my windows. Much needed anyway. Overnight ventilation should be much improved and hopefully solar gain reduced. Unfortunately this came after the heatwaves so it'll be next summer before I know what difference it makes. After that I could probably do a lot with awnings, though very much not in character for my house.

Daikin do seem to do some quality units and I'm impressed with how low yours modulate. Your experience with how the spread of heat becomes more of an issue at low outside temperatures is particularly useful. I'm resigned to another winter on gas while I investigate. Fortunately I fixed my gas price back in April. Using fans to help distribute heat wasn't one I'd thought of so I will add that to the mix.

I did look into the Multi+ a bit. However I think it's oversized for my needs. Also quite expensive (Daikin have an online tool that gives a rough estimate) especially as the £2,500 BUS grant for air to air is still a work in progress and not available yet. Also my hot water use is really low so I won't get much benefit from the greater efficiency there.

For context my total gas consumption is very low. For the last three years it's been around 3200kWh a year. I think I mentioned extreme thrift. Being a terraced house helps. Good insulation and high thermal mass combine to mean I might be able to spread the heat around for more of the winter. The big question is will it work in the coldest weather because I don't want to have to use gas as a back up.

The standing charge for gas is over 30% of my gas bill and 10% of my total energy bill. So it's the only significant option left to reduce my bills. Plus a certain feeling of resentment. Mostly though it's about how I want to stop burning stuff. 

To digress, that's why, a couple of years back, I got an electric car. It was the single biggest thing I could do to reduce my emissions. Life without petrol stations and with a smart electricity tariff is considerably less expensive. I should add that total energy bill includes 10,000 or so miles a year of electric driving. The revelation has been how bad combustion engines are at actually propelling vehicles. The skill I had to learn in using a gearbox and clutch (what a horrid engineering bodge that is) just to compensate for the undesirable power, torque and efficiency characteristics put me in mind of when, as a teenager I learned to use a slide rule. 

The relevance is that I also want life without gas piped into my home. In the same way electric cars are better I'm sure heat pumps are too. Moving heat energy rather than just burning stuff is a smart way to do it. My science and engineering instincts tell me air to air is the smarter way. Your SCOP of 5.2 supports that.

Posted by: @springswood 

I'm working on passive ways of reducing indoor temperatures. 

Good quality black out blinds are very effective at reducing solar gain. That was our first step before re-glazing with low U-value, low g-value double glazed units, and then finally installing A2A. We still make good use of the black out blinds in summer in the west facing bedrooms, and also at either end (east or west) of the living room depending on the time of day.

Posted by: @springswood 

Daikin do seem to do some quality units and I'm impressed with how low yours modulate.

The low power output modulation is very good on these units, not only on the spec sheet, but also in practice. I can see it on the electricity consumption app for my Solar PV. On mild autumn nights, the electricity consumption due to the A2A sits at a low level with no signs of short-cycling.

Posted by: @springswood 

Your experience with how the spread of heat becomes more of an issue at low outside temperatures is particularly useful. I'm resigned to another winter on gas while I investigate.

Being a terraced house helps. Good insulation and high thermal mass combine to mean I might be able to spread the heat around for more of the winter. The big question is will it work in the coldest weather because I don't want to have to use gas as a back up.

The heat transfer may work sufficiently well in your house. I think the only way to really know would be to try it.

My house is a 4-bed detached with an L-shaped downstairs layout around an integral garage. To get from the living room to the kitchen/breakfast room, the air from the A2A indoor unit has to travel across the living room, out of the open door into the hall, turn right, then through another open door into the kitchen, avoiding going up into the large stairwell in the process. Now it doesn't quite do that, however, on a cold night when the closed off utility room (off the breakfast room) ends up at 15C, the kitchen will be at 17.5C and the living room 20C (the set point of the A2A) in the morning. So the heat is moving around reasonably well, it's just not completely even. There is likely some useful heat transfer through the internal stud wall between the living room and the kitchen that is helping. The warm air also reaches upstairs from the stairwell and warms the nearest bedrooms similar to the kitchen.

Since you have an electric car and are already on Octopus Intelligent Go (from your profile), you have some really good options open to you.

Firstly, your overnight electricity price is comparable to standard gas prices, hence you could heat your hot water overnight with a simple immersion heater (100% efficiency) as opposed to gas at 88% efficiency at best. There is already a small cost saving there.

Secondly, you can afford to overheat the house using the downstairs A2A unit during the cheap rate overnight period, effectively using the whole house and it's large thermal capacity as a form of storage heater. Even if the temperature loss and hence the energy consumption is a little higher due to higher indoor temperatures, this is more than made up for by the much cheaper rate electricity used. A couple of degrees over temperature i.e. 22C rather than 20C is probably enough, but you can obviously experiment with that.

Lets' assume your boiler is 92% efficient on space heating and 88% on hot water. You will be paying around 8p a unit for gas once your fix ends, so let's say 3200 units = £256. Perhaps (and I'm guessing here) 800 units go into hot water heating and 2400 units on space heating. By going to A2A and an immersion heater, you would get efficiencies of 5.2 on space heating and 1.0 on hot water heating. You would need outputs of 704kWh for hot water and 2208kWh for space heating, which would require 704kWh and 425 kWh input respectively. Let's say the prices are 8p overnight and 26p during the day, and half of your space heating is during the day that gives £56.32 for hot water and £72.25 for space heating, a total of £128.57. So you could halve your heating bill and also do away with the gas standing charge of about £105 as well. That is quite a compelling economic argument to go with the environmental one.

A2A installations have a big advantage over A2W in that the systems are self-contained. There is no trying to match up 2020s technology with ancient hidden pipework and radiators of dubious vintage and output. As a result, the SCOP on the box is most likely what you get. There is also a lot less chance of ending up with a system that doesn't work properly. That's not to say there aren't cowboys out there, but the reputable air-conditioning companies have been doing these simple and effective installs very well for many years. In short, if you pay for A2A yourself and take care to choose a good installer, there are no hoops to jump through and the process is likely to be far smoother than for an A2W system. The downside is of course no BUS grant,

Our two single split systems install was done by two people in a day with minimal disruption. One hole drilled in the living room wall at the back of an electricity socket to hook up the power to the outdoor units, one hole drilled in each wall to take the refrigerant pipes, condensate pipe, and electricity cables through to each indoor unit. Trunking on the outside to cover the pipe runs, and an electrical isolation switch and junction box. Mounting points attached to the walls to hang the indoor units, big rubber anti-vibration feet on the outdoor units. Connect everything up and show us it working in both heating and cooling modes. Each indoor unit comes with a simple remote control and the whole lot integrates with a phone app that provides remote control over an internet connection, allows heating and cooling schedules to be set up, and provides data on input and output energy.

One option would be to go for the A2A system and immersion heater, but retain the gas boiler, pipework and radiators, and see if you can simply avoid using it. It's burning no gas when it is not in use and hence producing no CO2. Doing so would still cost you the gas standing charge, which may grate, but it provides useful redundancy and a fall back if the weather gets really cold or you have guests who like to be at 20C plus all the time. Further, if you ever come to sell, it is likely that the house is worth significantly more with the gas heating system still in place, and would sell much more easily. The average person may not be comfortable purchasing a house where the only heating is A2A and there is only an electric immersion heater to heat the hot water. Much of the cost savings for you are due to the time of use tariff that you have access to because of your electric car. That may not be the case for the next owner. They may be old and need the house to be at an even higher temperature all of the time. You don't want your pool of potential buyers to be limited to only those with an EV.

If you have traditional suspended flooring / floorboards downstairs and have not already done so, then insulation under there e.g. 150mm Rockwool netted up between the joists, would make quite a difference to the heat loss.

Finally, Solar PV with a battery is highly cost effective, with a typical return on investment in 7 or 8 years, while lasting 20-25 years. That would be perfect alongside an A2A system, since you could then run your heating during the day on electricity directly from solar production or from the battery charged at the cheap overnight rate. That would drop your average cost for space heating considerably. You would also get most of your other household electricity usage at a lower cost and make some credit on export. (It's still better to charge your EV overnight on the cheap rate grid electricity and get a higher rate, e.g. 12p, for your solar export during the day).

All very good advice, thanks again @rob-of-york 

You're right those numbers look very good.

It sounds like getting heat into my kitchen isn't as difficult as yours.

I should add that I've been here 25 years and hope to stay here for a good while longer. I don't suppose a gas heating system will be such an asset in, say, 20 years. I know a great plumber who installed and services my boiler and he's interested in it for spare parts.

You're right I could install the air to air then see how I get on with it before removing the old gas system. I know disconnecting the gas is free, I wonder if there would be a charge for reconnecting it later?

@springswood

Gas connection / disconnection is the responsibility of your energy provider i.e. Octopus, so you could ask them. I know they do disconnections for free, but I expect re-connection would cost. In the interim, you could see if there is a no standing charge gas tariff that you could move to. The unit rate would likely be much higher, but if you plan to use almost nothing then that would not be an issue.

Worst-case if you got rid of your gas boiler and found that you needed more space heating, then you could always get some electric radiators. Another alternative, given that you have the central heating system pipes and radiators there already, would be to replace your gas boiler with an electric boiler. There are some low cost heat-only ones available for £600 to £700. They would only be 100% efficient, unlike a heat pump. However, being electric there would be no burning gas, no flu needed, no CO2 emissions, and you could use cheap overnight electricity to add heat the rooms not covered by the A2A and supplement that during the day on the rare occasions that you needed to. The A2A would do the majority of the heating very efficiently and the electric boiler would add to that only when necessary. It's an unusual hybrid, but as an electric-only solution it is perhaps worth a thought. The electric boiler could take care of your hot water too either using the existing (?) hot water tank, or you could get a (more expensive) electric combi boiler with no need for a hot water tank. 

@rob-of-york 

More worthwhile options to consider. Thanks.

For hot water a tank is my preferred option. Mostly because a long hot bath with bubbles and a book is one of life's joys I'm not willing to forego.

@rob-of-york @springswood 

I did exactly this combination of direct electric and a2a in three domestic scale community centres that I had converted from gas to ashp.  A2A for the large main rooms, electric resistance (underfloor mats or electric rads) for the small rooms, immersion heater.  It worked out well.  Obviously that's a slightly different use case but some of the principles are the same.  

That said I really would look at improvong the insulation of the kitchen as suggested by another up thread.  

Thanks @jamespa 

The last hard to fix draughts in the kitchen are behind the units. "Fortunately" changing to an induction hob means new worktops (at least) so I'll finally be able to get in there.