@temperature_gradient I've had air-to-air for just over 3 years and once you realize how best to use it, its great.  Mine is a Mitsubishi with 2 2.5kW wall units, one in the kitchen/utility area and one in the main bedroom and a 3.5kW floor standing unit in the sitting room/dining area.  I have a fairly standard 1970s 3 bed semi.  I have also kept some of my original electric radiators, thinking I might need to use them if it got very cold.  For the first 2 winters I tried to run the heating like a gas boiler, programming it to come on morning and night with a manual boost in between if I needed it.  I was using the electric radiators during cheap rate at night (on Intelligent Octopus Go). I found the temperature of the house fluctuated enormously, heating up very quickly, but cooling down fast too.  During the winter just gone I changed the strategy and got a very pleasant surprise.  Instead of trying to heat each individual room, I used the 3.5kW heater in the living room to heat the whole house, with all the doors open and left it running 24/7.  Once the ground floor had got up to temperature the system ran on 400-600W most of the time, only ramping up further if it was really cold outside.  A surprising amount of heat made its way into the upstairs rooms. As the kitchen area is single storey with 3 outside walls, I also ran the unit in there for a few hours at night, and I sometimes had to put the bedroom unit on for 20 minutes or so before I went to bed.  Other than that, on the coldest nights the 2 1kW radiators in the small bedrooms came on and I have an electric towel rail in the bathroom.  The ability to switch to cooling in the summer is a real bonus.

Hot water is provided by a Sunamp thermal store, which can be charged with cheap rate electricity in winter and charges via a solar diverter in summer.  This fits into the cupboard that used to house the combi boiler - no need for extra tanks.

There is one other bonus to air-to-air - a decent reduction in humidity.  Before I got the system most of my rooms were above 60% all the time and the kitchen and bathroom were often in the 80%s.  While I never actually got mouldy walls, I did find I had to run a dehumidifier a lot of the time to try to keep the humidity down.  Now the whole house has settled into the 40s-50s and even if the kitchen or bathroom gets steamy it is easy to dry it out again, unlike before.

@anne-smith Glad to hear you know the benefits and its working so well for you, we've been installing these for 30+ years (and A2W) They always work well and everyone is happy, there are best ways to use them which is low and slow but constant, I've had mine on 24/7 since 1st November and barely noticeable that they are on except whole house at a comfortable level. Had two of mine on AC this weekend too!

I'm fairly new to the forum and have not voted. 

Background - have 1* Mitsi 7.2 KW and 2* Daikin 3.5 KW A2a heat pumps for just over 4 years, our main heating used to be oil.  We have just installed a Valliant Arotherm + 10Kw and Vaillant Unistor HW cylinder.  

The Mitsi was our main source of heating throughout the year, with added input from the oil boiler (installed 4 years ago!)when it was very cold.  Over the four years, we have tweaked things a bit.  When the temp dropped to near zero, the mitsi would go into defrost mode every 45-50 minutes. Our oil usage had dropped by 1000L per year.  

We never liked the feel of warm air being blasted, but the COP was around 3.5 during the winter months.  One of the Daikins is used pretty much year-round, except in the summer, for heating a bedroom, and it works well; the other is mainly for cooling, except in the winter months.  I wish we had a VRF unit!  

Let's see how the Vaillant stands up to them.  Before anyone asks, yes, I did jump through the hoops of Planning permission. 

Posted by: @temperature_gradient

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There's another technology option, sat between A2A and A2W which is a hybrid of both which looks quite attractive if it takes off and becomes more widely available - Panasonic have a system called Aquaria Loop, intended for apartments/large buildings, which uses in-room W2A units (powered with their own integral heat pump) that can provide both heating and cooling, which are connected to the building's wet pipework system with an outdoor unit, which maintains the loop at something around room temperature, above dew point to prevent condensation.

I missed this @temperature_gradient . I wonder if any other manufacturer has this as an add-on..

Posted by: @batpred

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I missed this @temperature_gradient . I wonder if any other manufacturer has this as an add-on..

It's the only product I've seen that does this and I couldn't see anyone advertising it yet in the UK.

It looks like you could achieve something similar with a regular heat-pump with a cooling capability, with diverter valves to have a separate loop to some fan-coil type units to provide the cooling, but then there's issues around condensation on pipework which needs managing. Plus, using A2W heat pumps for cooling in homes seems to be pretty rare/unknown in the UK currently.

Posted by: @temperature_gradient

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It looks like you could achieve something similar with a regular heat-pump with a cooling capability, with diverter valves to have a separate loop to some fan-coil type units to provide the cooling, but then there's issues around condensation on pipework which needs managing. Plus, using A2W heat pumps for cooling in homes seems to be pretty rare/unknown in the UK currently.

We may be lucky, since our home is on a concrete basis and so we have no wood rot concerns at ground level. The idea of the diverter valve may work, as a large part of our ground floor is served by a single circuit. 

And given the way that Arotherm controls seem to justify so much labour may mean that after all, there will be something unique in handling a diverter valve! Of course we may also need a fan dehumidifier to get the complete feel.   

I’m going to be recording a podcast episode next week dedicated fully to A2A heat pumps. If you have any questions for my expert panelist, please drop them below.

@editor when will the MCS site have any certified installers for air-air heat pumps? And when will it admit the technology is by no means new! 🙂

Posted by: @djh

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And when will it admit the technology is by no means new! 🙂

Air-to-Air Heat Pumps

There are currently no installers certified for this technology.

Air-to-air heat pumps are a new technology that will be available to installers later this year. Please check back soon for updates.

😂😂

When our relatives bought a flat in the historic centre of Lisbon over 15 years ago, they were happy the previous owners had installed aircon, handling the usual complications due to the preserved views etc. What they did not realise was that it would double as an (air to air) heat pump!

i guess even 20 years ago it was not new technology! 

I have a few questions for your expert. I'm looking to replace my gas combi boiler with an air to air system, though I'll wait until the BUS grant becomes available.

My first concern is how to run a system for best comfort and efficiency? My house is well insulated but old, meaning it has high thermal mass (from brick internal walls) and high ceilings (2.6 to 2.8 m). The last thing I want is to feel a blast of warm air.

I understand this can be avoided by running the system at a constant low output. I understand this is best achieved by weather compensation. Is this available on air to air systems?

The impression I get is that the controls are based around room thermostats and that rooms can be programmed individually. Now in general I'm a fan of reducing energy consumption by only heating when and where it's needed, or in the jargon set backs and zoning. How is this achieved without temperature fluctuations.

Also is cycling a problem? For instance my house has well separated rooms and will need four cassettes. Add in the need for hot water and I'm looking at the Daikin Multi+. That has a minimum output (off the top of my head) of about 3kW. The heat loss in my house is under 5kW when 21C indoors and -3C outdoors. This means for most of the heating season the demand is well below output. Does this affect the efficiency or life of the system?

My next question is any real world performance data available? That is how much electricity is used for the actual heat delivered. Whenever I've asked I've been quoted the manufacturers figures under test conditions. Can that really be translated into running costs?

In air to water systems SCOP is often measured. It seems very dependent on system design. This is very sensitive to flow temperature, radiator sizes, pipework and so on. It seem to me that's where air to air is very different. It seems possible that any air to air system delivers the maximum efficiency from the hardware. 

Still that efficiency must vary with heating demand hence my question, what is the real world performance data?

And a supplementary question. I've noticed that different cassettes (indoor units) can give higher efficiency with the same outdoor unit. So am I right in thinking it can pay in lower running costs to get better cassettes?

I hope some of those, at least, are interesting.

Posted by: @editor

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I’m going to be recording a podcast episode next week dedicated fully to A2A heat pumps. If you have any questions for my expert panelist, please drop them below.

Here's a few from me:

1. For homes with existing central heating, looking at heat pump solutions and wanting a solution which can provide both heating and cooling, could an A2A solution cover the whole home, or would it need to be A2A/aircon in selected rooms in conjunction with a regular A2W heat pump?

2. If using A2A for whole-house heating/cooling, how do you handle more difficult rooms, like small rooms - cloak-rooms, ensuites which need some heat but where a regular A2A indoor unit is too big, or simply not feasible to provide such a large number of indoor units? Can the indoor units be used in bathrooms with high humidity? Kitchens where there more dust, grease smells etc from cooking?

3. In 2 storey houses, does it make sense to have a central ducted indoor unit covering the upstairs rooms, with the heat exchanger and ducts inside the attic? While using the regular in-room units in the downstairs rooms?

4. Condensate drainage - does this need to be discharged into a sewer, or can it go into surface water drains and soakaways?

5. Cosmetics / appearance - a lot of the indoor units are very plasticky and stand-out visibly, they're rather ugly compared to a discrete radiator. Are there models which can be painted or boxed-in and painted to make them blend into the room?

6. Control systems - is it possible to run an multi-room A2A system like a conventional heating system, with a master controller setting temperatures for all of the rooms? Common timer settings? Smart controls like Hive/Tado style functions to control the whole house remotely?

Some excellent questions - thanks guys! Keep 'em coming!