In practise, compared to Air to Water and Ground Source heat pump systems, are Air to Air heat pump systems a) (most importantly) more efficient for domestic heating and b) cheaper to install?

I think they should be, making them perhaps the best renewable heat solution, but I can find no research to confirm or refute these propositions. Can anyone help? More background below.

-----------------------------------------------------------------------------

The Energy Saving Trust field trial found that A2W heat pumps average system efficiencies of 2.2 (3.2 at best). Ground Source manages 2.4 (high of 3.3). That’s too low to be worthwhile vs. most other heating sources, and nowhere near vs. gas. You need 3.0+ to be better (for cost and carbon) than a high efficiency gas boiler.

And A2W and GS are very expensive to install. You are looking at £8k for A2W and £13k for GS.

So you pay a fortune to install something dirtier and more expensive to run (not to mention service) than gas. You can generate your electricity greenly, or pay for ecotricity, but your heating costs will be even higher.

Now, as far as I can tell, A2A should have the following advantages over A2W and GS, leading to significantly higher COPs:

1. Fewer heat transfer stages (ambient heat to refrigerant, then refrigerant back to air). A2W & GS have: ambient heat to refrigerant, refrigerant to water, water back to air via big radiators or underfloor heating.

2. Lower required output temperatures: A2A will pump directly to heat a room to 20 degrees. A2W or GS have to heat water to much higher temperatures. The larger difference between ambient and output temperatures will make it harder for the system.

3. More direct and efficient transfer of heat to internal air. With A2A refrigerant heat is transferred back to convected air in internal wall unit. A2W & GS use big radiators or underfloor heating and natural convection. I believe this the latter will be a less efficient means to transfer heat where it is needed, with greater heat losses.

4. Less heat lost to sub structure. Air to air will blow heated air straight into the room. Radiators tend to heat the wall behind them, and underfloor must heat the floor substrate, with some lost around the ends and in the pipes between the radiators and hot water tank.

5. Faster heating. Not really an efficiency point in itself, but A2A will start pumping forced convection hot air quickly. A2W/GS will take time for radiators to heat, and once they do, will rely only on natural, not forced, convection, so take much longer to heat spaces. This means:

6. No unnecessary heating: A2A can come on when required (or just before when required) if they will heat space faster, and only heat rooms where internal units are switched on (i.e. where they are needed). A2W and GS will have to heat water in the tank even if it will not be used for some rooms (otherwise it would take forever for the water tank to heat, then for that water to heat the radiators, then for the radiators to heat the room). Additionally, some heat will be lost as the water sits in the tank.

Disadvantages:
1. Air to Air internal units will use electricity, lowering the overall efficiency. But from what I can tell, this shouldn’t be much. Can anyone confirm this?

If the above factors (or some of them) are correct (and ignoring downsides such as internal noise for the moment), the real world COP and system efficiency of an A2A system should be significantly higher than A2W and GS. If A2A manages efficiencies of over 3 (and manufacturers’ ratings can be 4-5), then it is much better than the alternatives, and cheaper and greener even than natural gas (you might even pay for expensive green electricity and go carbon neutral, without it costing more than natural gas).

Moreover, an A2A system with 6 indoor units should cost £4.5k to buy and install. That’s 56% of the cost of A2W and 35% of GSH, and frankly much nearer to what people can afford. Add this in, and you have a system which is much cheaper to install AND run than A2W/GS.

It’s what I’m inclined to go for, but here’s my problem. I can’t find research which will confirm or refute my propositions above. The EST field trial only looked at A2W and GS, and they have no plans to look at A2A. If I’m correct about A2A being more efficient AND much cheaper to install, the lack of definitive research on this will cause the system to languish when it could be the holy grail of renewable heating. Moreover, I’d really like to know if what I’m going to go for is silly or not. Does anyone know of any research on this question, or have observations on whether they think I am likely to be correct or not?

P.S. I have deliberately ignored domestic hot water. I know A2A won’t do this, but space heating is 75% of total domestic heating energy requirements. If I can get my head around the real world COPs of A2A for heating, I was then going to consider how best to ‘add in’ hot water. It might be best to have an alternative heat source for water (LPG, gas, electric, solar thermal etc.) or to use a heat pump to pre-heat water to a combi or something. Probably a ‘follow up’ topic.

You'll need Google translate to make some sense of it but here are test results on 9 Air - Air heat pumps from a Swedish magazine.

http://www.polarpumpen.se/test-av-luftvarmepumpar

I know the consumer agency here in Sweden do test every few years on different groups of heat pumps I will try to dig out some reports.

Posted By: GarethCYou need 3.0+ to be better (for cost and carbon) than a high efficiency gas boiler.

It's quite a bit lower than that for carbon:

Gas boiler (90% efficient) = 0.2239kg CO₂e per kWh heat delivered.
At 0.5246kg CO₂e for grid electricity the heat pump would hit par at CoP = 2.34

Posted By: Paul in MontrealOver here Anything-to-water heatpumps are pretty rare as they're useless for air conditioning…

Quite handy for domestic hot water, though.

Posted By: Paul in MontrealTrue, but people don't tend to combine heating and hot water over here.

Someone mentioned that it is better to split them on another thread. I tend to think the same as they serve different functions and different times. Has implications for thermal stores as well.

Air-to-air can achieve far higher efficiencies than air-to-water:

http://www.hitachiaircon.com/argws/groups.do?action=getGroups&categoryId=173&rangeId=1&langcode=en

I'm not sure that your view on efficiencies stacks up Tony

The efficiency of any heat punp is set by the compressor and the type and mix of refrigerant - propane is an excellent refrigerant but we don't use it in other than specialist systems in favour of azeotropic mixes that go a little way to acknowledging global warming and ozone depleton potential.

What changes the efficiency of different systems is when you need to use it and what mans (if any) of storage that you deploy. Given that ASHP operates on an annual air temperature that averages to the ground temperature and often air temperature is above ground temperature it's when you use it that sets the efficiencies - ie the seasonal efficiency or annual efficiency - ditto for the comparison to water source - what's the MWT of the water and what is the range of temperatures - if it varies by a reasonable amount, running the heat pump when the water is warmest is most efficient.

Basically - the heat pump is most efficient for comparable machines when the difference between the source and sink temperatures is as small as possible.

What then further changes the efficiencies is the sink temperature - a heat pump only generating a flow temp of say 25c for UFH or large size emitters will be more efficient than one generating 35C for forced air ventilation - regardless of the type of source medium

The design needs to optimise the timing of when heat or coolth is required depending on the source and what method of delivery/distribution is needed

Regards

Barney

Posted By: SteamyTea but the energy in and the energy out would be pretty similar with only the GSHP being a bit different if it used a bore hole. Is there any data on this?

Geographical location would be the decider - highlands of Scotland with a great part of the winter at below zero temperatures would likely realise a GSHP performing the best; more input energy would be required with the other two due to regular defrosting. However, compared to say Cornwall where they hardly ever have a frost then an air to air, followed by air to water with GSHP being the worst. Air to air units through their very design have higher COPs as the condensing temperature (typically no higher than 32'C) is lower than that of both ASHP & GSHP.

Hi St - yes, it's the seasonal efficiency of the particular technology in a particular system that's the true metric - but that's not what Tony said.

for identical buildings with identical demand an ocupancy you have identical gains and losses. The eficiencies now are set by when you have to run the HP and how you distribute it.

We did a quick and dirty comparison between a bore hole GSHP running to UFH and an air source HP running to a buffer store and then to UFH and if you limit when the ASHP can run (ie a minimum air temp) and have a large enough store with a limit on upper temperature there isn't actually much in it. These of course a long lag systems.

If you use ASHP to an all air distribution system with basically the refrigerant being distributed to fan coil units in the space ( basically variable refrigerant volume flow) and only heat when required in a fast response building then again, not much difference.

Whilst a pretty raw tool you could do the comparison with something like SBEM - we used IES which is a little more dynamic and by adjusting building construction, glazing etc for each type of system you can get a reasonable comparison.

The key is to look at the strengths and weakness of all three sources and then optimise both the storage (or not) and the delivery medium to the optimised emitters for the strengths and weakness of the source.

At the end of the day it's the same amount of energy needed - the efficiencies are just in the tinkering with rates, times, tariffs and temperatures and medium of delivery.

Classic dilemma is say ASHP - is it more efficient to run it during the midmorning/afternoon period into storage to minimise the source sink delta t of to say sod it and run it at night with the biggest delta T and use a cheap off peak electricity tarrif - or even run it from a gas engine rather than an electric motor and that way you get higher temps for DHW and lower temps for UFH but without the 3:1 carbon penalty of electric v gas

que PV again and is it better to generate electricity and return it to low grade heat or capture solar thermal and try to use that.

Regards

Barney

Would it be a good idea to run an air to air heatpump as an addition to an existing wet radiator system especially during the coolish spring and autumn days when firng up the boiler to charge the thermal store is a bit uneconomic. More especially if the following day outside air temperature is 18-20 C and you don't need that 2500l of hot water. Also to supplement winter rad useage.

Posted By: TimPowys-LybbeIs there any control system that avoids this waste?

Of course - it all depends on your thermostat. Over here, you can get both manual and automatic switch-over stats. The manual ones allow one to select either heating or cooling, but won't switch from one to the other. The automatic ones will - but you can still program completely different set points for the heating and cooling functions.

Paul in Montreal.

You get minimal radiant heat from radiators (more accurately: convectors) and underfloor heating (convector/conductor) - only a fire or an infra-red heater will give you radiant heat.

On the flipside the A2A unit de-stratifiies and homogenises the temperature in the room, which can require a lower temperature for the same level of comfort.

Personal preference, usually derived from personal experience, dictates which "feels" warmer or cooler or simply "nice" in general. UFH is the most universally accepted. Old people like radiant heat or (hot) forced air because their brains are wired to associate a roaring fire with winter warmth. Younger europeans like the subtle air movement from wet radiators. Americans like the instant response of forced air.

A2A units that "feel unhealthy and dry" are often blowing all the dust in the room around. Their major major major advantage is that it is far more difficult to f**k up the installation of a "packaged solution" than it is f**k up the specification and installation of source, heat pump, and emitter with an A2W or GSHP. This is why they perform relatively well in the EST trials, or rather the A2W or GSHP units in those trials perform so spectacularly badly.



GarethC - your thinking is right in my opinion. The A2A units aren't in the RHI because they're already economic for base load heating. You won't see them in new build because they're not the lowest capital cost solution. You won't see them being pushed by grant-hunters as they're not being subsidised to buggery. You won't see them in high end builds because UFH is nicer (quieter, primarily) not because it is more efficient or better value. Where they ought to be is in every mainstream heated property that's off-gas.

They should have non ozone depleting refrigerants. They should be installed alongside the best practicable insulation and airtightness measures.


I'm also of the opinion that space heating and hot water are radically different in their requirements and ought to be supplied by separate systems. Historically it didn't much matter with non-condensing boilers and rads being all the rage and space heat being dominant. Hot water now dominates space heat requirements (if indeed there are any after hot bodies and electroncial toys turning electricity to heat are accounted for) in modern low energy builds and we'll be looking into this again.

Solar-electric has its attractions: the initial connection fee, standing charges, and annual maintenance you avoid by not having gas appliances pays for a lot of hot water. Drainback thermal store with panels sized for 3-season performance with a 3-phase electric flow-boiler for 4th season top-up on peak rate electricity, or predictively preheat part of the thermal store using an immersion on lower CO2/price night rate electricity, or both for flexibility and redundancy. Worth the additional complexity of a heat pump? Debatable. Preheat your cold water (except kitchen tap) to 20C too. This is safe, makes solar collectors or heat pumps hyper efficient by minimising deltaT, will further reduce the electricity consumption of washing machines running 30C washes, will make your expensive hot water go further in showers and baths, and in most cases this "tempered" cold water means that you don't need to add hot/fire a boiler/draw off a tank in order to wash your hands at basins. Instant electric boiling water tap for the kitchen, and use the dishwasher rather than washing up by hand.

<blockquote><cite>Posted By: Paul in Montreal</cite><blockquote><cite>Posted By: TimPowys-Lybbe</cite>Is there any control system that avoids this waste?</blockquote>

Of course - it all depends on your thermostat. Over here, you can get both manual and automatic switch-over stats. The manual ones allow one to select either heating or cooling, but won't switch from one to the other. The automatic ones will - but you can still program completely different set points for the heating and cooling functions.

Paul in Montreal.</blockquote>

Sounds good news. Can you give me the name of the suppliers of that sort of kit, please?

Tim

Are there any A2A heat pumps designed to fit MVHR ducting and do you think this is something that can be easily done? I imagine so but I haven't seen it.

<blockquote><cite>Posted By: RobinB</cite>Are there any A2A heat pumps designed to fit MVHR ducting and do you think this is something that can be easily done? I imagine so but I haven't seen it.</blockquote>

Yes and no. You can use the return ducting for the SUPPLY side of the MHVR, but you still need to run separate extract ducting. A2A ducts are typically 10x the volumetric capacity of HRV ducts due to the much large energies that need to be moved around.


<blockquote><cite>Posted By: GaryB</cite>The controllers are supplied with the A2A units and are tailored to the manufacturer.</blockquote>

They don't have the ability to add an external thermostat? I'm very surprised if this is the case.

<blockquote><cite>Posted By: Tim_Powys-Lybbe</cite>Sounds good news. Can you give me the name of the suppliers of that sort of kit, please?</blockquote>

http://www.amazon.com/Honeywell-RTH3100C1002-Digital-Heat-Thermostats/dp/B000VL440U for example - this does multi-stage heating and cooling as well as control of auxiliary heat - though not automatically. We have a White Rogers thermostat that is semi-automatic (you have to manually switch between heating and cooling). Here's a link to their line of 4-wire thermostats:

http://www.emersonclimate.com/en-us/products/thermostats/Pages/programmable_universal_thermostats.aspx

Paul in Montreal.