Further to Dave’s comment in this thread https://rangerovers.pub/topic/3016-ac-leak-test-at-home?page=2#pid38452, I’ve had a bit of time on my hands today. The car is ready and loaded up so I’ve spent the time waiting for Dina to finish work so we can set off before driving to Spain to write this.
I would like to think that at least some of you understand how the AC system in your car works, but in case you don’t, a little explanation. Everything has 3 states, solid, liquid and gas, the only thing that differs is the temperature that they change from one to another. Water, as we all should know, has a boiling point of 100C, the temperature where it changes state from liquid to gas (and 0C when it changes state again from liquid to solid, aka ice). However, if it is under pressure, the boiling point increases. That is why your cooling system has a pressure cap and the increase in pressure means it doesn’t boil (change from liquid to gas) until around 120C. That is why, as long as you don’t have a leak anywhere, your cooling system can run at 105-110C without boiling. In the same way, if you run on LPG you fill your tank with a liquid (Propane in this case) at around 10bar (145psi) so it remains a liquid but, as Propane has a boiling point of -44C, as soon as it is no longer under pressure, it becomes a gas.
So, what does this have to do with AC? Because it uses this to move heat from one place to another. It is filled with R134a gas, Tetrafluoroethane (CF3CH2F) with a boiling point at atmospheric pressure of -26.1C. Your system also has what are termed a low side and a high side signified by the pressure in the system. Starting at the low side, the system is full of a gas at a pressure of around 2.6 bar when operating. That gas passes through a compressor which raises the pressure to around 10.3 bar (at the High side) at which point it is fed to the condenser (the one in front of the radiator, the one that leaks with monotonous regularity) where, it condenses and becomes a liquid. That generates heat which is dissipated by the airflow though the condenser. This liquid then passes through a small orifice where it vaporises as the pressure drops on the other side of the orifice and goes through the evaporator (see how the names of the various components start to make more sense now?). At this point it gets very cold (although strictly speaking, in thermodynamics there is no such thing as cold, only a lack of heat, so the correct terminology is that it ‘draws heat’, something it took me ages to get my head around when I did the FGas course) in the evaporator, air is blown through it and that is the nice cool breeze you should get out of your vents. At that point the cycle starts again as the gas gets to the compressor to be compressed and turned back into a liquid. This is just the same as a Calor gas bottle getting condensation or even ice forming on the outside if you have your barbecue/patio heater/ blowtorch running flat out for a while. The liquid in the bottle is vaporising so is getting cold (sorry, drawing heat).
OK, so that is an automotive AC system and a domestic AC system works in exactly the same way. You have the compressor and condenser in a box outside your house (along with a load of control electronics). That is linked by two copper pipes (liquid and gas) to the indoor unit. These come in a variety of forms but the most common ones are the wall unit, the rectangular box on the wall up near ceiling height, or the ceiling cassette, the square units set into the ceiling that (usually) have 4 outlets blowing the cold air out in different directions. There’s multiple different designs but they all work in the same way. The big difference between a domestic system and that in your car is that they are reversible. The flow of the refrigerant can be reversed so the condenser becomes the evaporator and vice versa. That way, when the flow is reversed, the indoor unit gets hot and the outdoor unit gets cold. That way they can provide heating as well as cooling. Different units differ in how they achieve this, with some of the cheap Chinese made systems you have to manually set them for heating or cooling. The better systems, like the Fujitsu units I prefer and install, have an Auto setting. You put it on Auto, set the temperature and it reverses the flow as and when it is required. That way you can set 21C and no matter if the ambient is -5C or 30C (or higher as it has been recently in some areas), it will automatically maintain an indoor temperature of 21C.
On a decent quality system working correctly, with an ambient temperature of 25C when set for maximum cooling, the air coming out of the indoor unit will be down to 2-3C. When set for maximum heating, it will achieve 55-60C. If the ambient is cooler, this will be a bit lower but the Fujitsu systems will still provide 50C down to an outdoor temperature of -15C. This is spread around the room by a fan so is much the same as using an electric fan heater. The big difference is that it can achieve the equivalent of 3kW of heating (or cooling) while only drawing sufficient electrical energy to power the compressor, which will normally be around 600W, making it a cheap way of heating a room. Multiply that by the number of rooms in your house and it adds up to a considerable saving. The outdoor units can supply a single indoor unit or up to 8 but the install does get pretty complex…….
That brings us on to air source heat pumps (or ground source heat pumps for that matter, they work exactly the same). They are configured just the same as an AC system when using it for indoor heat as they aren’t reversible. The difference being that instead of giving a source of heat which then has air blown over it to distribute the heat throughout the room, a heat exchanger is used so that heat is used to heat water which is then sent around the existing pipework to your radiators.
This is where the problems start. First of all you have in inherent loss in the heat exchanger dropping the water temperature down to around 50C, many of the more modern houses have 10mm microbore pipework, perfectly adequate when the water is being pumped around from a gas boiler at around 70 degrees but too restrictive for water at a lower volume and 20 degrees cooler. Then a radiator will have been specified for the size of the room. Radiators tend to be specified in Btu’s (British Thermal Units), as are many AC units. You take the volume of the room in cubic feet, multiply by 5 and that gives the size of the unit or radiator you need in Btu. So an average living room or bedroom of 5m x 4m with conventional ceiling height is 16.4 ft x 13ft x 8ft, giving a volume of 1,705 cubic feet. Multiply by 5 gives 8,528 so I would install a 9,000 Btu AC unit. Radiators are also rated in Btu but that figure assumes they are fed with water at 70C and at the sort of flow rate achieved by a central heating pump. As the water from a heat pump is cooler and the flow is lower, in virtually all cases larger radiators (and often pipework) need to be installed. This is why people are having to spend in excess of £20k for an installation only to complain that the house isn’t as warm as it was with a conventional gas boiler. Radiators aren’t that efficient anyway, as all you have is a hot spot in one place in the room and the heat is spread mostly by convection with a little by radiation. That’s why it is recommended that the insulation properties are improved, to keep what feeble amount of heat you have from escaping.
Is it cheaper to run though? No not really. Let’s take the average 2 storey, 3 bedroom house. You’d be looking at a footprint of roughly 30ft square, so that is 900 square feet per floor so 1,800 square feet floor area. With 8ft ceiling height, that means you need around 72,000 Btu in total heating capacity or 28kW equivalent. OK, so AC units and air source heat pumps are pretty efficient so won’t be drawing that amount of power, but they will still draw in the region of 6kW as there’s one serious compressor in there (anything over a conventional house system will need a 3 phase supply). At today’s average electricity costs of around 30p per kWh, that’s £1.80 for every hour it is on. Not that cheap compared to a gas boiler, even at today’s prices, without taking into account the purchase price and the modifications needed to what you already have.
The only time a heat pump system will work adequately is if you have underfloor heating (and walking on a floor at 50C is a little more comfortable than one at 70C!). Although even then I have installed an AC unit into a house with underfloor heating fed by a heat pump as it would kick in as soon as the temperature dropped but if, after a couple of days the sun came out, it would switch off again. The owner of the house wanted AC to fill in the gap between the weather getting cold and the underfloor heating starting to work properly and also to give him the benefit of cooling in summer.
The irony of the whole thing is that you can get a Government grant of up to £5k to install a heat pump and they have a lower VAT rating too reducing the cost to buy and install. Although as they are so damn expensive in the first place there’s still a considerable outlay, particularly when you consider you can replace an existing gas boiler with a more efficient, modern one, for a couple of grand. But, even though it works in the same way and is probably better in many cases, you can’t get a grant for AC (and the systems are still rated at 20% VAT) as it gives you cooling as well as heating, so you are getting a bonus which the Government won’t pay for.
Personally I think Hydrogen fuelled boilers are the way to go but technology moves a lot faster than Governments so when they first made their recommendations that we should all be going for heat pumps, Hydrogen fuelled boilers weren’t around. Much like how they advocate we should a be driving battery electric cars when hydrogen fuel cell powered ones , or even a near conventional internal combustion engine running on Hydrogen, seem a much more viable, and ecologically friendly, option.