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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.

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That's really interesting Richard, thanks for that!

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It is an interesting subject.

I wondered about setting up a makeshift system for the house or caravan using car parts... possible? I could work it out but if you have the answer already you'll save me the job lol - How powerful an electric motor would be needed to drive a car AC compressor? I know another motor or 2 would be needed to push air through the evaporator and reducer. Maybe use Hanson quick release fittings to make for a portable system?

For the domestic systems (and I suppose EV systems which can also supply cold air or heat?) how is the system reversed... Surely not as simple as reversing the pumping action / spinning the motor the other way?

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Lpgc wrote:

It is an interesting subject.

I wondered about setting up a makeshift system for the house or caravan using car parts... possible? I could work it out but if you have the answer already you'll save me the job lol - How powerful an electric motor would be needed to drive a car AC compressor? I know another motor or 2 would be needed to push air through the evaporator and reducer. Maybe use Hanson quick release fittings to make for a portable system?

For the domestic systems (and I suppose EV systems which can also supply cold air or heat?) how is the system reversed... Surely not as simple as reversing the pumping action / spinning the motor the other way?

I would guess that it would only work if you had a very small room although a caravan might be OK. When you consider the volume of air in a car versus a house or caravan you will know why the compressor is so tiny compared to that in a domestic A/C unit.

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Your usual excellent techno-commercial intro/analysis: Thanks Richard !

'For my sins' I have discussed such issues - or at least tried to - with local Council officials and even MPs; Almost universally they seem unable to grasp the basics but are 'happy' to implement whatever "the Law/Rules dictate/decide" of course... Oh dear: Might send them your summary instead now though....

In the meantime some of my over-heated neighbours are happy it seems to buy a big plastic box with a fan in it.... merely cooled by 10L of cold water. That's TV ads. for you though, and 'only' £200 (!). A few have the free-standing (proper) A/C units with a tube out of the window (but then they panic about night-time security). Some even have A/C with an 'ugly' external box.. Accordingly the type of heat-pump with large vents through the wall (ie. nothing external) seems ideal but even then they think an enclosure 1X0.5X0.5m is 'too intrusive'... There is definitely a shift in such thinking since 40C hit us recently though... as aesthetics 'go out the window' (!) when folks can't sleep.... ?

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Lgpc: As a fixed A/C installation does not have the advantage of forward motion (ie. cooling air over the condenser so you need a big fan instead, as you mentioned) then an electric motor of 1 or 2 kW would probably be needed (or say about 2HP in old money) ?

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davew wrote:

In the meantime some of my over-heated neighbours are happy it seems to buy a big plastic box with a fan in it.... merely cooled by 10L of cold water. That's TV ads. for you though, and 'only' £200 (!).

My daughter had one of those, a total waste of money, even if you fill them with ice.

A few have the free-standing (proper) A/C units with a tube out of the window (but then they panic about night-time security).

Not proper by any stretch of the imagination. They also seem to use the Italian horsepower/Chinese stereo makers output rating type of information in their specifications. I measured up a bedroom for a customer and it came out at needing a 7,000 Btu unit which surprised the customer as he had used a 9,000 Btu portable and it hadn't been capable of keeping the room cool in hot weather. Not only did the unit I installed cool the room but if the door was left open it cooled half the house.....

Some even have A/C with an 'ugly' external box.

So how else do you house the condenser? Even if the customer wants it, I will not fit the outdoor unit on the front of a house. Hide it round the back or down the side where it can't be seen.

Accordingly the type of heat-pump with large vents through the wall (ie. nothing external) seems ideal but even then they think an enclosure 1X0.5X0.5m is 'too intrusive'...

That isn't a heat pump but a heat recovery system where the heat is extracted from the room and used to provide hot water. The external box for a heat pump is considerably larger, usually with two fans, than the one for a smaller AC system

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davew wrote:

Lgpc: As a fixed A/C installation does not have the advantage of forward motion (ie. cooling air over the condenser so you need a big fan instead, as you mentioned) then an electric motor of 1 or 2 kW would probably be needed (or say about 2HP in old money) ?

Big fan not needed, think about it, there's very little airflow though the condenser when sitting in traffic, and it doesn't really matter how hot it gets (within reason). The place where I did my FGas course also did the automotive course and had a test rig for training that contained everything you would find in a car except for an electric motor to drive the compressor instead of the engine. I'll give then a bell and see what motor it needs when I'm back in the UK.

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Some of those heat recovery systems (Greenwood make one of them fitted to new builds) are designed to provide ventilation whilst minimising heat loss, they consist of some pipework, a heat exchanger and a large polystryene covered box with a condensation drain on the bottom of it. I don't think theres any moving parts involved in them from what I remember, though I think they must have a fan somwhere in the system.

Typical use for them is in new builds to suck air out of the bathroom/kitchen and replace with fresh air whilst keeping as much of the heat as possible through the heat exchanger. The box on them would typically be installed either in the airing cupboard area, or loft space.

Edit - should have made clear, most of the reason for this system appears to be avoiding damp by improved ventalation rather than heating, whilst minimising heat losses.

I briefly did some work on them on a contract a long time back between other work, fitting a modifiication to them on some new build properties.

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How are the systems 'put into reverse' to switch between heating/cooling a room? Does the motor spin the other way / done by changing which jets the refrigerant flows through / valve arrangement on the pump and/or evap / condenser? Are they by design more efficient at heating or cooling a room (and jumping the gun a bit maybe - are the evaporator and condenser the same size, even perhaps the same part number?).

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Richard: Thanks: Briefly, some clarification:

That 'only £200 cold water box/fan' nonsense is of course 'well dodgy' - probably as far as the ASA is concerned of course; You may as well just fill a hot water bottle with ice water.... ?

By proper A/C above I probably should have said "proper" ( ie. with inverted commas ) - simply as it has cold out one end and hot out the other.... and that is is at least externally vented

As for the 'heat pump / conditioner' I mentioned without external box it was something very similar to this (and only 1K fitted ...): https://www.appliancesdirect.co.uk/p/a1%2fiqool-smart12hp/electriq-a1iqoolsmart12hp-air-conditioner

Of course I agree that an external box will be (functionally) best, again it is just that aesthetics certainly affect folks' buying decisions.... not just my neighbours. Ask a woman !? Fan noise is key too. Some expect if to be almost silent "like the one in my office building..." (!!)

As BrianH said New Builds/New Regulations may mean house design needs to change.... along with expectations.

The main point is that there is definitely a real need for Customer/Consumer education on all this though: I have lost count of the number of folks asking me questions like 'why their whole garden needs to be dug up' for a Heat Pump Installation...? usually because they have (only) heard about Ground Source kit: Perhaps the BBC could oblige instead of all their celebrity baking/dancing/etc twaddle; I won't be holding my breath.....

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davew wrote:

That 'only £200 cold water box/fan' nonsense is of course 'well dodgy' - probably as far as the ASA is concerned of course; You may as well just fill a hot water bottle with ice water.... ?

It's basically a fan that blows the air over cold water or ice to cool it, better than nothing but not by much and certainly not worth the money.

By proper A/C above I probably should have said "proper" ( ie. with inverted commas ) - simply as it has cold out one end and hot out the other.... and that is is at least externally vented

As for the 'heat pump / conditioner' I mentioned without external box it was something very similar to this (and only 1K fitted ...): https://www.appliancesdirect.co.uk/p/a1%2fiqool-smart12hp/electriq-a1iqoolsmart12hp-air-conditioner

Those things are noisy as they have the compressor, condenser and evaporator all in the one box inside the house. They work well enough (although not the ones with ElectriQ on the front), better than the portable units, and are an option if you can put up with the noise and have a reason why you can't site an outdoor unit.. The ElectriQ units are sold almost exclusively by Appliances Direct (and their other identity, Aircon Direct) and are horrible Chinese made units that look good (ish) but aren't particularly good or well made. They are cheap for a reason.

Of course I agree that an external box will be (functionally) best, again it is just that aesthetics certainly affect folks' buying decisions.... not just my neighbours. Ask a woman !? Fan noise is key too. Some expect if to be almost silent "like the one in my office building..." (!!)

They are almost silent as they are the same units as fitted in office buildings. Often, the units installed in offices are more industrial so noise level is considered less important anyway so are noisier than ones intended for a domestic install. Indoor unit noise level of the units I fit is 20 dB(A), while the outdoor units are about the same.

As BrianH said New Builds/New Regulations may mean house design needs to change.... along with expectations.

Rather than putting an indoor unit into every room making the install complex (and expensive), the way to do it on a new build would be to incorporate ducts (or use the void space between joists) so a ducted system could be installed. This uses a single indoor unit (or one per floor anyway) that lives in the attic or in a service cupboard and outputs via vents installed in each room.

The main point is that there is definitely a real need for Customer/Consumer education on all this though: I have lost count of the number of folks asking me questions like 'why their whole garden needs to be dug up' for a Heat Pump Installation...? usually because they have (only) heard about Ground Source kit: Perhaps the BBC could oblige instead of all their celebrity baking/dancing/etc twaddle; I won't be holding my breath.....

A ground source heat pump will work much better than an air source one as the ground temperature remains reasonably constant a few feet down. So while the air temperature can vary anywhere between -10 and +35C, 10 feet down, the temperature will probably be between 5 and 10C and won't change much at all. So when you are using this as a source for the evaporator (in the case of a heat pump installed for heating), it is far more reliable as the temperature differential is constant. Downside of course, is that you need to dig up a large quantity of the garden (if you have a garden in the first place, not the postage stamps that most new builds come with these days).

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@LPGC. Having remembered the question and waiting for the delivery of the units for my 51st install since starting this lark, I called the tutor at the training place where I did my FGas. He had a look at his automotive test rig and the motor used to drive the compressor is a 2.2kW, 3 phase motor, so pretty substantial (which probably explains why the idle speed drops on some smaller engined cars when the AC clutch kicks in).

He was sceptical that a system from a car would be sufficient to work in a caravan. You have to remember that the important thing when sizing a unit is the volume of air it is expected to cool. So a system removed from even a large car like a Jag, is still only intended to cool a much smaller volume than a caravan. If you have a system removed from a campervan that would probably be OK (as the volume will be much the same).

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Thanks Gilbert

How are the systems reversed (heat / cool a room) ?