UPDATED: Air Source Heat Pumps/Air Con - Full Info & Guide, is it cheaper to run than mains gas?

John_Pierpoint

Surely the point is that an economic heat pump system is running with water temperatures that are ideal for breeding up the legionella germs:

http://en.wikipedia.org/wiki/Legionella

• Above 70 °C (158 °F) - Legionella dies almost instantly
• At 60 °C (140 °F) - 90% die in 2 minutes (Decimal reduction time (D) = 2)
• At 50 °C (122 °F) - 90% die in 80–124 minutes, depending on strain (Decimal reduction time (D) = 80-124)
• 48 to 50 °C (118 to 122 °F) - Can survive but do not multiply
• 32 to 42 °C (90 to 108 °F) - Ideal growth range
• 25 to 45 °C (77 to 113 °F) - Growth range
• Below 20 °C (68 °F) - Can survive but are dormant, even below freezing

TiredGeek

Mikeeuropa, I think I know where you're coming from here.


You have a large tank of water sitting at 50'c with a coil through it which the mains water runs and then ends at your taps, cold water goes thru the coil and ends at the taps hot. Correct?


Good in theory but when I looked into this I was told the size of the coil would have to be massive to allow the cold water to get hot in the amount of time it travels thru the tank. What you'd get it is a shot of hot water that's been sat in the coil followed by a cool flow....


The tank is full of water that goes to the taps, the EcoDan (or whatever) just pushes hot water thru a coil in the tank for an hour or so to bring the whole lot up to 50'. Then there's a separate immersion heater to sterilize when you feel like it

Mikeeuropa

lovesgshp: I think you're maybe still misunderstanding me here. I'm not worried at all about the Legionella risk. Having read this thread I was worried about the cost of the sterilisation routine. What I was suggesting is that using a thermal store in the way I suggested would completely remove the need to run sterilisation routines. Mains water is only heated for a few seconds and so there is no elevated Legionella risk and as the stored hot water never comes into contact with humans there's no need to sterilise it.

John_Pierpoint wrote: »

Surely the point is that an economic heat pump system is running with water temperatures that are ideal for breeding up the legionella germs:

I'm not sure if you're agreeing with me or not there John. Yes I agree that the system is running with water temperatures ideal for breeding Legionella - so my point is that instead of letting that buggy water come out your taps you transfer the heat from it to fresh water that isn't full of bugs. And as far as I understand the way to do that would be with a thermal store approach. Or to be more precise my original question was "why isn't the thermal store approach used in all ASHP systems as that would remove the need for sterilisation cycles and make the systems more efficient". I was hoping someone could come back with a reason this approach doesn't seem to be used, and maybe TiredGeek has done that.

TiredGeek wrote: »

Mikeeuropa, I think I know where you're coming from here.


You have a large tank of water sitting at 50'c with a coil through it which the mains water runs and then ends at your taps, cold water goes thru the coil and ends at the taps hot. Correct?

By golly I do believe someone's got it! Yes, exactly what I was saying.

TiredGeek wrote: »

Good in theory but when I looked into this I was told the size of the coil would have to be massive to allow the cold water to get hot in the amount of time it travels thru the tank. What you'd get it is a shot of hot water that's been sat in the coil followed by a cool flow....

So we're getting towards an answer to my question here. It seems that maybe there's issues getting the volume of hot water out. I know thermal stores are used to produce hot water at the taps but maybe that is on other fuel sources where the stored water is held at higher temperatures (exactly what we're trying to avoid with ASHP as they don't run efficiently at high output temps)? Or maybe the volume of the thermal store has to be significantly bigger in order to store sufficient energy (so you'd need to have room for an enormous store to sit somewhere without falling through the floor boards)?

I have seen diagrams of some thermal stores where the stored water is pumped over the "DHW" heat exchanger, maybe that allows sufficient heat transfer to keep the water running hot at the taps but then of course you've got the energy used by the pump to take into account and that perhaps costs more than a periodic sterilisation of a hot water tank?

Unfortunately I suspect it would need someone with far more figures at their finger tips and knowledge of the calculations/formulas than I have to make to come up with the answers to those questions.

John_Pierpoint

I have lived in a small house with a gas Combi - trying to fill a bath in mid winter was a dribbling pain in the neck; as was using a low powered electric shower.

matelodave

John has pointed out one of the differences between a thermal store or stored hot water system.
As said the domestic hot water for a thermal store system requires the water to be pumped through a heat exchanger of some sort - either a coiled pipe in the tank or a flat-plate type which is more efficient. However you need the water flow to be quite slow so that it exchanges enough heat from the store. The faster the flow, the lower the temperature rise - that's why combi boiler outputs are usually rated a lot higher than the house heating requirements to give sufficient heat to the domestic hot water as it flows through the boiler

A stored water system ensures that the water from the tank flows at what ever pressure it is fed to it.

Some systems still have a tank in the loft and the pressure is about 0.5 bar. Others have the tank pressurised by the incoming cold water main which gives higher pressure to the hot water - up to 2bar or more. This makes showers work a lot better than open vented systems.

Cardew

Mikeeuropa wrote: »

I have seen diagrams of some thermal stores where the stored water is pumped over the "DHW" heat exchanger, maybe that allows sufficient heat transfer to keep the water running hot at the taps but then of course you've got the energy used by the pump to take into account and that perhaps costs more than a periodic sterilisation of a hot water tank?

Have you got a link or specification for such a system?

ASHP systems are designed to keep the water temperature fed to radiators/under-floor heating(UFH) as low as possible to increase their efficiency(COP). Temperatures of 30C to 35C are quite common.

Under your 'unusual' system may we assume that the 'heat store' contains the water that will be pumped to the radiators/UFH?

The minimum temperature required for DHW is, say, 50C. If your 'DHW heat exchanger' is contained in that heat store then the water obviously that has to be kept at 50C.

In winter Mains water can be below 5C. So to have a heat exchanger capable of raising the heat of a, say, 10 litres a minute flow of water by 45C would require a heat exchanger to be absolutely huge and cost a fortune.

Bear in mind a large gas Combi boiler with a 40kW output couldn't raise that flow of water by 45C.

When you want DHW in the summer(but no central heating) you still have to have the same system with the heat store water at 50C?

Mikeeuropa

Cardew wrote: »

Have you got a link or specification for such a system?

Hi Cardew, the document "Performance Specification for Thermal Stores" on this link http://www.greenspec.co.uk/files/energy/storesperformance.pdf goes into a lot of detail about the different types of thermal stores both for individual buildings and group heating schemes.

Cardew wrote: »

ASHP systems are designed to keep the water temperature fed to radiators/under-floor heating(UFH) as low as possible to increase their efficiency(COP). Temperatures of 30C to 35C are quite common.

Under your 'unusual' system may we assume that the 'heat store' contains the water that will be pumped to the radiators/UFH?

I'm not sure just how "unusual" this is, admittedly not many on here over the last few days seem that familiar but on this page of the Energy Savings Trust website http://www.energysavingtrust.org.uk/Generating-energy/Choosing-a-renewable-technology/Thermal-stores (which contained the link to the "Performance Spec" document mentioned above) it states...

Thermal stores for heat pumps

An air source or ground source heat pump will work more efficiently with less wear on the pump and compressor if it does not have to continually cycle on and off (short cycling) when the demand for heat is low. This is more likely to happen if your heat pump is relatively large and less likely if it is relatively small and running continually to meet demand. It is also less likely if you have an air source heat pump with a motor that can modulate its output.

One of the ways to avoid the short cycling of a heat pump is for it to be linked to a thermal store (usually referred in this instance as a buffer tank). However, there are other ways (such as leaving a part of the heating system permanently open) and for this reason the need for and sizing of a buffer tank linked to your heat pump is something that your installer will decide upon in conjunction with the recommendations of the manufacturer.

So it seems to be an accepted design philosophy by the EST. As to whether the heat store contains the water for the radiators/UFH this again is a design choice. Page 6, figure 2.4 shows the "Direct integrated thermal store (DITS)" so the store providing heat to both DHW and to heating - typically the heating feed is from the lower part of the store so takes lower temperature water. Figure 2.6 shows the "Direct hot water only thermal store (HWTS)" - as it says, the store for DHW only, so the system operates like a traditional system with a 3 way valve to direct the heat source either to the heating or the store. Incidentally figures 2.5 and 2.7 show the same 2 setups respectively but with the forced/pumped convection exchanger for the DHW (mentioned in my previous post) which I presume increases the effectiveness of the heat transfer and allows greater volumes of reasonably hot water to be achieved.

Cardew wrote: »

The minimum temperature required for DHW is, say, 50C. If your 'DHW heat exchanger' is contained in that heat store then the water obviously that has to be kept at 50C.

Yes I would agree with that but if we want DHW at 50C then the ASHP has got to produce some water at 50C whether it's putting it into a "tank" or a "store".

Cardew wrote: »

In winter Mains water can be below 5C. So to have a heat exchanger capable of raising the heat of a, say, 10 litres a minute flow of water by 45C would require a heat exchanger to be absolutely huge and cost a fortune.

Yes, you could well be right. Please lets not forget that I'm not here saying this thermal store design is the solution to all problems and the best design since sliced bread. I simply thought it would be a solution to the "sterilisation" costs/inefficiencies and, presuming there are plenty of people out there more intelligent than I am I was hoping for an explanation of why it didn't seem to be used - certainly not amongst the contributors to this thread.

QUOTE=Cardew;64015881]
Bear in mind a large gas Combi boiler with a 40kW output couldn't raise that flow of water by 45C.[/QUOTE]

Yes and this is perhaps the crux of the issue - getting sufficient heat transferred into the incoming mains water. I'm suspecting the the cost/practicalities of doing that are the reason this approach doesn't seem to be used. It's just that, as yet, nobody has come on here and said with cast iron confidence (maybe because they are aware of tests carried out etc) that that is the case. We can all of us surmise but without proof it is only an opinion or best guess.

QUOTE=Cardew;64015881]When you want DHW in the summer(but no central heating) you still have to have the same system with the heat store water at 50C?[/QUOTE]

Yes agree again, just as you would with a tank instead of the store, unless you had an alternative such as solar thermal to provide the heat source during summer on either design.

[Deleted User]

Since when are thermal stores/buffer tanks/header tanks used for domestic hot water? Never heard of a ASHP Combi Boiler :rotfl:

Cardew

Hi Mike,

The principle of a heat store for Heat Pumps is well established. Given the relatively low output of a heat pump(compared to gas/oil boilers) they act as a buffer when the heat pump cannot cope with the space heating demand.

Page 6, figure 2.4 shows the "Direct integrated thermal store (DITS)" so the store providing heat to both DHW and to heating - typically the heating feed is from the lower part of the store so takes lower temperature water.

I am completely baffled by that 'system'. Unless I have missed something, it appears to show the water supplied to radiators and to DHW in the same tank?? The water in the radiator heating 'circuit' quickly becomes black and oily, not to mention the(toxic?) rust inhibitor that is routinely added.

The thought of that water mixing with the DHW tap water is just mind boggling.

Although that first link shows a system that you suggest i.e. a heat exchanger in a heat store raising the temperature of inlet mains water from ambient to ???C for delivery to taps, I had never heard of such a system. I cannot begin to imagine how large such a heat exchanger would need to be to raise a decent flow by 45C or so. I even tried to work out how many square metres a plate exchanger would need, but gave up when my calculations indicated it would be bigger than the Albert Hall!

Anyway the system you envisage is in effect an ASHP combi i.e. mains water for DHW directly heated and not stored. For the reasons given in the earlier post I cannot see such a system existing.

Cardew

In addition to the above, I believe that your assumption/understanding that the prevention of Legionella is the 'Achilles heel' of ASHPs is incorrect.

In a normal heat pump system, even with a buffer heat store, it is only the separate DHW tank that needs raising occasionally to 60C.

My understanding is that a far greater problem on many systems are the 'defrost cycles'.