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

peat

My ASHP is out of warranty, anyone know if it is possible to get insurance cover

[Deleted User]

peat wrote: »

My ASHP is out of warranty, anyone know if it is possible to get insurance cover

Watching with interest...

I looked myself a while ago but no joy

Still under warranty in the meantime, got a spare 14kw EcoDan in storage anyway

Cheers

Mikeeuropa

Am I being daft here? Having read through the majority of this thread (phew!) I noticed that one of the killers for ASHP is the frequency of heating the hot water tank sufficiently so as to kill Legionella.

I'm thinking that if you had a heat store type tank (ie. where the hot water for the taps is fresh mains water flowing through a coil in the tank to extract the heat rather than the water in the tank coming out of your taps) then there'd be no need for the Legionella kill cycle surely? It's always fresh water coming out the taps and the water in the tank is a sealed system that doesn't come into human contact.

Wouldn't that save you having to raise the temp to 60? I'm sure there's something I'm missing as I don't assume for one minute this is a eureka moment, it's just I can't see what!

matelodave

Why not get the sterilisation bit into perspective - it's done once a week for about 30mins - so it's only got to raise the water temperature by 10-15 degrees for half an hour. Costing far less than heating a tank to 60 degrees all the time.


A tank at 60 degrees loses more heat than one at 45-50 degrees, so the overall system losses of a conventional system or immersion heater are far higher. Not only that but when you heat water to 60, you have to dilute it with cold to make usable, so heat loss through the system pipe work is greater as well.


It's easy to focus on specific aspects but in the end a heat pump is part of an overall heating system as is a conventional boiler and the whole system should be taken into account when making comparisons. I asked in a previous post, what allowances are being made with gas boilers to take into account the electrical power requirements of their pumps, controls & valves, combustion fans etc rather than just quoting the efficiency of converting gas to heat.
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lovesgshp

I agree with Matelodave, that the legionnaire problem can be overhyped. Here the system (GSHP), goes into disinfection mode once a month. Legionnela breeds in a 20-45C range, so if you keep the hot water above that then the risk starts to minimise.
Here, the tank temperature is set @ 48C, but the delta brings it up to circa 54C, as soon as it drops to 46C. 7 years on no problems!!
If you are using a system, that is only on a periodic basis, then drain a large amount of water out through taps, as the bacteria can build up in a low use environment.
If that concerned, then you could set the DHW temp @ circa 52, in which the delta would take it to about 56C on the heating cycle, reducing the possibility even further.

Mikeeuropa

matelodave, lovesgshp, I'm not disagreeing with anything you're saying, yes the Legionella problem probably is overhyped, yes bringing up to 60 once a week/month is cheaper than having it at 60 all the time. No arguments.

But neither of you answered my question I'm afraid. Why is there a need to do the sterilisation at all when it's fresh water running through the heat exchanger?

lovesgshp

Sorry, will try to specify a little better,
In the winter, the ground temperatures are quite low, so risk of the legionnela breeding is reduced. In summer however the temperature in the supply lines can rise and therefore increase the bacterial multiplicating. Therefore the need for higher temperature ranges to kill them off.
Just as a example, imagine you have a pool shower, that is not used for months, but is run from the heat pump. In the Spring months, the water in the pipes may get to + 20C, but is not used, so the bacteria breed quite happily. You use it for the first time without making sure that it is cleared with fresh water, risk can be high.
The need for sterilisation is a safety factor, built into the system, but you can switch it off. Most prefer it to be there, although my view is that if you use enough DHW and have level set about 50C it is not always required. My opinion only, but we have had no clients getting Legionnaires here in Italy from heat pumps in the last 15 years.

Mikeeuropa

I appreciate your reply lovesgshp and I don't want to sound like I'm arguing with you but...

I understand what you're saying on the pool shower example, but sterilising the "tank" will still not solve the issue of Legionella in the pipes. Periodic/infrequent usage is a problem of it's own dealt with by running the water out of the system I assume.

In a normal household situation with daily usage and a thermal store where the DHW runs through the heat exchanger I still can't understand the need to sterilise. How many litres does a heat exchanger coil hold? And how many litres of DWH a week/month will actually run through it? I don't have those figures but I'm pretty sure that the percentage of sterilised water coming out the taps will be almost negligible.

matelodave

It depend what type of system you've got - some have a buffer tank/thermal store (Ecodan do it like this I believe) and freshwater is pumped through a heat exchanger to heat it and you are correct you shouldn't need to sterilise the tank.
My system (Daikin Altherma) has a stored hot water tank in which the water in the tank is heated via a heat exchange coil like a conventional system, but it's only heated to 45 degrees. It's the stored water that which is used at the taps (again like a conventional hot water system).


It's this type of system that has a sterilisation function (I think it's a building regs requirement if you don't heat or store the water at a higher temp).

lovesgshp

Ok.
Will try to simplyfy it even more.
When you run the water through the heat store, then it heats it to the temperature in that store, i.e. 50C. so that is the feed temperature to your DHW system. The thermal store is at the temperature preset by whoever installed the system.
We use more specialised DHW tanks (twin layer, up to 300Ltr), as against thermal stores, so is more difficult for me to answer, especially as it depends on the volume of DHW you expect to use.