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

John_Pierpoint

Let's call it a store within a store rapid response ASHP system.
My real practical experience of heating technology was back in the first central heating pre natural gas boom of the 1960's. Like aircraft we tended to hear about the ones that crashed; like aircraft one does not want to be involved in such a system how ever unlikely the crash; because for an owner of such a system the crash it's a 100% result.

I all too often found myself reporting on yet another "Primatic" failure and a furred up boiler.
Somehow we still seem to be seeing examples of bad design in the heat pump market and the triumph of hope over experience.

http://www.gasboilerforums.com/primatic.html

[Primatics worked on paper, and very often it was the handy man plumber egged on by the know nothing home owner, who was the root of the problem: "just put three extra radiators in the new loft extension"................
But the same could be said of Heat Pumps and Photo Voltaic panels - ignorance is not bliss.]

Cardew

Mikeeuropa wrote: »

OK, at the risk of being accused of splitting hairs .

Really it doesn't matter what 'your' system is called, because as far as I am aware such a system with an ASHP doesn't exist; and given the reasons above - buffer store water temperature, summer use etc - never will exist.

matelodave

Everyone seems to be getting their knickers in a knot about what happens to the hot water after it's come out of the heatpump.


Effectively a heatpump is a boiler or means of making the water hot (or warm if you want to be pedantic) and all its got is a flow & return of hot water. It's what you do with it after its heated that everyone seems to be arguing about. Whether it goes into a thermal store, convention hot water tank or gets pumped around the central heating system isn't actually a function of the heat pump - it's a function of the system that's attached to the heatpump in a similar way to a gas or oil boiler. The difference is effectively the lower temperatures of the system water and how it's dealt with in the system - not the heat pump.


When I was investigating the different "systems" before we chose ours it became apparent that there were several ways of dealing with the hot water. Some seem to be everso complicated - thermal store, multiple pumps, heat exchangers, motorised valves and even immersion heaters etc. Some just had a hot water tank and a three port valve which either heats the tank or supplies the heating circuit. That's what I chose deciding that fewer components should mean fewer things to go wrong.

In the end you should be looking at the overall system design rather than focussing on a single aspect of one of the components and getting hung up about it


Now, how about getting a discussion going about defrosting;)

Cardew

matelodave wrote: »

Now, how about getting a discussion going about defrosting;)

A more interesting subject, but very difficult to find out lots of information.

There are lots of reports on MSE and elsewhere of frequent defrost cycles, and I have seen it written on 'plumbers' forums that 'everyone knows that defrost cycles are the 'killer' for ASHP'

What I find interesting is the latest(phase 2) EST 12 month trial of heat pumps. A quote from the Phase 1 trial

The study found that only 13% of all sites in the trial achieved a level of efficiency the EST considers to be a 'well-performing' system, with the rest scoring below this benchmark

(that included GSHP)

After the disappointing - to say the least - initial 12 month trial, a further 12 month trial was carried out on some of the original systems and included some new installations to the new MCS guidelines.

http://www.energysavingtrust.org.uk/Organisations/Working-with-Energy-Saving-Trust/The-Foundation/Our-pioneering-research

The manufacturers were heavily involved and carried out some major and minor alterations, even down to replacement of the heat pump itself.

Now the average system COP(SPF) for ASHPs was 2.45. The detailed DECC analysis does give some detailed breakdowns of individual systems. On one ASHP

There is very significant defrost at this site (544 kWh, or 7%
of the total heat delivered, taken as reverse heat flow, rather than direct electric consumption).

Defrost is observed during every month of the year, reaching 11% of total heat delivered in July, as shown in Figure 73. This illustrates the importance of ensuring that defrost functions are configured correctly.

It makes quite interesting reading - the consumption of a pump can be considerable.

With reference to earlier discussions in this thread, one chart shows the reduced system COP on sterilisation days.

The most important lesson IMO is that you need to ignore the manufacturer's data on the ASHP itself. e.g. a COP of x.x with the outside temperature at yC. There are a host of factors that drag the annual system COP down to that 2.45 and for some systems as low as 2.0. It should be stressed that is after manufacturer's 'interventions'.

A surprising statistic to me was that one ASHP with underfloor heating achieved a system COP of 3.6! the next highest was 3.0 and the majority at 2.0 to 2.2.

zeupater

matelodave wrote: »

Everyone seems to be getting their knickers in a knot about what happens to the hot water after it's come out of the heatpump .....

... Exactly, in a thermal store system as described it's a case that all heat sources share the transfer fluid and charge the TS, then all loads share the same fluid and deliver the heat to where it's needed (radiators, under-floor, DHW etc), moderated by mixer valves if & where necessary .... the whole kit forms a 'system', which is what should be the main point of the discussion ...

I could be mistaken, but I was of the impression that the original query was whether it was necessary to heat the TS on a regular basis in order to reduce the risk of legionella and other bacterial growth within the DHW flow, in which case post #1315 stands ....

Ourselves, we don't have HP wet heating, but with solar thermal there really isn't an issue for most of the year because the whole DHW cylinder regularly sits well above 60C, in the shoulder months that becomes less regular and in the winter only the top portion (feed) of the cylinder is heated to ~60C+ by the GCH .... however, the whole cylinder is copper, as are the house pipes, which is why the materials used were raised in a previous post .... however, if we were to use a low-temperature solution for heating the DHW I would certainly ensure that there was a regular sterilisation regime in place without going overboard on the frequency ...

HTH
Z

Cardew

zeupater wrote: »

...

I could be mistaken, but I was of the impression that the original query was whether it was necessary to heat the TS on a regular basis in order to reduce the risk of legionella and other bacterial growth within the DHW flow, in which case post #1315 stands ....



HTH
Z

My impression was that the query was based on the assumption that 'sterilization' was a big negative factor in the running costs of an ASHP.

The contention was to overcome that negative factor, that an 'ASHP combi';) (i.e. a heat exchanger in the thermal store directly heating mains water for distribution to DHW) could be used.

zeupater

Cardew wrote: »

My impression was that the query was based on the assumption that 'sterilization' was a big negative factor in the running costs of an ASHP.

The contention was to overcome that negative factor, that an 'ASHP combi';) (i.e. a heat exchanger in the thermal store directly heating mains water for distribution to DHW) could be used.

Hi

I agree ... same point different path ....

Effectively it resolves to ... 'if I save money will I get sick ?" ... the answer is therefore something like ... "If you save too much money & have a stainless steel heat exchanger & plastic pipe feeding your shower head, you'll be more likely to get sick than someone saving less and/or has a copper heat exchanger and DHW pipe feeding the shower head" ... in which case it's a personal decision, potentially a massive effect on health balanced by a minor impact on wealth (it must be remembered, the sterilisation cycle is different to maintaining ~60C at all times) ....

In terms of cost what would it be ... raising a 300l SS TS by an extra 20C would consume somewhere around 7kWh (at a COP of 1.0), so about £1/cycle, which would mean 50-70p more than if the heat was supplied by the heat-pump (~COP 2-3) ... the only question then is how often do you spend the extra ~50p, or so, after actually investing around £6k - £10k on the heat-pump ??

... to me it's a no brainer, have you seen the cost of prescriptions, or worse, funerals these days .....

HTH
Z

Mikeeuropa

Hi Cardew,

Cardew wrote: »

Really it doesn't matter what 'your' system is called, because as far as I am aware such a system with an ASHP doesn't exist; and given the reasons above - buffer store water temperature, summer use etc - never will exist.

Personally I think there's a merit in giving something a name that doesn't mislead people, but that's just my thoughts you have every right to think otherwise.

Have you seen Peats post? #1317

And at the risk of repeating what has been said before, summer use is bad with either system because you're heating a tank full of water to temperatures that aren't ideal for a heat pump, doesn't matter whether it's a "store" or a "tank".

lovesgshp

A normal installation here, is a GSHP/ASHP .... DHW tank, with electric immersion if needed. Buffer tank (normally for radiator systems, or dual floor controlled systems, due to the higher temps/stability required). DHW is always given priority over the heating system and has its own set temperature, with a heat delta of normally 5C, i.e. temp set @ 50C, heat switches on at 48C and off at 53C. Most heat pumps can easily operate in those ranges, without electrical immersion heater input.
The sanitation cycle, will sometimes use the backup, but not always. I only know the models we use, but even here on 7 years use, the electrical heater has only operated for 30 hrs over this period and it is set at 3Kw. If the pump cannot reach the required temperature for the heating within 60mins of starting, then it switches on. Do not think that 4 hours a year is that bad.
Running time for DHW in the summer is only about 3 hours a week, (double shelled hot water cylinder of 160 ltrs), off times from 20.30 - 06.00 daily. Temp set @ 48C with 30 day disinfection cycle. We can still run a large bath and water is back to normal temperature within about 15 mins
Most here do not use thermal stores, unless they are on woodburners, pellet stoves etc

Mikeeuropa

zeupater wrote: »

I could be mistaken, but I was of the impression that the original query was whether it was necessary to heat the TS on a regular basis in order to reduce the risk of legionella and other bacterial growth within the DHW flow, in which case post #1315 stands ....

Cardew wrote: »

My impression was that the query was based on the assumption that 'sterilization' was a big negative factor in the running costs of an ASHP.

You're both right to a large extent. As a non-professional in this area I read this thread to build on my limited knowledge of ASHP systems. The impression that started to build was that there were a number if people complaining of the cost of the regular sterilisation cycles. I pondered this issue and thought that if a thermal store was used rather than a hot water tank then there would be no direct human contact with the water that was potentially carrying Legionella and hence there would be no need to run the sterilisation cycles, hence these sterilisation costs could be avoided. Now, being a non-expert in this area I felt sure I couldn't have been the first person to think of this solution. So I assumed somebody had already thought of it and it had been dismissed for some reason.

Hence I put out the question "what is the reason this idea has been dismissed?" Unfortunately the discussion seemed to take on a life of it's own far bigger than I had intended by asking the original question

And just to say that #1315 doesn't really stand with the Gledhill TS mentioned as there isn't a slug of mains water sitting inside the TS growing bugs. What does dismiss it, in my eyes, is the fact you have to keep the TS contents at 75C in order to get sufficient heat transfer into the mains feed. Why use a HP to keep the TS at 75C if you can use it to keep a "tank" at 50C and just occasionally sterilise it?