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

[Deleted User]

Cardew wrote: »

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

Straight forward question with no strings attached...

I hear defrost cycles mentioned now and again about being problematic?

Looking at my own situation it would appear to be a normal but occasional function of an ASHP and in the context of overall runnings costs is similar to mains gas.

Put me out my misery

Cheers

Mikeeuropa

Hi Cardew

Cardew wrote: »

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.

Yes exactly, I agree, which is why I don't understand these references to ASHP Combi boilers - what I've been asking about is a system with a big tank full of water, you don't have tanks full of water in Combi systems. I don't get why people are drawing a parallel to a Combi, at no point am I suggesting the ASHP directly heats the water "on demand", maybe I'm just being thick

Cardew wrote: »

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.

Yes, respectfully, I think you've missed something Figure 2.4 does show the "water" (full of inhibitors etc) in the tank flows around the heating system but it doesn't mix with the DHW. DHW is provided by taking fresh mains water in at point 5 on the diagram, through a heat exchanger coil, and out to the taps at point 4. So the DHW never mixes with the contaminated water.

Cardew wrote: »

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.

As said above, I don't understand why it's being referred to as a combi, the heat for the DHW comes out of a tank full of water not directly from the ASHP on demand.

As for systems existing, I just did a quick Google on "thermal store" and top of the list (ignoring the ads) was this link http://www.gledhill.net/page/180/Thermal-Storage.htm. I think Gledhill are a fairly well known company but of course I should point out "other suppliers are available" and I have no connection to Gledhill, this is not a sales pitch for their products. There's an animation on that page that explains how it all works and, although it concentrates mainly on other heat sources such as wood burning stoves and solar thermal it does briefly mention ASHP towards the end. Further down the page it also states...

The domestic hot water is generated via a highly efficient plate heat exchanger. As hot water is demanded, it is mains cold water which is instantaneously heated as it passes through the heat exchanger and delivered to the taps. This method of generating the hot water completely removes the risk of legionella. It also has the added advantage of the stored water not requiring pasteurisation provided by carbon-intensive electricity for legionella protection, as the homeowner never comes in contact with it.

I'll see if I can ask them the question of what temperature the water needs to be in the tank for this "highly efficient plate heat exchanger" to produce sufficiently hot water at the taps.

I'll report back.

lovesgshp

Minimum temp in their thermal store, needs to be 75C according to their brochure (see the notes). Flow rate is 15 ltrs/min for DHW.

http://www.gledhill.net/page/184/Torrent-GreenHeat-HP.htm

zeupater

Hi All

The issue is pretty simple ...

The thermal store is integrated with all heat sources which together form a complete sealed system and therefore share the fluid (water/fernox etc).

The DHW enters the TS cold, flows through a finned heat exchanger, and exits at a raised temperature.

The questions which need to be asked are ...
(i) - Can the bacteria multiply in the HE
(ii) - What conditions can prevent this
(iii)- What do I prefer to do

So (i) ... Yes, there is a likelihood that the bacteria will multiply as long as the correct conditions exist. The discussion so far hasn't addressed the issue of where multiplication will take place, which would be most likely on the inside walls of the heat exchanger pipe, so HW demand introduces a flow which could dislodge some of the build-up .... this is completely different to expecting that the bacteria would only be in suspension in the water and therefore the rate of change of the DHW would prevent infection.

.. (ii) - heating the contents of the thermal store to recommended levels heats the heat exchanger walls and kills any bacteria which are attached, as well as any in suspension in the HE pipe through sterilization , therefore reducing the probability of onward infection.

.. (iii) - You personally decide to either take the risk, or not ....

Of course, there's also the question of what materials the TS heat exchanger or DHW cylinder are as this will have an effect on the likelihood of infection in the first place .... but that's another story !!

HTH
Z

Mikeeuropa

lovesgshp wrote: »

Minimum temp in their thermal store, needs to be 75C according to their brochure (see the notes). Flow rate is 15 ltrs/min for DHW.

http://www.gledhill.net/page/184/Torrent-GreenHeat-HP.htm

Brilliant lovesgshp, that answers my question then. What's the point in producing a thermal store full of water at 75C when you can just produce a "hot water tank" at 50C with an occasional boost to 60C? There is none hence everyone uses a hot water tank and not a thermal store.

We got to the answer between us all in the end Thanks everyone who has contributed.

peat

Mikeuropa,

Thats how my system works - thermal store directly connected to rads and ASHP. DHW from mains through a heat exchanger. Result - mains pressure hot water (potable).

Peat

Mikeeuropa

peat wrote: »

Mikeuropa,

Thats how my system works - thermal store directly connected to rads and ASHP. DHW from mains through a heat exchanger. Result - mains pressure hot water (potable).

Peat

Well that's interesting Peat. What temperature is your thermal store at then? And can you run a bath from it? And if you don't mind me asking what make/model is it? Do you have a link to info on it?

Cardew

Mikeeuropa wrote: »

Hi Cardew



Yes exactly, I agree, which is why I don't understand these references to ASHP Combi boilers -

As said above, I don't understand why it's being referred to as a combi, the heat for the DHW comes out of a tank full of water not directly from the ASHP on demand.

The reference to a 'Combi' is as I stated below:

Originally Posted by Cardew


Anyway the system you envisage is in effect an
ASHP combi i.e. mains water for DHW directly heated and not stored.

A Combi directly heats DHW, other systems store DHW in a tank. Thus 'your' system is analogous to a Combi.

Anyway we agree that even if an ASHP system uses a thermal buffer store, many/most would not have that water - which is destined for radiators - at 50C or more - simply to provide DHW.

Also to provide DHW in summer(when heating is not required) it would be necessary to heat that buffer store to 50C or more.

[Deleted User]

peat wrote: »

Mikeuropa,

Thats how my system works - thermal store directly connected to rads and ASHP. DHW from mains through a heat exchanger. Result - mains pressure hot water (potable).

Peat

Are you sure?

ASHP heats thermal store @ ~75c - does not sound right to me...

Perhaps confusing thermal store with a convention DHW tank that has a coil in heated by the ASHP with an immersion booster for sterilisation routines. The ASHP typically will heat water twice a day taking 30 odd mins each time.

Obviously hot water would be an mains pressure be it showers or baths.

Mikeeuropa

Cardew wrote: »

The reference to a 'Combi' is as I stated below:

A Combi directly heats DHW, other systems store DHW in a tank. Thus 'your' system is analogous to a Combi.

OK, at the risk of being accused of splitting hairs but for the benefit of others like myself who have only a developing understand of all things Heat Pump related, I think you would be better calling it a Thermal store Combi then as it's the Thermal store that directly heats the DHW. With a Gas Combi the Gas boiler kicks in immediately and directly heats the water when a heating or hot water demand is placed, with an Oil Combi the Oil boiler kicks in immediately and directly heats the water when a demand is placed.

So, to me (excuse me please if it is only me but I am self confessed far from an expert in these matters), the phrase "ASHP Combi" implies the ASHP kicks in immediately a demand is placed and directly heats the water - which certainly isn't the case in what we have been describing, so I think the term leads to confusion. It certainly led to my confusion. That's perhaps why people were laughing at the idea being ridiculous because with a fairly common rated output of 8.5kw or 14kw they're nowhere near powerful enough to provide direct hot water - even I know that

Final thoughts, and then I will shut up , with the thermal store system I was describing the ASHP could actually be swapped out and replaced by any other heat source, for example a Gas/Oil/Wood boiler, would you then describe it as a Gas/Oil/Wood Combi system?? I doubt it very much which is why it's misleading to refer to this as an ASHP Combi.