Is it going to be let much in the winter (e.g, over Christmas)? I'm wondering if peak heating load would happen at a different time from peak DHW (domestic hot water) load so the peak heating required would be less than the sum of these. If the peak DHW requirement is in the summer would some solar input help?

Posted By: Nick ParsonsSo your space-heating requirement is 4.2kW?

As I read it, that's to the radiators. There's also the UFH.

“DHL” (apart from the parcel company) is not one I've heard before: domestic heat load? Would that include the DHW?

''also trying to reign in the budget''

I think the Chancellor of the Exchequer tries to do that!

I seem to remember someone on here mentioning that the outdoor units of ASHPs have a very short service life on the Scottish islands do to the effects of the salt air.

Something to ask about locally.

How much is your system set to cost you to buy and install?

And Arran's particularly wet, so damp air is more likely to cause problems for the ASHP's efficiency. Also, as someone on this forum pointed out, air source heat pumps in principle perform worse for heating new, well insulated buildings, since such buildings only need to be heated when the outdoor temp is colder, which is when ASHP's COPs drop off.

That's less of an issue for hot water, but as ST points out, they just don't seem to be particularly good for hot water, due to a combination of the high temperatures they need to supply (bad for COP) and, I think, standing losses in the tank.

Anyway, ASHPs in the UK seem to average system performance factors of about 2.5 on average. That means associated emissions are only about 10-20% lower than LPG (and natural gas). I suspect, given your location and insulation, that you risk getting a bit less than that (over-sizing the system might help, but I'm not sure with air to water systems).

On the west of Scotland, you might want to consider the following options, depending what's important to you (I don't have enough info to give a strong recommendation, perhaps others can - it's just interesting to consider from my point of view too):

1. Oil boiler. Currently very cheap to run (probably as cheap, if not cheaper, than natural gas?), and installation costs -should- (although I don't know) be lower than an ASHP I imagine. But not very green. But if you're well insulated, you won't need much heating, offsetting this.

2. LPG. I think -a bit- more expensive to run than ASHP, but not that much. And pretty green (as good as natural gas) and similar install costs to oil boiler (IF you can get LPG delivered at a reasonable price to Arran).

3. Storage heaters for heating. Very cheap if you can get economy 7 (as cheap as natural gas, so quite a bit cheaper than an ASHP), although not very green (but you won't do all that much heating), and cheap to install.

Presumably you can get pretty cheap hot water with E7 too. If not, you could use LPG or propane cylinders -just- to provide on demand hot water. Get a couple of big cylinders that can easily be switched between. Will be pretty green, not too expensive to run, and the capital costs should be far lower than heat pump systems (no tanks etc.).

Hmm, I suspect there might be a few holes in my thoughts here. Someone will be able to correct!

You don't say what the floor coverings are..but these can play havoc with low temperature UFH..Rads win here as do embedded heat in walls....

Posted By: Chris P BaconI seem to remember someone on here mentioning that the outdoor units of ASHPs have a very short service life on the Scottish islands do to the effects of the salt air.

There are plenty of ashp installed on Scottish Islands and on Arran in Particular.I am planning on Blygold anti-corrosion coating.

Posted By: SteamyTea

quote

I don't think that there is any real problem for the space heating. That is not related to the 'amount' needed, that is just a case of sizing the ASHP correctly. Not as if you need the UFH to be higher than 35°C. You can look at your heat pump CoP/Temperature curve and local temp and RH data to see what is needed.

It is the hot water that is the real problem, here you do need a high temperature. Disregarding the legionella problem, you really need hot water at about 50°C. An ASHP can deliver at that temperature, but the longer it is running, the more likely it is to frost up. This is often worse around 0°C ambient temperature as the RH can be close to 100%.

That is one of my worries.I have already purchased 1 Nibe f2040 12 KW Pack IB.I realise that the main problem is the hot water delivery,If I add 16KW unit giving me 28 Kw in total plus 500l cylinder with immersion heater for legionella there should be less recycling.The the en-suites will have showers not baths, valves factory preset @38°C,this is what we have at present and no one in the house feels the need to alter this .

Posted By: GarethCair source heat pumps in principle perform worse for heating new, well insulated buildings, since such buildings only need to be heated when the outdoor temp is colder, which is when ASHP's COPs drop off.

Do you really think they perform better in old drafty buildings? What buildings don't require heating when the temp is colder,I'm afraid I don't understand your logic.

Heat pumps work best when the heating system can heat the building using "warm" rather then "hot" water. This is hard unless the building is well insulated.

Posted By: Ed Davies

Posted By: newnightDo you really think they perform better in old drafty buildings?

It would be reasonable to expect that a heat pump which is running for more of the year, (e.g., one for an old drafty building) would have a higher COP averaged over the year than one that only runs during a few coldest months. In that rather limited sense they would perform better.

Yes, this is kind of what I meant. Essentially, if your home needs to be heated when it's 15 degrees outside, the ASHP will at least do so at a stratospheric COP. All other things being equal, that's not a bad thing (e.g. if it does so far more efficiently than even a high efficiency natural gas boiler would)

I do take ST's point though. In theory, both a well insulated (big) home and a poorly insulated (small) home could need the same sized heat pump with the same efficiency performance.

But in practise, I've always assumed that well insulated homes tend only to switch the heating on at all at significantly lower outside temperatures because they are less draughty, so feel comfortable at lower temperatures, and also other sources of heat, not least people but also appliances, contribute meaningfully to the heat demand. Is that not the case? If it is, the heat pump for the well insulated house will perform with a lower COP.

On reflection though, I realise it might not make as much difference as I thought, even if that is correct. If the draughty house switches on the heating when it's 15 degrees outside, and the well insulated one switches it on at 10 degrees, then I don't think the difference in performance will be all that much... Not sure. If the well insulated one only switched heating on when it was five degrees outside, it -would- make a difference, but that's clearly not going to be the case.

GarethC
Yes, it is always a problem to work out what is the best system. Not a case of 'one size fits all'.
So a bit of arithmetic needs to be done.

If you take a house that needs 1000 kWh/year for heating and 1500 kWh/year for DHW (or what ever you think you need).
Assuming that the only thing that is different is the 'boiler', then it becomes a bit easier.
Just plug in the relative numbers and away you go.

The one big thing that is unknown is future energy costs, but history can help a bit, as can future fixed prices i.e. CfD on Nuclear, Wind and Solar.
Generally across the EU, and historically in the UK, domestic energy prices have been about 5% of household income.

Posted By: newnightCan anyone advise, if I go for 2 outdoor units, are they better as one connected system or should I run 2 separate systems 1 for heating and 1 for hot water

Do I need 2 ASHPs 12+ 12kw or 12 + 16kw

or could I manage with one 16 KW Pump if the Heat loss for the building is 12.5KW

If the maximum heat loss is 12.5 kW, then a 16 kW ASHP will do you.

Posted By: djh

Posted By: SteamyTeaIf the maximum heat loss is 12.5 kW, then a 16 kW ASHP will do you.

He wants DHW as well ...

Firstly it depends on how the maximum heat loss is worked out and how many days every 5 years it will be cold enough outside to get it. Then you need to consider how long you are willing to wait for the house to warm up when it is very cold outside and the heating has been off for some time.

The average kW over the complete day that is needed for hot water is not that high, so allow 3.5 kW may well be enough, but how much more will a bigger ASHP cost and would a larger ASHP be happy when the demand is less....

Posted By: djhBut more often than not there are reasons to not want to average it over the whole day.

Yes. But space heating can be averaged over longer periods, particularly if you have a UHF slab, so if there's a simple way to divert all/most of the heat pump output to DHW when recovery (reheating the tank) is wanted then that should be fine. Running the ASHP at DHW temperatures for only a short period will help with the long-term average CoP.

Posted By: ringiIt depends how the “head demand” was worked out in the first place.

We live in the North West, if the heat demand is worked out assuming it is -1 outside, and there is a long cold patch like a few years ago when it is -5 outside AND you are trying to heat a house from cold…..

System sizing should be done from the max heat load, not the average heat demand. i.e. instantaneous power required at -10°C rather than annual energy demand at say -1°C. But yes there wil be some times when the outside temperature drops to -20°C. In that case you rely on three things: (1) the averaging of space heating mentioned by Ed, (2) additional heaters such as I'm suggesting for warming from cold and (3) an acceptance that conditions are outside spec and you'll need to wear extra jumpers whilst it is so cold outside.

The ASHP may already be taking a lot of the incoming power to the building, so you can’t run many electric heaters. The ASHP would also be giving less than its rated output due to it being -5 outside.

Agreed that heat pump sizing needs to be done carefully. If you've sized your electric supply so that there's not much more than the ASHP takes then I think you've made a major design mistake. The house is presumably all electric since otherwise the answer to heating it would be a gas fire. So there's likely at least 10 kW of additional demand in the kitchen alone. Then there's whatever else you've allowed for.

So I agree that it is possible to cock up heat pump system designs and it gets done scarily often, but I don't think there's a problem if it's done right.