Heating: thermal stores, ASHPs, solar thermal, oh my!

Smiley_Dan

I moved into a large detached house a few months ago. This house is off the gas grid, and currently heated with an oil boiler.

Solar PV is due for installation next month; I'm hoping this will cover a large portion of our hot water requirements with an Optimmersion using spare 'leccy for the immersion heater in one of our (!) hot water tanks.

Thoughts now turn to heating. I'm looking for a solution that makes financial sense long term. I am no eco-warrior... I'm doing this to de-risk our family's financial future. As such, moving away from oil seems like a good idea, plus if RHIs can be gained for any new installation, so much the better.

GSHPs are out of the question: I don't have enough garden space, plus the conversion of radiators etc would be a mighty pain.

The obvious replacement for the oil boiler in our case is a biomass one. However, I'm concerned about the viability of biomass long term. First, this simply makes us dependent on wood pellet production rather than oil production. In theory this should be a good compromise; the costs of wood pellet production is far lower than oil, with far fewer regulations encouraging a more open market and therefore hopefully lower prices.

However I'm concerned how far wood heating scales in terms of supporting more and more people... I know many new housing estates using biomass boilers. At what point do we (or wherever the wood comes from) run out of woodland? At that point, the price goes up again.

I'd like to investigate the use of a thermal store as a way of combining PV, solar thermal, an ASHP, maybe a wind turbine, and having the oil boiler as a backup. But how feasible is it? I have no handle on the numbers required... how much energy would have to be generated to heat a 5-bed house.

The other advantage of this setup is the redundancy... if one thing breaks we don't go cold!

If anyone has any resources where I can begin to work this lot out that'd be great. I'd rather understand the numbers before beginning to engage installers etc.

Cardew

If one of the reasons you are ruling out a GSHP is because conversion of radiators, the same would apply to an ASHP.

One of our posters is a great advocate of fan assisting radiators to make up for their under-sizing in existing houses. Not that I think ASHPs are a great solution given the cost of installation and their performance is not great IMO. Albeit RHI will soften the financial blow!

Smiley_Dan

Ok, scratch all the heat pumps PV + solar thermal + turbine + oil boiler as backup all heating a thermal store... could that work?

Cardew

Smiley_Dan wrote: »

Ok, scratch all the heat pumps PV + solar thermal + turbine + oil boiler as backup all heating a thermal store... could that work?

Solar thermal gives good output in the summer months for Domestic Hot Water(DHW) but far less output in the winter when you need copious amounts of hot water in your thermal store for radiators and hot water.

To a lesser extent that applies to Solar PV, the output in winter is much lower; as you can see from the data posted on this MSE Board. Even with a large system producing 4,000kWh pa, I suspect you will be fortunate to get more than 1,000kWh diverted(via a device) during the 'heating season'.

Not widely advertised by the solar industry(for obvious reasons) the Government commissioned a report on Solar Thermal(by Cranfield). A lot of systems(both types) were tested in the South of UK(Richmond) and the average annual output of those systems was approx. 1,000kWh - again mainly in summer. Obviously you could double or treble the size of the display, the winter output would still hardly make a dent in the heating requirement for 'a large detached house'.

OK - we have dismissed Solar Thermal, Solar PV, and ASHPs for heating purposes;) Now wind turbines!***

If you have £20,000 or so, live in a suitable area and you(and neighbours) are not bothered by the noise - that could work!

*** I am NOT disputing the financial viability of Solar Thermal/PV with RHI and FIT, merely giving my opinion that they are not a solution for winter heating.

Smiley_Dan

In terms of kWh then, what *are* the heating requirements for different sizes of houses then, as a rough guide? I can't find any numbers anywhere.

EricMears

Smiley_Dan wrote: »

In terms of kWh then, what *are* the heating requirements for different sizes of houses then, as a rough guide? I can't find any numbers anywhere.

That's rather in the category of "How long is a piece of string ?"

However, providing you've kept them all, just look at your last year's worth of energy bills for your previous house and convert them all to kWh (electricity already will be, I believe gas bills are converted these days too but oil & coal will need to have a factor applied and I'm afraid I'm not sure off the top of my head what factors to use).

If you have a neighbour with a similar house, perhaps they'd show you their bills ?

That's far more useful than anyone telling you what 'a typical house' might use.

Martyn1981

Hiya Dan, interesting situation. I also think that bio-mass may be the obvious choice, but as you say, long-term costs could be an issue, hard to say.

Have you looked thoroughly into the ASHP issue? I know very little, but I have picked up on the fact that the technology is improving, and could well be suitable for locations without gas (as seen in much colder parts of the world) but will depend on your heating needs and insulation levels.

Only a couple of days ago I was reading (on another forum) about the claims being made by a company about their ASHP designed for the UK market (climate). If you can wait a few years, you’ll be able to see how these and others do in testing and application.

Crucially, they aim to match ‘our’ temps, hotter DHW, and the hotter heating water to make use of existing radiators for retro-fit. So have a thunk about your current rads, are they inadequate, adequate or slightly over-sized? Could they be made adequate/over-sized by improving insulation, air tightness, consumption patterns/needs etc?

Also how does the cost of leccy compare to oil (or biomass) and the cost of outlay for the system. ASHP is not cheap (yet?) so as you said earlier, need to think about RHI contributions.

Depending on the size of the PV system, and orientation, and pitch, it may well contribute a lot to the heating needs (during daylight) in the shoulder months (Mch/Apr & Sept/Oct), but probably won’t contribute much during the bottom 4 months when excess generation is lower, and for less hours, more heating is needed and the ASHP CoP will be lower. My PV systems being off-south and shallow pitch heavily favour the BST months over the GMT ones.

Heating your DHW with excess generation should work well in the BST months, and you could even go further using an ASHP water cylinder, that requires far less leccy and are growing in popularity in the US …… but not such a good idea for the winter if you did install ASHP house heating, having one heat the house, and the other cooling it. Bring on Harry Hill “Fight!”

You’ll find more about ASHP’s elsewhere on MSE, but this thread may be of interest looking at the combination of PV and ASHP (in the form of small A/C units) during the shoulder months.

https://forums.moneysavingexpert.com/discussion/4715287

Mart.

Smiley_Dan

EricMears wrote: »

That's rather in the category of "How long is a piece of string ?"

However, providing you've kept them all, just look at your last year's worth of energy bills for your previous house and convert them all to kWh (electricity already will be, I believe gas bills are converted these days too but oil & coal will need to have a factor applied and I'm afraid I'm not sure off the top of my head what factors to use).

If you have a neighbour with a similar house, perhaps they'd show you their bills ?

That's far more useful than anyone telling you what 'a typical house' might use.

I kind of expected a "how long is a piece of string" answer but given cubic capacity, existing insulation, a target temperature and geographic location can't a ballpark figure for heating requirements be calculated? My concern is going into this project blind. With PV it's easier because although there's uncertainty as to how much you'll generate, at least you have other people with their experiences.

We've just moved into this house, it's more than twice the size of our old terraced house so *heating* I think will be used a LOT more. Looking at our old bills won't help.

Given we use an oil boiler and take deliveries of oil, I can wait until the tank is empty I suppose and do a litres/days used to work out the kWh based on the efficiency of the oil boiler (89% atm according to last service). Let me know if this is wrong...

Smiley_Dan

Martyn1981 wrote: »

Have you looked thoroughly into the ASHP issue? I know very little, but I have picked up on the fact that the technology is improving, and could well be suitable for locations without gas (as seen in much colder parts of the world) but will depend on your heating needs and insulation levels.

Only a couple of days ago I was reading (on another forum) about the claims being made by a company about their ASHP designed for the UK market (climate). If you can wait a few years, you’ll be able to see how these and others do in testing and application.

Sounds interesting.

One of the interesting points here is the role of the thermal store. By "virtualising" radiator hot water into the store, and running the heating system from there, I understand that multiple sources of heat can be used in heating up the store.

This is attractive in my case because I could start with the oil boiler. The rads at that point are decoupled from the source of heat and other sources can be added, slowly reducing our exposure to oil. PV, I understand, can even be used for the heating system (not just hot water) in this configuration.

That's my crazy plan, but I'm totally naive as to the technicalities, thus my request for NUMBERS!

Martyn1981

Smiley_Dan wrote: »

Sounds interesting.

One of the interesting points here is the role of the thermal store. By "virtualising" radiator hot water into the store, and running the heating system from there, I understand that multiple sources of heat can be used in heating up the store.

This is attractive in my case because I could start with the oil boiler. The rads at that point are decoupled from the source of heat and other sources can be added, slowly reducing our exposure to oil. PV, I understand, can even be used for the heating system (not just hot water) in this configuration.

That's my crazy plan, but I'm totally naive as to the technicalities, thus my request for NUMBERS!

Hiya Dan, can't help with any specifics, but can suggest a few pointers.

Firstly, do you have an EPC, as you recently bought the house? Does that suggest the amount of kWh's of heat needed, and also crucially the potential of the property? You can also go to the EST website and do a DIY EPC. I tried it once and it was very accurate. Actually I did it twice as I didn't believe the first, but after PV extension, the 'real' EPC was very similar.

Know little about thermal stores, but Zeupater (on here) does, and might be along if you're lucky, but I note in this discussion (link attached below) about mixing inputs, a thermal store is also mentioned and this paragraph referencing radiators (as opposed to UFH):

http://www.navitron.org.uk/forum/index.php/topic,21137.0.html

"Ideally you need a buffer tank to separate all the flows, you may get away with a low loss header. This way you can keep the existing rads and depending upon outside temp run then at 35-40-45-50 as required if 50 isn't coping then swap over to the LPG. (With a buffer tank you'd run the rads at Δt10 and the heat pump at Δt5)"

Here's a reference to an ASHP that might tick all your boxes, but it's 'advertising' hence my suggestion to watch and learn for a bit. Not a recommendation, just a pointer:

http://www.glendimplexni.co.uk/news/53-launch-of-the-a-class-air-source-heat-pump

Sorry to be boring, but before you do anything, don't forget the first three steps - insulate, insulate and insulate.

Mart.

thenudeone

Several "high temperature" ASHPs are available (eg Daiken Altherma HT) which should work without having to change radiators, although it still won't be as hot as water from an oil boiler, so you may have to leave the heating on for longer periods.

If you install before March 2014, HT pumps are still eligible for RHI at a legacy SPF of 2.5. After then, they won't be eligible.

You can always leave the oil boiler in place and have a bivalent system in which the boiler is used only occasionally when the ASHP can't cope in very cold weather, but if you do that, you will have to install a heat meter, and the RHI payments will be based on actual green heat generated (with a cap of the theoretical requirement), rather than automatically at the theoretical heat requirement.

I'm planning exactly that.