Solar Thermal, Heat Pump, or what....?

[JamesBlonde]

Theoretically this could be in the plumbing forum, but it'll become clear why it isn't in a second.

We had solar PV installed in the winter on a SE facing roof. At the same time, we tried to ensure that there was sufficient space on the roof to install solar thermal (evacuated tubes). This could plumb in to our existing 210L (70L dedicated solar) Gledhill cylinder. That was the plan anyway....

We got a quote from the people who installed our PV. they were also offering a quote for Air Source Heat Pump, so i thought it was worth asking about that as well.

The quotes came back - about £4K for the solar tubes (tubes and plumbing into cylinder only), and about £9K for the ASHP, not including radiator upgrades. The solar thermal was a LOT more expensive than expected, particularly as I'd previously had a quote from another supplier last summer for £4500 including fitting a new cylinder! So I kind of expected the quote to be around £2000 - £2500 (my budget).

So firstly, is £4K too expensive for tubes and fitting only?

Secondly, Whilst the ASHP was expensive and beyond current budget, the projected payback was in around 6 years. This was surprising, and looked like a far better option ignoring the price. It could provide heating as well as water, and potentially replace our 10-15 year old Potterton boiler. The problem is that I've not found anyone with any experience of heat pumps to radiators. So are they overselling this?

Finally, given that both quotes are outside of my original budget anyway, what would you spend £2500 on to make your house more efficient? I have 10m x 2m on each side of the house uninsulated eaves, although the attic is super insulated, so I will be buying 50 or 75mm PIR to go over the rafters as at least a budget attempt at insulating all of the eaves.

Any other suggestions? New telly? Gas rather than leccy cooker? save it?

 

[PrenticeBoyofDerry]

I think you answered your own question.
Improve the thermal insulation but remember it is an advantage to allow air to circulate in the loft space, so do leave spaces around the eaves to allow this.
I assume you are up to your 16A max on the PV array, so cannot add on to that?
As I understood it there where concerns regarding ASHP and the noise levels they offer, they may have improved though.

 

[JamesBlonde]

PrenticeBoyofDerry, the ventilation was why I was thinking of putting the PIR over the rafters rather than in between, but I'll leave an air gap at the bottom. Does that make sense?

PV is 15 x 265w panels - I'm thick, so dunno how that computes? It would balance the roof out to have another panel, so could be a consideration.

Any thoughts on the solar thermal?

 

[PrenticeBoyofDerry]

If your insulating you roof rafters, leave a 50mm ventilation gap between roof and PIR.
Leave ventilation gap around eaves allows air to circulate.
Solar thermals are okay, if the weather is like this, could save up to 90% on your bills. But you will need to do some calcs on the roof load.
Simple calcs on PV suggests you are up to your allowed max.

 

[JamesBlonde]

Reason I was going for a thin (50mm) layer of PIR was partly cost but partly so that I could just shove it straight over the top of the eaves without having to worry about cutting it up or leaving a gap between the PIR and the felt (there are ventilation channels up into the attic). Rafters are probably 100mm so ventilation gap will be plenty, and the option is there to fill the rafters if I want to. 50mm PIR wouldn't encroach on the eaves too much either. By ventilation gap around the eaves, what do you mean? Is my gap at the bottom going to be sufficient or are you suggesting more?

Will have a think about roof load - not something anyone has mentioned to date!

 

[JamesBlonde]

How cold is really cold? Are we talking waaaaay below freezing, or <5c or something?

There is a temptation to look at a new boiler, but ours appears to be fine so there is a temptation just to plow on with it.

 

[SimonH2]

The quotes came back - about £4K for the solar tubes (tubes and plumbing into cylinder only), ...

Sounds a bit high to me - but then it does rather depend on how much work is involved.

You can get an idea of what the hardware should cost at Navirton's online shop, then you have to work out how much work is involved in installation.

As for heat pumps, consider carefully the performance - look at the spec sheets for yourself and don't accept what a salesman tells you without checking. Performance may sound great, until it gets cold when performance drops and many units revert to just using an inline immersion heater (ie you're running electric heating which is quite pricey).

But you have to think carefully, how much heat do you use, what savings could you make, and will the investment actually pay for itself (ever) ? With solar thermal you get most heat when you don't need it, and generally not too much when you do need it (in winter). Similarly, as mentioned above, when you really need the heat, the performance of ASHPs generally drops off alarmingly.

You may be able to get round (or at least reduce) the ASHP performance issue by running it during the day to charge the store, and run the heating off the store overnight, but that would mean more complicated controls. However, less than 100l of water (what's available in that cylinder for the heating) won't actually store all that much heat.