Advice on energy efficient system please

[chrisreboot]

OK, thanks for the replies chaps!

Update as of today:
I’ve had a local plumbing/renewables firm round for a chat and gone through ASHP to boilers to cylinders and here’s the rough conclusion, subject to a few variables.

1. South facing roof will not support adding thermal panels due to lack of space after PV installation. (There is however a west facing gable end roof that we didn’t discuss, but in light of the previous posts above, perhaps I should have asked about that?)
2. Solar Thermal may not therefore be cost effective as a result of west facing roof, but a new double/triple(?) coil cylinder *could* be added to the existing combi in case we decide to add thermal later on. Otherwise use use gas as a top-up as required at the end of the day to boost the PV injected heat from the immersion.
3. Add a Kingspan cylinder, probably 300L, with single immersion and coil for feed from combi (and keep existing combi for now, but replace with either a condensing combi or system boiler in due course).
4. Keep combi feed to kitchen hot tap, and run all other taps from cylinder (i.e. for daytime washing up etc we use gas, and bathing at night runs off the cylinder).
5. ASHP ruled out on the basis of cost/performance for now. The viability and disruption of adding UFH to get the most out of such a system isn’t going to happen in this house – better for a new build or renovation project.
6. For now, the potential siting of a new cylinder is likely to be in the garage, which is planned for conversion into a coat/utility room, so the floor will be raised etc before any plumbing work can be done. I don’t see the point in putting in pipes and a cylinder now, only to have to lift all that up a few months down the line.
7. It may also make sense to use this time to gather some usage stats from the PV to see exactly what the spare energy is, to see if the cost of implementing a cylinder is actually worth it (assuming we only do a PV fed immersion with it, and not solar thermal).
8. I do very much think there is mileage in this concept, but with unknown spare energy it’s difficult to know what the payback would be. Rough example though based on yesterday’s stats: we produced approx. 7.5Kwh and probably sent somewhere in the region of 4-5 of that back to the grid unnecessarily. A 300L cylinder would have the temperature raised by approx. 12-15C. Now that is based on a sunny day in January, so in summer we could potentially be very close to getting a whole cylinder to where we would need it to be by the end of the day. If not, we top up with a much smaller amount of gas.
9. Likely cost is going to be somewhere in the region of £2300 inc VAT parts and labour. That includes the PV solar immersion hardware and fitting.

 

[chrisreboot]

Baxi DuoTec Advance system combi with solar PV immersion heater in thermal store. Winning.

If I understand correctly, you are meaning a combi (that isn't condensing?) that is similar to the Alpha product I originally mentioned (with GasSaver) for pre-heat - that I get.

Then to couple that with a thermal store (not cylinder?). I get how a thermal store would work - but why wouldn't I go for an unvented cylinder?

From what I think I understand though, in summer the system would be primarily pumping heat into the store via the PV powered immersion. In winter, the central heating would be acting as a preheat to incoming mains to the temporal store, before then passing that energy into the thermal store?

I'm missing some of the finer points here - sorry, noob questions.

 

[TBServices]

You're almost right. You won't need a thermal store (the temporal store is a mini thermal store). How many bathrooms and hot outlets do you have? 300l is a big old cylinder.

The baxi system is the same as the alpha system, but the DuoTec is undoubtedly a better boiler. Check the link I posted above.

This system sounds like it could be better for you to be honest. Reduced cost of install, lower running costs than stored water, works with solar pv and a lot more compact.

 

[Ermintrude]

Plus one. You could also configure combisave aswell not sure it's entirely cost effective with long runs but I would be fitting it before DCW inlet to store. As it will pre heat in the winter much more effectively. Combisave goes on flue .
The thermal store needs to be sized off the. Collectors.

 

[Worcester]

OK, So for £2,300 what's you're payback period?

How much gas will you save?

£2,300 sounds like 50,000 kWhTh to me, say you're current Hot water consumption of gas is 5000 kWhTh per annum and you mange to half that, it would give payback in the order of 20 years....

However deduct off the cost of the fact that you're boiler needs replacing anyway, and you'll probably get a payback nearer 10 years..

 

[chrisreboot]

Thanks again for the replies, very useful!

Our total gas consumption per annum is approx. 25000 kwh, but I don't know the split of CH vs DHW.

We have one bathroom, but we usually run 2-3 baths a night (some of the kids are luckily at the age where they will share!). I take a shower on a morning, but everyone else bathes at night. The only other HW demand is from the kitchen sink. Washing machine is cold feed as is dishwasher.

Worcester is probably right about the payback period of the cylinder on it's own, but I wanted to look at the system as a whole, factoring in the dependencies. So for example, if I replace the boiler later, can I introduce the cylinder now, or do I have to replace the boiler first/at the same time as introducing the cylinder.

Anyway, that last part seems like I can do the boiler later and split the project into two costs which is easier to swallow.
As I said at the start, we are planning to live here for a long time, so even a 20 year payback is acceptable.
That said, the GasSaver technology coupled with a more efficient boiler and the PV savings ought to reduce that to perhaps nearer the 10 year mark.

The GasSaver might save me up to 25% and a new boiler up to 25-30% (as mine is 70% efficient) - if you believe the hype. If I can then add another 25-50% reduction by pre-heating with an immersion, I *should* be quids in (eventually!).
The boiler *does* need replacing due to age and efficiency in the not too distant future anyway which is what prompted the whole thing.

I'll definitely look at the Baxi system and the temporal store idea next.