Advice on energy efficient system please

[chrisreboot]

Hi all, it's my first post, so please be gentle!I currently have a standard gas combi boiler to supply my central heating and hot water.It's about 15-20 years old and I'm told about 70% efficient. So I am now in the process of looking at what the best replacement solution is.Add to the mix the fact that we have recently installed a solar PV system into the house which at times will over-produce energy.My current plan of thinking is to harvest that spare energy by putting it to use in heating water for us. This would involve having some kind of storage cylinder and immersion heater.This sounds a little like old school heating systems (which the house previously had before the airing cupboard had the water cylinder ripped out at the time the combi boiler was installed).However, I am thinking of the following system (and here's where I need critique from experts):1) Replace combi boiler with new more efficient condensing system boiler. (Not sure what I would need exactly, but we have a 4 bedroom house with one bathroom, and 6 kids to wash so our bathing hot water requirement is high!)2) Add a thermal store cylinder that would allow me to heat the water using a mixture of the gas boiler, and the immersion.3) To ensure best energy efficiency, I've seen on the market solar immersion switches that power the immersion when it detects that the Solar PV output is greater than the household consumption (i.e. diverts the excess power from going back to the grid), so the spare electricity I produce would go to heating the thermal store.I also very much like the idea of energy recovery, so something like the Gas Saver Product from Alpha Innovation. It looks good, but I guess that would have to be coupled with one of their own boilers.If there are any other similar pieces of kit out there that fit the bill I'd be happy to hear about them. Preferably an integrated system.Note: at present the solar PV installation is very new and I don't have the full data on over-prodcution to see if this plan is viable, so if anyone has any supporting evidence either way to this, again I'd be glad to hear about it. It could be that our spare output would not justify the installation. However, the fact that these products exist tells me there is mileage in this.I have no experience in this whatsoever, all I know is that I have spare power going to the grid that could lower my gas bill potentially.Am I shooting in the right direction with this kind of setup?Cheers,Chris

 

[Killy Bing]

Regarding a thermal store my knowledge is limited but it is just that, to store heat from multiple sources.
you would need an additional cylinder for your bathing water putting costs up further unless you have one which acts as a combi to give instant hot water but these are prone to scaling.

however it is ideal if you intend to add solid fuel, a heat pump or water based solar in the future.

the immersion idea is a good one but how big is the pv and what is the output?

as For the alpha gas saver it is a good idea but the parts for alpha boilers are considerably more.
the additional repair costs in the future will far outweigh money saved.

personally, unless you're going to stay in the house for life.

i would go for an intergas combi boiler with an unvented cylinder (my preference is oso ho****er super series or even better ACV cylinder)

Have the combi feed the kitchen tap for hot water
if you wish to add solar in the future get a cylinder with a second coil.

and insulate insulate insulate!!
have the floors up and insulate all hot pipework
upgrade loft insulation
look at cavity wall insulation
where cavity wall is not possible consider "dot and dab" insulated plaster board for those walls.

 

[efficiencyman]

Do you have more roof space for solar thermal ? If you do then a multiple coil cylinder could be heated for free or partly heated during the cooler months, ( I did that 23 years ago when my teenagers were at home, I reckon it`s paid for itself 4 times over )
In 2012 I fitted a 15 Kw heat pump to heat the house, under floor, Koi pond and pre heat the DHW very pleased with the performance. Needs a careful set up & control to obtain a good COP factor !
I would fit a condensing SYSTEM Boiler for storage cylinder top up and for heating ( as mine is, when in HYBRID mode if you fit a Heat Pump)
These are my points, Killy Bing has answered well, especially insulate everything.

 

[chris watkins]

What is the rated output from the PV set in watts, in full sun ?
I thought the Gas Saver was only for use with combi's & you were talking about having a system boiler ?
Welcome to the forum Chris.

 

[chrisreboot]

thanks for the feedback already guys!

So I'll try and answer the points in order. Since posting this earlier, I've taken other advice, and it may appear that perhaps this may not be the most obvious way to go, but I'll get back to that in a minute.

The PV system is 3.4 kwh output. I won't be adding solid fuel, but there is the possibility of adding solar thermal on the roof, but I have no experience of this as yet. We have no plans to move for a long time yet, so I'm prepared to invest for the long term.

Perhaps a cylinder with a second coil is a good shout so I can add solar thermal as well as power an immersion from PV.

Insulate is great advice. We need to lag pipes which is on the cards. We recently had a Green Deal assessment done, and have an EPC. Since then we have changed lights to low power/LEDs all round, and already had cavity wall and loft insulation. PV was next and has been in for 4 weeks now. Reducing the gas bill is the next target.

Not 100% on the Gas Saver being compatible with the system boiler, I have pinged the manufacturer to check.

I have a visit booked with a renewable energy fitter next week to see about boiler/cylinder options and also air source heat pumps and solar thermal. More research to do...!

 

[Worcester]

OK, here's some ideas (we install ALL renewable technologies except wind..) - you have a LARGE hot water demand, and depending upon wether you have to space to put large storage cylinders there are a couple of solutions

1)
Replace Combi with a high efficiency condensing gas boiler.
Add in unvented twin coil hot water cylinder to loft space if you've got it (keep your airing cupboard) - you'll need to have the pressure and flow checked to make sure that an unvented cylinder will work. based on the house occupancy you'll need a DHW cylinder of AT LEAST 6*45x2 = 540 litres. (45 litres per person, x 2 because you'll want a dedicated solar thermal element) - OR you could go to a twin tank set up - 2 x 250ltr - 500 ltrs in the loft is a LOT of cylinder = 600kg
Add in an 'ImmerSun' type device PV Water Heating, Solar Hot Water Switch and Controller Supplier in Lincolnshire, UK | immerSUN to the Immersion heater in the twin coil tank
Then you'll want at least 8m2 of solar thermal panels

OR: (my recommendation)
Use new high efficiency combi boiler that accepts pre-heated water. So you've got as much water as you could ever want.
Install a thermal store with a solar coil and immersion heater to capture the excess energy from the PV - ImmerSun as above.
Add in 8m2 of solar thermal panels (South or West facing - East gets the morning mist) (1 m2 / person)

Your chances are with that set up that the combi does very little from May - September (our heating is completely turned off in those months)

Example - there are variations (no drain back on the panels, unvented store etc ) this is to illustrate the basic configuration


ASHP waste of ££ if you're on mains gas .. (and we fit them )

 

[TBServices]

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

 

[TBServices]

http://www.baxiknowhow.co.uk/campaigns/heatsaver.htm

 

[TBServices]

And weather comp.

 

[Worcester]

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

Good solution, if he doesn't want to add thermal later..

 

[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.