changing boiler to make better use of solar.

crucian

Hi everyone,

We have a combination boiler which is coming to the end of its life.we have a had a pv system fitted since January 2014 and will need a new boiler,we have 11 panasonic 240 panels south facing and 5 west facing.I appreciate there are many variables but would it be worth us getting an immersion tank and fitting a diverter to it, our plumber is a friend and he rarely charges us for any repairs even though we try! He will obviously bill us for fitting a new boiler.I will work out some accurate figures nearer the time I am just after some thoughts from you knowledgeable folks.Thanks in anticipation.

Ectophile

My immediate thought is that with the cost of installing all the new kit, you may never get your money back.

I already have a hot water tank and immersion heater, and still never even bothered getting a solar diverter fitted. The cost of heating the hot water by gas every day is only a few pence, so I couldn't justify it.

pinnks

This is an interesting one. If you already had a tank etc the maths does pan out unless you have no heat losses in your pipes between boiler and tank. Mine are unlagged and about 22m round trip. I turn my gas off in April or May and back on in October. I save about £70-80 per year on gas and get all my hot water. I then get a small contribution to the rest of the year. Perhaps £10-20 but no way to actually measure.

So, that's your potential saving. I guess it then comes down to space and disruption considerations as you need to run pipes and put the tank somewhere and then the question of whether you prefer hot water from a tank or combi. I have used both and prefer the tank but that is pure personal preference...

Exiled_Tyke

I had a similar dilemma. I have cylinder heated from the boiler but no immersion. I calculated that to fit an immersion heater and a diverter would not be cost effective (i.e. unrealistic payback). So I've left it alone and hope that batteries become viable for storage in the next year or two instead.

Martyn1981

There are add on products to work with combi's like the Sunamp heat battery and the Viridian Pod, but again, I doubt they are economically viable.

Mart.

movingon

So are you saying that on the assumption there is no other equipment other than the solar panels themselves, there is no advantage over a combi boiler versus conventional boiler and water tank arrangement?

Cardew

I would agree with post 2.

I think at times the lure of getting something 'free' makes people forget basic financial common sense.

Gas costs between 2p/kWh and 3p/kWh. How much solar generated electricity could be diverted to a hot water tank? 1,000kWh? 2,000kWh?
With a new boiler you will have pretty high efficiency, so realistic annual savings would probably be in the order of £50. To save quibbling about savings let us say £100 pa.

To convert a house with a combi to a conventional HW system with a HW tank, a cold water tank in attic* would cost? £1000? £1,500?; plus the cost of an Immersun or similar.

* You can get away without the cold water tank and get a pressurised tank but that will add £hundreds

Ben84

Using electricity to heat water is currently a poorer use of the fuel at the moment for environmental reasons, as gas makes less pollution than electricity. Less CO2, sulphur and other materials. But the gap isn't as big as people might think, and it's reducing, so that may not always be true for a couple of reasons.

Gas boilers drop greatly in efficiency when heating the hot water, making this a specific situation. Unlike radiators which can do their job much cooler if they're sized big enough, the temperature of the hot water is higher. The more you heat something the worse its ability to absorb more heat (Fourier's law of heat conduction). Even the new condensing boilers that are typically well over 80% efficient for radiator use (well, ideal radiator use with big radiators) will often drop to around 50% and send a lot of heat up the chimney when heating the hot water. People often are mistaken and think their new 90% efficient boiler will be 90% efficient at hot water, and I have often seen energy comparison sites using these figures for hot water, but that's not true.

Also, now that the UK grid has less coal/oil plants, the CO2 emissions from heating the hot water with electricity is going down. In the future when more nuclear plants are built, the gap may close or even change in favour of electricity, but that's not likely to happen any time really soon.

So for now, using electricity to heat water is more polluting than gas, even if you're making it with your own solar panels as you can always feed the electric in to the grid to reduce electricity use from other sources. I know people find the idea of reducing their personal CO2 emissions attractive, but if it's environmental benefits you want, sending your solar electricity in to the grid is better than reducing your gas use.

Ectophile

movingon wrote: »

So are you saying that on the assumption there is no other equipment other than the solar panels themselves, there is no advantage over a combi boiler versus conventional boiler and water tank arrangement?

It's all down to payback period.

On a sunny day, like today, a solar diverter will give you a full tank of hot water at no cost. But in the winter, it probably won't - so you still need some other way to heat the water.

My decision was that I don't use much hot water in a day. The amount of gas I use re-heating the water every morning barely even registers on the gas meter. So I decided that the cost of fitting a diverter wasn't worth it for me.

For other people, it may make sense. Especially someone who needs a new boiler anyway. And it's even more likely to be cost-effective for someone who doesn't have mains gas.

zeupater

Ben84 wrote: »

Using electricity to heat water is currently a poorer use of the fuel at the moment for environmental reasons, as gas makes less pollution than electricity. Less CO2, sulphur and other materials. But the gap isn't as big as people might think, and it's reducing, so that may not always be true for a couple of reasons.

Gas boilers drop greatly in efficiency when heating the hot water, making this a specific situation. Unlike radiators which can do their job much cooler if they're sized big enough, the temperature of the hot water is higher. The more you heat something the worse its ability to absorb more heat (Fourier's law of heat conduction). Even the new condensing boilers that are typically well over 80% efficient for radiator use (well, ideal radiator use with big radiators) will often drop to around 50% and send a lot of heat up the chimney when heating the hot water. People often are mistaken and think their new 90% efficient boiler will be 90% efficient at hot water, and I have often seen energy comparison sites using these figures for hot water, but that's not true.

Also, now that the UK grid has less coal/oil plants, the CO2 emissions from heating the hot water with electricity is going down. In the future when more nuclear plants are built, the gap may close or even change in favour of electricity, but that's not likely to happen any time really soon.

So for now, using electricity to heat water is more polluting than gas, even if you're making it with your own solar panels as you can always feed the electric in to the grid to reduce electricity use from other sources. I know people find the idea of reducing their personal CO2 emissions attractive, but if it's environmental benefits you want, sending your solar electricity in to the grid is better than reducing your gas use.

Hi Ben84

I understand and basically agree with the underlying theory that electricity could and should be used to provide a 'higher' form of energy than thermal where possible, but really don't follow the logic in the above concerning solar pv & the CO2 environmental impact and believe that there is a fundamental accounting flaw being employed ...

The basis of logic applied seems to be that although purely diverting solar pv to thermal has absolutely no CO2 impact for the household with solar pv, it results in less energy available for export to the community as a whole, which then needs to be replaced (for now) mainly by CCGT (gas) centralised generation and therefore the environment suffers ... however, earlier you propose that the alternative of providing the DHW heatload by a domestic gas boiler would only be 50% efficient ... this introduces a simple CO2 balancing question based on providing the required amount of DHW from the various methods - if the centralised CCGT generation efficiency after distribution losses resultant is higher than that of a GCH boiler (your 50% boiler efficiency, which probably wouldn't be atypical for DHW alone) then the CO2 argument above falls apart, if not there is only a net difference environmental gain from CO2 reduction - however, a third alternative also exists in that satisfying the DHW requirement by solar thermal you'd be directly utilising lower grade energy (heat) with no export possible, so logically resulting in a 100% CO2 reduction ....

HTH
Z

Ben84

Hi zeupater,

I've gone back and done the maths again. You're right, I made a mistake in the numbers! Gas would in many cases produce more CO2 than using electricity in the UK to heat water.

CO2 from electricity from the grid is currently quite low this afternoon at 268g per unit (https://www.ecotricity.co.uk/our-green-energy/energy-independence/uk-grid-live), but tends to be somewhere in the 300s when I've looked at it. Whatever it is, we can assume it's 100% efficient at turning a kWh in to heat in your hot water tank. Meanwhile, using a boiler that's around 50% efficient would be somewhere in the range of 400g per unit of energy that actually heats the water. Of course CO2 isn't the only pollutant to consider, but it is a big one.

So, I calculated it the wrong way. I might go back to using the immersion heater elements in my hot water tank after all.