Then I decided to use the MyEnergi eddi device to control the hot water tank.

PV feeding an immersion made financial sense when exports were "deemed" (e.g. 50% of generated under the old FIT arrangement), but now I think many people are on an smart export guarantee tariff, so every unit not exported means a reduction in income - it effect it costs about 15p/kWh to heat water that way (compared with maybe 6p ish to do it by gas (depending on boiler efficiency etc)). Environmentally it's swings and roundabouts of course, but quite likely your kWh might be better spend displacing a kWh of gas fired electricity generation (at perhaps 60% efficiency)  than gas fired local hot water generation (at perhaps 90% efficiency). Depends on your gas boiler of course - if yours still has a pilot light, I guess it's not in the efficient condensing category...

Another approach (which I've got) - is to have solar thermal panels in tandem with PVs (if you've got the roof space of course) - simple thermal panels get many more kW out of a sq m of roof than PVs, even if it's only "low grade" energy as warm water, it's perfectly useful for heating (even pre-heating) a stored hot water system. In my case it feeds a thermal store that provides space heating as well as domestic hot water - so the thermal panels make a contribution to the underfloor heating in winter too.

   - Andy.

Before I had the hot water tank fixed we were using gas heating to heat the hot water from 5am and again at 3pm. This enabled us to monitor what was going on. The records indicate that a months worth of gas heating for the hot water tank cost £25. Albeit with the old water tank.

I don't yet have the same records for the eddi but one week uses 23.9kWh. This uses the battery or excess export, this cost/price of this is about 15p per kWh, which is about £3.60, which is about £15 for the whole month.

So using the eddi is both environmentally sound and probably saves money.

We run our gas boiler at 60 degrees centigrade. We also heat the water up to 60 degrees.

Although we have a traditional gas boiler is still has about 20 years of life left and while it maybe less efficient than a condensing boiler I believe that it is more environmentally sound than a condensing boiler. The technicians inform me than a condensing boiler lasts on average 7 years, while a traditional boiler last about 40 years. So the environmental assessment needs to include the disposal of the previous condensing boiler and its replacement with one which had gone through design, development, delivery and installation.

I think that condensing boilers need engineering input to extend their lives either by improving the design or make it based on standard components so they can be replaced when they fail.

I agree with you on standard component solutions. Although, It seems that many products are intentionally designed with a limited lifespan, which benefits manufacturers in terms of increased sales and profits.

I would expect a standard, non-combi, condensing boiler to last a lot longer than 7 years if you buy a decent make.  My Worcester Bosch was already in the house when I moved here 14 years ago.  The paperwork says it was installed in June 2007.

Just a comment:-  Heating water to 60 degrees is too hot for human use. The safety criteria we have in the US is 125 F (about 52 C) max..

Peter Brooks

Palm Bay   

Yes, but the risk of too low a temperature is bacterial growth, e.g. legionella. I set the boiler to 60 ºC, which means that it gets a few degrees hotter, and the water to about 55 ºC, which does not scald.

That's why I said on average. Some do last longer. My brother's lasts longer but he, as an engineer, repairs the problems. My daughter's, in London, lasts about 7 years. I was quoting what technicians told me who are involved in replacing dead condensing boilers

PeterRT said:

I think that condensing boilers need engineering input to extend their lives either by improving the design or make it based on standard components so they can be replaced when they fail.

Give me a cast-iron floor-standing boiler any day, even though thermocouples burn out every few years. Ours is still going after 42 years (or thereabouts). A relay in the control system left us without hot water a fortnight ago, but the tepid shower was acceptable in the hot weather. I was able to collect a replacement from RS (Radio Spares) on the same day. I do, however, worry about the cost of the pilot light.

AJJewsbury said:

it effect it costs about 15p/kWh to heat water that way (compared with maybe 6p ish to do it by gas

Which might alter the equation, but Andy's contribution does change my thinking. The hot water cylinder is a big battery.

I suspect that timing is all - even our 20+ year old dish washer can be set to start at any time. So perhaps a large battery is not necessary.

PeterRT said:

I think that condensing boilers need engineering input to extend their lives either by improving the design or make it based on standard components so they can be replaced when they fail.

Give me a cast-iron floor-standing boiler any day, even though thermocouples burn out every few years. Ours is still going after 42 years (or thereabouts). A relay in the control system left us without hot water a fortnight ago, but the tepid shower was acceptable in the hot weather. I was able to collect a replacement from RS (Radio Spares) on the same day. I do, however, worry about the cost of the pilot light.

AJJewsbury said:

it effect it costs about 15p/kWh to heat water that way (compared with maybe 6p ish to do it by gas

Which might alter the equation, but Andy's contribution does change my thinking. The hot water cylinder is a big battery.

I suspect that timing is all - even our 20+ year old dish washer can be set to start at any time. So perhaps a large battery is not necessary.

More pilot light is used during the summer when gas usage is lower. The average usage per day for our Potterton boiler is 3.6kWh which costs about £7.50 per month. So last year I discussed with our maintainer on how to turn if off and we did that this year.

Our boiler uses gravity feed to send hot water to the water tank, which is not very efficient.

PeterRT said:

More pilot light is used during the summer when gas usage is lower.

Eh? The pilot light is on 24/7. It does not extinguish when the main burners ignite.