[meldrewreborn]

@wizzo227

I'm not sure that I've said that I want more hot water or any heating from the solar. I was making the obvious point that its benefit is mainly in the summer.

Unless its a DIY job, or else a rather stupid give away by a local council, I'm not convinved that solar thermal is a commercial proposition if its replacing gas. Replacing electricity might be another story.

[wizzo227]

Once the heat is in the tank, other installations could use it for both hot water and radiators around a house, which is where my "desirable" 60kWh/day number is from and what the second coil is for in the rather expensive type of unvented tank. Yours is much smaller, perhaps for doing just some hot water on just the sunnier days in summer when you get sun on the West roof. Still, that 2m^2 mini demonstrator looks like being able to collect several hundred kWh per year of heat in summer, and that should make some small dent in the 12000 kWh/year of typical household gas/heat expense. Do you have better figures ?

Oversized solar thermal with a heat dump is my preferred route to cover requirements of about 2/3 of days of the year; you can't design anything solar for the coldest gloomiest week in midwinter, especially if you live north of Aviemore.

[meldrewreborn]

@wizzo227

I reckon it saves 2,500-3,000 kWh per year.

So at 7p per kWh its only max of £210 annual saving. Perhaps £1000 if it replaced electricity.

[meldrewreborn]

@wizzo227

My gas consumption look like this (units are '00 Cu ft): I only have one year of data from before the panel went in.

[retrotecchie]

Originally Posted by meldrewreborn

@wizzo227

I reckon it saves 2,500-3,000 kWh per year.

So at 7p per kWh its only max of £210 annual saving. Perhaps £1000 if it replaced electricity.

Maximum efficiency of solar thermal is around 80% for a domestic scale setup. Maximum efficiency of PV is about 20%. The same square meterage of collector nets four times as much energy from thermal as it can from PV, so all the time the ratio of electricity price to gas price is below 4:1, you get a better return from thermal than for PV. Watt for watt, solar thermal costs a third of the price of solar PV so all in all, solar thermal is a better investment in my book, given that heating and DHW is the biggest energy expense for most people.

[wizzo227]

@meldrewreborn
I'm not familiar with the unit '00 Cu ft. Please could you explain for the benefit of all readers what is charted. For example are those gas consumed after some random number of days when a reading got sent off, or are they an exactly regular quantity per month ?

My plain old meter outside displays cubic metres, as does the meter read line on my gas bill. After some messing with calorific value (38 MJ/m^3) and 3.6MJ/kWh, and volumetric correction factor always near 1.02(m^3/m^3) my conversion from cubic metres as displayed to billable units of kWh is "multiply by about 11". The length of time between meter reads is important.

Please could you explain what your brown chart is showing ? Am I right that in summer 2006 and summer 2007, your monthly gas bill never went below "22", and in every later year, after your small solar thermal went up, summer gas use decreased to mainly cooking only, less than about "5" of those units in any summer month ever and often as low as <I can't read that chart from here>.

[wizzo227]

Back to the original question about what to do about your 34 year old boiler. That you already have some summer renewable water heater changes the recommendation somewhat. I still favour getting another heat collector passing more warm water to your hot water cylinder, and a late autumn early spring preferance is my reason to want 70 degrees incline south facing for that, which is necessarily away from that house. Then maximum plain old solar photovoltaic. 16% efficient polysilicon usually costs less than the best slightly more compact monocrystalline silicon, and usually your installer will have a deal with a supplier which mostly removes your choices of that. Your options include small East facing roof mounted plus possibly extra south facing ground-mounted, which might be worth having if you can find an unobstructed place for that which gets sunny in winter. It might compete for the same south facing land as the best place for next additional solar thermal. Gardens don't grow so well where shaded, so that might limit how much you can have.

With renewables, there are never enough, which is why I found an off-site renewable supplier called Ripple. For example, if you didn't have a convenient space for quite enough solar photovoltaics, you could sign up to bring in more renewable electricity from Ripple, which you will no doubt have plenty of uses for. That will be much more useful in the long run than replacing a solid old gas boiler with a flimsy new gas boiler.

If you plan to buy one next thing, I'd get the east roof covered in solar photovoltaics.

[retrotecchie]

Originally Posted by wizzo227

If you plan to buy one next thing, I'd get the east roof covered in solar photovoltaics.

No. Change the boiler. You have a tank so only need a system boiler, not a combi. Gas boilers have been given a reprieve by the Gumment, and your payback time will be far quicker than investing in solar PV. A system boiler and swap for your old one will give you change out of £1000 whereas the cheapest 3.5kW solar PV system will set you back at least 5k and will take around 14 years to pay back. Without a smart meter, you won't be able to get any SEG for any export unless you have a separate export meter fitted.

[wizzo227]

Originally Posted by retrotecchie

No. Change the boiler.

Bad idea. Any new fossil boiler is a waste of money, it is not towards nil net carbon, and the price of heating oil is not always going to be cheap either. Descale the existing gas boiler and look after it sure, but every major spend can get a little bit more of your home renewable. That would make life a bit less miserable years from now. We are only another stupid oil crisis away from your having only your renewables at times, so even though your existing small solar thermal is an incomplete solution, at least it helps with nil running costs and nil dependence on imports.

[retrotecchie]

@wizzo227

We'll agree to disagree.