Mixing a combi boiler with "wet" solar panels

[Clive]

Hi,

I have bought a small two and a half bedroom end-of-terraced Victorian house. It has gas central heating with a floor-standing boiler that does the job. The boiler could be 20 - 25 years old. The system has a hot water tank with a jacket that seems as thick as a cardigan. The system supplies loads of hot water and some of the rads are absolutely scorching. I'm sure the rads could do with a "re- balance". I feel it is all very inefficient in fuel terms but it works and will cost a pretty penny to replace.

I'm thinking about doing / getting done kitchen, bathroom and loft.

I would like to stick some "wet" solar panels on the roof, even if they don't pay off financially, so I feel I am doing my "bit" for the environment. OK, money does come into it - see below.

At most, the house will be occupied by one or two people. So here is the idea. Replace the boiler with a condensing combi boiler. Then I get hot water "on-demand" and at mains pressure. I don't heat a tank and then let it cool for hot water I never use. I had a Vaillant combi boiler in a one-bedroom ground floor flat and (apart from the occasional wobbly) it would chuck out hot water at mains pressure within seconds. I want to retain that and avoid shower pumps and electric shower heaters.

The "clever" bit would be to have a hot water tank but only use it for the central heating. It would have two supply circuits, one from the wet solar panels on a south-facing roof - this would "pre-heat the water. The second supply circuit would be from the boiler. The idea being I could collect as much heat as possible on a sunny day on the solar circuit and top it up with gas-heated water. The tank would need to be well-insulated to store the heat from a sunny day and release it in the evening.

Could such a setup be installed and would the heat gained from the panels be worth the expense of the installation? Is there anyway of making a rough calculation on the heat / temperature gain to work out the money saved on pre-heating the water in the tank?

Thanks

Clive

[Stuart Noble]

Well, you could stick an old radiator on the roof as here

If you're serious, I'd look at prices in Greece where these things are on every holiday home and are bound to be a good deal cheaper

[David Hansen]

On 27 Mar 2007 06:04:18 -0700 someone who may be "Clive" wrote this:-

Yet you go on to suggest an expensive to buy and complicated boiler.

You can get mains pressure hot water, heated on demand, by using a thermal store.

Yet you go on to suggest using a hot water cylinder for "pre-heating".

A thermal store will do that.

No shower pumps or electric heaters with a thermal store. Note also that gravity hot water systems don't need them either, as one can fit a venturi shower.

Solar heating is necessarily a relatively slow process, as solar radiation is diffuse. Thus it involves capturing heat over relatively long periods. This then needs to be stored.

In this situation I would consider using a thermal store which is heated by a solar panel on a separate circuit and the existing boiler heating the shell water (until it dies, then replace with a condensing boiler). Heating would be taken from the shell by a separate pump. Hot water would be generated from a coil in the thermal store, or plate heat exchanger if you are in a hard water area.

Cost depends on how much you do yourself.

is a reasonable place for some DIY equipment.

[meow2222]

As David explained, you cant lose the water storage and go solar - not unless you put in solar batch heaters, which are storage and panel in one.

Combis are not something to choose unless you have no other option. Performance is poor.

A big storage tank would enable boiler heating of the top half, vacuum tube heating of top half, and flat panel heating of bottom half.

Some more info:

good payback energy saver is of course you need proper tank insulation.

With a heatbank you dont need to replace the tank, as an inbuilt exchanger isnt needed.

NT

[Stephen Dawson]

I will say it again, very simple system, using a Geldhill Torrent RE Thermal Store. see

for more info.

[manatbandq]

So you get the most hot water for heating on the sunniest days when you are least likely to use it. I think you have it arse about face. Use solar to augment the hot water which you will use year round.

MBQ

[David Hansen]

On 28 Mar 2007 06:46:38 -0700 someone who may be " [e-mail]" wrote this:-

Properly designed a solar system will produce almost all the hot water needed in summer.

In winter it is unlikely to produce more than a small fraction of the space heating requirement, unless it is grossly oversized for summer use.

In the spring and autumn it might provide a useful fraction of the space heating requirement on some days.

[meow2222]

If one wants to extend it to space heating, sensible thing is to use warm air panels for that, as theyre inherently much more efficient, have a much longer season and cost less.

NT

[Clive]

I see where you are coming from. Isn't that the conundrum with any domestic solar-based heat / energy colleciton in the UK? The conditions that will lead to "usable" heat from the panels will tend to be those days when you don't need the space heating. That's why in my original post I asked about heating water by solar and supplementing it with gas.However, one always needs hot water, even in summer for personal washing (unless that isn't your thing) and clothes washing.

Whether it's financially viable, I don't know. At least I will have the smug satisfaction of feeling I am "doing my bit" for the environment. I am sure there is an argument that the energy expended in manufacturing, transporting and installing the equipment needed for a "solar-supplemented" system far exceed any marginal savings in gas burned.

thanks for the link about the "thermal store". That's looks like the kind of system I might get. Looking at the diagrams and descriptions it seems broadly similar to what I described in my original post: dual "heating" circuit from solar and the boiler to "top-up". Substitutue "tank/cylinder" for thermal store and it seems a similar idea. Anyway, I now have the right terminology and a potential supplier - so much thanks for that.

Thanks for the good point challenging the idea of replacing the old boiler with a combi. With the thermal store, system as I understand it, the boiler is simply a source of heat to top up the thermal store.

Oh, I'll ignore the high-handed tone of much of the responses in this thread. I had a few ideas, I came to this group asking for information and advice in the knowledge others were better informed, otherwise why would I come here? The information provided has been very useful to me. Thank-you. I'll ignore the disparaging tone in which much of the information was delivered.

[Clive]

I also meant to write further, but Windows "sticky keys" got me. If you have never experienced that, consider yourself lucky.

Anyway.

The thinking behind installing a combi boiler was to achieve instant,

*mains pressure* hot water. The hot water in the house *is* hot (yes, I have fallen in love with the simple, little to go wrong, 25-year old boiler). Now, the thermal store literature claims it can deliver instant, *mains pressure* hot water. I can't understand *how* , since it's just a cylinder - sorry, thermal store. Won't it need a head of pressure i.e. loft installation to deliver hot water under pressure? I'll take that on trust unless anyone can explain how the thermal store system achieves the mains pressure output?

I can now see that my requirements for heating and hot water are contradictory. I guess the thermal store approach tries to gather as much usable heat as it can for both needs to reduce gas (or other fossil fuel) burned. This thread is helping me understand the options.

BTW, I have looked at that self-built heat-exchanger to reclaim heat from shower waste and pre-heat the cold supply to the shower. Looks really neat, simple to manufacture and plumb in - so seems surprising it's not mass-produced.

Clive

[meow2222]

Its often brought up, but is not a problem in reality. The system is sized to give useful heat for a sizeable amount of the year, and any excess harvest in midsummer does not cause any real problems.

I think thats true with systems that arent well designed. There are many about that will never pay their way financially or energy wise. To get one that does something useful one must properly evaluate the proposed system first, and try design after design until one proves good. It will be nice to see the day when this process is no longer necesary, due to future leaps in performance and economy.

The hot water outlet connects to the incoming water mains. No header tank is used. Hence its still at mains pressure. Its made hot along its travel by going through a heat exchanger. The tanked hot water is an entirely separate body of water that just heats the heat exchanger plates.

The downside of this is fouling of the heat exchanger, hence the need to use an exchanger with much higher power rating than will be needed initially.

Good question, why isnt it?

Its not fitted at new buid because its an unnecessary cost to the the builder and house buyer, and isnt required by regs yet. Also few house buyers even know about them so far.

Its not diy retrofitted because its too much upheaval/work for limited gain.

So there isnt much of a market at present. With increasing drive to get better energy efficiency I daresay they;ll become required some time soon.

NT