System for someone who got it wrong!

[pmagowan]

Hi guys,
I have gone about things back to front.  I was keen on gadgets and so I bought a 50kw atmos log gassifier and a 3000L thermal store with a DHW coil and a solar coil.  I have now come to my senses and am insulating the house to 'near' passive standards.  Obviously the kit I have is too large but I wonder if I can use it with solar thermal.  My needs will be mostly for DHW as heating should be minimal.  I will have a Lohberger stove running on logs and pellets and so in the winter the back boiler on this should top up my DHW.  In the summer the stove will not be used and I will rely on ST.  I am only one in a big house but that may change.  1 shower a day (morning) and a bit of dish washing is the need.

How do I set this up?  Do I keep the tank as a heat dump?  What size tank would be an alternative.  The atmos could be used if I need DHW on cloudy days in summer but don't need the Lohberger heat to the house.  This may be overkill and I may be better selling it and using an immersion heater for this, hopefully rare, occasion.  I am hoping to require no central heating other than to the MVHR duct and to the orangery (mostly for frost protection).

Any ideas?
Thanks
Paddy

[clivejo]

It maybe overkill, but it could save you time and money in the future.  Imagine being able to do a burn when you want to (nice cold evening sitting watching TV) and then using the heat as required.  You can do that with a thermal store, 3kL will hold a lot of heat.  I currently do a burn every evening and this keeps me cosy over night and a hot shower in the morning.  So I dont think you have been back to front at all!  I do recommend you put a lot of thought into how you configure and plumb it, make sure its well lagged and only extract the heat when you need it!  If you have a family or guests to stay, you can relax knowing there is plenty of hot water at your disposal.

[wookey]

So your hot water usage is maybe 60l/day, so it'll take you a month and a half to get through a fully-warmed store. That's unlikely to be efficient. Good stratification is important otherwise your ST will simply make all 3000l midly tepid and you won't have any useful hot water.

Go completely insane on the store insulation. You can put 50cm all the way round and that might be enough.

[pmagowan]

Thanks guys,
I had planned it all for when my losses where huge and central heating would be the biggest drain.  Now it should be negligible.  Are you guys thinking the 3kl tank can still be used.  Insulating it is not a problem.  Would ST manage to heat useful water for DHW or would I have a tank of tepid water?  How can I get stratification to work best.  I have experience of my original plans as i helped my dad install the same.

Would it be wise to have a smaller tank?

[CeeBee]

Not based on experience, but I find it hard to imagine stratification being so brilliant that ST could heat a few 100 litres of water at the top of a 3000 L tank to a good hot temperature while the rest remained cold. And no chance of any sensible amount of ST heating the whole 3000 L enough.

Just wondering if anyone has thought of (or even used) a system with a 'normal' small h/w cylinder equipped with solar thermal, but with the alternate heat supply (plugged into what would have been the lower 'boiler coil') coming from the large thermal store?

Come the Renewable Heat Incentive, if I wanted to swap from gas to biomass, then I'd be in this position of having a small h/w cylinder with solar thermal, but with space to add a large thermal store somewhere.

[billt]

You can certainly use solar panels to get usable hot water with a big store, but it's likely to be throwing good money after bad.

FWIW, I have a system similar to yours, a 2,500l store and a 25kW Atmos log boiler, but solar panels were part of the system design from the start. The system has 120 58mm evacuated tubes and it has kept the store hot enough for our hot water needs from the middle of April to the middle of October. A bit more attention to insulation and we'd get a couple more weeks out of it. It got up to 81 C last May after a few sunny days and took 13 days to drop to 47 C (a bit low by most peoples standards but acceptable to us). The solar coil is at the bottom of the tank so the whole tank is heated, a bit of stratification does develop but not a significant amount. Had the large Akva solar tanks been available I might have used one of those and it should work a lot better with solar panels.

[dhaslam]

If you can get a good price fpr the  large store and  boiler it would be better to sell it.  That is as long as you are sure that the  heat  requirement for the house reduced to very low figures.     There is no point in producing heat to be slowly lost in  a  large tank.  Generally an immersion will only need to top up a solar tank by  about twenty degrees on dull days.  If you adjust the temperature setting on the immersion to be just hot enough it saves a little extra.   

Have you thought about PV?     

[pmagowan]

Thanks guys,
I really fell for the idea of the large thermal store at the time.  It is such a good solution and I now have it in my head.  The idea of storing all your heat to use as required is  cool.  I am pretty confident though that my heating requirements will reduce to negligible.  I'm clutching at straws as I can't really see much use to the large store if I dump heat into it in the summer unless it works as a buffer for long periods of poor sunshine. 
I was thinking of weird ideas such as putting the solar through the top coil first etc but probably stupid.  I think I will get a smaller tank and plumb it in.  If the large one is needed I could add it in or sell it if not.  What type of tank would be best and do most people use a coil for DHW in a sealed system?
Thanks

[knighty]

if I were you I'd wait till all my insulation etc.. was sorted and then think about the heating...

I'll be very impressed if you end up needing as little heat as you're hoping for!

[clivejo]

if I were you I'd wait till all my insulation etc.. was sorted and then think about the heating...

I'll be very impressed if you end up needing as little heat as you're hoping for!

I second that! 

[JohnS]

One idea is to get a smaller hot water cylinder or store - say 150-200ltr depending on your demands (is it just one person or a family, now or in the future?).   Heat it from a solar coil with a back up immersion.  Use the existing thermal store as a dump for when the new cylinder is up to temp.  Feed the water into the new cylinder via the store so as to get the benefit of it until you know where you are.

John