Hi, I have an off-grid house running only solar (no mains AC). I have a 280L thermal store, which has heat input from 2 solar water panels and an input from a wood-burner with back boiler. We also have UFH installed in floors. My question is can I install a gas fired boiler to the thermal store to increase store water temp in winter, as we don't get enough hot water from store in winter to heat all the floors well enough. I am just not sure if connecting a gas fired boiler is possible or not. thanks for any advice on this.

You def can connect a gas boiler to a TS, but you need to make sure there is adequate head (water pressure in the gas boiler) so it doesn't trip on low pressure.
I have a more preliminary question about why you can't get enough heat energy from the LBS. It looks to me like the return to the LBS is some distance from the base, not that far below the ufh draw off. That means your working volume for ufh is very small, which is prob why you can't get heat into the floors. Plus, the ufh conxns are close, so prob getting short circuiting.
If it were me, I would first connect the LBS return to the lowest point which is the ufh return (basically add a'T'). You still won't have huge storage capacity, but you'll have twice what you currently have. This is a much better way to plumb your TS, question is will it do enough. 280 l is quite tight.
I'd do this, then consider the gas boiler.

Posted By: colin440As for the connections for the UFH in/out, i assume these are positioned at the optimum positions, as the storage tank came already with the sockets installed for each input/output and assume the manufacturer installs these at the optimum positions, but from wha you are saying it sounds that these are not!

It depends what you think the optimum is. As supplied the stove is plumbed so as to leave some unheated water at the bottom that the solar can heat, so as to maximise the 'solar fraction' which is a figure-of-merit used sometimes. I think it assumes that the stove will be firing the whole time the UFH is on. As greenpaddy says, you can rearrange to allow a bit more storage from the stove, at the expense of not getting as much from the solar thermal.

Its sounds like the back boiler isn't powerful enough for the size of the house.

Have you got mains gas available? if not then I think oil works out cheaper than LPG.

I have a Grant oil boiler feeding a 300L vented store and UFH. Sure a bigger store might work better but a 300L store isn't totally useless.

Thanks for all the replies. Sorry I should have said the house is located in Andalucia (at 1200m elevation) with strong sun most days! I realise this is probably a discussion board for UK houses/residents. So we get alot of heat from the 2 solar water panels - the other day the TS was reading about 70C, as not using the UFH in summer only DHW. In winter on sunny days the TS can get to 60-65C in the day, just from the 2 panels. But always when we put the UFH on, we have the wood-burner running.
On the question of the locations, the wood burner is on the ground floor and the TS is on the upper floor, with the pipes from the burner to the TS at 45 degrees (works with thermosyphon I think it's called, ie. no pump). Seems to work well.
So it's maybe designer with the in/out location to the TS in consideration that the solar water is creating more heat than, say in the UK.

The solar plumber (not the guys who made the original installation) said he would place a condensing type boiler on the first floor, in same room as the TS. He would just take the outlet flow for the UFH and make a connection to the gas boiler, then back to the outlow for the UFH.
Not being a plumber, this sounds like it makes sense - just heating the water that goes under the floors as it leaves the TS. He said the boiler can be set up to run automatically with cut off temps as required. I suppose that the temp of the TS will still fall (initially) as the cold water in the floor pipes returns to the tank.

CWatters - yes I also think the boiler isn't powerful enough for the size of the house. But adding a gas boiler would probably be the better option, as then there is always that power source, if for example we run out of firewood for a few days. And on really cold days (ave temp minimum in January is around 0C at 1200m elevation), we can run the wood-burner and gas boiler to create more temp.

Re. size of TS, yes it sounds like I would have been better installing a larger TS, but I took advice of the installers on all the technical aspects. They sent me that diagram above, but to be honest I need to post a photo of the actual TS with the connection as plumber, as I couldn't say 100% that the locations are exactly as on that diagram. I'll try and find a photo, as not at the house right now.
cheers

Here in England the cheapest way to buy LPG is to have an LPG tank. These are slightly smaller than an oil tank but still quite sizable. In England the supplier owns the tank and only they can refill it. However you can transfer ownership to another supplier if you want. With oil you own the tank and anyone can fill it up for you. Best find out what restrictions there are where you live.

You can buy LPG in 47kg cylinders but it's much more expensive that way.

Example images..

LPG tank..
http://www.northernenergy.co.uk/wp-content/uploads/2016/06/LPG-tank-Grass-2.jpg

Oil tank..
http://www.jwhinchliffetanks.co.uk/wp-content/uploads/2015/07/jwh1.jpg

Pair of 47kg cylinders..
https://www.mcldirect.com/8565-thickbox_default/senturion-247-gas-cylinder-storage-for-two-47kg-tall-cylinders.jpg

PS: Check if you would need 2 or 4 cylinders. I think in very cold locations there is an issue with two cylinders not providing enough flow rate for a boiler. Might have that wrong.

PPS: If you go for cylinders check there are plenty of people willing to deliver them as a 47kg cylinder is heavy. We use a pair to feed a hob.

Colin just reading this through and part of the issue might be the fact that the wood burner is connected via gravity to the store, so heat transfer occurs fairly slowly.

It would make more sense if you could set up a heat leak on the gravity side and pump heat from the wood burner to the store.

You would need a loading valve/return riser or a low limit stat to do this but the rate of transfer would increase.

Remember that condensing boilers don't work well if connected direct to thermal stores, due to the return temperatures straying too high as you try to achieve high temperatures to support hot water production and provide enough energy for the UFH.

If you add one it could be set up for weather compensation and feed the UFH direct and full temp to the store on a priority basis for the hot water provision alone.

Most modern boilers are capable of this either with a 3 way changeover valve or a 4 pipe arrangement (2 to heating, 2 to hot water).