Solar PV, combi boiler and wood stove with a back boiler?

[Hannah77]

Hi

 

I am building an extension and want to put solar PV on the roof, with a battery and water storage tank. I want to use bthe solar to heat water in a storage tank and then direct power to a battery. I also want to use a wood stove with a back boiler to heat water in the tank. I have a combi boiler at the moment with capacity for the whole house. Installing under a wet floor heating system in the ground floor of the extension. 

 

Is there a way to integrate this system so that the combi boiler backs up the hot water system when there is not enough electricity generation from the solar PV or hot water from the stove? Are there companies that can provide a design for this kind of intergrated system?

 

Thanks

 

Hannah

[SteamyTea]

How much PV you thinking of fitting and what size will the wood burner be.

 

I take it you are fully aware of the health risks of wood burners in the domestic setting?

[sharpener]

1 hour ago, Hannah77 said:

Is there a way to integrate this system so that the combi boiler backs up the hot water system when there is not enough electricity generation from the solar PV or hot water from the stove?

 

WBS should have the priority when it is operating so there is some outlet for its heat. So set the immersion heater stat quite low to hold off the PV when stove is in use. You might want to inhibit it manually on a sunny winter's day if you know you are going to run the WBS all evening, difficult to automate unless your usage pattern is very predictable. Bear in mind there won't be much PV in winter and it is more valuable as electricity to charge the battery than to produce hot water.

Check for limitations in the instructions for the WBS you are thinking of buying. For example our Woodwarm stove can only be used with an open-vented cylinder, which would require an expansion tank in the loft, and needs a thermal store if you are using it for underfloor heating.

 

Don't know if you can take the output from the h.w. tank via the combi so it will top up the temperature only if necessary, check the instructions for it. Or you could have a diverter valve operated by a tank stat that determines whether it is hot enough. Most competent heating engineers would be able to sort this.

[Hannah77]

1 hour ago, SteamyTea said:

How much PV you thinking of fitting and what size will the wood burner be.

 

I take it you are fully aware of the health risks of wood burners in the domestic setting?

The south facing roof is about 3m x 10m so that less the 3 velux. Don't know what that equates to.

 

Fire risks?

[sharpener]

15 minutes ago, Hannah77 said:

Fire risks?

 

Breathing in the high level of particulate matter in the smoke I think. Perhaps more of a risk to your neighbours than yourself so long as you have the requisite supply of combustion air from outdoors.

[JohnMo]

2 hours ago, Hannah77 said:

Hi

 

I am building an extension and want to put solar PV on the roof, with a battery and water storage tank. I want to use bthe solar to heat water in a storage tank and then direct power to a battery. I also want to use a wood stove with a back boiler to heat water in the tank. I have a combi boiler at the moment with capacity for the whole house. Installing under a wet floor heating system in the ground floor of the extension. 

 

Is there a way to integrate this system so that the combi boiler backs up the hot water system when there is not enough electricity generation from the solar PV or hot water from the stove? Are there companies that can provide a design for this kind of intergrated system?

 

Thanks

 

Hannah

I would be very careful about going complex. All sounds great until it doesn't work or never works as you think it should.

 

Issues are some manufacturers of combi boilers allow pre heated water most don't because they are not really sure about it. So their easy answer is no.

 

PV and battery, let the PV charge the battery. Use the electric as you need

 

Really would not bother with the complications of a back boiler. Why, you now have a combi boiler doing DHW and doing CH, you have a cylinder you have to charge up to get CH started, this could be heated via combi (via a coil as combi's need a sealed system) or heated by back boiler. Your cylinder will be a thermal store and really need to be pretty big if coming off a back boiler. Bucket loads of heat loss as it will be vented and get to high temps.

 

Companies will do an integrated system and charge a pretty penny for their troubles.  My view your heat loss / waste will be pretty big.

 

I would

Small wood stove if you must.

Spend the money or most likely just time to get weather compensation working on the combi. Take as much gains in efficiency from condensing as you can. If you combi will take preheat, a small 50L cylinder is all you need. see attachedCombi-SuperFlow-White-Paper-v1-2-4.pdfCanetis-SuperFlow-Product-Sheet-WE-050318.pdf

Leave a few thousand pounds in the bank. Save money every day of the heating season.

[Hannah77]

My combi won't take heated water. I had been advised that you could feed the thermal store with hot water from the combi when needed intsead of having an immersion for when PV not supplying enough?

[JohnMo]

47 minutes ago, JohnMo said:

Small wood stove.

Spend the money or most likely just time to get weather compensation working on the combi

 

Then do this. The rest is just a waste of money. 

 

32 minutes ago, Hannah77 said:

been advised that you could feed the thermal store with hot water from the combi

So basically convert combi to system boiler, so in summer during a warm dull day (not much PV) you have to fill a big thermal store with hot water to enable you to have a short shower? Why?

[sharpener]

5 hours ago, Hannah77 said:

I had been advised that you could feed the thermal store with hot water from the combi when needed intsead of having an immersion for when PV not supplying enough?

 

Are you thinking of a big thermal store with the DHW coming through a hi-gain coil inside it? This would suit your WBS as well.

 

But not easy to see how you would control all this for max efficiency.

 

Previously we had an oil boiler feeding a conventional h.w. coil, we had the stat set 5K lower than the immersion stat so if there was enough PV the boiler would not cut in. In winter the boiler was timed to test this at 1700 which was after the PV had finished but early enough to provide for showers before dinner. In summer less of a problem and we rarely needed the boiler at all.

 

You could use the combi in a similar way. But as @JohnMo says it is all getting very complicated. Personally I wouldn't try and integrate the WBS unless you have a massive supply of free wood. Have heard they do not burn well with the cooling effect of a back boiler.

 

7 hours ago, Hannah77 said:

The south facing roof is about 3m x 10m so that less the 3 velux. Don't know what that equates to.

 

Sounds like 2kW or so. Midsummer Wholesale and others have PV roof planners on their web sites. IME that won't be enough to provide a regular supply of hot water and charge a useful size battery, we had nearly twice that to begin with and now have 4x.