Can I add a small electric boiler in parallel with my central heating?

[Catephatic]

Hi, I would really appreciate some advice on a proposed change to my central heating circuit. I am able to divert excess electricity from my recent solar PV installation to one (or two) hot water tanks and I would like to plumb in a small hot water tank in parallel with my central heating circuit. This will hopefully reduce the running of my oil boiler on sunny days in winter. I have created a flow diagram showing my proposed system, please see attached. I would really appreciate if someone with more experience than myself could pick holes in my plan, or hopefully, confirm it's viability.

 

[ShaunCorbs]

Not worth it you need 60l per kw load that’s per hour

Let’s say you have a 6 kw load that’s 360l for one hour heating even let’s say 2 kw load that’s 120l for an hour (so stnd cylinder)

Better off using an eddi just for your hot water

 

[Murdoch]

Your PV won’t heat the water anything like as much as you think it might, and that’s on a bright sunny day

if you have a hot water tank, then get a diverter so your hot tank is heated with surplus lecky

 

[Catephatic]

Not worth it you need 60l per kw load that’s per hour

Let’s say you have a 6 kw load that’s 360l for one hour heating even let’s say 2 kw load that’s 120l for an hour (so stnd cylinder)

Better off using an eddi just for your hot water

Thanks for your reply, very useful, I had not idea of those numbers. I am using Eddi for my hot water, but find that it has my tank up to temp in about 2 hours on average so I could potentially be pumping power into another tank for many hours. I really would like to be able to use this excess power for space heating. I will do a bit of research on the concept of 'load' as applied to CH, but thinking about it, why would my oil boiler be something like 12kw unless that is the sort of number required to drive all the radiators. Possibly one solution may be to buy something like an oil filled 3kw electric radiator?

 

[ShaunCorbs]

Or to buy and install a battery for excess then you can use it daily ?

 

[Knappers]

Just feed power to immersion?
Set system to heat your water to 60deg once a day and have solar immersion element fed from PV cells

 

[Chuck]

The 'small hot water tank' needs to be chosen carefully, it needs to be designed as a thermal store. The OP's diagram seems to show a standard DHW store but these have the HX coil at the bottom and rely on convection to transfer incoming heat from the coil to the water above it. This process is not reversible because, when cooling, stratification occurs not convection; if you use a heating coil to cool the tank you'll get a miserable amount of heat back out again.

@Knappers's suggestion is good. The only reason to run the boiler in the summer is to heat hot water, which the PV source will do nicely. PV's don't produce much useful power in the winter but if you schedule the boiler to heat the DHW towards the the end of the night then any surplus power the PVs do produce during the day can be stored in the DHW tank. The programme can be tweaked to best match the household's use of DHW.

 

[gmartine]

Is your HW storage heat up time a 2 hour average across the year?

 

[Catephatic]

Or to buy and install a battery for excess then you can use it daily ?

Yes, that is the most sensible solution because in summer I am not going to want any CH, and unless I feed back to the grid (poor prices offered), my excess power will be wasted. But a battery kit will double the price of my installation and push the pay period back to about 8 years.

 

[Catephatic]

Is your HW storage heat up time a 2 hour average across the year?

No, to be fair I don't think the average will work out anywhere near 2 hours across the year. This month when there has been any kind of sun around it has heated in 2 hours, but the average over the year in all different sunshine conditions will probably change to "tank gets up to temp on most days". I should have said "in sunshine conditions, tank generally heats up in about 2 hours". That may increase mid-winter.