Idea: Add-on solar diverter hybrid

[yngndrw]

Hello all, I should start off by stating that I'm not in the industry - I'm just a DIYer with a "what-if" idea.

Hybrid systems and solar diverters are nothing new, but as far as I can see the current hybrid systems all require a change of boiler and I'm going to propose a system which could use an existing boiler. I believe my design ensures that Legionella bacteria is killed while maintaining a minimum return flow temperature so that the boiler will run in condensing mode.

I'll try to keep it quite generic, but will ground it in a real system to give some context and explain some issues with other options. So for context we are moving into a rural house which has an oil-fired boiler and an unvented cylinder. It already has solar water heating for the DHW and has some solar PV which we intend to increase dramatically. (15-20kW of solar PV is the target) The house an old cottage with very low ceilings so insulation is a problem, we'd like to switch to under-floor heating at some point but we're not sure we'd be able to give up the extra floor height. Essentially the goal is to off-set as much of the heating as possible with solar, but an ASHP wouldn't really be ideal due to the poor insulation. We'd like to retain the oil boiler as a backup and to supplement anything we do. We may also add air conditioning which we'd also use for heating, but that's not likely to cover the heating requirements of the whole property.

So with the context out of the way, here's a quick diagram of my proposal:

Imagine that the left-side is connected to the rest of the house with a conventional S-plan configuration.

The general idea is that you have a large buffer tank with a high-power (~9-12kW) immersion heater. The immersion heater is powered through a solar diverter and is configured to try to heat the buffer tank up to around 80C with any excess solar PV generation.

The heating system's call for heat triggers the pump which by default will pull hot water directly from the buffer tank through a 3-port valve. When the buffer tank is up to temperature, this will heat the house using the stored thermal energy and the boiler will not run.

If the buffer tank temperature drops below 65C while calling for heat, the cylinder stat will operate the 3-port valve (Using just the grey input which will switch the valve to the A+B position), which in turn will fire the boiler. The boiler will then primarily heat the house with some heat bleeding off into the buffer tank which should keep it above 60C for the Legionella bacteria.

To ensure that the heat does bleed off into the buffer tank in order to heat it to the minimum temperature, I've included a thermostatic mixing valve in the return line with the hot input connected to the heating return pipe, the cold input connected to the bottom of the buffer tank and the output connected to the boiler. This would ensure that the return to the boiler is as cool as possible by drawing cooler water through the buffer tank when the heating load is low.

As for some calculations, with the buffer tank's thermostat set to 65C and the solar diverter's thermostat set to 80C, assuming a 200l buffer tank it would store about 3.49kWh of energy - That's nearly 3x the storage of an Enphase AC battery (1.2kWh) which would cost around £1,860 * 3 = £5,580. Buffer tanks have a much lower energy density and therefore use much more space, but in terms of cost I think it would make sense if you have that spare space.


Any thoughts on this idea? Have I missed something or made any silly assumptions that are incorrect?

 

[ShaunCorbs]

Dig down and insulate and then you can fit ufh quite common in older houses to have to do this

As for buffer you need around 65l per k load per hour

Let’s take a stnd 12kw property so that’s 780l per hour required buffer capacity around 6 hours normally around 4000l required

 

[yngndrw]

Dig down and insulate and then you can fit ufh quite common in older houses to have to do this

Thank you we'll have to look into this but it's good news that this is quite a common thing to fix. Sadly for the upstairs also has a particularly low part where a gully in the roof runs across the house which we already need to raise so that we can walk through that area without ducking - Will have to see what the planning say about raising that gully to see if we can also gain enough space upstairs for this. (And check the costs of course! Might need to look at the systems which fit under the floorboards.)

As for buffer you need around 65l per k load per hour

Let’s take a stnd 12kw property so that’s 780l per hour required buffer capacity around 6 hours normally around 4000l required

Doh, I had the numbers right there in front of me and missed the glaring flaw in the scheme - Thank you for pointing that out. In that case this idea becomes solely about a cheaper way of diverting to offset some fuel usage without having to worry about flow temperatures and heat pump compatibility, rather than any storage. It does however rely on a large immersion heater and excessive amounts of solar PV - Enough to produce a decent amount if electrical energy early in the morning or during the winter months.


For our situation I'm not sure we'd have the space for such a large storage tank, at least not in a location which was practical for the piping. Could possibly bury one as we need to dig up some areas anyway but would need to find a suitable tank which would survive underground so I suspect that's a non-starter.

Looks like there's at least a path towards lowering the flow temperature and then adding a heat pump which might be the most practical thing for us. That or possibly just installing air conditioning and using that as the primary heat source throughout the day.

 

[ShaunCorbs]

Do you get much sun light / energy you could try solar tubes it’s more efficient and can run the heat straight away eg in the morning to heat the property

They make in short ufh t and g panels that replace the flooring eg

OneBoard® structural system | Continal Underfloor

OneBoard® is a suspended floor underfloor heating system that can be installed directly on any type of joist system replacing the normal structural boards.

www.continal.co.uk

It’s one of them budget is key and if your going to do ufh down stairs best to do it before you move in

 

[yngndrw]

Do you get much sun light / energy you could try solar tubes it’s more efficient and can run the heat straight away eg in the morning to heat the property

Do you mean the evacuated tubes or something else? The property already has those (one panel facing east and one facing west) for the DHW but I've only ever seen them used for DHW and never for the central heating - Will have to look into that one further.

They make in short ufh t and g panels that replace the flooring eg

It’s one of them budget is key and if your going to do ufh down stairs best to do it before you move in

Thank you, that looks ideal. We will be removing some walls downstairs anyway so the added disruption of adding underfloor heating won't be an issue - Or should I say it won't be any worse than we'll already have to put up with!

 

[ShaunCorbs]

Yes

 

[yngndrw]

Yes

Thank you, will have a look into that then.

I really appreciate your time and help.