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?
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?