Heating Design Circuit - will it work?

[Ian_H57]

Hello,

I'm looking at installing a new central heating system with UHF on the ground floor and normal rads on the 1st Floor. The design was initially to have at its heart an Ideal System Boiler and a Telford Tempest 500l Indirect Cylinder but I'm considering changing/modifying this design to use cheap rate electricity from Octopus Go/Agile to heat up the cylinder during the night and/or solar PV in the summer months with the System Boiler giving back up in perhaps the depths of winter. Octopus Go/Agile provides very cheap rate electicity for a 4 to 6 hour window in the night and, although not now, has in the past paid the consummer to take the energy during that period. I'm sure when we get through the present difficulties those days of cheap rate will return.

Anyway back to the system. The system is composed of;

Ideal System Boiler
Telford Tempest 500l tank (used as a heat bank)
2 x 3kw Immersion Heaters
2 Plate Heat Exchangers
2 Way Motorised Valve (Hot water supply to Plate Heat Exchangers)
Pump (Hot water supply to Plate Heat Exchangers)
2 x Non Return Valves (for Plate Heat Exchangers)
Flow Switch/Detector (for Domestic Cold Water supply to Plate Heat Exchanger)
Pump (DHW Loop Pump)
Pump (Central Heating)
Motorised Valve (Radiator Circuit)
Motorised Valve (UFH Circuit)

Operation;

The water in the Telford Tempest 500l Cylinder is heated by 2 x 3kw Immersion Heaters using Octopus Go/Octopus Agile cheap rate tariff, Energy from the Solar PV System and the Ideal System Boiler. The hot water from the System Boiler is on a closed loop, passing through the heating coil in the Cylinder before returning to the System Boiler. The Cylinder has an expansion tank attached!

The hot water, ~ 70°C, is pumped through a two way Valve, either to the DHW Plate Heat Exchanger or the Central Heating Plate Heat Exchanger depending on which circuit is calling for heat. The DHW circuit has a flow switch to detect the flow of water when a tap is opened. The Central Heating Circuit has a Thermostat for the Radiators and a Control Signal (Boiler Enable) for the Under Floor Heating Circuit. There is also a pump for the DHW loop circuit for instant Hot Water at the taps, although the pump will be on a timer/clock, this will also activate the flow switch.

You will of course realize that I am not a plumber, I've probably used incorrect symbols in my drawing but I am interested. Is this an idea worth pursuing and will it work? I would have preferred to have had both the DHW and central heating passing though coils in the top of the cylinder rather than using plate heat exchangers but don't know if such a cylinder is available?

Your thoughts would be most welcome
Ian

 

[ShaunCorbs]

It will work but only for 40 ish minutes (around 4-5 hours of heating up) then the gas boiler will have to kick in as the store is depleted

 

[Ian_H57]

Hi Shaun, Thanks for your reply, I'm not sure that I understand such a short time frame. The 2x 3kW immersion heaters will put 24 to 36kW into the Cylinder and on reflection this is not enough. In the depths of winter I use a little over 60kWh of gas/day, so if the system was upgraded to 2x 6kW immersion heaters that would give 48 to 72kW into the system.

Thanks
Ian

 

[ShaunCorbs]

You require around 60l store per kw per hour of use

So let’s say you have a 12 kw Heatloss / boiler you need 720l stored upto temp for 1 hour of heat so 1440l for 2 hours etc

As for kwh your 2x 6kw would be still the same as your store would just heat up faster eg 2 hours instead of 4 for example

 

[Ric2013]

You can get multiple coil cylinders (look at buffer tanks/thermal stores which will usually include a heat exchanger for your hot water to taps). Not quite sure why you feel a need to have the cylinder separated from the radiator water using a plate heat exchanger. Unless you're just hoping to save on the cost of inhibitor?

I would suggest that the only plate heat exchanger is required to separate the DHW to taps from all the rest. The cylinder in this case needn't even have a coil.

 

[Ian_H57]

Well that doesn't help at all with the question