Hot water cylinder as buffer tank

[Little Clanger]

I need to change the heat source for two wet radiators. According to one of the online radiator sizing calculators, together the two rooms need about 2 kW, so presumably for 6 hours running, I need 12 kWh. I have a spare 250l Santon Premier Plus Economy 7 cylinder and an Economy 7 tariff with Octopus. The cylinder is rated to lose about 2 kWh per day, although presumably if I add more insulation, this would reduce. I had thought of heating the cylinder on the cheap rate and then pumping the heated water through an in-line electric flow boiler. The DHW would not come from this cylinder. Would this work? Not sure how to calculate the kWh of the stored heat and not sure how hot can the input be for the boiler.

 

Does this sound basically feasible?

 

Thanks

[SteamyTea]

  On 10/06/2024 at 17:38, Little Clanger said:

According to one of the online radiator sizing calculators, together the two rooms need about 2 kW, so presumably for 6 hours running, I need 12 kWh

Expand  

The heat load (energy) will be 48 kWh\day.

To pump in that much energy in 6 hours you need a radiator that can deliver 12 kW (power).

[JohnMo]

Simple calculator 

https://www.omnicalculator.com/physics/water-heating

 

From 40 to 75 degC is about 10kWh.

35 to 75 is 12kWh

 

From what you explained the cylinder would need to comply with G3 rules for unvented cylinders. You require a mixer on the outlet. Then flow low and slow all day.

  On 10/06/2024 at 17:38, Little Clanger said:

together the two rooms need about 2 kW,

Expand  

Do a proper calculation most the radiator suppliers calculators are rubbish.

 

If you size the radiator for dT 15, flow temp would be about 35 degC on the coldest day, that would increase the storage capacity to 12kWh.

 

Then set up a secondary thermostat to active the cylinder immersion to keep the cylinder at 35 degC. No external heater required. The secondary heating would only occur on the coldest day.

[Little Clanger]

Thank you both for your reply 

[Little Clanger]

  On 10/06/2024 at 17:59, JohnMo said:

From what you explained the cylinder would need to comply with G3 rules for unvented cylinders. You require a mixer on the outlet. Then flow low and slow all day.

Expand  

Although it's an unvented cylinder, I was planning to run it vented, with the existing F&E tank 

  On 10/06/2024 at 17:59, JohnMo said:

 

Then set up a secondary thermostat to active the cylinder immersion to keep the cylinder at 35 degC. 

Expand  

Do you mean to set up a secondary thermostat on the higher 'boost' immersion, or on the lower 'Economy 7' one?

[JohnMo]

  On 10/06/2024 at 19:22, Little Clanger said:

Do you mean to set up a secondary thermostat on the higher 'boost' immersion, or on the lower 'Economy 7' one?

Expand  

Use the lower one on E7 and if an upper one, just set that thermostat to 35.

 

But as @SteamyTea says if your radiators need to kick out 2kW that's 24 x 2kWh a day, not 6 x 2kWh. Unless you are heating an uninsulated shed 2kW sounds a lot.

Edited June 10, 2024 by JohnMo

[Little Clanger]

  On 10/06/2024 at 19:27, JohnMo said:

But as @SteamyTea says if your radiators need to kick out 2kW that's 24 x 2kWh a day, not 6 x 2kWh. Unless you are heating an uninsulated shed 2kW sounds a lot.

Expand  

I don't normally have the heating on 24hrs, but obviously, I need to do a better calculation of how much heat I need. Is there a reasonably straightforward method?

[JohnMo]

The basics are here

 

https://www.open.edu/openlearn/nature-environment/energy-buildings/content-section-2.4.1

 

You need to understand how you external walls are constructed and insulated 

 

A cheat sheet is here

https://www.heatgeek.com/how-to-size-my-heat-pump-or-boiler-heat-loss-cheat-sheet/

[Little Clanger]

  On 10/06/2024 at 20:03, JohnMo said:

The basics are here

 

https://www.open.edu/openlearn/nature-environment/energy-buildings/content-section-2.4.1

 

You need to understand how you external walls are constructed and insulated 

 

A cheat sheet is here

https://www.heatgeek.com/how-to-size-my-heat-pump-or-boiler-heat-loss-cheat-sheet/

Expand  

That's great.

Thanks very much 

[sharpener]

1 kWh is 860 litre-degrees.

 

So for a 250 l tank you need 3.44 C change in temp to store/release 1 kWh of heat. So for 12kWh you need a delta T of 41.3 C.

 

Hence as others have said if you heat the tank to 70C it will put out 12kWh in falling to 30C approx.

 

But if your radiatiors are sized to output the 2kW at 70 deg they will emit virtually no heat at all at 30. If they are sized for the low end you will need TRVs (or some more sophisticated controls) to prevent the rads from overheating the rooms when the tank is at max temp. But if you are heating them intermittently they will require less heat once they are up to temp initially so you might get away with rads sized for some intermediate temp.

[JohnMo]

  On 11/06/2024 at 22:54, sharpener said:

are sized for the low end you will need TRVs (or some more sophisticated controls) to prevent the rads from overheating the rooms when the tank is at max temp

Expand  

Or a simple mixer on the cylinder outlet, mix in return flow to out flow to produce a fixed flow temp and use a simple thermostat to start or stop circulation pump. Same as you would do for UFH.

[sharpener]

I would have thought TRVs would succeed in metering the flow to control the room temp just as effectively at lower cost and less to go wrong.

 

What the OP is planning is analogous to my scheme for running rads off a thermal store charged at off-peak times, so I will know more when my HP is installed. Has just been put back by another three weeks though and I won't really know until winter anyway.

[Little Clanger]

  On 11/06/2024 at 22:54, sharpener said:

1 kWh is 860 litre-degrees.

 

So for a 250 l tank you need 3.44 C change in temp to store/release 1 kWh of heat. So for 12kWh you need a delta T of 41.3 C.

 

Hence as others have said if you heat the tank to 70C it will put out 12kWh in falling to 30C approx.

 

But if your radiatiors are sized to output the 2kW at 70 deg they will emit virtually no heat at all at 30. If they are sized for the low end you will need TRVs (or some more sophisticated controls) to prevent the rads from overheating the rooms when the tank is at max temp. But if you are heating them intermittently they will require less heat once they are up to temp initially so you might get away with rads sized for some intermediate temp.

Expand  

Thanks, sharpener

[Little Clanger]

  On 12/06/2024 at 08:12, JohnMo said:

Or a simple mixer on the cylinder outlet, mix in return flow to out flow to produce a fixed flow temp and use a simple thermostat to start or stop circulation pump. Same as you would do for UFH.

Expand  

Again, thanks, JohnMo

[Little Clanger]

In my situation, a simple solution would be to have the cylinder (AKA thermal store!) in a small shed adjacent to the wall of the house, where the present pipework is. I've got loads of Cellotex to insulate the shed with. Would that be realistic? If so how much Cellotex would be advisable?

[JohnMo]

  On 16/06/2024 at 17:23, Little Clanger said:

In my situation, a simple solution would be to have the cylinder (AKA thermal store!) in a small shed adjacent to the wall of the house, where the present pipework is. I've got loads of Cellotex to insulate the shed with. Would that be realistic? If so how much Cellotex would be advisable?

Expand  

That would work, I would use around 50mm as a minimum. Insulation on pipes 25mm.

[Little Clanger]

  On 16/06/2024 at 18:05, JohnMo said:

That would work, I would use around 50mm as a minimum. Insulation on pipes 25mm.

Expand  

Yet again, thanks for your advice

[Little Clanger]

Wondering if it would be simpler to just have the 250l Santon Premier Plus and run both the DHW and CH off that by adding an external flat plate heat exchanger. Less cheap overnight heat storage than having both tanks, but only 2 immersions instead of 4