Heat leaking out of hot water tank?

[tonymorris24]

Hi everyone

Would love some advice! We have a hot water cylinder which lost 14 degrees (48 to 34) in four hours, with no demand/usage. So I think the heat is escaping. The circulation pump (which you can see in the photo) is off, so could passive convection be the reason? The pump is always warm despite not running.

Hot water seems to come out remarkably soon from the taps. The cylinder is on the ground floor, recharged by a GSHP. Should a heat trap (found phrase on the internet!) on the internet?

Should I try turning off one of the taps/valves to see what impact that has, as a test? Which one - the black handle connected to the top of the tank or the red handle (by the circulation pump) connected to the bottom?

Thank you!!

 

[Murdoch]

FWIW your hot water should be much hotter than 48 degrees - how are you doing these tests?

 

[tonymorris24]

FWIW your hot water should be much hotter than 48 degrees - how are you doing these tests?

The temperature is measured from the ground source heat pump - it does a c. 30 min cycle and the water is heated to 50 ish. That's all we need really.

 

[Murdoch]

The temperature is measured from the ground source heat pump - it does a c. 30 min cycle and the water is heated to 50 ish. That's all we need really.

Sounds rather low imho

 

[Chuck]

The temperature is measured from the ground source heat pump - it does a c. 30 min cycle and the water is heated to 50 ish. That's all we need really.

You've had a legionella risk assessment done for your system? If the above is all you are doing currently, further measures are almost certainly needed.

How what's the tank capacity, make and model? Looks like about 300 litre but it's not one I recognise.

 

[tonymorris24]

You've had a legionella risk assessment done for your system? If the above is all you are doing currently, further measures are almost certainly needed.

How what's the tank capacity, make and model? Looks like about 300 litre but it's not one I recognise.

Hi, I think the GSHP has a legionella cycle.

The tank is the HPC300. 275l nominal storage capacity. Thanks

 

[Chuck]

The tank is the HPC300. 275l nominal storage capacity. Thanks

In which case I make your loss rate about:

275 [litre] * 4200 [J / litre °C] * 14°C / ( 3600 [s/hr] * 4 [ hr ] ) = 1122 [J/s] = 1.1 kW

I'd expect it to be closer to 50 W, i.e. it's twenty times too big, which needs investigating.

That level of loss is going to require a flow of hot water out of the cylinder being replaced by cooler water flowing back in. If you have an FLIR camera figuring out what's happening should be straightforward but always start with the basics. E.g. do you have a leak or dripping hot tap somewhere? Do you have a secondary circulation loop (for 'instant' hot water at outlets) and is it properly insulated?

 

[tonymorris24]

In which case I make your loss rate about:

275 [litre] * 4200 [J / litre °C] * 14°C / ( 3600 [s/hr] * 4 [ hr ] ) = 1122 [J/s] = 1.1 kW

I'd expect it to be closer to 50 W, i.e. it's twenty times too big, which needs investigating.

That level of loss is going to require a flow of hot water out of the cylinder being replaced by cooler water flowing back in. If you have an FLIR camera figuring out what's happening should be straightforward but always start with the basics. E.g. do you have a leak or dripping hot tap somewhere? Do you have a secondary circulation loop (for 'instant' hot water at outlets) and is it properly insulated?

Thank you for the details! Yes, I agree it's far too high. The system was put in by the previous owner (underfloor heating throughout with separate buffer tank, all done about 6 years ago) so it's probably insulated but unfortunately I can't be sure. Perhaps I am wrong. The only secondary hot water pump that I know of is the one connected to the hot water return (seen in the photo), which isn't running. There could be a hidden one but that's a bit of a mystery.

The pipe connected to the top of the tank is 40 degrees and the pipe at the bottom is 31 degrees. What I don't know is the flow rate! I'm not sure where to turn next so any ideas are appreciated! Thanks

 

[tonymorris24]

Thank you for the details! Yes, I agree it's far too high. The system was put in by the previous owner (underfloor heating throughout with separate buffer tank, all done about 6 years ago) so it's probably insulated but unfortunately I can't be sure. Perhaps I am wrong. The only secondary hot water pump that I know of is the one connected to the hot water return (seen in the photo), which isn't running. There could be a hidden one but that's a bit of a mystery.

The pipe connected to the top of the tank is 40 degrees and the pipe at the bottom is 31 degrees. What I don't know is the flow rate! I'm not sure where to turn next so any ideas are appreciated! Thanks

Oh, and no leaks or dripping taps that I can find or hear.

 

[ShaunCorbs]

Turn the secondary pump off via elec and red lever valve and retest

 

[tonymorris24]

Turn the secondary pump off via elec and red lever valve and retest

Thank you - will try that and report back!

 

[tonymorris24]

Thank you - will try that and report back!

Turning off the red tap has definitely cooled the pump casing, so heat must have been travelling through the pipes. The tank base is still 30 degrees and top is still 40 degrees, but that might be from conduction from inside the tank. I guess I need to wait and see how long the water stays hot in the tank!

 

[ShaunCorbs]

Yep pump might be running it’s a low flow / rpm pump so might not be able to tell if it’s running etc there should be a controller for it

Also bad news as it sounds like the secondary loop isn’t insulated good enough

 

[tonymorris24]

Yep pump might be running it’s a low flow / rpm pump so might not be able to tell if it’s running etc there should be a controller for it

Also bad news as it sounds like the secondary loop isn’t insulated good enough

The on/off for the pump is controlled by the GSHP (it's wired into it directly) but I'm thinking about putting in an inline switch and just turning it off to be sure (for now I've pulled the live wire out).

BUT I think it's not running anyway, I really think it's just convection/gravity that's causing the circulation!

We'll see how slow the warm water comes through without the secondary loop but perhaps we can just live without it.

 

[ShaunCorbs]

Shouldn’t be wired to the gs system

Also won’t be gravity as there should be a non return valve best to test if it’s go power as I suspect it’s wired always on

 

[tonymorris24]

Shouldn’t be wired to the gs system

Also won’t be gravity as there should be a non return valve best to test if it’s go power as I suspect it’s wired always on

It's definitely wired to the Vaillant heat pump and controlled from there. I've tested with a volt meter and no power to the pump when the circulation is off (which it is).

But I can't find a non-return valve. I've not removed all the lagging but I suspect that's the problem! Would a missing NRV allow circulation? I thought it was to stop water going the 'wrong way' not just circulating without the pump on.

 

[ShaunCorbs]

Yes but only if the line / loop is un insulated eg cools and flows back to the pump

 

[tonymorris24]

Yes but only if the line / loop is un insulated eg cools and flows back to the pump

Ok thanks - sounds like that's the problem then. What's the approx cost of getting one fitted (in London) do you know? thanks!

 

[ShaunCorbs]

Tbh should be around 1 hours labour plus a drz non return valve (which you could supply)

 

[tonymorris24]

Tbh should be around 1 hours labour plus a drz non return valve (which you could supply)

Sorry if I'm being stupid but I can't see how a non return valve would stop water flowing FROM the top of the tank TO the bottom of the tank. Presuming it's going that way.. or do you think it's going from the bottom (30 deg) via the loop to the top (40 deg)?

 

[ShaunCorbs]

It will add resistance so it can’t gravity flow and yes as there’s nothing to stop it every time you open the hot tap it can pull cold water from the return circuit as it’s teed into the cold main

 

[tonymorris24]

Turn the secondary pump off via elec and red lever valve and retest

I just wanted to say a quick thank you for your help with this. In the end I turned off the red handle and the convection stopped. As a result the hot water tank only loses a few degrees over the course of the day and not the entire volume of hot water. Excellent!

 

[benjycov]

I have the same problem as the original poster. 500ltr tank heated by GSHP plus immersion situated on the ground floor, i am monitoring cylinder temperature, inlet, outlets and the secondary return temperature so I have a pretty clear picture of what's going on. I'm losing 30C passively over 24 hours which equates 17.4kwH vs the manufacturer standing loss of 2.81kwH. I know the loss is convection because the secondary return pipe is pretty much always 30C (and hotter while the secondary pump is running).

My first problem is that there is no check valve between the secondary pump and the tank, however even when I close the valves around the secondary pump (thus closing the secondary return to the tank), I still get 30C temp on the secondary return (the boiler cupboard itself remains around the same ambient temp as the house).

Thus the problem is convection, the question is how do i fix it?. Googling tells me that heat traps/loops but all the results seem to be US related, and no references to the UK... I assume physics works the same in the UK thus the solution should work but I find it strange that there are no references to people doing this in the UK?!

 

[ShaunCorbs]

If you close the pump valves the pipe will stay warm for a bit might be best to turn the pump off and shut the pump
Valves for a week to see your Heatloss stays the same or drops