Hot water circulation when pump is off

[wytco0]

Hi All I am after a bit of advice about our hot water system, we have a Daikin low-temperature Air Source Heat Pump providing DHW and heating, we also have solar thermal which provides additional heating into the hot water tank. It all works well, however, I am pretty sure we are losing heat from the hot water tank.

Our hot water is circulated by a circulation pump which is normally switched off as we know it takes a lot of heat out of the tank, we switch the pump on for about 1 hour in the morning when we have showers.

The hot water tank is downstairs but our house is upside down with the bedrooms downstairs and the living rooms upstairs, I have noticed that the upstairs taps have hot water almost instantly even though the circulation pump is off. I think that there is some sort of circulation going on even when the pump is off, I suspect this is the hot water trying to rise and ending up at the highest points in the house, but we use very little hot water upstairs as we have a dishwasher so we don't need instant hot water and if we do need it quickly we can always switch on the circulation pump. I would prefer that the hot water stayed in the tank and did not circulate unless we want it to as this also causes DHW demand on the heat pump even when we are not in the house.

Does anyone have any idea why this is happening and is there anything we can do to stop it?

Thanks

[Countrypaul]

What is the arrangement of the HW pipes from your HW cylinder to the upstairs? If the pipe comes off the top of the cylinder and then up to the first floor it is highly likely you are getting gravity circulation - the same as is used for many back boilers and even oil boilers (don't know about gas) to heat the tank upstairs.

One way you can possibly combat that is for the pipe leaving the tank to go down to level with the bottom of the tank before going back up, that way the lower density hot water cannot rise that easily to your upstairs but the pump would be unaffected.

You say you have a pump switched on for an hour in the morning which sounds like a lot depending on the pump performance, do you have a themostat on the return to turn the pump off when the returning water is hot enough - that could save you a significant amount of energy.

[wytco0]

What is the arrangement of the HW pipes from your HW cylinder to the upstairs? If the pipe comes off the top of the cylinder and then up to the first floor it is highly likely you are getting gravity circulation - the same as is used for many back boilers and even oil boilers (don't know about gas) to heat the tank upstairs.

Yes there is a pipe from the top of the cylinder which is normally warm to the touch.

One way you can possibly combat that is for the pipe leaving the tank to go down to level with the bottom of the tank before going back up, that way the lower density hot water cannot rise that easily to your upstairs but the pump would be unaffected.

This sounds interesting and should be fairly simple to implement. Would this cause any other issues?

You say you have a pump switched on for an hour in the morning which sounds like a lot depending on the pump performance, do you have a themostat on the return to turn the pump off when the returning water is hot enough - that could save you a significant amount of energy.

Yes the pump is on for an hour but it can be changed with a timer, we set it for an hour because it normally when we are having showers. I don't think there is a thermostat on the return. If I installed a thermostat what temperature should I set it to I assume it would always be slightly cooler that the temperature of the tank hot water? The pump is connected to a pipe nearer the middle of the tank, I assume that it pumps cooler water into the tank and that the hot water goes out of the pipe at the top.

[brackwell]

Our shower pump only comes on when the pressure reduces ie when the tap is opened.

Are you describing the system where you have a loop of hot water circulating round the house with the individual outlets spurring off.  I have never thought this appropiate for a house leading to a lot of heat loss. I would always have a direct outlet from tank to shower by the shortest means posssible in 15mm plastic pipe.
Ken

[Countrypaul]

What is the arrangement of the HW pipes from your HW cylinder to the upstairs? If the pipe comes off the top of the cylinder and then up to the first floor it is highly likely you are getting gravity circulation - the same as is used for many back boilers and even oil boilers (don't know about gas) to heat the tank upstairs.

Yes there is a pipe from the top of the cylinder which is normally warm to the touch.

One way you can possibly combat that is for the pipe leaving the tank to go down to level with the bottom of the tank before going back up, that way the lower density hot water cannot rise that easily to your upstairs but the pump would be unaffected.

This sounds interesting and should be fairly simple to implement. Would this cause any other issues?

Not that I am aware of, but I am not a plumber or heating expert. I believe the appraoch I have suggested has been mentioned on this forum in the past as a way of reducing losses (I think that is where I first learned of it).

You say you have a pump switched on for an hour in the morning which sounds like a lot depending on the pump performance, do you have a themostat on the return to turn the pump off when the returning water is hot enough - that could save you a significant amount of energy.

Yes the pump is on for an hour but it can be changed with a timer, we set it for an hour because it normally when we are having showers. I don't think there is a thermostat on the return. If I installed a thermostat what temperature should I set it to I assume it would always be slightly cooler that the temperature of the tank hot water? The pump is connected to a pipe nearer the middle of the tank, I assume that it pumps cooler water into the tank and that the hot water goes out of the pipe at the top.

I think the temperature for the return (assuming it is just for circulating DHW) will need to be set by trail and error, but there are some "obvious" considerations. How hot do you need the water at the taps, if only at 38C for hand washing or 42C for a shower then there is not point in returning water at that temperature to the tank. do the basins, baths, showers have thermostatic mixing valves as there is no point in circulating hot water that more than that needed at the outlets. Putting the thermostat at the return near the tank is probably easier, but putting it further back along the pipe would be more beneficial, such as just before the last outlet (assuming it is used - if not the one before that) as there is no point in just heating the pipe.

Make sure you HW pipe is fully insulated including any drops from it. Is it copper or plastic as copper can also drain away heat if it is in contact with other things that can act as heat sinks.

Whether you are better off with seperate feeds to each outlet, or with a larger pipe with separate drops does depend on a number of issues.