Posted by: @mjr

Posted by: @kev-m

At the moment, my ASHP is cycling every 30-40 minutes, I presume because its minimum would heat the water more than the weather compensation curver says it should. Maybe the smaller ASHP would run constantly?  Mine seems to when it's a little bit colder. However, it's very mild atm and when it gets much colder, my ASHP should be working less hard than a smaller one and COP should be better. It's a bit of a balancing act I think.

Hi @kev-m (and maybe @derek-m will have a view on this too) I'm now bumping into a similar problem. Weather compensation seems to mean my heatpump is trying to operate below its minimum on warmer days (3.1kW @ 15°c ambient, 35°c water, for example) so it cycles often, which seems bad for CoP.

I know it's against the orthodox view, but I'm wondering if it could be worth raising the high-outdoor-temperature end of the heat curve, so deliberately warming the house with relatively cheap heat when it's above 13°c or so out (instead of aiming to hold a constant temperature as the curve currently does), but telling the smart thermostat to impose a maximum temperature in those conditions and let it switch the heating off for an hour (or more) when it gets too warm. In a way, it's still cycling, but with a much longer period, possibly even only one on-off-on cycle per afternoon.

Then as everything cools in the evening, the thermostat can go back to its high maximum and get out of the way, weather compensation takes over again, the heat pump is running above minimum and all is good like before.

Funnily enough, this is exactly what I'm trying now. My heat curve is a little high so that left unchecked it heats the house to 22 deg.  I've got the thermostats set to 21 deg.  I've got to go out for the evening but I'll post some data to show what happens tomorrow.  

Posted by: @mjr

Posted by: @kev-m

At the moment, my ASHP is cycling every 30-40 minutes, I presume because its minimum would heat the water more than the weather compensation curver says it should. Maybe the smaller ASHP would run constantly?  Mine seems to when it's a little bit colder. However, it's very mild atm and when it gets much colder, my ASHP should be working less hard than a smaller one and COP should be better. It's a bit of a balancing act I think.

Hi @kev-m (and maybe @derek-m will have a view on this too) I'm now bumping into a similar problem. Weather compensation seems to mean my heatpump is trying to operate below its minimum on warmer days (3.1kW @ 15°c ambient, 35°c water, for example) so it cycles often, which seems bad for CoP.

I know it's against the orthodox view, but I'm wondering if it could be worth raising the high-outdoor-temperature end of the heat curve, so deliberately warming the house with relatively cheap heat when it's above 13°c or so out (instead of aiming to hold a constant temperature as the curve currently does), but telling the smart thermostat to impose a maximum temperature in those conditions and let it switch the heating off for an hour (or more) when it gets too warm. In a way, it's still cycling, but with a much longer period, possibly even only one on-off-on cycle per afternoon.

Then as everything cools in the evening, the thermostat can go back to its high maximum and get out of the way, weather compensation takes over again, the heat pump is running above minimum and all is good like before.

Hi Mjr,

One of the suggestions that I have made previously, is to boost your indoor air temperature by 1C or 2C during the warmer period of the day, when a heat pump is operating at its most efficient, and in so doing reducing the heat demand later when the heat pump is less efficient.

I can't remember what manufacture of heat pump and controller that you have, but it may be possible to schedule your controller to increase the indoor temperature over a set period of time each day.

We still have a gas boiler, but also an A2A heat pump. The later I run when our solar PV system is generating sufficient power, which in turn shuts down the gas boiler for a number of hours. Whilst running the heat pump, it raises the indoor air temperature up from 20.5C to somewhere between 22C and 23C, which in turns delays the point at which the gas boiler needs to kick in. The other day the gas boiler was off for a period of approximately 13.5 hours, thereby considerably reducing our gas consumption.

This buffer tank question is an interesting one. I read an old paper on this subject (dated 2013 I think) that said that provided the circulating volume remained sufficiently large (I.e. no trvs in the main heated area etc.) and the heat pump was an inverter type that could modulate its output down dependent on deltaT demand, a buffer tank was not necessary. (Aren’t all the modern heat pumps inverter type that can modulate down?)

Then I also see discussion of systems that basically have two pumped circulating loops with a heat exchanger/buffer in between so that the rad water doesn’t circulate through the heat pump.  This was said by the installer to be necessary as a condition of the heat pump warranty. (surely that is less efficient with the loss through the buffer and the extra pump?)

There seems to be a whole lot of conflicting information out there.

Posted by: @cycleneil

This buffer tank question is an interesting one. I read an old paper on this subject (dated 2013 I think) that said that provided the circulating volume remained sufficiently large (I.e. no trvs in the main heated area etc.) and the heat pump was an inverter type that could modulate its output down dependent on deltaT demand, a buffer tank was not necessary. (Aren’t all the modern heat pumps inverter type that can modulate down?)

Then I also see discussion of systems that basically have two pumped circulating loops with a heat exchanger/buffer in between so that the rad water doesn’t circulate through the heat pump.  This was said by the installer to be necessary as a condition of the heat pump warranty. (surely that is less efficient with the loss through the buffer and the extra pump?)

There seems to be a whole lot of conflicting information out there.

Hi Neil,

I have looked in detail at buffer tanks, and applying the basic laws of physics I have come to the conclusion that installing one in a heat pump system is not a good idea.

From what I gather, a buffer tank in its most simple form is merely a cylinder, with an input and output connection at the top end and an input and output connection at the bottom end. The heated water flowing from the heat pump unit enters the top of the buffer tank at one side, and is required to be drawn out of the top connection on the other side of the buffer tank by a secondary water pump, which is required to pump the heated water to the various heat emitters. The return water from the heat emitters enters the bottom section of the buffer tank at one side, with this cooler water being drawn out of the buffer tank at the other side by the water pump in the heat pump unit. This system would possibly work fine if the flow rate from the water pump within the heat pump, is matched by the flow rate of the secondary water pump. But what happens when any zone valves open or close or any TRV's open or close? You could have a situation where there is 25l/min of water flowing through the secondary water pump, but only 20l/min coming from the heat pump. In that case the extra 5l/min would need come come from the return flow from the heat emitters, which would mean that you would have cooler water mixing with the heated water that is feeding the heat emitters.

One doesn't need to be Einstein to realise that this would reduce the heating capacity and probably the efficiency of the system. If the heating system does not have the required volume specified by the heat pump manufacturer, the a capacity or volume vessel should be installed rather than a buffer tank. This may also remove the need for any secondary water pumps to be installed.

Posted by: @kev-m

Posted by: @mjr

Posted by: @kev-m

At the moment, my ASHP is cycling every 30-40 minutes, I presume because its minimum would heat the water more than the weather compensation curver says it should. Maybe the smaller ASHP would run constantly?  Mine seems to when it's a little bit colder. However, it's very mild atm and when it gets much colder, my ASHP should be working less hard than a smaller one and COP should be better. It's a bit of a balancing act I think.

Hi @kev-m (and maybe @derek-m will have a view on this too) I'm now bumping into a similar problem. Weather compensation seems to mean my heatpump is trying to operate below its minimum on warmer days (3.1kW @ 15°c ambient, 35°c water, for example) so it cycles often, which seems bad for CoP.

I know it's against the orthodox view, but I'm wondering if it could be worth raising the high-outdoor-temperature end of the heat curve, so deliberately warming the house with relatively cheap heat when it's above 13°c or so out (instead of aiming to hold a constant temperature as the curve currently does), but telling the smart thermostat to impose a maximum temperature in those conditions and let it switch the heating off for an hour (or more) when it gets too warm. In a way, it's still cycling, but with a much longer period, possibly even only one on-off-on cycle per afternoon.

Then as everything cools in the evening, the thermostat can go back to its high maximum and get out of the way, weather compensation takes over again, the heat pump is running above minimum and all is good like before.

Funnily enough, this is exactly what I'm trying now. My heat curve is a little high so that left unchecked it heats the house to 22 deg.  I've got the thermostats set to 21 deg.  I've got to go out for the evening but I'll post some data to show what happens tomorrow.  

results so far

https://renewableheatinghub.co.uk/forums/renewable-heating-air-source-heap-pumps-ashps/kev-ms-ashp-performance/paged/12#post-7574