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Should Our Water Circulation Pump Be Configured to Run All The Time?

GrahamF · 2025-10-14T09:55:53Z

Our system is open loop with just one zone and FRVs on most radiators. The TRVs in the bedrooms and one sunny lounge are all wide open. We are running weather compensation, which seems to be tuned well. The water circulation pump in our Grant Aerona 290 15.5kW heat pump is configured to run all the time, regardless of whether the compressor is running. At this moment, the compressor is off and the water is pumping at 1.2m3/hr (20l/min). When the compressor is on, the water flow rate increases. The Grant support engineer explained that the heat pump has to keep the water flowing around the system, even when the compressor is turned off, so that it knows if the water is at the correct flow temperature. When the water temperature drops too low, it will restart the compressor. I can configure the water pump to turn off when the room temperature reaches the target set on the room thermostat, but then I won't be getting the full benefits of weather compensation and the compressor will cycle more. My concerns are that this is wasting a bit of electricity and it is imposing unnecessary wear and tear on the water pump. On a hot day, the compressor might need to run only 2 hours per day to heat the hot water, but the water circulation pump will still run 24 hours per day. I would be interested to know how other people's systems handle this and what you would do if you had our system. Technical Details The Pump Blockade parameter is set to No, which makes the pump run all the time. When the room reaches the temperature target set on the room thermostat, the pump is supposed to reduce the flow temperature to maintain that room temperature. We could change Pump Blockade to Yes. In that case, the water pump and therefore the compressor would turn off when the room reaches the emperature target set on the room thermostat. Here is the relevant section from Grant Aerona Smart Heat Pump System Controller Installation & Operating Instructions 7.8.1 THERMOSTATIC PUMP BLOCKADE Thermostatic pump blockade is a hydraulic control feature within the heating circuit setting that enables the Grant Aerona Smart controller to either switch a circuit pump/valve OFF (Thermostatic Pump blockade ON) or keep the circuit active (Thermostatic Pump blockade OFF) based on the status of an installed thermostat monitoring the circuit. If a thermostat has been installed on any heating circuit, you have the following choices in how to control that circuit: With pump blockade 'ON' - Each heating circuit thermostat will control the circulating pump or motorised valve (whichever is installed) for that circuit based on the air temperature the sensor/thermostat is monitoring. With pump blockade 'OFF' - The heating circuit will continue to have circulating pump or motorised valve enabled (whichever is installed) but the smart controller will drive down the flow temperature to the minimum value to maintain the target air temperature within the circuit i.e the circuit remains active. This option would be best suited for open loop room optimisation.

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Air Source Heat Pumps (ASHPs)

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Unsure

(@unsure)

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Joined: 4 months ago

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26/10/2025 3:29 pm

I signed up here due to finding this question via a Google search. I've been looking into this with my Aerona R32 for some considerable time. I've had long conversations with Grant and tried various settings. I've been running with 'Pump Blockade On' for over 9 months without any issue except now that winter is upon us. Because Pump Blockade also turns off the 2 port valve on the rad circuit, when the outside temperature drops below 4 degrees the circulator pump kicks in for frost protection, and is restricted by the closed 2 port valve - which can't be good for it. Therefore, I've had to turn 'Pump Blockade Off' over the winter.

Despite speaking with Grant and showing them a range of graphs and scenarios, I feel they don't fully understand what the concerns are over a pump running almost 24/7 (these pumps are not cheap). I was one of the first to adopt the smart controller, so very few Aerona owners understood the issue I was having last year. Now more of these are being fitted, this question keeps coming up across the net.

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GA3_USR

(@ga3_usr)

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Joined: 3 months ago

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29/11/2025 4:22 am

We recently (Aug 25) had a Grant Aeorana 3 installed under a Eco grant.

Initially set with 48degC fixed flow temp for space heating no schedule just to be controlled by turning room stat up/down as required, but I've since changed this to WC.

DHW at 50degC which reheats when it drops to 45degC, again no schedule.

I've noticed the following regards the regards circulating pump (integrated in to the ASHP unit );

For space heating in 'day' or 'night' mode the pump appears to run all the time regardless of Pump Blockade setting.

For space heating in 'holiday' mode the pump stopped once the room stat was satisfied, but I don't remember the blockade setting at the time.

In Aug/Sep with only DHW enabled 24hr no schedule and the heating set to off the following seemed to happen;

DHW drops to 45degC

ASHP Target goes to 55degC

ASHP (and circulating pump) start up, valve goes to DHW

ASHP flow/return get to 55degC (tank stat ~49.6degC)

ASHP shuts down

Valve goes to CH

Circulating pump runs indefinitely on the CH circuit.

Tank drops to 45degC

ASHP fires up and the cycle repeats.

I noticed that after a Legionnaires' cycle the circulating pump had stopped.

I checked the stat in the the DHW tank and it was just dangled in the pocket so I packed it with a little fluffy insulation and the following happens;

DHW drops to 45degC

ASHP Target goes to 55degC

ASHP (and circulating pump) start up valve goes to DHW

ASHP flow/return aim for 55degC

Tank stat hits 50degC

ASHP shuts down

Valve goes to CH

Circulating pump runs for a few minutes then stops

Tank drops to 45degC

Cycle repeats.

My conclusion after all that, the ASHP decides when it needs the circulating pump running and it's dfficult to work out what triggers it or the logic behind it.

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damonc

(@damonc)

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Posts: 23

30/11/2025 10:26 pm

there's a detect cycle setting (4200) on the old grant controller which controls the internal pump. It means the internal pump only runs when the compressor is running then runs a 'detect cycle' every 5 mins or so to circulate the primary water to allow a temp check, if it's below what the curve setting requires the compressor starts up. That's my reading of it anyway, has anyone tried this setting on an Aerona3?

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