Call for heat on open loop weather comp system

Air Source Heat Pumps (ASHPs)

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Patrick

(@patrick)

Active Member Member

Joined: 2 months ago

Posts: 5

Topic starter 22/02/2026 7:09 pm

Hi all, is there anyone familiar with how the Grant R290 is wired?

While discussing some of the details of my self designed heat pump system with our plumber, he mentioned something I'm not quite sure of.

I had stated that we would be better off going without an integrated/ pre-plumbed cylinder that Grant had suggested since the system will be open loop with no zones, so the two zone valves would be unnecessary. His response was "We can go open loop, but we will still need 1 thermostat upstairs and 1 downstairs as a “call for heat” for the heat pump, so we will need the zone valves."

As far as I know, if running on weather comp then that serves as the call for heat right? Which means we shouldn't need any stats or zone valves?

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ASHP-BOBBA

(@ashp-bobba)

Noble Member Member Professional Installer

Joined: 1 year ago

Posts: 408

22/02/2026 7:26 pm

Posted by: @patrick

↑

Hi all, is there anyone familiar with how the Grant R290 is wired?

While discussing some of the details of my self designed heat pump system with our plumber, he mentioned something I'm not quite sure of.

I had stated that we would be better off going without an integrated/ pre-plumbed cylinder that Grant had suggested since the system will be open loop with no zones, so the two zone valves would be unnecessary. His response was "We can go open loop, but we will still need 1 thermostat upstairs and 1 downstairs as a “call for heat” for the heat pump, so we will need the zone valves."

As far as I know, if running on weather comp then that serves as the call for heat right? Which means we shouldn't need any stats or zone valves?

I don't know Grant system but most ASHP are very simular. The grant controller should act as the only stat on an open loop for best performance and you can probably opt for something like a wireless wifi dynamic stat (linked below) that gives you weather curve and a bit more, then open loop the system to that control and no 2nd zoning upstairs. You need someone that can balance the heating circuits correctly, this is important for it to work correctly.

Open loop should be as much of the house you can balance and then add some kind of rad controls such as simple TRV's to rooms that run a risk of parasitic heat from plant rooms below or solar gain on the side of the building. These TRV's will act simply as stop points, you should set these around 1 deg higher than the desired temperature so they do not interfere with the open loop until the heat is above comfort desired. Ensure non parasitic rooms stay open looped and no TRV's often its recommended the flow minimum is near the Kw rating of the unit (6l/pm for 6kW and 7 for 7 as an example, exact minimums are all available in the manuals, having around 50% of the circuits open is normally enough to ensure minimum flow.

https://www.grantuk.com/professional/products/air-source-heat-pumps/aerona-controls/#:~:text=Features%20load%20compensation%20which%20uses,heating%20curve%20every%20few%20seconds

AAC Group Ltd covering the Kent Area for design, supply and installation of ASHP systems, service and maintenance, diagnostics and repairs.
Professional installer. Book a one-to-one consultation for pre- and post-installation advice, troubleshooting and system optimisation.

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