Another 17kw Grant Heat Pump - Advice Needed!

bn06amp · 2022-12-13T12:25:28Z

Hello, I'm new to the forum and also new to heat pumps as well, so please do forgive me if some of my knowledge isn't quite up to scratch. I appreciate there is a similar topic on here but I didn't want to distract from the great advice that Sonik is being given on that thread. We had a Grant heat pump installed in Oct this year, our house is approx 250sqm and is underfloor heating throughout (upstairs and downstairs). I accept that I'd (up until yesterday) had the ASHP set up completely wrong. Every single room in the house was zoned, with some zones completely off and other completely open. After reading useful forums, I've now completely removed all of the actuators from the manifolds and turned the central thermostat up to 30. Prior to doing this we were finding that the house was getting nowhere near to temperature and our overall consumption per day was 120 - 130 KWH, which is quite simply just not affordable, given the house was not heating up (it was really cold) and the system was still draining so much consumption, yesterday we simply just turned it off till we could work out the problem. So last night we made the changes and turned everything back on, thermostat turned up to the max, house was still pretty cold this morning, but has now, very slowly started to heat up and is definitely going in the right direction. Everywhere seems to be heating pretty evenly (even the areas where we'd anticipate a high heat loss) with the exception of my daughters bedroom, which is still cold and I really don't know why. Our heating curve is set to 50 degrees at 0 external temperature. I used the advice on the other thread to look at flow, return and consumption Flow - 45 Return - 42 Power 4600 - 4700 So with the above in mind, I had a few questions which I was hoping to get some advice on: - Given the pump is running the flow at 45 and the house is definitely heating up, should I adjust the curve down slightly - given the still pretty high consumption I'm wondering if this is because the pump is trying to reach a temperature of 50 which is just not realistic (and a flow of 45 seems to be heating the house up ok). - I was also advised that having a higher flow could be causing the heat pump to go into the defrost cycle more often, and turning the flow down slightly could reduce the amount of time the unit goes into defrost and draws heat from the system? Is this something people would advice. - Given the heat pump had been off, I appreciate it will now take some time to get the house to an even temperature, once this is achieved and I'm happy with the ambient temperature, would you expect the running costs to drop (as again running at 100kw a day just isn't rally sustainable!) - Any ideas why there is one room not heating up? Given the actuators have been removed, the valves to this room are fully open! - Our installer has offered to come connect the immersion up to the Pump. We use Octopus intelligent so we get a cheap overnight rate of 7.5p, would it be adviseable to have the immersion connected up and run this a couple hours overnight? Any help and advice really would be hugely appreciated. Ash

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Air Source Heat Pumps (ASHPs)

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bretix

(@bretix)

Estimable Member Member

Joined: 3 years ago

Posts: 79

20/12/2022 5:37 pm

@allyfish Thank you, I've learnt so much more I'm the last week than the time the ashp were installed.

I'm assuming both pump adjustments would have to be like for like?

On a side note since the milder weather I'm finding the radiators warmer than ever.

2 10kw Grant Aerona3
Heat loss calc 16.5 kw @ -2.8 degrees
4.32 PV

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AllyFish

(@allyfish)

Noble Member Contributor

Joined: 3 years ago

Posts: 493

20/12/2022 6:01 pm

@bretix we're all learning on these systems 🙂 I would set the secondary pump to full flow, always. If the pipe distance between your low loss header (white rectangular box in photos above) and the outdoor unit is quite short, and is larger 28mm pipe, a lower speed setting on the pump inside the ASHP might better balance. Most of the system restriction is on the secondary (radiator and house side) of the system, there's normally much less pressure loss between the low loss header and the ASHP. ASHPs need more flow than fired boilers, because they have a lower deltaT temperature difference between flow and return. For half the delta T you need to double the flow to transfer the same heat energy in kW.

There's not too much to worry about having too high a flow on the primary side, but best to balance primary and secondary if possible. What happens with the low loss header is that water can pass down through it, short circuiting the secondary flow and return. They are designed to do this for installations where the primary ASHP side flow is constant, such as a fixed speed primary pump, like as used in Grant's units, but the secondary side flow might vary due to on/off zoning controls or zone modulating controls like TRVs or UFH control valves. The ASHP sees a slightly higher return water temperature then, but it just modulates down in response. The opposite, having higher flow on the secondary side than the primary, is more of a problem, as the secondary pump starts recirculating return water up through the header, into the secondary flow pipe, and you see a temperature drop - the flow to the radiators on the secondary side is colder than the flow from the ASHP on the primary side. That's not so good, as all the heat energy generated by the ASHP is then not flowing into the emitters.

Check that weather compensation is enabled and set to something sensible, the Grant default settings are a good starting point.

This post was modified 3 years ago 2 times by AllyFish

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