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Volumiser required on Grant R290 air source heat pump?

Patrick · 2026-01-18T19:20:18Z

Hi all. After years of planning and researching, our renovation is finally happening, as part of which I aim to have an ASHP installed.We're in Ireland, and I have come to the conclusion that while I'm sure there are knowledgeable engineers and installers out there, it's too much of a minefield to try and find them. So my plan is to design the system myself as much as I can, before handing the spec over to a competent plumber to execute. (side note: if anyone is able to help me or recommend how to sanity check my heat loss and emitter sizing, please let me know)I have settled on the Grant R290, as there's now some good results from various installs, and it's readily available in the Irish market. I contacted Grant with my plans, who recommended a 6.5kW unit (matches my own numbers).What has me a little confused is their recommendation of a 30L volumiser. We'll be running UFH from JK throughout the ~61m2 ground floor. Some quick ChatGPT assisted maths says this will be at least 33L in the UFH loops, before the manifold, other pipework, and 4 rads upstairs are taken into account. So easily above the minimum 32.5L system volume from the datasheet.So are Grant just being cautious here? Would it be a bad idea to omit the volumiser? Conversely, are there any downsides to including it?I should add, I thought Grant would be doing a much more detailed design, but it seems like it was more of a cursory glance at my plans and numbers, followed by a quick generic spec of parts.I'm committed now to the HP journey, but a lot of small doubts and uncertainties are still keeping me up at night 🤪

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Air Source Heat Pumps (ASHPs)

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Max Abbey

(@max-abbey)

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31/01/2026 12:09 pm

@patrick That is great news, must chase that up, Thanks

👍

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Judith

(@judith)

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Posts: 489

01/02/2026 5:31 pm

Posted by: @patrick

↑

Great news this week too. The heat pump grant here has increased from €6500 to €12500.

Thats generous! Does the small print of the grant allow DIY? In UK it has to be a MCS registered professional( doesn’t guarantee competence) and replacing an existing fossil fuel boiler.

2kW + Growatt & 4kW +Sunnyboy PV on south-facing roof Solar thermal. 9.5kWh Givenergy battery with AC3. MVHR. Vaillant 7kW ASHP (very pleased with SCOP 4.7) open system operating on WC

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Max Abbey

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01/02/2026 7:05 pm

If it follows other grants here, then, as before, you have to use a registered contractor. And like in the UK there is no guarantee as regards quality etc
It puts me off using grant assistance in most cases but at that level of support I will look more seriously into it.

There is one other really good scheme here for vacant homes. If the house is over 2 years not lived in then there is a max of €50k for bringing it back into habitable use. This grant does allow for DIY (with local authority oversight) so that sum can be pretty much parts only support.

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GrahamF

(@grahamf)

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01/02/2026 7:39 pm

@jamespa I agree that it is important to avoid a 4 port buffer or low loss header, due to the impact on efficiency.

My understanding is that volumisers should be installed on the return circuit, not the flow?

Grant Aerona 290 15.5kW, Grant Smart Controller, 2 x 200l cylinders, hot water plate heat exchanger, Single zone open loop system with TRVs for bedrooms & one sunny living room, Weather compensation with set back by room thermostat based load compensation

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GrahamF

(@grahamf)

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01/02/2026 7:42 pm

I have a Grant Aerona R290 15.5kW heat pump, which is running very nicely. Certainly don't go for the Aerona3 R32 heat pump or its antiquated controller.

I would be happy to help you with how to use the Aerona R290 Smart Controller.

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JamesPa

(@jamespa)

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01/02/2026 7:44 pm

Posted by: @grahamf

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My understanding is that volumisers should be installed on the return circuit, not the flow?

There is a thread from a year ago where this was debated at length and the conclusion, well argued imho, was flow is better but it's likely only to make a difference in a relatively small minority if cases, and the difference will be a marginal comfort one.

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

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Max Abbey

(@max-abbey)

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01/02/2026 7:59 pm

@grahamf Indeed looking at R290 models only. It will be a good while until I actually get to interpret the controls but thank you in advance

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Patrick

(@patrick)

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Topic starter 12/02/2026 8:32 am

@judith Indeed it's similar to the UK. It has to be an SEAI registered installer(no guarantee of competency) and the house must reach a minimum level of energy efficiency after the works are completed.

I spoke to our plumber who said he can't even install a heat pump on his own house as that won't qualify for the grant. So he needs to get another registered installer to do it.

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