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Is a Valliant Inline 6kW heater a BUS buster?

GuydeB · 2025-07-15T16:27:03Z

Like many newbies desperately trying to ASHP an old house from oil fired, I have been down many rabbit holes - in brief, three phase not possible (£44K quote from National Grid), 18kW heat loss (Good Energy onsite survey), new ASHP-ready UFH in kitchen extension.One recent suggestion was the (very expensive) Grant VortexAir 15/26kW Oil Boiler and 17kW Heat Pump Hybrid - definitely NOT BUS compliant, but at least a start.However I might have cracked it - or have I?I am considering a Vaillant aroTHERM plus 12kW combined with a Valliant Inline 6kW back-up heater. Looking initially for 45-55 degs flow temp to start (separating the UFH) while we change our rads over the next 12-18 months when finances allow. Plus keeping the existing Joule 300L tank (which we COULD use with just an immersion if ever required).Is this BUS compliant?? We are replacing our old Grant Oil burner - we're rural and not on the gas network (yes we have wood burners but let's not go there!!).Good Energy told me in their opinion this was NOT BUS compliant, my installer said it was but was told by the authorities that until he sends in the documentation they can't tell him (!), Chat GPT is convinced it is as well (!!), but surely I am replacing the fossil fuel with electricity throughout?I'd welcome any thoughts....many thanks this is the inline heater:

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Air Source Heat Pumps (ASHPs)

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JamesPa

(@jamespa)

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Joined: 2 years ago

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12/09/2025 11:20 am

Incidentally this is the sort of thig you can do if you can measure your consumption, in this case from half hourly gas meter readings. Its how I sized my heat pump. Two professional surveys (3 hrs each, one costing £300 the other free but still worthless) said 16kW, actual is 7kW.

Hence why I am always very cautious about surveyed heat loss most particularly in older buildings that have had miscellaneous fabric upgrades and other difficult to determine factors.

Measured consumption - Copy.pdf

PS - Back to cascades - I think Mitsubishi are one of the go to manufacturers. If you got the 11.2kW and 8kW R290 - the latter of which has 2 compressors 2kW fixed and 6kW variable - you could go all the way from 19.2kW to 2kW without cycling, given the right control. Now that would be a smart way to run a cascade heat pump and its more or less guaranteed to cover your heat loss both at top and bottom ends.

This post was modified 2 hours ago 6 times by JamesPa

This post was modified 1 hour ago by JamesPa

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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GuydeB

(@guydeb)

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Joined: 4 months ago

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Topic starter 12/09/2025 1:20 pm

So this is from Grant - can't fault them for speed of response to an email about 2 x 10kW:

When using two R32 Aerona³ heat pumps connected in a cascade configuration, one of the two (the supplementary unit) will be controlled by the other (the lead unit).

Under this hydraulic arrangement the lead unit can be used to provide both Heating and DHW, but the supplementary unit will be used only for Heating.

Unfortunately, as per the above, it is not possible to supply two separate flow temps simultaneously. Normal operation would be to target the higher temperature circuit ( Rads ) and use a blending/mixing set to control the UFH circuit.

*Please be aware, Grant UK R290 heat pumps are not currently suitable for cascade configuration.

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