Welcome to the forums @juddmay.

We have a bivalent system running an ASHP alongside an oil boiler. We never really used the oil in any significant quantities - it was a back up and did the odd defrost. This year, we are trialing a new biofuel in the oil boiler and so we’ve been using our bivalent system.

The major takeaway is that many ASHPs don’t have sufficient controls to cooperate with an oil boiler. Even with our system we’ve had to tweak settings to trick the ASHP into relinquishing control. We addressed these points in a video we posted recently which may help you, and have also spoken about efficiency. Even though the video is about the fuel, we speak at length about our system.

In closing, hybrid/bivalent systems are definitely possible - it’s about finding the right heat pump (from a settings and controls perspective) that will allow them to work seamlessly, and for the system to be designed accordingly.

I’ll ask installers which ASHP brands they recommend for hybrid setups.

The  largest ever trial of heat pumps in the UK which is currently underway has installed a lot of hybrid heat pumps.

Not yet published much in the way of findings which will come later. 

https://es.catapult.org.uk/news/electrification-of-heat-trial-finds-heat-pumps-suitable-for-all-housing-types/

I expect many are mains gas/heat pump setups but some of the learnings would be relevant. 

@editor 

Many thanks for your very prompt reply and I looked at your Home Farm site. A few things pop out

Firstly, did you insulate your property given its age, or change rad sizes given you ran first year on just ashp (sorry, couldn’t see this detail)?

Secondly, there was an extensive Dept of Business report in 2017

which highlighted that under many conditions a HHP system could be the most practical solution for properties like mine, though the main driver of the report was to consider electricity usage and emissions....but a lot of its conclusions were ahead of available technology at the time...which is why I’m chasing experiences now. There are off the shelf systems ready to go (Grant for example) but again detailed performances references are scant.

Lastly, I wondered what SCOP you’re getting from your HP in single or bivalent mode given the report I mention summarised “If the heat pump contributes to meeting DHW demand, this would be likely to reduce the overall efficiency of the heat pump, as well as increasing the average and peak electricity demand. If the DHW demand is spread throughout the day, for a continuous heating schedule the DHW and space heating demand will coincide, resulting in an increased load and an increased flow temperature compared to that required to meet the space heating demand. This will reduce the COP of the heat pump. Also, the temperature for DHW is at least 40 degrees Celsius7, which is above the optimal flow temperature of a typical air- source heat pump, meaning that even if there is no space heating demand, the heat pump will be operating to meet the DHW demand under non-optimal conditions.
Due to these factors, several manufacturers selling HHPs shared the view that in a hybrid configuration, using the boiler to provide DHW is the most practical option, and that using the heat pump for this would not be recommended.”

We’re all on a journey to learn.....By the time I get there I’ll be in my dotage. Cheers. Judd 

@juddmay, I’ll reply to your post above later this afternoon.

In the time being, I received a message about the best heat pump options for hybrid systems from Graham Hendra: “RED is easiest, Hitachi is the most programmable, it can do literally anything. Samsung is horrible. Mitsubishi is worse. Midea is OK.”

The guys at Daikin sent us some manuals explaining the controls on Daikin systems for hybrid systems. Look from page 72 onwards, and it'll hopefully provide you with a few more answers.

For Grant the heatpumps are made by Chofu who provide a much more detailed manual.  See section 8.3, page 111 onwards:

GITHUB.COM "https://github.com/aerona-chofu-ashp/docs/blob/main/Chofu/R32/New-Zealand/Chofu-Operating-Installation-Manual-R32.pdf"

In particular, 8.3.2 describes 'Backup heater in Supplementary mode' where it'll run both at the same time, using the oil burner to boost up the temperature when the ASHP can't do it all.

Note that the heatpump has its own room temperature compensation/control system which Grant completely ignore by using classical on/off thermostats as if it's a traditional boiler. It is possible this supplementary mode doesn't work if you use them as Grant indicate.

I don't have a hybrid setup so no direct experience.  In general, when people talk about hybrid setups are they running both at the same time, or just turning off the heat pump and firing up the boiler?

Oops, missed the edit window. It's section 8.4 on page 120 (External Heat Source) not 8.3 (electric backup heater) - it appears you can have both and they work in the same way, just the settings for each can be different.

@prunus Many thanks for that

the schematic on page 118, suggests that the EHS has no direct impact on DHW, and access to it is via a buffer tank. That flies in the face of the govt report which highlights DHW is best done by traditional heating source (oil in my case), but that doesn’t look like an out the box solution from Grant. Will speak to them Monday

@editor Many thanks for this. I’m finding not quite the flexibility of installation I would like. Worcester Bosch offer hybrid solution but you’ve got to add an additional interior unit to achieve it. Theoretically I’m well placed in that my oil boiler required hot Water tank....rather than condensing route...but practically just adding ashp and software controller is not so easy. By using the govt recommended route of DHW from oil boiler means I could keep my existing HW tank, but the setup for most of the systems look like adding either a sizeable piece of kit I don’t want, or moving away from the out the box install that it comes with. I’m preferring the Grant option in that everything can move outside and into one box if required, but that means overriding the default setup options...so I’m exploring that today with a Grant installer. Keep you posted.thanks again.

@editor hi Mars. How much is HVO per litre?

@julianc, last year it was £1/per litre, but as with all things it's gone up to around £1.30/litre. As far as I know, it's not really available everywhere at the moment though. The reason it's so bloody expensive is because it's loaded with levies, duties and taxes – if they were stripped away, it would be same price as kerosene, about 51p/litre (that's our rate for this week).