Going bivalent: ASHP + HVO

by Mars

This is the third winter that we’re heating our home with an air source heat pump and the experiment continues. As regular followers of our blog and YouTube channel will know, this winter we’ve been trialling a new biofuel, HVO, alongside our air source heat pump.

We discuss some of the issues running our air source heat pump in conjunction with our boiler as part of a bivalent system, and we share our honest advice with homeowners that are considering making the transition to a heat pump from an oil boiler.

During the video we mentioned Renewable Heating Hub and the forums and we’d like to invite you to participate, take part and join the conversation.

We discussed the carbon emissions between our HVO boiler and our air source heat pump, and the data in this post that we published in January might be of interest to you.

We also promised to a share a link to the app we use to monitor electricity generated CO2 emissions. It’s called When To Plugin In, and you can download it here.

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Jennifer Ross
Jennifer Ross
1 year ago

Great blog, thank you. Here is my response received today from a query I left with them a couple of days ago. Please note their ‘new scheme for heat pumps’. The increase in our tariff is not 50% but 250%.

Thanks for getting in touch and apologies for the delayed response. Here is some information on our new scheme for heat pumps,


These will be the rates if you choose to stay on Octopus go 

Unit rate (04:30 – 00:30):
30.83p/ kWh
Unit rate (00:30 – 04:30):
7.50p/ kWh
Standing Charge:

We also have the Octopus agile tariff which may be the best for you, you get access to half-hourly energy prices, tied to wholesale prices and updated daily. So when wholesale electricity prices drop, so do your bills – and if you can shift your daily electricity use outside of peak times, you can save even more.
Here is a link with more information, you can also sign up through the link 

Jennifer Ross
Jennifer Ross
1 year ago

Sorry guys – not very well written post from yours truly. ‘Them’ refers to Octopus Energy. I just copied and pasted their response from an email I received today. It doesn’t take much to work out what our annual usage of 10000 kWh will convert to in £ sterling as we only use the heat pump to heat water from midnight to 4pm.

1 year ago

Thanks very much to both of you for this interesting round-up. I’m keen to learn more about the control possibilities with bivalent systems and about HVO – is the trial still running do you know? Both our oil boiler and ASHP are Grant.

Having kept the boiler for the same reasons and made the same assumptions about our control of it, my suspicions were raised when the installer suggested that the boiler should be set to take over when the outside temperature was something like 2 degrees C. Given our hilly location, that seemed unacceptable, so it was set at 0 degrees and we decided that we’d see how things went this winter, having spent a fortune on oil the winter before. We have recently had solar panels and a battery installed.

I haven’t got to grips with the data side of it all yet, just watching the smart meter, solar generation and battery use is fairly preoccupying for the moment and there are so many questions and variables to consider, but frustration is the most commonly experienced feeling here:-
The oil boiler can be running when the solar panels could have been powering the heat pump. Similarly, later in the day, when the battery has charged up again from the solar panels. Also, the boiler seems to run for longer than expected after the temperature has risen again. If this is the case, is it a necessary function of a bivalent system, or another control complication?

Sometimes the radiators generally just don’t seem to get as warm as usual – not sure whether this is related to when the ASHP has taken over again after the boiler has been on, and/or whether it is just colder outside. One or two radiators never seem to get warm.
On a related point I thought it was supposed to be most efficient to leave the ASHP on all the time. Why do some people refer to turning it on and off to save money? (I think Octopus refer to this on one of their heat pump videos?) just wondering what the considerations were and how to balance these out, with costs being the determining factor for us. Our insulation levels are improving, but far from modern day standards.

Getting the RHI application accepted took a long time with meter issues and we’ve finally submitted our first meter readings. Unfortunately Ofgem says these readings are not at expected levels and are being reviewed.

So no, not a smooth journey, but having said that, our house is at a much more comfortable temperature than last year………

Barrie Harbott
Barrie Harbott
1 year ago

I had a new gas boiler fitted last year. How can i get help and information about adding an ASHP so that I have a “BIVALENT’ system?
My detached house has 4-5 bedrooms and was built early 1970’s. Good double glazing, cavity wall and loft insulation. We already have Photovoltaic roof panels.
What are the subsidies and likely tariffs, as well as an approximate cost?
Huge thanks,
Barrie Harbott

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