Posted by: @nigel-w

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Is all that because I should only be running either/or boiler/HP but not both together?

That’s exactly what you should be doing – but it all comes down to how your controller is set up. We have a 16 kW hybrid system too, and honestly, the controller is less than ideal. I’ve had to tweak it quite a bit to get it to do what we want. If I forget to disable certain settings, the system runs both the boiler and the heat pump at the same time, which is counterproductive (and an extra cost). You really want them operating independently –either one or the other.

In our case, when the temperature gets close to 0°C, the boiler takes over because the heat pump can’t deliver enough heat to the index circuit. Without making that switch, we’d end up paying a lot more for electricity while still not getting enough heat, which is neither efficient, comfortable or sustainable. Using the boiler in those cases makes more sense for us

For context, we have a 400sqm home, and the 16 kW heat pump alone just doesn’t cut it during the coldest days of the year. Some might not like this approach, but relying on the boiler for 10-14 days annually – when the heat pump struggles – strikes a better balance. It keeps our overall system costs down while ensuring we maintain comfort during extreme conditions.

That said, it’s essential to figure out how to separate these operations on your controller. Running both simultaneously isn’t correct or efficient. Was your system originally designed as a bivalent setup, or is this something you’ve had to adapt to post-installation?

Many thanks Mars, that's really interesting and helpful.

Our gas powered CH system was subjected to an add-on ASHP installation with a buffer tank which is fed by the HP and(/or) the boiler. The Midea controller makes provision for the boiler as an "Additional Heat Source" AHS but from what I can see of the controls back end, it only assumes the boiler will kick in when ambient falls below the chosen parameter but not that the HP then shuts off. However from a quick look at the manual it may be I can set the HP to switch off below an ambient level so if I set that as the same as the boiler kick-in, that might work.

I'll also ask our installer when I see them next week but although I'm no engineer, I like to be able to tinker myself within reason.

It sounds like you might be in similar position to us - we didn't want to go for a >16kW ASHP at the outset as it would probably have involved 3-phase etc. Maybe in the long term, having a supplementary second HP might be a good thing but of course more expense and complication. Meanwhile I'd like to see how much gas we'd actually burn annually if we get the current hybrid system configured properly.

Thanks again, every day's a school day!

Nigel

Posted by: @nigel-w

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However from a quick look at the manual it may be I can set the HP to switch off below an ambient level so if I set that as the same as the boiler kick-in, that might work.

That’s exactly what we do with ours – I think we’ve set it to 1C. Additionally, if we’re expecting a prolonged cold snap lasting a few days, I’ll lower the heat pump’s maximum flow temperature to 10°C. This ensures it doesn’t kick in unnecessarily (which it sometimes has a tendency to do), as the flow temperature will always remain above 30°C in those conditions. You just need to turn it back up when temperatures get back above 1C.

We’ll be recording another podcast on hybrids in the next couple of weeks, where I plan to address a lot of these points in more detail.

Posted by: @nigel-w

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Meanwhile I'd like to see how much gas we'd actually burn annually if we get the current hybrid system configured properly.

Given that we have similar-sized properties and setups, your boiler shouldn’t need to operate for more than about two weeks in a calendar year, unless we experience an extended period of very cold weather. This approach keeps your gas consumption minimal while maintaining comfortable temperatures and manageable running costs.

Do your gas boiler have weather compensation?  If so, enabling it will improve efficiency.  I assume the boiler is huge, and can operate at high power hence high temperature, the ashp keeping going but at reduced COP due to the high operating temperatures.  I must admit I’m confused why the ashp wouldn’t switch off as likely it’s exceeded its WC set point due to the gas boiler high flow temperature.

In general though, if your aim is carbon reduction (rather than cost) I’d expect leaving the ashp on while the boiler is on is the lowest CO2 emissions technique.  Even an inefficiently running ashp will easily beat a gas boiler in CO2e/kWh(heat).  Direct electric, COP=1, is now lower carbon than gas.

Thanks Robi.

No my gas boiler doesn't have any weather compensation. I've just run it for past 36 hours to test at 55 deg ie the most efficient condensing boiler temp. It seems (as expected) to mess with the heat pump but nevertheless -- somewhat surprisingly -- the HP still delivers about the same average COPs of about 1.8 at these low ambient/high flow temps as it did without the boiler boost last week in similar conditions. 

It's used a chunky 90 kWh of gas per day in this mode but has achieved the aim of keeping the house warm enough during the cold spell and I reckon will only neecessary for maybe 20 days per year which will reduce my carbon saved per year (by my own calcs) from about 7.6 to 7.2 tonnes versus the pre-HP gas-boiler-only set up.

I would however still want to trial of an either/or setup where the HP stops when the gas boiler runs. I need to poke around the controls to see how I can do that.

I look forward to seeing Mars' forthcoming pod on hybrids. 

As the boiler is just on/off at a given running temp, can you drop the temp down to the lowest acceptable, leaving it on most of the day (18hours+)?  The boiler itself will run more efficiently the lower its return temperature is, and the heatpump will be more efficient the lower its flow temperature is.    There’s no magic ‘best temp’ for a boiler - lower is always more efficient.  A gas boiler can even achieve over 100% !  This sounds like nonsense, but the gas calorific content is given assuming a certain flue temp - go below that and you get over 100% efficient🤔

I have a home brew hybrid system (didn't want to scrap a nearly new boiler). Found the best cost wise was is to switch over from ASHP to gas when you are likely to hit defrosting. So I basically don't let the ASHP go into defrost mode, but instead allow the gas boiler to fire up. Heat or gas boiler generally run overnight. So have a 6 hour average temp threshold set where the boiler is allowed to fire up.

Gas boiler running is most efficient if you can get it to run nonstop for at least an hour, (the longer the better) I am heating a slab via UFH, and the boiler runs for 6-8 hours non-stop. It runs about 38 degs, so gets plenty of running in condensation mode. Running any lower flow temperature the boiler cycles and gas consumption goes up by a surprising margin.

Thanks Johnmo that's very interesting. I ended up doing a test over a week in dual mode, now I want to try a run with boiler only but need to figure out how to allow that in the controls. The Midea controller only assumes you want to run boiler as an additional, not an either/or. But I'm sure I can find a way to fool it.

@nigel-w I have a bivalent system setup which comprises a 12Kw Samsung HP and 26Kw (ridiculous I know!) oil boiler.

When the HP was installed the oil boiler was only 4 years old and is very efficient plus with the high price of electricity and market volatility, I decided to keep the boiler and have the HP installed in parallel.

The Samsung controller has a backup boiler feature which when activated disables the HP altogether and emits an electrical signal that can be used to switch a relay. The relay can then divert the thermostat output to the oil boiler heat demand.

Like @editor I have set the threshold temperature to activate the oil boiler at 0C, when it becomes less economic to run on electricity. In my case it is merely cost saving, I actually tested the heat pump when we had -6C here and the house was still warm however the leccy meter was going mad with 4.5Kwh 😀 This is when the oil boiler is much cheaper 🙂