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Bivalent system questions - ASHP with oil boiler

Air Source Heat Pumps (ASHPs)

Last Post by Mars 4 years ago

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Mars

(@editor)

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Joined: 5 years ago

Posts: 3412

Topic starter 23/11/2021 10:23 am

As some of you may know, we have a bivalent system – our air source heat pump "replaced" our oil boiler, but we kept the oil boiler a a backup, and I'm glad we did.

We have now configured the system to work in the following manner: when temperatures drop below 7C outside, the ASHP hands over the heating duties to the oil boiler, as this is the point where our ASHP efficiencies start to drop, and with electricity tariffs of 24p/kWh we just can't afford to run the heat pump 24/7.

So the question is about the oil boiler. Because our flow rate is set to 40C, the boiler comes on frequently, for short bursts. As an example, it'll come on for 10-15 seconds at a time, and it can come on 2-3 times a minute. Is this a "healthy" way of operating the boiler?

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Derek M

(@derek-m)

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23/11/2021 1:09 pm

Posted by: @editor

As some of you may know, we have a bivalent system – our air source heat pump "replaced" our oil boiler, but we kept the oil boiler a a backup, and I'm glad we did.

We have now configured the system to work in the following manner: when temperatures drop below 7C outside, the ASHP hands over the heating duties to the oil boiler, as this is the point where our ASHP efficiencies start to drop, and with electricity tariffs of 24p/kWh we just can't afford to run the heat pump 24/7.

So the question is about the oil boiler. Because our flow rate is set to 40C, the boiler comes on frequently, for short bursts. As an example, it'll come on for 10-15 seconds at a time, and it can come on 2-3 times a minute. Is this a "healthy" way of operating the boiler?

Hi Mars,

I'm not an expert on oil boilers, but what I think is happening is similar to our gas boiler when it operates for a longer period of time. When the boiler initially fires up, it starts heating the water being pumped through, but because it can supply much more heat energy than is required, the water soon gets up to temperature. Because there is still a heat demand from your home, the water continues to be pumped around the system, so cooler water is still flowing into the boiler, and will cool the sensor measuring the water flow temperature and hence fire up the burner again.

I think that you will find that this is normal, particularly if you have the boiler controls set for a low water flow temperature, but you could ask the manufacturer if this is acceptable. It will probably become less pronounced when the outside air temperature is lower and the boiler has to work harder.

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MattEngineer

(@mattengineer)

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Joined: 4 years ago

Posts: 50

23/11/2021 3:06 pm

Hi Mars,

what you’re referring to is ‘short cycling’ and less desirable on a fossil fuel boiler. Also there is no efficiency gain for running the boiler at a lower flow temperature. As long as the return is below 55deg (assuming it is a condensing boiler) you benefit from the energy recovery of the exhaust.

What is the make and model? Check your manual for the modulation ratio or the minimum heat output. I presume you have the specs of your radiators, at a given temperature delta it will give you the output, add this together for the zone you have active.

or the easier way:

Set flow rate to max 55deg and use your thermostat to provide on / off control.

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Mars

(@editor)

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Joined: 5 years ago

Posts: 3412

Topic starter 23/11/2021 5:45 pm

Thanks guys.

Fortuitously, I had an oil boiler technician visit us today (that's a different story - watch this space) and he said that the short cycling is a no-no and won't do the oil boiler any good in the long run. Just got off the phone with Global Energy Systems and they're going to look into how they manipulate the settings to stop the short cycling.

I'd like to caveat this post by saying we were sold a bivalent system that would be "easy" to switch from ASHP to oil boiler and that the oil boiler would act as a standalone back up. That is certainly not the case – it involves tweaking settings to trick the heat pump into allowing the oil boiler to do the work, and right now the only way it can do that is to come for 4-5 seconds at a time, 3-4 times a minute. Not efficient. No helpful. Not ideal!

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