Renewable Heating Hub Heat Pump Forums

Search with Wattson
Forums
Renewable Heating
Air Source Heat Pum...
Bivalent system que...
 
Notifications
Clear all

Joining the Renewable Heating Hub forums is completely free and only takes a minute. By registering you’ll be able to ask questions, join discussions, follow topics you’re interested in, bookmark useful threads and receive notifications when someone replies. Non-registered members also do not have access to our AI features. When choosing your username, please note that it cannot be changed later, so we recommend avoiding brand or product names. Before registering, please take a moment to read the Forum Rules & Terms of Use so we can keep the community helpful, respectful and informative for everyone. Thanks for joining!

Bivalent system questions - ASHP with oil boiler

 
Air Source Heat Pumps (ASHPs)
Last Post by Mars 5 years ago
4 Posts
3 Users
4 Reactions
3,351 Views
RSS
Mars
 Mars
(@editor)
Illustrious Member Admin
Joined: 5 years ago
Posts: 4762
Topic starter 23/11/2021 10:23 am   [#176]

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?


New to heat pumps? Take the Heat Pump Fundamentals course and use INTRO50 at checkout for 50% off.
Rewritten and updated for 2027
📘 From Zero to Heat Pump Hero
🛠️ Bodge Buster
🎧 Prefer to listen? Heat Pumps Explained


   
Quote
 Derek M
(@derek-m)
Illustrious Member Member
Joined: 5 years ago
Posts: 4425
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.



   
👍
2
ReplyQuote
 MattEngineer
(@mattengineer)
Trusted Member Member
Joined: 5 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. 



   
👍
2
ReplyQuote
Mars
 Mars
(@editor)
Illustrious Member Admin
Joined: 5 years ago
Posts: 4762
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!


New to heat pumps? Take the Heat Pump Fundamentals course and use INTRO50 at checkout for 50% off.
Rewritten and updated for 2027
📘 From Zero to Heat Pump Hero
🛠️ Bodge Buster
🎧 Prefer to listen? Heat Pumps Explained


   
ReplyQuote
Forum Jump:
  Previous Topic
Next Topic  
Share:

SPONSORS

Join Us!

Installer Finder

Degrees of Separation

Latest Posts

SPONSORS

Members Online

Click to access the login or register cheese
Protected By
Shield Security PRO