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Flow rate issues?

 
Air Source Heat Pumps (ASHPs)
Last Post by Peterr 2 years ago
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 Peterr
(@peterr)
Estimable Member Member
Joined: 4 years ago
Posts: 67
Topic starter 14/12/2022 3:48 pm  

When our 11.2KW Ecodan was serviced back in October the engineer remarked that the flow rate was a little on the low side, approx 20 litres/min.  He didn't think it would cause any major issues, but just advised us to be aware of it.

Roll forward to the current cold snap, and we've a couple of issues, and I'm just wondering if they could be attributable to the low flow rate.

1. When running a water cycle (set to 45C) the flow temp rises to about 47C and stays there until the tank starts to get close to the target temperature and then seems to drop to about 45C.  Sometimes the flow temp might creep up again to 46C, but generally it just sits at 45C still running the water heating cycle, presumably because the tank hasn't reached a high enough temperature for the controller to be satisfied.  I now work around this by using the iBoost to give a quick 15 minute burst on the immersion heater when the tank temperature reaches about 40C.  Probably ought to mention that we don't have an immersion connected to the controller.  If I don't use the iBoost the water cycle will continue until it has reached the 60 minute heating time set by the controller, then go back to space heating for 30 minutes before starting the water cycle again.  If left to its own devices it will just continue in this loop.

2. We have radiators upstairs and underfloor heating on the ground floor.  Upstairs temperature is fine - sits at a fairly steady 19-20C, but ground floor rooms are a bit too low for our liking (17.5 - 18C).  We are running weather compensation, set to 42C at -3C, 20C at 18C.  With the outside temperature below 0C I am seeing a water temperature of about 30C at the UFH manifold, mixer valve is set to maximum (65C).  UFH has its own circulating pump.

The main pump and the UFH pump are both set to their maximum speed.

Any thoughts before I call an engineer out to charge me £££££'s for diagnosing the problem?

Thanks


   
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 AllyFish
(@allyfish)
Noble Member Contributor
Joined: 3 years ago
Posts: 487
14/12/2022 5:02 pm  

For a typical 5degK delta T between flow and return, to provide 11kW you ideally need 11/(5x4.2) = 0.524l/s or 31l/min. Suggest the flow needs to increase or check and adjust the deltaT setting on the Ecodan if possible. Generally, the lower you set the weather compensated maximum supply temperature, the higher the flow needs to be because the deltaT between flow and return decreases. That's just how the physics of heat loss works.

What flow temperature have you the hot water boost set to? Medium temperature ASHPs may struggle to reach 50degC flow temperature or higher at the moment due to the very cold weather and also periodically defrosting. The flow temperature goes up like a saw tooth blade. It keeps dropping back during each defrost cycle.


   
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 Peterr
(@peterr)
Estimable Member Member
Joined: 4 years ago
Posts: 67
Topic starter 14/12/2022 5:44 pm  

Posted by: @allyfish
↑

For a typical 5degK delta T between flow and return, to provide 11kW you ideally need 11/(5x4.2) = 0.524l/s or 31l/min. Suggest the flow needs to increase or check and adjust the deltaT setting on the Ecodan if possible. Generally, the lower you set the weather compensated maximum supply temperature, the higher the flow needs to be because the deltaT between flow and return decreases. That's just how the physics of heat loss works.

What flow temperature have you the hot water boost set to? Medium temperature ASHPs may struggle to reach 50degC flow temperature or higher at the moment due to the very cold weather and also periodically defrosting. The flow temperature goes up like a saw tooth blade. It keeps dropping back during each defrost cycle.

I don't think the deltaT can be adjusted on the Ecodan - it's not something I can see in the controller manual.

The hot water flow temperature is, I assume, set by the controller on the basis of the desired tank temperature.  Again, I can't see anything about setting it manually.  It seems to reach about 2C above the desired temperature and then drop back down as the tank comes up.

We're currently defrosting about every 60 to 80 minutes, and it seems to take about 15 minutes from the start of a defrost before the flow temperature returns to what it was. 

 


   
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