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Ecodan not getting to set flow temperature

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 Gary
(@gary)
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Hi all,

My 8.5KW Ecodan has an issue, it works fine in DHW mode and the flow temps get up to 55C.  When in space heating mode it can only get up to 40C with all of the zones open.

See images from last night, DHW flow temp up at 55C, after finishing flips to space heating at fixed 45C (for test pupose).  Rads upstairs and UFH downstairs all calling for heat.

image
image
image

 2 hours later it still hasn't got to 45C, during this time the heat pump is pulling 3kw of power, that max current on the 8.5kw unit is 22A which is over 5kw so it doesn't seem to be trying hard enough.

This isn't an issue with the current ambient temps but when it gets to 0C then the house needs to a higher flow temp.

When UFH zones turns off when they hit their max temp the flow temp does get upto 45C so is the issue with flow rate or the mass of water its trying to heat?

Pump speed is set to 5 (23L/min)

Any thoughts appreciated.

 


   
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(@derek-m)
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Posted by: @gary

Hi all,

My 8.5KW Ecodan has an issue, it works fine in DHW mode and the flow temps get up to 55C.  When in space heating mode it can only get up to 40C with all of the zones open.

See images from last night, DHW flow temp up at 55C, after finishing flips to space heating at fixed 45C (for test pupose).  Rads upstairs and UFH downstairs all calling for heat.

image
image
image

 2 hours later it still hasn't got to 45C, during this time the heat pump is pulling 3kw of power, that max current on the 8.5kw unit is 22A which is over 5kw so it doesn't seem to be trying hard enough.

This isn't an issue with the current ambient temps but when it gets to 0C then the house needs to a higher flow temp.

When UFH zones turns off when they hit their max temp the flow temp does get upto 45C so is the issue with flow rate or the mass of water its trying to heat?

Pump speed is set to 5 (23L/min)

Any thoughts appreciated.

 

I would suggest that you check the setting of the upper limit for the LWT in the controller.

Do you have any of the following in your system? Buffer Tank, Low Loss Header or Plate Heat Exchanger.

What is the heat loss calculation for your home?

 

This post was modified 7 months ago by Derek M

   
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 Gary
(@gary)
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@derek-m Thanks Derek, the max LWT is set to 50C, there is no buffer tank, but as it was fitted with a preplumbed Ecodan tank I think they all come with LLH and it has an external plate heat exchanger next to the HWT.


   
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(@derek-m)
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Posted by: @gary

@derek-m Thanks Derek, the max LWT is set to 50C, there is no buffer tank, but as it was fitted with a preplumbed Ecodan tank I think they all come with LLH and it has an external plate heat exchanger next to the HWT.

Looking at the data tables for a 8.5kW Ecodan, your heat pump was probably running at full output and developing something in the order of 10kW of thermal energy.

Unlike a gas or oil boiler, that could probably supply 30kW or more of thermal energy at maximum output, a heat pump is much more limited.

Heat pumps therefore respond much slower, and can take lengthy periods of time to raise the Indoor Air Temperature (IAT). The rate at which IAT increases is also dependent upon the thermal mass of your home, along with the heating demand (heat loss) at varying Outside Air Temperature (OAT).

A further factor is the size and capacity of your heat emitters in relation to the calculated heat loss for your home.

So in relation to your charts, the LWT starts to increase more rapidly when its value is lower, but as more of the thermal energy produced is absorbed by the heat emitters as the LWT increase, then the rate of increase in LWT becomes slower. It is like accelerating in a car, initially the car accelerates quite quickly, but as the speed builds the rate of acceleration reduces.

Because you have UFH, if the slab is large it will take time to heat up.

Setting a desired LWT of 50C does not mean that the actual LWT will immediately jump to 50C. Heat pumps are often set to operate at a Delta T (difference between LWT and RWT) of 5C, so if at the start the RWT is 25C, then the LWT will be limited to approximately 30C. As the heat emitters and building start to warm, the RWT will start to rise which in turn will allow the LWT to increase.

As you have stated, as the heat demand reduced, the LWT increased to the requested 45C.

What is the heat loss calculation for your home?

 


   
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 Gary
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@derek-m Thanks - heat loss calc was 7.2KW.  The UFH is retrofit so has a relatively quick heat up time as its thin.

I understand what you are saying but I'm sure the HP can pull more than 3KW but its not trying to.  I have seen the house pull 4.5KW overnight when only the heat pump is on and coincidentally at that time the flow temp got to 45C, for whatever reason after 2 hours it reverted to 3kw and the flow temp dropped back to 40C.


   
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(@derek-m)
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Posted by: @gary

@derek-m Thanks - heat loss calc was 7.2KW.  The UFH is retrofit so has a relatively quick heat up time as its thin.

I understand what you are saying but I'm sure the HP can pull more than 3KW but its not trying to.  I have seen the house pull 4.5KW overnight when only the heat pump is on and coincidentally at that time the flow temp got to 45C, for whatever reason after 2 hours it reverted to 3kw and the flow temp dropped back to 40C.

Based upon the data table for an 8.5kW Ecodan, running at maximum output at a LWT of 45C, it should require approximately 2.7kW of electrical energy. Your objective should be to minimise your electrical energy consumption, not increase it.

I would suggest that you run your heat pump in WC mode with settings that maintain the desired IAT.

 


   
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 Gary
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@derek-m Sorry I may have confused, I'm not trying to run it efficiently, this is a test to determine whether there is a fault with the unit or if there is some setting I need to change to get the flow temp up.

In normal running I run it in WC and it works fine, but when its -2C outside WC will be at 45C and if it can't get there when its 8C outside I dont see it doing that when its -2C.


   
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(@fazel)
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Posted by: @gary

but when its -2C outside WC will be at 45C and if it can't get there when its 8C outside I dont see it doing that when its -2C.

I'm no joking, put some ice(make an (big enough)ice cube with a string in it) on the air temperature sensor at the back of the unit, and watch what it does(HP).

The compressor speed is set depending on multiple factors, one is the outdoor air temperature.

 


   
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 Gary
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@fazel Thanks I was starting to wonder if this might be a factor, maybe I will just wait till it gets colder 🙂


   
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(@harriup)
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Looking at your flow temps, when the space heating starts the flow and return rise quickly in parallel, there is then a slowing of the increase but more so in the return temp. As the dT has increased to what the hp wants to work with it starts to wait until the return temp has risen before increasing the flow temp. As these rise the wait increases until it is around 15 minutes between each alternate step. So it is the slowing extraction of heat that is preventing the hp raising the flow temp more quickly. Derek-M has given you the theory, I think this is what it looks like in graph form!

If you are supplying this water to both a radiator and UFH circuit I would suggest that while the radiators can take heat out the UFH is at its limit and the return water is barely losing any. You may just have to open doors and windows to cool the floor to simulate the heat loss of 0°C in order to get the flow temp up.

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