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Flow temperature up and down like a yo-yo

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(@greekgoddj)
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Hello all, I have started having an issue with my Mitsubishi Ecodan ASHP, whereby every few minutes it gives up trying to run even though the thermostats are asking for heat the entire time. You can see this in the attached 3 hours worth of flow temperature data from unit. You can see the temperature climb and then after a few minutes it gives up, lowers, and tries again. Periodic enough.. there is a pattern there.

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Mitsubishi noted that the flow rate is lower (16l/min) than it needs to be (20-32l/min) (pump is running at max, external unit only behind the adjacent wall, so waiting for a new pump - they already sent the wrong item out once).

However,  I would expect a lower flow rate to affect efficiency of the unit, not necessarily result in the heat pump giving up every few minutes.

Does anybody have any thoughts? Thanks

 

 

Mitsubishi Ecodan 11.2kW ASHP


   
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(@derek-m)
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Posted by: @greekgoddj

Hello all, I have started having an issue with my Mitsubishi Ecodan ASHP, whereby every few minutes it gives up trying to run even though the thermostats are asking for heat the entire time. You can see this in the attached 3 hours worth of flow temperature data from unit. You can see the temperature climb and then after a few minutes it gives up, lowers, and tries again. Periodic enough.. there is a pattern there.

2
1
0

Mitsubishi noted that the flow rate is lower (16l/min) than it needs to be (20-32l/min) (pump is running at max, external unit only behind the adjacent wall, so waiting for a new pump - they already sent the wrong item out once).

However,  I would expect a lower flow rate to affect efficiency of the unit, not necessarily result in the heat pump giving up every few minutes.

Does anybody have any thoughts? Thanks

 

 

It would appear that your heat pump is cycling.

Is your heat set for a fixed Leaving Water Temperature (LWT), in Weather Compensation (WC) mode, or Auto Adaptation (AA) mode?

Do you have any room thermostats?

 


   
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(@greekgoddj)
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Topic starter  

Hi, it is in weather compensation. Water heater is not turned on, so it isn’t the cause of the interruption. I have also checked that it isn’t defrosting that is the interruption. There is a thermostat for each UFH zone/room, and one for upstairs radiators. But again the interruptions to the heat pump are not a matter of a lack of thermostat asking for heat.

So, can the lower than ideal flow rate be causing the heat pump to cycle? 

Mitsubishi Ecodan 11.2kW ASHP


   
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(@derek-m)
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Posted by: @greekgoddj

Hi, it is in weather compensation. Water heater is not turned on, so it isn’t the cause of the interruption. I have also checked that it isn’t defrosting that is the interruption. There is a thermostat for each UFH zone/room, and one for upstairs radiators. But again the interruptions to the heat pump are not a matter of a lack of thermostat asking for heat.

So, can the lower than ideal flow rate be causing the heat pump to cycle? 

What are the settings for the Weather Compensation (WC)? What is the water flowrate? What is the Leaving Water Temperature (LWT) and Return Water Temperature (RWT)? Do you have a buffer tank or Low Loss Header (LLH)?

 

This post was modified 3 months ago by Derek M

   
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(@greekgoddj)
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43C @ 0C to 25C @ 12C.

Flow rate is 16l/min (it should be 20-32l/min, waiting for new pump)

LWT and RWT, are those not the same values showed in the graphs (flow temperature & return temperature)?

I am afraid I don’t know the answer to the last question 🙂 What should I be looking for to answer this one?

 

THanks!

Mitsubishi Ecodan 11.2kW ASHP


   
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(@derek-m)
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Posted by: @greekgoddj

43C @ 0C to 25C @ 12C.

Flow rate is 16l/min (it should be 20-32l/min, waiting for new pump)

LWT and RWT, are those not the same values showed in the graphs (flow temperature & return temperature)?

I am afraid I don’t know the answer to the last question 🙂 What should I be looking for to answer this one?

 

THanks!

Increasing the water flowrate should reduce the DT (the temperature difference between LWT and RWT), but may not stop the cycling. Cycling is normally caused by the thermal energy demand, being lower than the minimum continuous output of the heat pump, which can occur during milder weather conditions. What is the Outside Air Temperature (OAT) at the moment?

Cycling can also be caused if the thermal energy demand is reduced because not all the heat emitters are being utilised. What heat emitters do you have and are they all being used?

If you have room thermostats I would suggest that you set them all at 1C or 2C above the desired temperature.

A further cause of cycling is requesting a low Indoor Air Temperature (IAT), which also causes thermal energy demand to be reduced.

From the point of view of low water flowrate, have you check that the filter is clean?

 


   
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(@sunandair)
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Posted by: @greekgoddj
Does anybody have any thoughts? Thanks  

 

Hi Greekgod

I thought you might have known…. All heat pumps are made by the Greek goddess of Mischief, Delusion and Folly. Have you or your installer upset her? Goddess Ate’ is her name.

As Derek is suggesting there are quite a few possible causes. But the first thing is to ensure your flow rate can reach the upper part of the range quoted by “Mischiefbishi”. Once you can achieve that range you may then be able to identify other causes. 

Im a bit surprised you’re not getting a flow rate error message tbh.

 


   
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(@greekgoddj)
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Topic starter  

@derek-m Outside temperature was 0 🙂 thermostats inside were already 1-2 degrees higher than normal to rule out the system not calling for heat in the first place. That is all of downstairs UFH and all radiators upstairs.

The filter itself didn’t look dirty, but the technician drained some fluid, stuck it in a tube that checks how clean/dirty it is and it suggesting the system may need a clean as well (odd as the entire system/house is 3.5 years old). However the technician still believes we need to change the pump first (it doesn’t “sound right” to him). So I continue to wait for Mitsubishi to send him the pump. Will update this thread then with the new rate and whether it helps the issue.

Mitsubishi Ecodan 11.2kW ASHP


   
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(@greekgoddj)
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Topic starter  

@sunandair Yup, I wait for the new pump.. then see if that helps the flow rate. If not then a clean of the system. By that point the flow rate should be right and that will be the moment of truth on whether the flow rate is the cause of the cycling.

Will be back once I have some news. Thanks to you both 

Mitsubishi Ecodan 11.2kW ASHP


   
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(@glpinxit)
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@greekgoddj yours is like our Ecodan 11.2 looked like a year ago (but less regular). This year I've turned up the two room-stats (upstairs and down) to 2C over the day/evening target on our all-radiator system and turned all the TRVs to max. Now I'm tweaking the w/c settings ever downward. COP has improved from 3.0ish to over 4 most days and the house is a little on the warm side. Here is this morning at 4C OAT- not always quite so flat as there are occasional defrosts:

IMG 1751

   
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(@greekgoddj)
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Topic starter  

@glpinxit yup, that is more of less like what mine looked like last year.. so clearly something has gone wrong at the moment. I am very much looking to get it back into health! 🙂

Mitsubishi Ecodan 11.2kW ASHP


   
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(@harriup)
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Have you tried lowering the WC curve to see if that has an effect?

The pump is stopping because the flow temperature is rising to its desired limit - as the return is only a couple of degrees below the flow this is happening very quickly. If you reduce the target the heat pump won't put as much heat into the exchange unit and so the water will heat up leww quickly, and even if it does continue to cycle it should be less frequent.

With a higher flow rate from a new circulation pump then that flow should be able to take the heat out of exchanger with a lower rise in temp. Your dT is quite narrow, usually slowing the pump speed creates a greater dT, but perhaps returning to a faster speed will get your system back in balance - as currently the pumps through the UFH won't have slowed to allow more heat to be given up and reduce the return temp.

Mitsubishi EcoDan 8.5 kW ASHP - radiators on a single loop
210l Mitsubishi solar tank
Solar thermal
3.94kW of PV


   
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