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How to set the warm end of the weather compensation curve when it's not warm

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(@dockray)
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Hi, I am very new to all this!

How do I set the warm end of the weather compensation curve when it’s not warm outside?

We’ve got a Mitsubishi Ecodan 8.5kW with UFH (plus 2 radiators on the mezzanine). We live in a newly converted industrial building that is well insulated, it has big windows so we have good solar gain when it’s sunny but we live 400m above sea level on an exposed fell in The Lakes so we have a lot of wind wash.

After commissioning, WC was left set at factory settings. I’ve watched the Heat Geek videos and read lots of threads on here and got the cold end of the curve set when it was minus two outside. We are now down to a flow temperature of 28* at -2 to give us an indoor temperature of 19.5. 

This month the heating COP is 3.4 and the DHW is 2.2. 

Now the weather is beginning to improve I am getting some cycling (but at other times the LWT & RWT appear to be nearly the same) so I’m guessing I need to alter the warm end of the curve or possibly the pump speed?? I've currently got the warm end of the WC curve at 20 degrees with an outdoor temperature of 10 degrees (because that is the highest the outside temperature has got so far) but it’s still cycling when the outdoor temperature gets above 7 or 8* (and I’ve got the room stats capping at 22* whilst I am trying to find the sweet spot, which is higher than I need). 

Our flow temperature seems to be lower than most people’s and I don't want to find I’ve got a low flow temp but something else in the system is working to keep the house warm - if this is even possible? I can’t compare energy usage as we now live in a different part of the country, in a different house, with an ASHP and UFH rather than gas CH and radiators.

I’ve got SO many more questions but I’ll leave it at this for now.

Until a few weeks ago I knew nothing about ASHPs so please go gently and simply with your replies. Thank you in advance.


   
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(@derek-m)
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Posted by: @dockray

Hi, I am very new to all this!

How do I set the warm end of the weather compensation curve when it’s not warm outside?

We’ve got a Mitsubishi Ecodan 8.5kW with UFH (plus 2 radiators on the mezzanine). We live in a newly converted industrial building that is well insulated, it has big windows so we have good solar gain when it’s sunny but we live 400m above sea level on an exposed fell in The Lakes so we have a lot of wind wash.

After commissioning, WC was left set at factory settings. I’ve watched the Heat Geek videos and read lots of threads on here and got the cold end of the curve set when it was minus two outside. We are now down to a flow temperature of 28* at -2 to give us an indoor temperature of 19.5. 

This month the heating COP is 3.4 and the DHW is 2.2. 

Now the weather is beginning to improve I am getting some cycling (but at other times the LWT & RWT appear to be nearly the same) so I’m guessing I need to alter the warm end of the curve or possibly the pump speed?? I've currently got the warm end of the WC curve at 20 degrees with an outdoor temperature of 10 degrees (because that is the highest the outside temperature has got so far) but it’s still cycling when the outdoor temperature gets above 7 or 8* (and I’ve got the room stats capping at 22* whilst I am trying to find the sweet spot, which is higher than I need). 

Our flow temperature seems to be lower than most people’s and I don't want to find I’ve got a low flow temp but something else in the system is working to keep the house warm - if this is even possible? I can’t compare energy usage as we now live in a different part of the country, in a different house, with an ASHP and UFH rather than gas CH and radiators.

I’ve got SO many more questions but I’ll leave it at this for now.

Until a few weeks ago I knew nothing about ASHPs so please go gently and simply with your replies. Thank you in advance.

Welcome to the forum.

Your problem is not unique, and will no doubt be suffered by virtually all ASHP owners during milder weather conditions. My original advice when initially setting the WC curve, was to set the warm end at a LWT of 25C, at an outside temperature of 20C, then adjust the cold end to achieve the desired indoor temperatures. I believe this approach would work fine when the weather is cold and the heat pump is required to run almost constantly, but may not be ideal in milder weather conditions. Much depends upon your particular home and system, as to which is the most appropriate operating technique to use.

As more detailed information has become available, I have refined my thoughts of how best to operate a heat pump in milder weather conditions. Just like gas or oil boilers, a heat pump has a minimum heat energy output, below which it cannot go. So in milder weather conditions, there will come a point when the heat pump is supplying more heat energy than is required to meet the heat loss of the home, with the heat loss itself reducing due to the milder weather.

To prevent the indoor temperature from rising too high, the heat pump must therefore stop operating for a period of time, so that the average heat energy supplied is sufficient to meet the heat loss. Frequent switching on and off of a heat pump is described as short cycling, which is not a very efficient and effective way of operating.

I have been giving some thought as to the possible ways in which the frequency of cycling can be reduced to the lowest level.

It would appear that the way heat pumps from different manufacturers can vary, so allow variation of the DeltaT setting, whilst others apply a standard DT of 5C, some control the speed of the water pump using the heat pump controller, others have pumps that have to be adjusted manually. Some systems have been set with a fixed LWT, other use WC, whilst a few others use WC and indoor temperature measurement in the control process.

So to try to answer your question. You have an 8.5kW Ecodan, which probably has a minimum heat energy output in the region of 2kW. Your calculated heat loss is of the order of 4.75kW at an outside temperature of -3.4C, so a heat loss of 2kW should occur when the outside temperature is around +10C.

In an ideal World, in theory, your heat pump should not start to cycle until the outside temperature increases to 10C, but as you have probably noticed we do not live in an ideal World. In the real World you will probably find that cycling starts at lower outside temperatures due to solar gain and human activity, or it may be that the way your heat pump has been configured, it cannot pull back to its minimum operating parameters, so is always producing more than 2kW of heat energy.

Lowering the LWT will indeed reduce the amount heat energy absorbed by the heat emitters, but it may not fully reduce the amount of heat energy produced by the heat pump, particularly if the heat pump controller cannot vary the speed of the water pump. So to help with your problem, check to see if the water pump is operating at the minimum speed which maintains the minimum required water flow rate.

I think that I am reasonably correct in stating that cycling at a frequency of 3 or less per hour is to be expected as normal, but should never be more than 6 cycles per hour. A possible way of slowing the cycling rate would be to bring any thermostats back into play, by setting the thermostats to a 'switch off' temperature setting of 1C higher than the desired indoor temperature, and if the hysteresis of the thermostat is adjustable, also set this to 1C.

This will hopefully produce the following effect in mild weather conditions. With the heat pump operating and supplying more heat energy than required, the indoor temperature will start to increase, until it reaches the 'off' setting of the thermostat, at which point the heat pump will be stopped. The heat pump will not restart until the thermostat has reset when the indoor temperatures has fallen by at least 1C. Because the weather is mild, it will take longer for the indoor temperature to fall, therefore extending the period that the heat pump is stopped. When the heat pump restarts it will take a little longer to raise the indoor temperature until once again stopped be the thermostat. You could give this a try and note the effect.

There are settings within the 'Room Temperature Control' mode which can also help to extend the cycling period. Have a look at the manual and come back with any questions.

 


   
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(@dockray)
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@derek-m Thank you for replying. I'll try reducing the water pump speed first as that too had been left on factory settings and let you know the effect.

I hadn't appreciated having an ASHP would be like having an additional teenager in the house - confusing, unpredictable, limited instruction manual... 😀 


   
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 mjr
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I think the 8.5kW ecodan minimum output is actually nearer 3kW not 2, but I don't have the data book handy.

My insulation and installation is detective at the moment, so 12 outside before we need no heating. My weather curve flattens above 10 to avoid cycling, the ecodan controller will cut power on flow temperature overshoots, and the room thermostats are set 2 degrees above target to send an 'off' signal as a last resort. That prevents most short cycling. I've tried some other things to catch the remainder, such as watching melcloud for low but not zero power usage, but I've no correct test yet.


   
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(@fazel)
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Posted by: @mjr

I think the 8.5kW ecodan minimum output is actually nearer 3kW not 2, but I don't have the data book handy.

My insulation and installation is detective at the moment, so 12 outside before we need no heating. My weather curve flattens above 10 to avoid cycling, the ecodan controller will cut power on flow temperature overshoots, and the room thermostats are set 2 degrees above target to send an 'off' signal as a last resort. That prevents most short cycling. I've tried some other things to catch the remainder, such as watching melcloud for low but not zero power usage, but I've no correct test yet.

 

Indeed - Page 61 of the pdf file

https://library.mitsubishielectric.co.uk/pdf/book/Ecodan_ATW_Databook_R32_Vol5_5

 


   
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(@derek-m)
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Posted by: @mjr

I think the 8.5kW ecodan minimum output is actually nearer 3kW not 2, but I don't have the data book handy.

My insulation and installation is detective at the moment, so 12 outside before we need no heating. My weather curve flattens above 10 to avoid cycling, the ecodan controller will cut power on flow temperature overshoots, and the room thermostats are set 2 degrees above target to send an 'off' signal as a last resort. That prevents most short cycling. I've tried some other things to catch the remainder, such as watching melcloud for low but not zero power usage, but I've no correct test yet.

You are correct. There is no common solution that meets all situations.  I presume that the main limiting factors will be the minimum operating speed of both the compressor and the water pump, both of which affect how much heat energy is produced and at what temperature. There will also be slight differences from one heat pump to the next, even if they are both the same model from the same manufacturer.

My objective has been to try to propose a simple solution, that can be used in most instances, to try to prevent short cycling, without causing large fluctuations in the indoor temperature.

If anyone tries my proposed method I would welcome feedback, good or bad, the more the better.

I am of the opinion that a unit containing a larger and a smaller compressor (say 67% output and 33% output) would help resolve many of the operational problems, but don't know of any yet being produced.

 


   
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(@dockray)
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@mjr Thanks for the reply. I'm hoping we won't need the heating on for too much longer. 

We've not done a Spring in this house yet. This might seem like a really basic question but once the house doesn't need heating and the WC curve is as low as it will go, do I effectively 'turn off' the heating by reducing the temperature on the room thermostats right down?

 


   
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 mjr
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Posted by: @dockray
once the house doesn't need heating and the WC curve is as low as it will go, do I effectively 'turn off' the heating by reducing the temperature on the room thermostats right down?

Only if you want to force it off, and I'm not sure whether it's better to do that or set the FTC to stop for heating, or set a summer schedule on the FTC with a low target temperature, just in case and so it restarts itself automatically in September or October.


   
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(@dockray)
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@mjr Thank you, I'd not even considered a summer schedule and this makes the most sense as the weather round here is very up and down even in the summer


   
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(@dockray)
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@derek-m I reduced the pump speed from 5 (maximum) to 3 and the house has been half to 1 degree under the target temperature but this is probably because the outdoor temperature has dropped. I'm going to try the pump at 4 whilst the weather is cooler, to get the house back to its target temperature and if necessary turn it back to 5. I'll reduce the pump speed again when it gets to around 7 or 8* outside as that seems to be when I get increased cycling.

Reducing the pump speed has improved Delta T a bit but there are still long periods ( an hour to two) when the LWT and RWT are virtually the same. This hasn't just started, it was happening before I reduced the pump speed too. Should it be doing this?

Delta T query

 


   
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(@derek-m)
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Posted by: @dockray

@derek-m I reduced the pump speed from 5 (maximum) to 3 and the house has been half to 1 degree under the target temperature but this is probably because the outdoor temperature has dropped. I'm going to try the pump at 4 whilst the weather is cooler, to get the house back to its target temperature and if necessary turn it back to 5. I'll reduce the pump speed again when it gets to around 7 or 8* outside as that seems to be when I get increased cycling.

Reducing the pump speed has improved Delta T a bit but there are still long periods ( an hour to two) when the LWT and RWT are virtually the same. This hasn't just started, it was happening before I reduced the pump speed too. Should it be doing this?

Delta T query

 

It takes a little practice to interpret the data to ascertain what is happening. Looking at your Some Delta T pdf, where the LWT goes below the RWT would indicate a defrost cycle taking place, when the heat pump takes heat energy out of the water to help melt the ice that will have formed on the Evaporator. The periods when the LWT and RWT are approximately the same, would indicate that the heat pump and water pump are not operating, because the system has reached the desired temperature. I think that the controller display indicates when the heat pump, and possibly water pump, are operating, so watching it for a while and monitoring the temperatures may prove my assessment to be correct.

If your controller cannot vary the speed of the water pump, you may have to lower it manually if you get short cycling, which would be your heat pump stopping and starting more than 3 times per hour.

 


   
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(@dockray)
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@derek-m Thank you for your help and time. You're right about it taking practice to read the data - I haven't found any information on this other than info Ive picked up reading threads on here.

"Looking at your Some Delta T pdf, where the LWT goes below the RWT would indicate a defrost cycle taking place, when the heat pump takes heat energy out of the water to help melt the ice that will have formed on the Evaporator."

I've looked at the Monthly Operations Mode Report on MELcloud and it says 4.1% of the energy used in the last 31 days has been on Freeze Stat and 7% for all the DHW  - so it is defrosting quite a lot.

"The periods when the LWT and RWT are approximately the same, would indicate that the heat pump and water pump are not operating, because the system has reached the desired temperature."

It makes sense that the LWT & RWT are similar during times when the system has reached its desired temperature as it tends to be during the night when we have a setback of 2 degrees and also mid afternoon when the house has heated up through solar gain. Does this mean I still need to be altering the WC curve or is it just a symptom of setback and solar gain?

"If your controller cannot vary the speed of the water pump, you may have to lower it manually if you get short cycling, which would be your heat pump stopping and starting more than 3 times per hour."

I changed the pump speed from 5 to 3 through the FTC (I could hear the power change) & it only cycled 3 times in the hour but it reduced the room temperature too much so we are trying it at 4 today to see if it is better.

Can I ask about my DHW or should I start a new thread?

We've got a 210l tank and the DHW has 2 x 90min heating cycles -  1 early morning & 1 evening (we've not been able to reduce the time of the heating cycle as the water didn't consistently reach its target temperature of 48*). The DHW is on 'normal' setting (it couldn't heat the water to target temperature on 'eco' in 90 mins). There are 4 of us in the house and 2 shower in the morning and the other 2 in the evening. The DHW tank is on 'large' setting' ( 2 heat sensors).

The problem is that unless we literally follow each other into the shower, the water goes tepid. We aren't having long showers. It holds it's temperature ok if nobody uses the hot water, the problem arises after one person has drawn some water off.

Any suggestions? Thanks.

 

 

 

 

 


   
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