Slope weather curve and questions

Reed_Richards

My (LG) heat pump can do what it calls "Air + Water" control.  Water refers to the Leaving Water Temperature and Air refers to the air temperature in the room where the controller is sited.  The Water temperature is normally determined by the Weather Compensation setting but if the Air temperature in the room is a long way short of the set air temperature it will boost the power input to the heat pump which could cause it to increase the water temperature or the flow rate or both. Conversely if the actual room temperature is close to the set room temperature it will decrease the power to the heat pump and so reduce the water temperature or the flow rate or both.  This "override" feature is known as Load Compensation.  My explanation contradicts an assertion made by @matt_drummer and indeed there are other ways of doing something similar but having the heat source know the difference between the actual and the set air temperature in the room is the most accurate way (and plenty of gas boilers will do exactly this).    

michaels

matt_drummer said:

Another way to think about this is from the point of view of the heat pump.

In two different scenarios producing the same amount of heat it can either heat 10 litres of water a minute by 10c (that is at a dT of 10c) or 20 litres of water a minute by 5c (at a dT of 5c)

Both scenarios result in the same amount of energy being required.

But as the amount of energy consumed by the heat pump is variable, it generates more energy than it consumes by extracting energy from large volumes of air and transferring it to a refrigerant and then the water, it may take more electricity to heat a smaller volume of water by a larger temperature than vice versa.

A dT of 5c is considered by most people to be the optimum. Lower the dT and you have to heat a larger volume of water less, and raise the dT you have to heat a smaller volume of water by a larger amount to get the same result.

There is a point where the best efficiency is achieved which is why Octopus set it to fan coils with a dT of 5c even though you have radiators.

I think mostly your explanation is clear and helps with understanding of efficiency.

Not sure if it is true that heating half as much water by twice as many C is less is less efficient if the 'heat to' flow temp is the same (eg 100l from 20 to 30 does not take more energy than 200l from 25 to 30).  I think it is less efficient to have the bigger delta T simply because it means a higher flow temp for the same rad output - for example 35 average rad temp can be either flow 40, return 30 or with twice the volume, flow 37.5, return 32.5, the efficiency comes from only having to get the flow to 37.5 rather than 40.

I could be wrong of course

matt_drummer

Reed_Richards said:

My (LG) heat pump can do what it calls "Air + Water" control.  Water refers to the Leaving Water Temperature and Air refers to the air temperature in the room where the controller is sited.  The Water temperature is normally determined by the Weather Compensation setting but if the Air temperature in the room is a long way short of the set air temperature it will boost the power input to the heat pump which could cause it to increase the water temperature or the flow rate or both. Conversely if the actual room temperature is close to the set room temperature it will decrease the power to the heat pump and so reduce the water temperature or the flow rate or both.  This "override" feature is known as Load Compensation.  My explanation contradicts an assertion made by @matt_drummer and indeed there are other ways of doing something similar but having the heat source know the difference between the actual and the set air temperature in the room is the most accurate way (and plenty of gas boilers will do exactly this).    

Hi Reed,

There is no contradiction, the Daikin does it the same, the load compensation monitors how fast the room temperature changes.

But this is nothing to do with what you quoted earlier.

Somebody said that the dT between flow and return temperature is related in some way to the room temperature, remember we are talking about Daikin heat pumps here, not other manufacturers.

It is not, the room temperature may influence a change in flow temperature to give more or less heat but with a Daikin the dT doesn't change.

If more heat is required the flow temperature increases. That would mean that the dT between flow and return would increase so to maintain the set dT a Daikin heat pump increases the flow rate so that the return temperature increases to maintain the set dT.

The opposite happens if less heat is required.

The room temperature has no affect on the dT with a Daikin heat pump, it just changes the flow temperature and then as a consequence, the flow rate.

That remains true until you reach the limits of the circulation pump, which is what I do for reasons I have explained before.

All current Daikin heat pumps fitted with a Madoka have load compensation that responds to the rate of change in room temperature detected in just the same way that your LG heat pump would if you fitted the LG controller.

The difference is that your LG runs a fixed flow rate (I am guessing like most others do) and changes in flow temperature (and therefore room temperature as that affects flow temperature with load compensation enabled) change the dT between flow and return as that is how the heat output is changed.

I am repeating myself but Daikin heat pumps are different, they change heat output by changing flow rate, either with or without a change in flow temperature, but the dT is fixed.

They just work differently to pretty much everybody else.

I hope this makes sense Reed @Reed_Richards

matt_drummer

michaels said:

matt_drummer said:

Another way to think about this is from the point of view of the heat pump.

In two different scenarios producing the same amount of heat it can either heat 10 litres of water a minute by 10c (that is at a dT of 10c) or 20 litres of water a minute by 5c (at a dT of 5c)

Both scenarios result in the same amount of energy being required.

But as the amount of energy consumed by the heat pump is variable, it generates more energy than it consumes by extracting energy from large volumes of air and transferring it to a refrigerant and then the water, it may take more electricity to heat a smaller volume of water by a larger temperature than vice versa.

A dT of 5c is considered by most people to be the optimum. Lower the dT and you have to heat a larger volume of water less, and raise the dT you have to heat a smaller volume of water by a larger amount to get the same result.

There is a point where the best efficiency is achieved which is why Octopus set it to fan coils with a dT of 5c even though you have radiators.

I think mostly your explanation is clear and helps with understanding of efficiency.

Not sure if it is true that heating half as much water by twice as many C is less is less efficient if the 'heat to' flow temp is the same (eg 100l from 20 to 30 does not take more energy than 200l from 25 to 30).  I think it is less efficient to have the bigger delta T simply because it means a higher flow temp for the same rad output - for example 35 average rad temp can be either flow 40, return 30 or with twice the volume, flow 37.5, return 32.5, the efficiency comes from only having to get the flow to 37.5 rather than 40.

I could be wrong of course

Hi @michaels

I will try and explain a bit more clearly what I mean.

You are correct, as I said, the same energy is required to produce the same amount of heat whatever the volume is.

And if we were talking about something like a kettle that heats water with an heated element it wouldn't matter.

But a heat pump gets it's energy from two sources, the electricity used to make it work and however much energy it can extract from the air around it.

So that varies depending on different factors, a big one being the air temperature.

But there are also the mechanics of transferring that energy in the refrigerant to the water and it may be that there is an optimum volume of water that results in the most efficient transfer of energy in the heat exchanger.

I cannot explain it in any more detail as I am at my limit of expertise but the heat exchanger will have limits which is why minimum flow rates are quoted and required.

There will be a point when some of the energy generated is wasted if it cannot all be transferred to the water flowing through the heat exchanger

A really small dT would be best for the radiators as it would allow the lowest flow temperature but there are all sorts of limits to how much water you can flow around the system.

For whatever reason though, most people aim for a dT of 5c and for the person asking the original question, that's a good starting point and with no detailed data something that is best left alone.

ninjaef

I'm glad I made the OP... just reading the dialogue between everyone is most illuminating and educational.  I understand dT basics now thanks to Matt whereas it was mystery before  And I'm glad the ASHP does all the work to maintain 5dT rather than me have to mess about in in the MMI. 😁 

I've also reduced the LWT through the WDHC and run the system 1C less (offset) but for longer - in the morning. Will need to observe and compare. And maybe do another adjustment, each two days or so until I find the sweet spot.

Have to tip my hat to Matt who doesn't tell us what settings we should use but rather articulates the concept. unlike youtubers who recommended settings but this is flawed because each home is different, with volume,  rads, heat loss, pump, installation mechanics, design parameters and more 

Matt allows us to make informed decisions rather than listen to naff 'influencers".

matt_drummer

ninjaef said:

I'm glad I made the OP... just reading the dialogue between everyone is most illuminating and educational.  I understand dT basics now thanks to Matt whereas it was mystery before  And I'm glad the ASHP does all the work to maintain 5dT rather than me have to mess about in in the MMI. 😁 

I've also reduced the LWT through the WDHC and run the system 1C less (offset) but for longer - in the morning. Will need to observe and compare. And maybe do another adjustment, each two days or so until I find the sweet spot.

Have to tip my hat to Matt who doesn't tell us what settings we should use but rather articulates the concept. unlike youtubers who recommended settings but this is flawed because each home is different, with volume,  rads, heat loss, pump, installation mechanics, design parameters and more 

Matt allows us to make informed decisions rather than listen to naff 'influencers".

Thank you.

@reed_richards and @michaels know what they are talking about too amongst the others.

Daikins are peculiar and I understand why Reed doesn't always follow what I am saying. It is because our heat pumps do not work like his, they do the same job by heating the water but the controls are different.

Daikins vary their flow rate to maintain a set dT whereas most other heat pumps run at a fixed flow rate.

Daikins also have a lot more adjustments possible than most other heat pumps that I have any knowledge of.

It really is very difficult to tell you how to set up your heat pump for your house.

It is best to make small adjustments, keep notes and see what happens, the main thing is to keep warm first.

Monitor the electricity use, on a mild day you wouldn't expect one of these Daikins to use more than 500W once running but take note that the first 30 minutes will always consume more power.

Have a look at heatpumpmonitor.org where you will be able to look at heat pump data live.

It may help you visualize what is going on.

And if you are interested, here is mine

Emoncms - app view

ninjaef

matt_drummer said:

ninjaef said:

I'm glad I made the OP... just reading the dialogue between everyone is most illuminating and educational.  I understand dT basics now thanks to Matt whereas it was mystery before  And I'm glad the ASHP does all the work to maintain 5dT rather than me have to mess about in in the MMI. 😁 

I've also reduced the LWT through the WDHC and run the system 1C less (offset) but for longer - in the morning. Will need to observe and compare. And maybe do another adjustment, each two days or so until I find the sweet spot.

Have to tip my hat to Matt who doesn't tell us what settings we should use but rather articulates the concept. unlike youtubers who recommended settings but this is flawed because each home is different, with volume,  rads, heat loss, pump, installation mechanics, design parameters and more 

Matt allows us to make informed decisions rather than listen to naff 'influencers".

Thank you.

@reed_richards and @michaels know what they are talking about too amongst the others.

Daikins are peculiar and I understand why Reed doesn't always follow what I am saying. It is because our heat pumps do not work like his, they do the same job by heating the water but the controls are different.

Daikins vary their flow rate to maintain a set dT whereas most other heat pumps run at a fixed flow rate.

Daikins also have a lot more adjustments possible than most other heat pumps that I have any knowledge of.

It really is very difficult to tell you how to set up your heat pump for your house.

It is best to make small adjustments, keep notes and see what happens, the main thing is too keep warm first.

Monitor the electricity use, on a mild day you wouldn't expect one of these Daikins to use more than 500W once running but take note that the first 30 minutes will always consume more power.

Have a look at heatpumpmonitor.org where you will be able to look at heat pump data live.

It may help you visualize what is going on.

And if you are interested, here is mine

Emoncms - app view

500W Matt, omg that's impossible  

Mind you, the two spikes are my battery charging at cheap rates so knock off 8kw

here's a day when it was 1C outside all day

matt_drummer

ninjaef said:

matt_drummer said:

ninjaef said:

I'm glad I made the OP... just reading the dialogue between everyone is most illuminating and educational.  I understand dT basics now thanks to Matt whereas it was mystery before  And I'm glad the ASHP does all the work to maintain 5dT rather than me have to mess about in in the MMI. 😁 

I've also reduced the LWT through the WDHC and run the system 1C less (offset) but for longer - in the morning. Will need to observe and compare. And maybe do another adjustment, each two days or so until I find the sweet spot.

Have to tip my hat to Matt who doesn't tell us what settings we should use but rather articulates the concept. unlike youtubers who recommended settings but this is flawed because each home is different, with volume,  rads, heat loss, pump, installation mechanics, design parameters and more 

Matt allows us to make informed decisions rather than listen to naff 'influencers".

Thank you.

@reed_richards and @michaels know what they are talking about too amongst the others.

Daikins are peculiar and I understand why Reed doesn't always follow what I am saying. It is because our heat pumps do not work like his, they do the same job by heating the water but the controls are different.

Daikins vary their flow rate to maintain a set dT whereas most other heat pumps run at a fixed flow rate.

Daikins also have a lot more adjustments possible than most other heat pumps that I have any knowledge of.

It really is very difficult to tell you how to set up your heat pump for your house.

It is best to make small adjustments, keep notes and see what happens, the main thing is too keep warm first.

Monitor the electricity use, on a mild day you wouldn't expect one of these Daikins to use more than 500W once running but take note that the first 30 minutes will always consume more power.

Have a look at heatpumpmonitor.org where you will be able to look at heat pump data live.

It may help you visualize what is going on.

And if you are interested, here is mine

Emoncms - app view

500W Matt, omg that's impossible 

here's a day when it was 1C outside all day

I said on a mild day, I don't class 1c as mild!

I was careful not to predict what you might need in the weather we have just had as there are too many variables.

But all the ones I have seen will run all day at 500W or less when it is 10c outside.

Mine is using 566W at the moment at 6c outside.

Look at some of them on heatpumpmonitor.org.

If you spend a bit of time looking at similar systems to yours you should be able to see what sort of electricity consumption to expect. It will be a good starting point in judging how well your settings are working.

At an average of -0.4c I used 17.6kWh for heating. That was at Octopus design temperatures indoors. My house is 100m2 with a 15m2 all glass conservatory that I heat.

My heat pump runs all day and night.

I find it most efficient, you can run lower flow temperatures and put in heat continuously rather than large dollops.

But also bear in mind your electricity tariff. I run on batteries that are charged overnight on Intelligent Octopus Go so my electricity cost is fixed all day. Your tariff may well be different so it may affect how you use your heat pump.

ninjaef

sorry Matt, thought you meant 500w  ..  ALL DAY ... you mean operation draw.  sorry. just had a look on my dashboard , 400w is max draw since 1pm. You're spot on again 😁

EDIT. and that's total house draw not just the pump heating.

matt_drummer

ninjaef said:

sorry Matt, thought you meant 500w  ..  ALL DAY ... you mean operation draw.  sorry. just had a look on my dashboard , 400w is max draw since 1pm. You're spot on again 😁

So, if you're warm enough then nothing is a million miles out.

Nothing much to worry about.

Only one other thing if you are running a weather dependent curve.

These Daikin heat pumps are not very happy below 30c flow temperature, particularly when it is warmer, so no real point setting your low point below 30c.