Weather Compensation Modelling and Actual results

[JamesPa]

As I promised I have created a first-cut model of weather compensation, attached.  Its based on the Mitsubishi PUZ-WM112 performance data, which is pretty comprehensive.  I have modelled the CoP with 5 different W/C schemes and none, estimating the CoP as a function of flow temperature and ambient by linear interpolation.  Weather data is average daily temperature for 2022 from the Met Office Central England database.  The average daily temperatures are collected into bins 1 degree wide, the number of days in each bin counted, and then the load and consumption for the conditions calculated and multiplied up to get the total annual for that 'bin' according to the w/c scheme selected.  The totals for all temperature 'bins' give the total consumption over the year.  This is compared with the total demand calculated the same way to get an average CoP, and the total for any particular w/c scheme compared with scheme 'none' (ie no weather compensation) to estimate how much w/c saves.

 

The 'ideal' W/C curve is based on adjusting flow temperature to precisely match demand at ambient temps other than the design temp, using the heat output curve of a typical radiator (which varies as (flow temp-room temp)^n where n=1.3).  If someone can tell me how UFH output behaves as a function of flow temp I can model that.  Other curves are variants of this,

 

Simply put, it suggests that the various weather compensation schemes I modelled save between 11% and 15% over no compensation at all, which is, frankly, disappointingly small (so I am a little nervous a mistake has been made - but I cant find it).  The model takes no account of inefficiencies due to cycling, only the improvement in CoP.  This might be significant, if anyone has any figures it might be possible to add them in. 

 

I will write it up in more detail over the next few days and - health warning - there may still be errors so, until its been peer reviewed or checked against real results, treat with extreme caution.

 

If anyone wants to critique, contribute suggestions, discussion or actual results of comparing the effects of weather comp, I suggest to do so here so as not to hijack other threads.

 

 

 

WC Simulation.xls

Edited January 23, 2023 by JamesPa

[JohnMo]

That's a good piece of work

 

Think UFH curve is just 1 for 1 instead of 1.3

 

Just plotted outside temp from -10 to 20 in one column and the formula next column

 

Added my WC curve to your spreadsheet and due to lower flow temps of UFH ended up with a projected CoP of 4.82 on WC, but the savings against a fix flow temp were low only 5%.  CoP changes to 4.59.

 

[JamesPa]

39 minutes ago, JohnMo said:

Added my WC curve to your spreadsheet and due to lower flow temps of UFH ended up with a projected CoP of 4.82 on WC, but the savings against a fix flow temp were low only 5%.  CoP changes to 4.59.

Yes, I had a quick play and it seems WC makes greater difference where the design flow temp is higher and less if its lower.  Thats not an obvious result, but not implausible.  Will explore it more over next days.

 

40 minutes ago, JohnMo said:

Think UFH curve is just 1 for 1 instead of 1.3

Well thats a lot easier!

[Kevm]

Nice work.  The results are similar to my own observed measurements.  I was surprised it didn't make much difference then. 

 I'll plug my numbers in later and see how it compares

 

Some of the horror stories you hear are for higher flow temps, e.g 50 deg so the difference will be more. 

 

@JamesPa

 

In answer to your question on the other thread, the data below is from my MMSP logs and is total ASHP electricity used including HW (I did say it was flawed!) and average outside temperature as recorded by the ASHP.  HW use was very little.  

 

 

Edited January 24, 2023 by Kevm

[ReedRichards]

@JamesPa In the calculation I made to derive a weather compensation curve for my radiators, the temperature drop across the radiator was a parameter.  I cannot see that as a listed parameter; is a particular value used?   

[JohnMo]

5 minutes ago, ReedRichards said:

temperature drop across the radiator was a parameter.

How do you model the radiator DT, with a variable temperature system, as DT would continually be changing in the WC system.  I know on my boiler that if I set the flow temp at 30 degrees the pump will be modulated to manage a DT of 4, if I increase the flow temp to 70 the pump flow rate is managed to give a DT of around 20.  Your DT is only really fixed at a set design temperature, form then on it moves, based on flow temp and room temp, the closer the room temp gets to radiator temp the less work the radiator does so the DT changes.

[ReedRichards]

You are absolutely correct, @JamesPa, I was just thinking that myself and realising I had done this incorrectly.  However the pump that circulates the water around my heating system operates at a fixed speed so I should be able to scale DT according to Delta T, I think.  I will look at that and see if it makes a difference.

Edited January 24, 2023 by ReedRichards

[JamesPa]

3 hours ago, ReedRichards said:

@JamesPa In the calculation I made to derive a weather compensation curve for my radiators, the temperature drop across the radiator was a parameter.  I cannot see that as a listed parameter; is a particular value used?   

To calculate the 'ideal' WC curve I assume that the emitter design and flow temperature is correct at the system design temperature and then, at other temperatures, use the radiator output curve to work out what the new flow temperature must be to give the correct output power.  This relies on the fact that most radiators have an output power which goes as (Trad-Troom)^1.3.

 

Of course there are various second order effects, eg as the flow temp decreases (assuming the pump speed remains constant) the delta t across the radiators may decrease and thus the average radiator temperature (which determines the output power) will not fall quite as quickly as a calculation based on flow temp would indicate.  Also the HP CoP varies a bit with the HP total output which is also not accounted for.  The Mitsubishi databook gives some figures for this so could be factored in, but its small so I doubt it will make much difference.

[JamesPa]

3 hours ago, Kevm said:

Some of the horror stories you hear are for higher flow temps, e.g 50 deg so the difference will be more. 

 

Definitely - its instructive to plug in various flow temps.  At 35C 'ideal' WC makes about 5% difference.  At 55C 'ideal' WC makes 25% difference.  My initial calculations were at 45C where 'ideal' WC makes 15% difference.

[JamesPa]

3 hours ago, JohnMo said:

How do you model the radiator DT, with a variable temperature system, as DT would continually be changing in the WC system.  I know on my boiler that if I set the flow temp at 30 degrees the pump will be modulated to manage a DT of 4, if I increase the flow temp to 70 the pump flow rate is managed to give a DT of around 20.  Your DT is only really fixed at a set design temperature, form then on it moves, based on flow temp and room temp, the closer the room temp gets to radiator temp the less work the radiator does so the DT changes.

I don't and cant. I model the flow temperature needed to achieve sufficient heat output from the radiator, on the assumption that the flow temp (and thus the average radiator temperature) is correct at the system design temp.  As I mention above the DT across the rad may change as the flow temp changes, and if it does the required flow temp wont change precisely according to the radiator power curve, but in most reasonable systems I would be pretty certain, unless someone can show otherwise, that is a second order effect, so wont materially affect the comparative WC results. 

 

Your pump seems to be set deliberately to mess around with the delta T, which obviously is a special case and no system will require a flow temp to vary over the range you quote.  I suspect it does this because it is 'set up' for radiators at 70, and UFH at 35, not because the pump manufacturers anticipate that this variation in flow temp to occur in actual operation.  A pump could equally well be set to achieve a constant delta T over the actual range of flow temperatures (I think some controllers do exactly that) in which case the 'second order' effect is completely nullified.

[JamesPa]

Just thinking a bit more on the above my 'ideal' WC curve effectively assumes that the pump modulates to achieve a constant delta T across the radiators.  Some I believe do, others don't.   In the case of a pump which doesn't, the 'ideal' WC curve will be a bit closer to linear (I think). That won't affect the results much, but will (I think) mean that a linear WC curve, which is what most HPs provide for, is less of a compromise.

[JamesPa]

Im attaching version 2 of the model which addresses a factor previously omitted and mentioned earlier in this thread, namely that the delta T across the emitters will vary as the load varies, unless the pump speed adjusts to compensate.  As I surmised this makes a small difference, but doesn't affect the general trends.  The main findings in summary are this:

 

• WC makes about 25% difference at 55C, 20 at 50C, 15 at 45C, 10 at 40C and 5 at 35C (all flow temps)
• A linear approximation to the perfect WC curve degrades the performance by 2% or less
• The 'Lizzie' adaption to a linear WC curve (whereby the flow temp never falls below 37C) degrades the performance by between 1% (at 55C) and 6% (at 40C).  Obviously this adaption makes no sense at 35C
• A 1C uplift to the WC curve degrades performance by 2-3%
• The degradations above can more or less be added together

The simulation is based on radiators, it can be adjusted for UFH by changing the emitter coef. (to 1 I am told).

 

Hope this is of interest

WC Simulationv2.xls

[Kevm]

@JamesPa, thanks, this is really useful.  Would you mind if I shared the link on another forum (renewableheatinghub.com)?  There are folks there who would find this really useful. I would of course credit you with its creation. 

[JamesPa]

8 minutes ago, Kevm said:

@JamesPa, thanks, this is really useful.  Would you mind if I shared the link on another forum (renewableheatinghub.com)?  There are folks there who would find this really useful. I would of course credit you with its creation. 

Please do.  

[Kevm]

Just now, JamesPa said:

Please do.  

thanks.  One question.  Once the heat demand is low and the flow is fixed and high (e.g. 55 deg) the ASHP will only be heating for a fraction of the day.  In practical terms, it's going to be stopped and started by a thermostat throughout the day rather than be on for a single period.  Do you think that the heating and cooling involved would add significantly to the power consumption? Based on my observations of my own system I'm not sure it would btw.  

[JamesPa]

1 hour ago, Kevm said:

Do you think that the heating and cooling involved would add significantly to the power consumption? Based on my observations of my own system I'm not sure it would btw. 

Honestly no idea.  Everyone says that cycling, particularly short cycling, is bad, but I don't know of data to prove it and haven't seen any heat pump specs which allow the penalty to be inferred.

 

What is certain, however, is that on/off at 55C is worse that on/off at a lower temperature (assuming the same average heat output).  Operating at a constant 55C (ie without weather compensation) is very bad news

[ReedRichards]

Although on/off at 55 C may not be quite what it seems.  My heat pump appears to target a 5 C temperature difference between flow and return as its top priority.  If the heating has been off for a while the water inside the system will have cooled and since most of this resides inside the heated fabric of the building, most of the cooling provided useful heat.  Suppose it has cooled to 30 C.  In my case the heat pump starts off at low power so gently raises the temperature.  Although it is trying to get back to 55 C it may never get there if it's mild outside and the heating demand from the building is low and the room thermostat reaches temperature.  So maybe the average water temperature is 40 C during a cycle and you get a "natural" weather compensation even when this is not engaged.   

[JamesPa]

48 minutes ago, ReedRichards said:

Although on/off at 55 C may not be quite what it seems.  My heat pump appears to target a 5 C temperature difference between flow and return as its top priority.  If the heating has been off for a while the water inside the system will have cooled and since most of this resides inside the heated fabric of the building, most of the cooling provided useful heat.  Suppose it has cooled to 30 C.  In my case the heat pump starts off at low power so gently raises the temperature.  Although it is trying to get back to 55 C it may never get there if it's mild outside and the heating demand from the building is low and the room thermostat reaches temperature.  So maybe the average water temperature is 40 C during a cycle and you get a "natural" weather compensation even when this is not engaged.   

Could well be, and I agree that this would to some extent mitigate the efficiency loss of the higher flow temp. 

 

Its still better to adjust the flow temp so that a constant flow matches the demand though, thats the lowest possible flow temp = highest efficiency, and also the most comfortable building because there is no hysterisis.

[PhilT]

2 hours ago, JamesPa said:

 Its still better to adjust the flow temp so that a constant flow matches the demand though, thats the lowest possible flow temp = highest efficiency, and also the most comfortable building because there is no hysterisis.

Interested to know if anyone has ever attempted AND achieved this?

[JamesPa]

1 hour ago, PhilT said:

 

 

1 hour ago, PhilT said:

3 hours ago, JamesPa said:

 Its still better to adjust the flow temp so that a constant flow matches the demand though, thats the lowest possible flow temp = highest efficiency, and also the most comfortable building because there is no hysterisis.

Interested to know if anyone has ever attempted AND achieved this?

Isn't that what WC aims to do.  I get the impression that several here have WC set up well and other controls are temp limiters, to kick in only eg when solar gain takes over, which comes jolly close.

[JamesPa]

On 27/01/2023 at 08:52, ReedRichards said:

Although on/off at 55 C may not be quite what it seems.  My heat pump appears to target a 5 C temperature difference between flow and return as its top priority.  If the heating has been off for a while the water inside the system will have cooled and since most of this resides inside the heated fabric of the building, most of the cooling provided useful heat.  Suppose it has cooled to 30 C.  In my case the heat pump starts off at low power so gently raises the temperature.  Although it is trying to get back to 55 C it may never get there if it's mild outside and the heating demand from the building is low and the room thermostat reaches temperature.  So maybe the average water temperature is 40 C during a cycle and you get a "natural" weather compensation even when this is not engaged.   

my initial response  was  "Could well be, and I agree that this would to some extent mitigate the efficiency loss of the higher flow temp."

 

Thinking about this again, I'm now not so sure.  Whilst the flow temp may never reach 55C, it quite likely that the refrigerant is at whatever temperature/pressure corresponds to a flow temp of 55C.  It depends on how the HP operates.  I imagine (but don't know) that there is a target refrigerant compression ratio for any given target flow temp and that, as soon as you ask for a higher flow temp, the compression ratio is hiked up.  If this is the case then the HP will be operating at the CoP corresponding to a flow temp of 55 even though the flow temp itself has not yet reached 55.

[ReedRichards]

My heat pump seems to ramp up its power output until it reaches a plateau.  Most of the time it then stays at this plateau level until something causes it to stop.  This is certainly the return water getting too hot when it is heating up just the smaller zone in the morning.  Or perhaps when both room thermostats are satisfied later in the day.  The plateau levels it stops at appear to be quantised and to depend on the outside temperature.  So when it is mild out it goes to power level 1, when it is around zero outside it goes straight to power level 3.  This is the mode of operation with Weather Compensation but it seems similar to what you describe; the compression level and hence the input power is selected on the basis of the outside temperature.  My Weather Compensation is not the fine-tuned type but something that gives me enough headroom to slowly raise the inside temperature during the day, as I have always liked to do.