Weather compensation wtf?

[waxingsatirical]

Please tell me I am wrong with my understanding of weather compensation.

 

A compensation curve is setup which relates the outside temperature to the target flow temperature. Cold outside, high flow temp. Warm outside, low flow temp. 

 

So the internal temperature is not taken into account? If so, what happens when I light my 4kW wood burner? Does the heating system just merrily keep inputting as much heat as it assumes the house needs?

[SteamyTea]

2 minutes ago, waxingsatirical said:

Cold outside, high flow temp. Warm outside, low flow temp

I think it also takes the ΔT between the flow and return into account.

So it is really measuring two temperatures, and adjusting the flow rates/output temperatures to govern the power.

I may be wrong on that though.

[Dan F]

1 hour ago, waxingsatirical said:

Please tell me I am wrong with my understanding of weather compensation.

 

A compensation curve is setup which relates the outside temperature to the target flow temperature. Cold outside, high flow temp. Warm outside, low flow temp. 

 

So the internal temperature is not taken into account? If so, what happens when I light my 4kW wood burner? Does the heating system just merrily keep inputting as much heat as it assumes the house needs?

 

This the the case with basic weather compensation.  Most manufacturers have some kind of "room temperature modulation" option which wil adjust the curve based actual room temperature.  This approach helps adjust for solar gain and high wind as well.

Edited November 25 by Dan F

[JohnMo]

Radiators or UFH?

 

UFH as it's close to room temperature, will stop outputting heat, so self compensates to a decent amount. Radiators maybe less so, or not at all. If you had a trv on the radiators in or close to the stove, they would modulate flow down in response to room temperature increase.

[Beau]

3 hours ago, SteamyTea said:

I think it also takes the ΔT between the flow and return into account.

So it is really measuring two temperatures, and adjusting the flow rates/output temperatures to govern the power.

I may be wrong on that though.

Thats how our old GSHP works

[G and J]

3 hours ago, SteamyTea said:

I think it also takes the ΔT between the flow and return into account.

So it is really measuring two temperatures, and adjusting the flow rates/output temperatures to govern the power.

I may be wrong on that though.

So if the woodburner or solar gain is heating the room the return temp will be high enough to stop the ASHP pumping?  
 

If so will the return from overheated rooms effectively redistribute heat a little to colder rooms?

 

[Dan F]

5 minutes ago, G and J said:

So if the woodburner or solar gain is heating the room the return temp will be high enough to stop the ASHP pumping?  
 

If so will the return from overheated rooms effectively redistribute heat a little to colder rooms?

 

Ours modifies flow temp based on room temp, not return temp!

[G and J]

1 minute ago, Dan F said:

Ours modifies flow temp based on room temp, not return temp!

Return temp is effectively, I presume, a weighted average of all rooms served by the UFH.  So if you have a fire in one room do the others then suffer a temperature drop?

[ReedRichards]

11 hours ago, Dan F said:

Most manufacturers have some kind of "room temperature modulation" option

 

Called "Load Compensation" in the context of gas boilers.

 

9 hours ago, G and J said:

So if the woodburner or solar gain is heating the room the return temp will be high enough to stop the ASHP pumping?

Not with radiators.  You need a room thermostat set a couple of degrees higher than your desired room temperature to act as a backstop if you don't have Load Compensation.

 

 

Edited November 26 by ReedRichards

[ProDave]

Against popular current trends, we have a room thermostat in the 2 main downstairs rooms, so when you light the stove, if that room is not already up to temperature, it's thermostat will soon turn it off as the stove heats.

 

We also have a conventional boiler time clock as a master control, if I am lighting the stove, I go and turn the heating off at that.

 

Surely even the fans of pure WC operation must have some kind of master controls so your system is not running all the time in the shoulder seasons pumping water 1 degree warmer than the target temperature around the pipes?

[JohnMo]

1 hour ago, ProDave said:

Surely even the fans of pure WC operation must have some kind of master controls so your system is not running all the time in the shoulder seasons pumping water 1 degree warmer than the target temperature around the pipes?

I shoulder season mine would circulate but just blast a bit of heat when needed. But ran a single thermostat in the hall as a temp limit.  But if you are running weather comp on the correct curve, there should be no need to fire up the stove, because the house should be toasty enough already.

 

Big fan of WC, but starting to migrate back to batch charging the floor, as using 30p electric when the battery runs dry over the last couple of cold weeks, started to get more expensive than it needed too.

 

 

[ProDave]

19 minutes ago, JohnMo said:

there should be no need to fire up the stove, because the house should be toasty enough already.

This is of course the marmite subject on the forum.

 

On a cold day, and particularly on a cold, wet dreary day, we like to indulge and fire up the stove, it heats the whole house hotter than we normally have the main heating set for, and we like to indulge in a bit of free extra heat.  And then it is some time before the heating comes on again as the house cools down so a saving on electricity.

[Roundtuit]

2 hours ago, ProDave said:

This is of course the marmite subject on the forum.

 

On a cold day, and particularly on a cold, wet dreary day, we like to indulge and fire up the stove, it heats the whole house hotter than we normally have the main heating set for, and we like to indulge in a bit of free extra heat.  And then it is some time before the heating comes on again as the house cools down so a saving on electricity.

Agreed.  There isn't a 'need' as the ashp is more than capable, but lighting the stove and banging some extra heat into the house of a winters evening does lighten the load on the Smart meter the next morning.

[SteamyTea]

2 hours ago, ProDave said:

free extra heat

There are huge hidden costs though, even in the countryside.

[waxingsatirical]

Thank you all for your input.

 

Presumably then, my system will be doing some kind of load compensation based on the return flow temp.

 

If I set the thermostatic valves on the radiators, then as they turn off return temp will increase more quickly and the ASHP will be more responsive? Or is this a bad idea?

[ReedRichards]

6 hours ago, waxingsatirical said:

Presumably then, my system will be doing some kind of load compensation based on the return flow temp.

 

Proper Load Compensation requires that your heat pump knows the target room temperature and the actual room temperature.  When the actual temperature approached the target temperature it can modulate the heat pump down to a sufficiently low output to maintain the room temperature at the target value (or to minimum output if that is not possible).  If the actual temperature falls far short of the target temperature it can boost the leaving water temperature so something higher than the value determined by Weather Compensation to warm the house more quickly (but possibly at the expense of efficiency).  My Atag gas boiler bought in 1998 would do this; it's hardly a new thing.   

Edited November 26 by ReedRichards

[JohnMo]

50 minutes ago, ReedRichards said:

My Atag gas boiler bought in 1998 would do this; it's hardly a new thing. 

Definitely not new, but space age stuff to many (including the installer), where a boiler is installed ignoring all these features, because plumbers just wants to install S or Y plan with 70 Deg flow temps.

[JamesPa]

On 26/11/2024 at 14:25, waxingsatirical said:

Thank you all for your input.

 

Presumably then, my system will be doing some kind of load compensation based on the return flow temp.

 

If I set the thermostatic valves on the radiators, then as they turn off return temp will increase more quickly and the ASHP will be more responsive? Or is this a bad idea?

Quite probably not, depends on make and how it is set up.  With a heat pump you have to re-learn how to use heating controls because, stupidly (and unlike many more enlightened countries), the UK didn't implement weather compensation on our boilers, which has increased the running cost of our boilers by about 10% for many, possibly most people (I can explain why).  In general dont set the TRVs - see below instead.

 

if you tell us what make your heat pump is we may be able to give more specific help but...

 

Basically you have four options, of which 3 are reasonably efficient and the fourth is definitely not unless you have UFH and are already operating at a low flow temperature, say 30C or less.

 

1.  Operate pure weather compensation: 

 

Leave TRVs (or at least most TRVs/TRVs in the main rooms) and any room stat set at least 2C above the desired temperature so they operate as temperature limiters, and the main control is the WC.  Balance rads.   Adjust WC curve as low as possible consistent with keeping your house warm.  Leave alone  Trvs will shut down the room where you have the fire.

 

2. Operate room-sensor-tweaked weather compensation: 

 

Leave TRVs (or at least most TRVs/TRVs imn the main rooms) and initially any room stat set at least 2C above the desired temperature so they operate as temperature limiters,.  Balance rads.   Adjust WC curve more or less as low as possible consistent with keeping your house warm, aiming to get it within a couple of degrees of the 'ideal' (which is scenario 1).  Then use one pure on/off room stat or better still the heat pumps internal sensor (but NOT any room stat that attempts to be smart by applying a pulse width modulated signal to the heat pump) to control.  This will result in long on cycles and short off cycles when its cold, and will be ~3-6% less efficient than (1) depending on how close the curve is to (10

 

3.  If your heat pump supports it, operate in adaptive/AI/Smart mode (terminology differs - and this feature is NOT available on some heat pumps including Samsung, Midea and several others)

 

Leave TRVs (or at least most TRVs/TRVs in the main rooms) and any room stat set at least 2C above the desired temperature so they operate as temperature limiters, and the main control is the Heat pump.  Turn off adaptive mode.  Roughly set up a WC curve according to the recipe in (1).  Make sure that the bit of your heat pump controller with the temperature sensor (usually the one with the flashy UI) is in a suitable reference room).  Turn on adaptive mode

 

4.  Operate at fixed flow temperature using a room stat to turn the heat pump on/off like a (UK) boiler

 

Unless you have UFH and are already operating at say 30C flow temperature, this is 20-30% less efficient than any of the above and may cause flow rate errors if you also have lots of TRVs.  Not recommended.

 

Hope that helps, Im sure if you tell us about your heat pump and which of the above you prefer people can give you some guidance.

 

Once you get used to it you will most likely find that the levels of comfort are higher with WC activated.  This is because there will be reduced temperature gradients across the room and less up and down of temperature with time.  Low temperature heating (which is the essence here and its not specific to heat pumps) is actually more comfortable for most, its unfortunate that our heating industry has been up till now/still is rather backwards compared to many other countries.

 

Edited November 29 by JamesPa

[G and J]

I’ll have no. 1. for our installation please.   
 

As far as possible we want a fit and forget type cosy heating system.

 

Can I do that alongside using three cheap rate periods of heating?

[JamesPa]

On 29/11/2024 at 13:05, G and J said:

I’ll have no. 1. for our installation please.   
 

As far as possible we want a fit and forget type cosy heating system.

 

Can I do that alongside using three cheap rate periods of heating?

Depends on your heat pump model.  Pure WC should be fairly close to fit, adjust and forget, but adjustment may take several weeks or more.  If your heat pump features an adaptive WC mode this should be closer with shorter adjustment time.  Some heat pumps can be paired with external control eg homely, havenwise or the like.  The first is, by repute, fit, forget and enjoy cost optimised operation.  The second is a newcomer but can deal with Vaillant heat pumps which homely can't (but then Vaillant heat pumps feature a native adaptive WC mode).  What do you have?

 

 

[G and J]

55 minutes ago, JamesPa said:

Depends on your heat pump model.  Pure WC should be fairly close to fit, adjust and forget, but adjustment may take several weeks or more.  If your heat pump features an adaptive WC mode this should be closer with shorter adjustment time.  Some heat pumps can be paired with external control eg homely, havenwise or the like.  The first is, by repute, fit, forget and enjoy cost optimised operation.  The second is a newcomer but can deal with Vaillant heat pumps which homely can't (but then Vaillant heat pumps feature a native adaptive WC mode).  What do you have?

 

 

We have a twinkle in an MCS plumber’s eye right now, we are planning ahead.  Our plans will be vetted by BCO and SAP assessor in the next couple of weeks, so still lots of decisions to be taken.  

[JamesPa]

For what it's worth my ashp was finally fitted a fortnight ago following a two year long and very painful planning saga.  Installer set it up in a mode combining wc and control based on room temperature, all using the heat pumps own system controller with no external controls.

 

After the first week I disabled the room temperature element of the control, and now have it set up on wc only, trvs fully open with heads unscrewed so they can have no effect.  The result so far, in my house and with my pump, is a more stable house temperature than with the mode in which the installer left it, and much, much more stable than was ever achieved with my gas boiler operating at fixed flow temp with a thermostat and trvs.  I have rads, not ufh with a massive slab BTW.

 

Obviously this is one case only, however its doing what the theory says (and what should be the most cost effective), so it's not really a surprise.   I would suggest that those who are sceptical of wc might wish actually to experience it if they haven't already!

Edited December 10 by JamesPa

[G and J]

Can’t wait. 
 

I also wonder if WC based heat input is even more vital with a UFH screed slowing down the response to heat input significantly.  
 

 

[SuperJohnG]

On 26/11/2024 at 11:40, ProDave said:

 

On a cold day, and particularly on a cold, wet dreary day, we like to indulge and fire up the stove,

This is the bit the forum misses I think. Everything is very exacting for the required heat load. My house, with a single zone, 7kW ASHP on WC with a room thermostat performs pretty much perfectly...I don't need a stove...but I'd sure like to add one for those days. 

 

We didn't add one as it'd probably bring in more cold then good.

[G and J]

5 minutes ago, SuperJohnG said:

but I'd sure like to add one for those days. 

I wobble every so often about the woodburner in our plans.   So thank you for sharing that.