You're quite right, @andrewj, that "always on" refers to the fact the heat pump is constantly running and adjusting the flow temperature rather than intermittently heating at a constant flow temperature. You're also quite right that generally the flow temperature is adjusted based on a weather compensation curve (outside air temperature drops, flow temperature is increased) to form a balance between thermal energy into the house and the house's heat loss. What is often overlooked, though, is that many gas boilers can be set up with weather compensation too and, indeed, that seems to be the norm on the Continent; this country is somewhat luddite in that respect. As a result, there would be very little if any difference in thermal comfort between a house serviced by a gas boiler with weather compensation and a house serviced by a heat pump with weather compensation; in both cases, the internal temperature of the home is constant.

When you regulate with room thermostats, that introduces several variables. Firstly, there is some inevitable thermal lag meaning a given room won't be a constant 21degC so much as constantly within a range between 20degC and 22degC. Secondly, since it takes more energy to raise a temperature than it does to maintain one, it means that when the boiler is active the radiators will be significantly warmer than the room and so occupants will feel a direct heat in addition to any ambient temperature. Thirdly, taking things to a greater extreme, the fact a boiler can warm a home pretty quickly by means of a high flow temperature, many people choose to turn the heating off when they're not at home and that means even when the house is warm again it's more direct heat than ambient heat since the building fabric is still not up to temperature.

All this not only affects what kind of heat is being experienced but also potentially changes in humidity of the air and the existence of warm or cold spots in the house. Always on with weather compensation means an even heat distribution throughout the house and a stable temperature as well. Whether that is more comfortable is personal preference but there are certainly many people who get on with it better.

As for the shoulder seasons - the edge cases you mentioned, you're quite right that if a heat pump can't modulate down enough then it'll start cycling - turning off for a while and back on again later. In this case, there will be a slight fluctuation of temperature in the home but not to any appreciable extent.

If you were to look a long way back to some of the forum's early videos you'd find one where Graham Hendra talks about warm radiators and gives the truism that you don't actually want a warm radiator; you want a warm room. If you can get a gas boiler to keep your radiators going low and slow, you'll get the same kind of thermal comfort as a well set up heat pump. If you run your heat pump like a boiler (flat out for short periods of time and turning off in between) you'll get a similar effect to a typical boiler-heated house, albeit at a high cost.  

Posted by: @majordennisbloodnok

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You're quite right, @andrewj, that "always on" refers to the fact the heat pump is constantly running and adjusting the flow temperature rather than intermittently heating at a constant flow temperature. You're also quite right that generally the flow temperature is adjusted based on a weather compensation curve (outside air temperature drops, flow temperature is increased) to form a balance between thermal energy into the house and the house's heat loss. ...

When you regulate with room thermostats, that introduces several variables. Firstly, there is some inevitable thermal lag meaning a given room won't be a constant 21degC so much as constantly within a range between 20degC and 22degC. Secondly, since it takes more energy to raise a temperature than it does to maintain one, it means that when the boiler is active the radiators will be significantly warmer than the room and so occupants will feel a direct heat in addition to any ambient temperature. ...  

This was my point though: the Heat Pump IS controlled by the Primary Pod (a thermostat by any other name) and is coming on and going off based on the reported temperature.  So surely the Heat Pump introduces the same issues as a gas boiler running under similar controls.

On a personal level, we had 9 thermostats governing UFH and TRVs on all the upstairs radiators with our previous lpg installation. These stats had 6 daily programmes each adjustable daily - so something like 350 combinations. Invariably at some point somewhere in he house would be cold and somewhere else hot. And we tended to run everything a bit cool and supplement with log burners. 

but with ASHP the whole space is the same temp. All the thermostats are set as limiters at the same temperature - our only issue is in the depth of winter when there is very limited airflow in the house - it can get musty. 

if I was to redesign a house for ASHP I would certainly use UFH (upstairs and downstairs) and I would dispense with internal doors downstairs (apart from WC!), and I would remove all controls, pumps, valves, thermostats and power everything from the ASHP. I might add some mechanical heat recovery to force some air circulation though

(our house is c. 450m2) 

Posted by: @andrewj

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Posted by: @majordennisbloodnok

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You're quite right, @andrewj, that "always on" refers to the fact the heat pump is constantly running and adjusting the flow temperature rather than intermittently heating at a constant flow temperature. You're also quite right that generally the flow temperature is adjusted based on a weather compensation curve (outside air temperature drops, flow temperature is increased) to form a balance between thermal energy into the house and the house's heat loss. ...

When you regulate with room thermostats, that introduces several variables. Firstly, there is some inevitable thermal lag meaning a given room won't be a constant 21degC so much as constantly within a range between 20degC and 22degC. Secondly, since it takes more energy to raise a temperature than it does to maintain one, it means that when the boiler is active the radiators will be significantly warmer than the room and so occupants will feel a direct heat in addition to any ambient temperature. ...  

This was my point though: the Heat Pump IS controlled by the Primary Pod (a thermostat by any other name) and is coming on and going off based on the reported temperature.  So surely the Heat Pump introduces the same issues as a gas boiler running under similar controls.

Yes, it probably does.

It's not that uncommon for some to have thermostats in a heat pump home with the thermostat temperature set to slighly higher than the target room temperature just so the thermostat acts as a backstop (with the heat pump doing its normal WC thing up to that point), so there are scenarios where a thermostat can have a legitimate use. However, ultimately the thermostat is only providing the heat pump with an on/off switch which is bypassing all that clever modulating programming that makes a heat pump efficient and cost-effective. Removing the thermostat from the equation, balancing the emitters properly and running the heat pump with weather compensation will put the heat pump back in control, make the living space more comfortable and, just as importantly, reduce your costs.

That said, I'm not familiar with a "primary pod", and if it is interacting with the heat pump in a more sophisticated way (i.e. doing a weather compensation kind of thing but based on internal temperature) then all bets are off; that's far from working as a thermostat. I'm off to do some research and see what these pods are all about.

**edit**

I've taken a quick squizz at the Octopus site and they talk about the pods as room sensors rather than thermostats. So far as I can see from the short poke around, Octopus set up the heat pump with weather compensation (i.e. the external temp defining the flow temperature) to meet a target internal temperature measured by the pod. That's the efficient way to do it, so I hope I'm right.

Posted by: @andrewj

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I heard this a lot: having a heat pump creates a home environment that is a consistent warmth throughout, because it's always on, and is a lot more comfortable than the heating provided by a gas boiler which is an on/off system.

Inherently it doesn't.  However a typical well executed heat pump installation will be more comfortable than a typical well executed boiler installation and the way this is done plays to the strength of a heat pump.

A typical boiler installation has the flow temperature fixed at 55C or even 75C and each room controlled by a room stat or TRV.  Thus the heat source in each room is (a) very hot and (b) cycles on and off.  This creates temperature gradients across the room and with time and tempts us to switch the heating on and off thus never properly warming up the fabric. 

A typical heat pump installation has a variable flow temperature that depends on the outside temperature (weather compensation) and is often 45C or less even when its really cold outside and perhaps only 27-30C when its mild - so the rads are luke warm.  The heat sources in each room are on most or all of the time and because of this and the fact that the water temperature is lower, temperature gradients are smaller.  Furthermore because you leave it on most if not all of the time the fabric warms up, so the walls are warm not cold and thus enhance the comfort.

You can operate a boiler with variable flow temperature (ie weather compensation) like a heat pump, although often they dont go down anything like as low so you cant achieve the full effect you get with a heat pump. As a matter of fact WC is mandatory in some mainland European countries which have more advanced heating industries than ours, which has largely shunned WC and low flow temperatures for boilers, and thus cost most of us 10% more than we should have been paying and degraded our comfort.

Before I got my heat pump (after I understood the blind alley that the industry has forced us down) I turned down my gas boiler flow temperature from the 75 it was set at by the installer to the lowest it would go (50C) and applied manual weather compensation (meaning that I turned it up to 55 when it was very cold).  In addition I operated 24x7 (as opposed to the typical two window pattern).  I had to retain TRVs because the flow temp was still higher than necessary to match the heat loss.  The reduction in flow temperature and consequently reduced gradients and increased run times resulted in a more comfortable house and lower bills.  When I got my heat pump I dispensed with the TRVs and operated at the much flow temperature necessary to just to match the loss.  This resulted in a more comfortable house still and a further reduction in bills.

As luck would have it running at the lowest possible flow temperature for a long time is the most efficient operating point for a heat pump, hence why I say this mode plays to their strengths.  Actually thats true of a boiler also but the effect is less pronounced.

As @majordennisbloodnok says, the other feature of WC is that it gets the signal to do something (ie a change in outdoor temperature - which of course drives the loss) before its too late.  Control based on room temperature alone relies on 'rear view mirror' operation, by the time the controller knows to change what the heating is doing it is too late to do so given the reaction time of the house.  This strength of this effect varies from house to house of course, some houses react very quickly, others take 12 hours or even a day to react.  In the latter case the outdoor conditions, which drive the heat loss, can change massively during the time it takes the house to react, leading to control instability.  This behaviour is something that experts in control theory will recognise but the rest of us find slightly counterintuitive.  Sometimes, however, the counterintuitive turns out to be the way the world works.

Posted by: @majordennisbloodnok

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**edit**

I've taken a quick squizz at the Octopus site and they talk about the pods as room sensors rather than thermostats. So far as I can see from the short poke around, Octopus set up the heat pump with weather compensation (i.e. the external temp defining the flow temperature) to meet a target internal temperature measured by the pod. That's the efficient way to do it, so I hope I'm right.

I couldn't find any information specifically about the pods - do you have a link please?  Mine is on WC but cycling on/off around the 22c temperature (I've checked, the unit is definitely off right now and the app states "heating" when it is on) and my expectation is that it would be staying on and modulating the flow temperature instead.  Even projecting forwards into the depths of winter, it would imply that once it deems the house up to temperature (determined by one sensor in one room!) it would be in a on/off mode. I'm finding it a bit confusing to be honest.

Posted by: @jamespa

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...

Before I got my heat pump (after I understood the blind alley that the industry has forced us down) I turned down my gas boiler flow temperature from the 75 it was set at by the installer to the lowest it would go (50C) and applied manual weather compensation (meaning that I turned it up to 55 when it was very cold).  In addition I operated 24x7 (as opposed to the typical two window pattern).  I had to retain TRVs because the flow temp was still higher than necessary to match the heat loss.  The reduction in flow temperature and consequently reduced gradients and increased run times resulted in a more comfortable house and lower bills.  When I got my heat pump I dispensed with the TRVs and operated at the much flow temperature necessary to just to match the loss.  This resulted in a more comfortable house still and a further reduction in bills.

As luck would have it running at the lowest possible flow temperature for a long time is the most efficient operating point for a heat pump, hence why I say this mode plays to their strengths.  Actually thats true of a boiler also but the effect is less pronounced.

As @majordennisbloodnok says, the other feature of WC is that it gets the signal to do something (ie a change in outdoor temperature - which of course drives the loss) before its too late.  Control based on room temperature alone relies on 'rear view mirror' operation, by the time the controller knows to change what the heating is doing it is too late to do so given the reaction time of the house.  This strength of this effect varies from house to house of course, some houses react very quickly, others take 12 hours or even a day to react.  In the latter case the outdoor conditions, which drive the heat loss, can change massively during the time it takes the house to react, leading to control instability.  This behaviour is something that experts in control theory will recognise but the rest of us find slightly counterintuitive.  Sometimes, however, the counterintuitive turns out to be the way the world works.

From my OP you can see I did something similar with my gas heating.  It was still, ultimately, controlled by the Hive thermostat in the Living Room though which controlled the boiler to come on/off as the Living Room required it.  To me that is an on/off system.

I could envisage the scenario for the Heat Pump to use a temperature sensor to modulate the Flow Temperature but to continuously provide heat into the house using the flow temperature to modulate around the set temperature.  That to me is an on system (i.e. it never goes off unless I turn it off.)

Although my Cosy 9 is using WC, it is "modulating" around the Primary Pod's reading by turning on/off as necessary.  I've double checked: the Pod is reading 22.3c and the heat pump is off; I have seen it turn on when the Pod was reading 21.9c.  This is the bit I'm confused about because this looks like thermostat controlled behaviour.  

Posted by: @andrewj

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Mine is on WC but cycling on/off around the 22c temperature (I've checked, the unit is definitely off right now and the app states "heating" when it is on) and my expectation is that it would be staying on and modulating the flow temperature instead.  Even projecting forwards into the depths of winter, it would imply that once it deems the house up to temperature (determined by one sensor in one room!) it would be in a on/off mode. I'm finding it a bit confusing to be honest.

Cycling depends on (a) how it is controlled - ie how it has been set up and (b) the outside temperature.

If it has been set up to be controlled by the room thermostat then it will cycle with the room thermostat, albeit with a longer period if WC is operating because the flow temperature is closer to the point at which the loss from the house just matches the heat supplied.

Even if it is controlled on pure WC it will still cycle when its mild outside, because it can only reduce its output so far, and if its warm enough outside the power required to balance the loss from the house is less than the minimum that the heat pump can go down to (boilers do the same).  Thus it switches on and off from time to time so that the average power supplied matches the loss from the house.

It sounds like you might be seeing the latter at present. although without knowing more detail of your control system (I dont know how the tentacled beast sets things up) I cant be sure.  Mine (which is on pure WC with no room influence at all) currently cycles during the day, when its warm, and runs continuously at night, when its cold.  Later in the season when its colder during the day it will run continuously 24x7.  In the depths of the season it will cycle in a different way and for another reason, namely it will 'defrost' between once per hour and once every 4-6 hours, which involves it changing mode to shed frost build up on the fins.  The controller sorts all of this out fortunately.

Best not to worry about this too much unless you are interested, in which case take time to learn and dont expect to understand everything on day 1!

The document I found was this one

A heat pump cycling at this time of year when it's still pretty mild is not really an issue as long as it's not too frequent. It's just that whilst the outside air temperature is reasonably high the flow temperature required to offset the house's losses is lower than the lowest flow temperature the heat pump can maintain. Once the outside air temperature drops a bit more, the house's losses will increase and the required flow temperature will rise to a point where the heat pump can remain constantly on. From that point onwards it'll be constantly regulating the flow temperature, only stopping periodically to defrost or to heat your hot water tank.

I can easily understand you feeling like you're getting a bit confused; you're certainly not alone here. That's one of the reasons this article was written to reduce the jargon and hopefully clarify. I know there'll be a lot in it that you do already know but the explanation about running the heat pump low and slow is worth reading and may fill in some gaps in your understanding.

Posted by: @jamespa

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Cycling depends on (a) how it is controlled - ie how it has been set up and (b) the outside temperature.

 

If it has been set up to be controlled by the room thermostat then it will cycle with the room thermostat, albeit with a longer period if WC is operating because the flow temperature is closer to the point at which the loss from the house just matches the heat supplied.

Even if it is controlled on pure WC it will still cycle when its mild outside, because it can only reduce its output so far, and if its warm enough outside the power required to balance the loss from the house is less than the minimum that the heat pump can go down to (boilers do the same).  Thus it switches on and off from time to time so that the average power supplied matches the loss from the house.

It sounds like you might be seeing the latter at present. although without knowing more detail of your control system (I dont know how the tentacled beast sets things up) I cant be sure.  Mine (which is on pure WC with no room influence at all) currently cycles during the day, when its warm, and runs continuously at night, when its cold.  Later in the season when its colder during the day it will run continuously 24x7.  In the depths of the season it will cycle in a different way and for another reason, namely it will 'defrost' between once per hour and once every 4-6 hours, which involves it changing mode to shed frost build up on the fins.  The controller sorts all of this out fortunately.

Best not to worry about this too much unless you are interested, in which case take time to learn and dont expect to understand everything on day 1!

and @majordennisbloodnok I just went to make a cup of coffee and ponder whilst that was happening.  I thought: "perhaps it is already running at the lowest flow temperature it can achieve and that temperature is high enough to heat above the requested temperature, in which case the only way it can bring it down again is by turning off."

I think that concurs with what you are both saying and that I will see behaviour in line with my expectations as the weather gets colder.  I suppose that also means some rooms are likely to be cooler than others simply because of this thermostat-like behaviour.  This is my first heating season by the way, it was actually installed at the beginning of May and has never had the heating function turned on.  As long as I can explain this to my wife I should remain on steady ground!!

The attached image shows its behaviour today. The other image shows the setup for WC.

Posted by: @andrewj

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I think that concurs with what you are both saying and that I will see behaviour in line with my expectations as the weather gets colder.  I suppose that also means some rooms are likely to be cooler than others simply because of this thermostat-like behaviour.

...

Yes, I think it does concur.

Regarding some rooms being cooler than others, I suspect that may have more to do with radiators not being as well balanced as they could be. There's another article - this time by @toodles - covering this and it's here. Balancing is often not done well by installers simply because it takes more time than they have available, but it's a straightforward process for a homeowner to do and gives good results once done.