Power consumption heat pump

shinytop

Verdigris said:

Is a slightly oversized ASHP not breaking a sweat more efficient than a "correctly" sized one working close to maximum?

Are you allowed to install an slightly oversized ASHP, under RHI rules, to, say (but perhaps not out loud), allow for a future extension?

Yes, I did.  I was borderline and went up a size with no problems.  I don't think Ofgem care if you do that because it's probably only going to affect your pocket; it will still heat your house.  What they don't want is an undersized heat pump where you have to use other forms of heating because that sort of defeats their object.

Reed_Richards

BikingBud said:

How are gas boilers smarter than oil boilers or heat pumps? It's an energy exchange mechanism, it follows the laws of physics. And we all know you canna change them!

The controllers are the smart element, some will have simple timers, turn heating on and off. Others may be configured and able to learn; to provide a room temperature of say 21degc in the bathroom at 07:30 they determine they need to switch on at 04:30 and others will be zoned for different rooms, have different temp curve for the increase in heat required for the drop in temp and further outside air temp compensation curves etc.

Okay, yes, what I meant is that gas boilers can do smarter things.  They can modulate their heat output down to very low percentages of their maximum.  Heat pumps cannot modulate down so much and oil boilers not at all.  Also there is a new control standard called Opentherm which enables a boiler to do clever things, both weather compensation and load compensation.  AFAIK this Openthem standard is only found on gas boilers.  My smart controller works with a heat pump or any conventional boiler, has all the features you mention but it cannot do load compensation because it does not communicate directly with the heat source. 

Reed_Richards

wittynamegoeshere said:...

Also... is there any harm in overdoing it?  If I just go a size or two bigger just in case then would it run at lower efficiency or would there be other downsides, besides the obvious one of having less money left in the bank?

What you have to remember is that a "correctly" sized heat pump will be under-utilised almost all of the time because they are sized for a low outside temperature that does not occur very often.  So, personally, I see no technical harm in going a size or two bigger.  The disadvantage is that the price goes up with size and I think it rises more steeply than would be the case for a gas or oil boiler.

BikingBud

Reed_Richards said:

BikingBud said:

How are gas boilers smarter than oil boilers or heat pumps? It's an energy exchange mechanism, it follows the laws of physics. And we all know you canna change them!

The controllers are the smart element, some will have simple timers, turn heating on and off. Others may be configured and able to learn; to provide a room temperature of say 21degc in the bathroom at 07:30 they determine they need to switch on at 04:30 and others will be zoned for different rooms, have different temp curve for the increase in heat required for the drop in temp and further outside air temp compensation curves etc.

Okay, yes, what I meant is that gas boilers can do smarter things.  They can modulate their heat output down to very low percentages of their maximum.  Heat pumps cannot modulate down so much and oil boilers not at all.  Also there is a new control standard called Opentherm which enables a boiler to do clever things, both weather compensation and load compensation.  AFAIK this Openthem standard is only found on gas boilers.  My smart controller works with a heat pump or any conventional boiler, has all the features you mention but it cannot do load compensation because it does not communicate directly with the heat source. 

I'd be interested to read your source for the assertions. These things can be achieved but not simply.

Again it's down to how the system is designed and the controller is set up rather than the energy source. If you can measure it you can control it.

What you appear to be saying is an electric kettle cannot be used to heat water to 60 deg! The issue is a kettle is generally automatic and set to turn off at 100 deg. A gas kettle runs until the open loop system kicks in ie you take it off the hob. An electric kettle has a fixed power element and a gas hob you adjust the gas flow rate via the burner valve and can lead to much waste. So you can see the effect of the control system is all down to how it is designed, or operated, not the heat source.

More complex control systems are introduced to ensure the optimum transfer of energy to achieve the demanded level of heat, as a general rule ASHP will have ext temp compensation as a standard capability to achieve this. Legacy, inefficient, gas boilers never had the complex control systems that are common now.

Complexity=cost so ASHP are more expensive than cheap nasty boilers.

BikingBud

Reed_Richards said:

wittynamegoeshere said:...

Also... is there any harm in overdoing it?  If I just go a size or two bigger just in case then would it run at lower efficiency or would there be other downsides, besides the obvious one of having less money left in the bank?

What you have to remember is that a "correctly" sized heat pump will be under-utilised almost all of the time because they are sized for a low outside temperature that does not occur very often.  So, personally, I see no technical harm in going a size or two bigger.  The disadvantage is that the price goes up with size and I think it rises more steeply than would be the case for a gas or oil boiler.

Incorrect size will lead to short cycles, unless it is an inverter ASHP.

shinytop

Reed_Richards said:

BikingBud said:

How are gas boilers smarter than oil boilers or heat pumps? It's an energy exchange mechanism, it follows the laws of physics. And we all know you canna change them!

The controllers are the smart element, some will have simple timers, turn heating on and off. Others may be configured and able to learn; to provide a room temperature of say 21degc in the bathroom at 07:30 they determine they need to switch on at 04:30 and others will be zoned for different rooms, have different temp curve for the increase in heat required for the drop in temp and further outside air temp compensation curves etc.

Okay, yes, what I meant is that gas boilers can do smarter things.  They can modulate their heat output down to very low percentages of their maximum.  Heat pumps cannot modulate down so much and oil boilers not at all.  Also there is a new control standard called Opentherm which enables a boiler to do clever things, both weather compensation and load compensation.  AFAIK this Openthem standard is only found on gas boilers.  My smart controller works with a heat pump or any conventional boiler, has all the features you mention but it cannot do load compensation because it does not communicate directly with the heat source. 

I’m not so sure about that.  I’ve just had MMSP monitoring installed and although it’s not quite up and running yet, it is giving some data. This is the heating only energy consumption from my ASHP for 3 hours last night.  It starts at 6pm.  It’s about 10 C outside and the flow temp is about room temperature in the high teens. I manually switched the thermostat up at just before 6:30. It seems to increase energy use in steps until it gets to the flow temp it wants.  It then lets it drop a bit before slowly increasing. It then seems to find a level at which it can maintain flow temp. Sometime after 8, the thermostat stops calling for heat and the flow temp drifts down. 

This doesn’t look like an unmodulated on/off situation.  I know the system is using input and output flows and outside temperature (it’s using weather compensation) to do what it’s doing.  I assume the red line is return flow temp but I don’t know what the energy units are;  the data points are every minute so I think they might be kJ but I need to find out.  It doesn't matter to illustrate relative usage.  I also know that room temperature is not being used for this as the 3rd party wireless thermostat I'm using does not communicate this back to the central ASHP controller; all it does is call for heat.  

I have been told that the system is capable of using room temps to further modulate output and the manual seems to confirm this.  I need to investigate this further but I would have to ditch the 3rd party thermostats.  The ASHP controller is a big box of complicated looking electronics (its about the size of a 1990s PC) so I'd hope it's doing something to justify its size and price.  

[Deleted User]

QrizB said:

DeletedUser said:

QrizB said:

DeletedUser said:

QrizB said:

DeletedUser said:

Is there a nice little graph somewhere that displays ASHP v GSHP electricity required at various outdoor temperatures to bring a room up to 21 degrees?

No, there are far too many variables. That's why a heat pump quote needs to start with a proper energy survey (not just the tick-box exercise of a EPC).

Really? Other than those i've listed, what are the other variables?

OK, your question was:

Is there a nice little graph somewhere that displays ASHP v GSHP electricity required at various outdoor temperatures to bring a room up to 21 degrees?

Factors that you haven't specified and which I can't reasonably infer:

1. What is the heat capacity of the room?
2. What is the rate of heat loss from the room?
3. What is the starting temperature of your room?
   
4. What is the humidity of the outside air feeding the ASHP?
5. How do you propose correlating air temperatures (ASHP) with ground temperatures (GSHP)?
6. Are you delivering the heat at 35C (UFH or fan coils) or at 55C (radiators)?
   

Sorry, i don't see how any of that is relevant to the question asked.

Nevertheless, it is.

Here, read this:

https://www.bbc.co.uk/bitesize/guides/zwrxsbk/revision/1

You obviously haven't understood my question. No matter, i've now found the relevant info. Thanks for your time.

Reed_Richards

shinytop said:

I’ve just had MMSP monitoring installed and although it’s not quite up and running yet, it is giving some data. This is the heating only energy consumption from my ASHP for 3 hours last night.  It starts at 6pm.  It’s about 10 C outside and the flow temp is about room temperature in the high teens. I manually switched the thermostat up at just before 6:30. It seems to increase energy use in steps until it gets to the flow temp it wants.  It then lets it drop a bit before slowly increasing. It then seems to find a level at which it can maintain flow temp. Sometime after 8, the thermostat stops calling for heat and the flow temp drifts down. 

This doesn’t look like an unmodulated on/off situation.  

That's very interesting; I wish I had such monitoring equipment.  It looks as if the heat pump is modulating to try to achieve a flow temperature of approximately 40 C when there is a demand for heat.  When you overshoot it corrects itself, in the first instance by turning off and in the second instance by modulating down to about 26 (%?).  My heat pump goes off completely when there is no demand for heat, that certainly includes the pump that circulates water around the central heating and, as far as I can tell, the pump inside the unit.  Is that true of your system?

The failing of this control system, as I see it, is that when the heat pump is modulating its output between 18:45 and 20:15 it does not know if it is adequately heating the room/house or not.  So it takes from 18:30 until 20:15 for your room to reach its desired temperature.  I would have it override the 40 C weather control setting to take the flow water hotter whilst there is a big difference between the room and the set temperature but then use a lower flow temperature than 40 C when the room reaches its set temperature in order to maintain that temperature.  

Reed_Richards

BikingBud said:

Legacy, inefficient, gas boilers never had the complex control systems that are common now.

I'm not actually saying half of the things you assert, @BikingBud. 

In 1998 I bought a gas boiler that was condensing, could modulate and also did both weather and load compensation.  It annoys me that in 2020 I bought a heat pump that was not capable of the latter control feature.  I subsequently found out that Viessmann heat pumps using a Viessman controller can do this but why on earth is it not standard?       

shinytop

Reed_Richards said:

shinytop said:

I’ve just had MMSP monitoring installed and although it’s not quite up and running yet, it is giving some data. This is the heating only energy consumption from my ASHP for 3 hours last night.  It starts at 6pm.  It’s about 10 C outside and the flow temp is about room temperature in the high teens. I manually switched the thermostat up at just before 6:30. It seems to increase energy use in steps until it gets to the flow temp it wants.  It then lets it drop a bit before slowly increasing. It then seems to find a level at which it can maintain flow temp. Sometime after 8, the thermostat stops calling for heat and the flow temp drifts down. 

This doesn’t look like an unmodulated on/off situation.  

That's very interesting; I wish I had such monitoring equipment.  It looks as if the heat pump is modulating to try to achieve a flow temperature of approximately 40 C when there is a demand for heat.  When you overshoot it corrects itself, in the first instance by turning off and in the second instance by modulating down to about 26 (%?).  My heat pump goes off completely when there is no demand for heat, that certainly includes the pump that circulates water around the central heating and, as far as I can tell, the pump inside the unit.  Is that true of your system?

The failing of this control system, as I see it, is that when the heat pump is modulating its output between 18:45 and 20:15 it does not know if it is adequately heating the room/house or not.  So it takes from 18:30 until 20:15 for your room to reach its desired temperature.  I would have it override the 40 C weather control setting to take the flow water hotter whilst there is a big difference between the room and the set temperature but then use a lower flow temperature than 40 C when the room reaches its set temperature in order to maintain that temperature.  

It's just the limitations of weather compensation; the flow temp depends solely on the outside temp.  Like I said, I think something cleverer is available if I lose the 3rd party thermostats.  The manual says this setting is available:

" In room temp. (Auto adaptation) mode the controller uses temperature sensors around the heating system to monitor space and flow temperatures. This data is regularly updated and compared to previous data by the controller to predict changes in room temperature and adjust the temperature of water flowing to the space heating circuit accordingly. By monitoring not only the outdoor ambient, but the room and heating circuit water temperatures, the heating is more consistent and sudden spikes in required heat output are reduced. This results in a lower overall flow temperature being required."

I could do this now but the wired in control unit with the room temperature sensor is currently in the cupboard with the tank so I'd have to move it outside or use Mitsubishi's own wireless stats, which apparently do communucate room temps back to the central unit.  They are a hundred quid each or so but with the price of electricity they might pay for themselves.  Whether this will heat the room quicker or be cheaper or both I don't know though.

I think when the thermostat stops calling for heat, everything stops part from a very little background stuff.  When the ASHP decides its flow temps are high enough (e.g. after my first high peak), it's still using some power - I assume it's still pumping water around but not heating it, hence the rapid drop in flow temp.