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The Mystery of the Weather Compensation Curve

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(@bob77)
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Posted by: @alec-morrow

@cathoderay put simply, waiting for things to get cold means you have to generate excess heat to raise temperature room temp quickly. That’s water in the system, pipes and radiators etc!

 

couple with keeping something warm uses less energy, at a lower temperature enhancing efficiency!

 

 

 

Is that actually true though? See the other thread entitled "heat pump myths". The cooler a house is at any given time, the less heat it is losing to the outside world. Weather compensation alone can't account for your usage of the home. For instance in my new extension I have a TV room that is generally only used in the evenings and weekends, so there is no point keeping it warm all the time. Surely the best option from an energy efficiency point of view is to use weather compensation on the heat pump combined with a timed thermostat to bring it up to temperature when I need it? In colder weather the room will have lost more heat but the WC will have increased the flow temperature so it will deliver more heat and hopefully warm it up fast enough. Though it may need some fiddling with the thermostat timings and WC curve to get it working as it should.

I would have thought the best way to conserve energy is not to heat spaces when they are not in use, but to be careful to avoid the "overshooting" which wastes heat. So carefully timed thermostats that can call for more gradual heat, right?

 


   
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(@alec-morrow)
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I’m not sure what you are questioning but keeping something warm must use less energy than heating up and cooling down. Of course if you bring usage into the equation then you can minimise hear loss

 

a heat pump though is least efficient at high load, but at low temp Cop should still be higher than 1

 

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cathodeRay
(@cathoderay)
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Posted by: @alec-morrow

keeping something warm must use less energy than heating up and cooling down

This is not necessarily the case. Which uses more energy: keeping my house at 18 degrees all day every day for a week, or heating my house to 18 degrees on Sunday, and leaving it cool for the rest of the week?

I suspect there is a complex break even point, the complexity added by the facts that (a) more energy is need to raise temperature rather than keep it steady and (b) heat pump efficiency is significantly inversely related to flow temps and (c) all the other things not yet factored in.  

Midea 14kW (for now...) ASHP heating both building and DHW


   
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(@scrchngwsl)
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Posted by: @alec-morrow
hence you thinking Nest is a good control!

To be clear, I didn't say that Nest is a good control, nor do I think that.

 

ASHP: Mitsubishi Ecodan 8.5kW
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(@ronin92)
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@alec-morrow A room and its contents has a given thermal content at given temperature.  When it cools, it's because it loses heat to the exterior through conduction, convection, air flow, etc.  If there were no loss, the room temperature would not change.  The comparison between heating to a constant temperature and allowing the room to cool when unused and reheating it is then just a matter of how much heat is lost from the room in each case. Heat loss is always higher for a higher room temperature.  The temperature profile against time of the momentarily unheated room is always going to be below that of the constantly heated room so the heat loss will always be lower in the former case.  So one would expect constant heating to incur higher energy losses and therefore energy use.

One might want a constantly heated room for practical reasons like the heating system takes too long to reheat the room so the heating is off too briefly to justify the fuss (in this, the temperature plummets quickly because of poor insulation and the emitters take forever to restore it).  I can certainly believe this to be true with some ASHP installations.  Or perhaps the sharply varying load of one room upsets the distribution of heating within the house.


   
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(@bob77)
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Posted by: @alec-morrow

I’m not sure what you are questioning but keeping something warm must use less energy than heating up and cooling down.

 

 

Why must it? Heat loss is proportional to the difference between indoor and outdoor temperatures.

Heat loss Q depends on the thermal transmission value U, the area A and the differential in temperatures Δt:

Q = U × A × Δt

U and A will remain constant, so the only variable is Δt.

Say it's a constant 0C outside, if you keep the house at 20C all week then Δt is 20C and the heat loss will be 20 x U x A.

If you let the house cool down to 10C and then heat it back up to 20C, then let it cool down to 10C again and so on, then the average temperature over the week would be 15C, so Δt is now 15C, and the heat loss will be 15 x U x A.

Obviously that's an extreme example, but the fact remains that a warmer house is losing more heat to the outside world, and that has to come from somewhere.

If you were using pure resistance heating, then there is no question it would be more efficient to let the house cool down and then heat it up as needed. Of course, with heat pumps, there is the added complication that maintaining a constant temperature can more easily be done with a low flow temperature than heating the house up. But depending on the efficiency of your emitters, it should still be possible to warm the house up using a low flow temperature without wasting heat by keeping rooms warmer than they need to be 24/7.

 


   
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(@alec-morrow)
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Posted by: @ronin92

@alec-morrow A room and its contents has a given thermal content at given temperature.  When it cools, it's because it loses heat to the exterior through conduction, convection, air flow, etc.  If there were no loss, the room temperature would not change.  The comparison between heating to a constant temperature and allowing the room to cool when unused and reheating it is then just a matter of how much heat is lost from the room in each case. Heat loss is always higher for a higher room temperature.  The temperature profile against time of the momentarily unheated room is always going to be below that of the constantly heated room so the heat loss will always be lower in the former case.  So one would expect constant heating to incur higher energy losses and therefore energy use.

One might want a constantly heated room for practical reasons like the heating system takes too long to reheat the room so the heating is off too briefly to justify the fuss (in this, the temperature plummets quickly because of poor insulation and the emitters take forever to restore it).  I can certainly believe this to be true with some ASHP installations.  Or perhaps the sharply varying load of one room upsets the distribution of heating within the house.

 

that’s not wrong, but it’s the plant that needs to be made efficient and that’s why weather comp is universal on appliances.

 

weathet comp controls also turn things on and off .. they are just glorified on-off thermostats with the benefit of higher comfort levels, more efficiency and more reliable machines (boilers or heat pumps) because that’s the way they are designed to be used

 

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(@oswiu)
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@bob77 this rationale is totally correct with a heating system that has a fixed efficiency eg a series of electric fan heaters which are always about 100% efficient. It may or may not be true with a heat pump or condensing boiler, the reason being that all other things being equal you would need to raise flow temperature (lowering overall efficiency) to provide for losses to a non heated room, and that the compressor in a heat pump is less efficient when working full pelt to bring a room up to temperature. The same is true but to a lesser extent with condensing boilers which can better condense at lower return temperatures, although the difference doesn't seem to be as great. 

Some say zoning can't really work with a heat pump. I imagine it would work fine if you design for it, but you would need very large emitters to get an unheated room up to temperature quickly without changing the flow temp. You would also need larger emitters in adjacent rooms due to internal losses to that room.

I don't think there is a general answer to this question and one would have to find it by experimentation with a specific need and heat pump. 


   
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(@alec-morrow)
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Yes that’s true, it’s about plant efficiency ( a route to economy) which is enhanced at low flow temps universally

 

low temps means poor recovery rate so best avoid things getting too cool

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(@alec-morrow)
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Yes that’s true, it’s about plant efficiency ( a route to economy) which is enhanced at low flow temps universally

 

low temps means poor recovery rate so best avoid things getting too cool

Professional installer


   
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(@ronin92)
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I'd add that air temperature is not the only determinant of thermal comfort in a room.  A major component of the perception of cold is radiative loss from the body.  When a room cools, the walls cool first and draw heat from the air.  When reheating, the air gets heated and that has to warm up the walls and that can be slow.  Cool walls make you feel colder by increasing radiative loss from the body even though the air temperature target has already reached.  I've occasionally wondered if a scannable IR heating beam could be controlled by a thermal imager to identify and optimally warm up the people without needing to heat the room itself.


   
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(@oswiu)
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@ronin92 I absolutely agree with this. A year ago I was allowing my home office to go unheated except during working hours, and whilst the air temperature was supposedly 21.5C when I went to work in the morning, I always felt cold probably from radiation, sitting on a cold chair, and having my arms on a cold desk. Now it's at 20C and with the same clothes on I feel a great deal warmer.

I've heard it said that people who have those IR panels don't heat their homes to as warm because it heats their bodies directly. I doubt a couple of degrees makes up for using 100% efficiency heat compared to a heat pump however. Also if you had "scanning" IR heaters to just heat people, I imagine it would still make life unpleasant having such cold air and everything you touch being cold, not to mention the risks of condensation, mold and pipes freezing, and then every time you were in the "shade" being thrown back into coldness again. Maybe it would be useful to just boost by a tiny bit, but it seems rather complicated to do so! 


   
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