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Basic ASHP Efficiency Questions

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15 hours ago, CallMeMeryl said:

The way the heat pump was 'sold' to use was that although electricity is dearer than gas, it would be much more efficient. Am I just misinformed do you think?

IMO yes. When you need the most heat they are usually the least efficient. I would only fit an ASHP in a well insulated airtight house and I would use gas or oil in an old, poorly insulated house. Whatever source of heating is used, it needs to be sized correctly.

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2 hours ago, PeterStarck said:

IMO yes. 

What a shame so much money is being invested in something that does not appear to work too well! My fault, I should have done my research rather than believe what I was told.

 

One final question (and I realise it is annoying being asked basic questions by uninformed people, so thank you for replying): Is there any setting you are aware of that I can change to make it a bit more responsive?  At the moment trying to effect a change in temperature is like trying to turn a supertanker.

 

BTW, we live about 10 miles away from you looking at your blog. :)  I am beginning to wish I had started from scratch.

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2 minutes ago, CallMeMeryl said:

 

One final question (and I realise it is annoying being asked basic questions by uninformed people, so thank you for replying): Is there any setting you are aware of that I can change to make it a bit more responsive?  At the moment trying to effect a change in temperature is like trying to turn a supertanker.

To continue your analogy, you won’t be able to make your supertanker handle like a ski boat, so don’t try. Use it in the way that it’s able to operate. 
I have an old and predominantly poorly insulated house and an ASHP. The heating isn’t responsive, but it is good at keeping a steady constant temperature. So I had to move away from thinking of the heating working like it did in other homes with oil or gas boilers.

My set up seems to be fine once up to temp, as my house is always at 21.5*c (even if away). 
 

It’s worth noting the amount of people who first turn on their system thinks something is wrong because the house is taking days to get up to temperature (some times 1 degree a day).. but that’s fairly normal. I thinking @ProDave said it took his house a couple of days to warm back up after going on holiday for a couple of weeks (in the dim and distant past).

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9 minutes ago, Gav_P said:

 

It’s worth noting the amount of people who first turn on their system thinks something is wrong because the house is taking days to get up to temperature

It is because people seem to have trouble grasping the difference between temperature, power and energy.

There is still a misconception that lower temperature means lower power, and therefore lower energy by definition.

This is wrong.

Not many people have an ASPH larger than 12kW. But even small houses often have a 20 kW gas boiler. Now some of that oversizing is to allow DHW to be instantly heated (and that is not a great flow rate). But it does mean that during cold spell, an extra 8kW or so of power is available for space heating.

 

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8 minutes ago, Gav_P said:

To continue your analogy, you won’t be able to make your supertanker handle like a ski boat, so don’t try. Use it in the way that it’s able to operate. 

I will settle for canoe mode :)

 

I understand all you say and it makes sense.  I feel confused by the installer and also the documentation that shows lovely weekly programming schedules with 6 different events in each day.  Indeed the installer asked me what temperatures I would like at different times of the day and set the schedule accordingly.  I soon found out this was nonsense, firstly when I got up to an icy cold bedroom in the morning!

 

Have you any comment on the setting the installer changed right at the start? 

 

We are where we are, so I will console myself with the hope that I am at least cutting carbon emissions. 

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25 minutes ago, CallMeMeryl said:

One final question (and I realise it is annoying being asked basic questions by uninformed people, so thank you for replying): Is there any setting you are aware of that I can change to make it a bit more responsive?

We have an EASHP providing warm air heating which only supplements our heating provided by three electric towel rails. I don't use a normal ASHP providing wet heating but I would follow what @Gav_P says as ASHPs can't be used as a direct swap for an oil or gas boiler and they aren't cheap to run when it's cold.

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4 minutes ago, SteamyTea said:

It is because people seem to have trouble grasping the difference between temperature, power and energy.

Guilty as charged!

 

20 hours ago, CallMeMeryl said:

the installer amended the curve from 55 to 45 @ -15 degrees, and 35 @+15 degrees.  Does this make sense?

 

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1 minute ago, CallMeMeryl said:

the installer amended the curve from 55 to 45 @ -15 degrees, and 35 @+15 degrees.  Does this make sense

Not on the face of it.

Would have to look at the spec if the unit and it's control system.

But at the moment I am looking at the tide going out, which warms up the town. 12 hours time, the tide will cool the tide down again.

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7 minutes ago, SteamyTea said:

There is still a misconception that lower temperature means lower power, and therefore lower energy by definition.

This is wrong.

So my OH suggested lowering the temperature we have set by 1 degree to see if it affects running costs.

 

If I understand something from your comment would this mean that whilst we might see a saving for a very short time, whilst the house cooled down, as soon asASHP started to maintain the new temperature it would be back to using the same amount of power?

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43 minutes ago, CallMeMeryl said:

If I understand something from your comment would this mean that whilst we might see a saving for a very short time, whilst the house cooled down, as soon asASHP started to maintain the new temperature it would be back to using the same amount of power

Not always.

If you lower the mean internal temperature by 1⁰C, you reduce the losses through the walls  floors, roof, windows and doors, and the ventilation losses.

But you have a colder house.

This is not the same as reducing the ASHP, or any other wet heating system flow temperature by 1⁰C.

With an ASHP, it may increase efficiency enough to counteract a longer running time. With a condensing boiler, it may mean you get less condensing and loose overall efficiency.

Energy (kWh) is Power (kW) X Time (h). We should really calculate in seconds (s), but the numbers soon get s lot of zeros.

Just to confuse things, the unit of energy is the joule (J), but there is a direct conversion to kWh.

A watt (W) is a joule per second.

So a kWh can be described as 1000 watts running for 3600 seconds or 3.6 MJ.

Probably notice there is no mention of temperature (T) in the description of power and energy. Temperature difference is the important bit. No temperature difference (0∆T) no energy transfer, so no power used. Called equalibrium. 

Increase the ∆T and there is an energy flow (Q). If you see the term flow, flux or Q, then you know there is a time element involved.  The usual letter for time is t, not to be confused with T for temperature (makes for a gun time in calculous ∆T/∆t).

To confuse even more, when dealing with heating systems, surface area (m²) becomes important. This is why a properly installed under floor heating system may run at 28 to 35⁰C. It has a lot of area to distribute the energy over time. It is why, with traditional radiators they are run at a high temperature (up to 70⁰C) to counteract their small surface area.

Heating an identical room, well the air in it, takes the same amount of energy. This is due to air needing the same amount of energy to be heated the same amount. This is known as the heat capacity (Cp measured by volume) or the specific heat capacity (SHC measured by mass).

Air has a SHC of around 1 kJ/(kg.K). We should really always work with the kelvin scale as it removes odd results when passing 273K, or 0⁰C.

 

Like all basic physics, the concepts are simple, it is the detail that gets confusing.  Why I have avoided radiative heating in the calculations. It is the radiative element that can make some rooms feel warmer than the air temperature leads you to believe.

Humidity levels can have the same effect.

They can both be calculated, as can latent heating affects, but a wall thermostat only measures temperature.

Edited by SteamyTea

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1 hour ago, Gav_P said:

To continue your analogy, you won’t be able to make your supertanker handle like a ski boat, so don’t try. Use it in the way that it’s able to operate. 
I have an old and predominantly poorly insulated house and an ASHP. The heating isn’t responsive, but it is good at keeping a steady constant temperature. So I had to move away from thinking of the heating working like it did in other homes with oil or gas boilers.

My set up seems to be fine once up to temp, as my house is always at 21.5*c (even if away). 
 

It’s worth noting the amount of people who first turn on their system thinks something is wrong because the house is taking days to get up to temperature (some times 1 degree a day).. but that’s fairly normal. I thinking @ProDave said it took his house a couple of days to warm back up after going on holiday for a couple of weeks (in the dim and distant past).

Yes we went away for a 2 week holiday last September. Not had any heating on before we went away, and it turned cold while we were gone.  And with no occupancy and no incidental heating, the house was 12 degrees inside when we got back late of an evening.

 

Heating on and WBS at full tilt warmed it up a bit for us to have a meal and go to bed but it was 2 days before the house was back to proper temperature and the UFH could then relax a bit.

 

This might be the one and only time I would concede that having your heating contollable over the internet would be useful, we could have turned it on 2 days before our return.

 

A feature of very well insulated houses is they hold their heat, so take a long time to cool down.  You don't want the rate of heat input when the heating is on to be too great otherwise the temperature will overshoot.  So it is set up to only heat the house gently and that works well to maintain a comfortable temperature  throughout the year, even when below -10 outside.  What it is not good at is heating the building quickly which is where having the WBS as a second source og heat helps a lot.

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@SteamyTea

Thank you for such a detailed explanation. I really appreciate it. I think I am going to leave all the settings as they are and accept that I have to change the way I use 'heating'.

 

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4 minutes ago, Temp said:

COP vs flow temperature graph

Aah, OK I think the penny has dropped. So a bit like thrashing my car 0-60 in 3 seconds uses more fuel than if I slowly accelerate at a reasonable pace?

 

Thank you ;)

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1 minute ago, CallMeMeryl said:

Aah, OK I think the penny has dropped. So a bit like thrashing my car 0-60 in 3 seconds uses more fuel than if I slowly accelerate at a reasonable pace?

 

Thank you ;)

Or more a case of driving at 50MPH instead of 90MPH.

The air resistance goes up with the square of the speed.

(Velocity and Speed are different)

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2 hours ago, SteamyTea said:

Or more a case of driving at 50MPH instead of 90MPH.

The air resistance goes up with the square of the speed.

(Velocity and Speed are different)

got it 

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5 hours ago, ProDave said:

Heating on and WBS at full tilt warmed it up a bit for us to have a meal and go to bed but it was 2 days before the house was back to proper temperature and the UFH could then relax a bit.

 

Sounds like the time we went away for Xmas from our terraced house in Sheffield. We only had gas fires in the downstairs rooms in those days - mid '70s.  It took a good 3 days or so to get some heat back into the house.  Chilly times - dressing in the morning in front of the electric fan heater....

 

Simon

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