My ASHP Journey in Bonnie Scotland.

matt_drummer

cannugec5 said:

matt_drummer said:

Here's a rough example of how radiators work.

If we had a radiator that was rated @ 1,000 watts output at a delta t of 50c and room temperature of 20c

That means if the mean (average) flow temperature was 70c (typical of many gas or oil boilers) and the room was 20c that radiator would deliver 1,000 watts of energy to the room, it could consume 1,000 watts from the heat source.

At a delta t of 30c, so a mean flow temperature of 50c and the same room temperature of 20c that same radiator can only deliver about 500 watts of heat to the room. This is a typical design point of many heat pump installations. That radiator can now only absorb 500 watts of heat from the heat pump.

At a delta t of 10c, so a mean flow temperature of 30c and the same room temperature of 20c that same radiator can now only deliver about 135 watts of heat to the room and it follows that it can now only absorb 135 watts of the heat produced by the heat pump.

So radiators need to be really, really big to run at low flow temperatures, not just to be able to heat the house but also to absorb the minimum amount of heat delivered by the heat pump.

If half your radiators are `closed' by the trvs then there is not enough capacity to deliver the heat produced and the heat pump has to turn off.

If the heat pump is now only flowing water through your downstairs radiators the heat pump will try and put in enough heat to achieve your desired room temperature. Depending on your settings this might involve the flow temperature increasing but you could find that you never get the temperature you want, the heat pump cycles on or off quickly costing you a fortune or that you end up consuming more electricity to heat just your downstairs than you would have done heating the whole house.

I know I’m coming across as really stupid, but I am struggling to understand this.

The downstairs temperature is easily reached and stays on target all day and night at the constant setting. 

When you (and others) use the term ‘cycling’ do you simply mean switching on and off? Isn’t that what the pump is meant to do? Surely the alternative is to stay on or off, neither of which will maintain the constant temperature. 

I do ( I hope) understand what you say about the upstairs radiators not using up the produced heat and therefore causing the pump to switch off. I think I might experiment with putting the upstairs thermostat the same as downstairs ( except in our own bedroom). That will ‘switch on’ an extra 6 radiators. 

You are not stupid. Some of this is very counter intuitive. It's catching me out and I am working through the process I am describing to you below. It is a bit overwhelming how much there is to take in and understand to get the best from a heat pump.

 Generally speaking, heat pumps consume the most electricity compared to the heat output in the first few minutes at start up.

If your heat pump runs for 5 minutes, switches off for a few minutes and then starts up again for another 5 minutes  and so on and so on then all your electricity will be consumed in the most power hungry part of the heat pumps heating cycle. This is called short cycling. It will cost you the most amount of money to run.
The heat pump will become more efficient the longer it runs as it is all up to temperature and it is only adding relatively small amounts of heat to the water returning from the radiators.

Of course, you don't want the heating pump running for longer than necessary as consuming more electricity than you need and producing more heat than you need is just a waste. It is a balance between efficiency and overall electricity consumption.

Your zones, trvs and room thermostats are masking what is going on with your heat pump.

What should happen is that your radiators are sized to match the heat loss of each room, none too big, and none too small, so whatever the water temperature flowing through them is gives the room temperatures desired. 

So to start with you try to run with no influence from any of these items, turn the room states right up and the trvs wide open.

Then set up a weather dependent curve on your heat pump controller where the temperature of the water flowing around you radiators is controlled by the outside temperature.

You adjust the curve as necessary so that whatever the outside temperature is the house is a warm as you want. You will then be running with the lowest and probably most efficient flow temperatures for the weather conditions.

Once that is set up you can introduce some influence as necessary from room thermostats and trvs.

Having significant influence from room stats and trvs will make it very difficult to get the heat pump running at it's most efficient.

This process takes time and may throw up areas of the design that are not perfect.

Try and look on other forums where people talk and discuss your particular heat pump.

As always, this is my view and I got this from people more knowledgeable on the subject than me.

Other people will have their own views.

Also look at Heat Geek videos on youtube, lots of useful information. There is a video about zoning and why you should avoid it if possible.

MultiFuelBurner

FreeBear said:

matt_drummer said: So on Cosy Octopus I need to run a higher flow temperature to generate the same amount of heat using the cheap periods so lets say it is now 200% efficient.

During the hours of 04:00 to 07:00, the air temperature is going to be quite a bit lower than it would be later in the afternoon. So the ASHP will have to work even harder, and you might not even get to 200% efficiency.

This is where the rumour mill is so suggestive people think it is fact. Normally suggested by those with gas central heating that really have no idea about heat pumps and their most efficient operating windows and configuration.

The golden rule for ASHP is not to use them like gas central heating and go low and slow.

The following attached table taken for our heatpump shows the effect of raising your outlet temp too high and the effect on the COP against the outside.ambient temperature.

Come winter time we will have put ASHP on 24/7 at the lowest outlet temp (weather compensation controlled) that is comfortably heating the whole house. Based on last year  that never rose above an outlet temp of 35oC

Anyone running their heatpump at 55oC outlet temperature for heating well they deserve their inefficiencies.

MultiFuelBurner

@cannugec5 how are you getting on with your Heatpump now?

Spoonie_Turtle

MultiFuelBurner said:

FreeBear said:

matt_drummer said: So on Cosy Octopus I need to run a higher flow temperature to generate the same amount of heat using the cheap periods so lets say it is now 200% efficient.

During the hours of 04:00 to 07:00, the air temperature is going to be quite a bit lower than it would be later in the afternoon. So the ASHP will have to work even harder, and you might not even get to 200% efficiency.

Anyone running their heatpump at 55oC outlet temperature for heating well they deserve their inefficiencies.

How you can one find out their outlet temperature?  (I'm hoping ours isn't as high as 55℃ as I'd assume that would make the radiators more hot than warm/lukewarm.)

cannugec5

matt_drummer said: 

cannugec5 said:

matt_drummer said:

Here's a rough example of how radiators work.

If we had a radiator that was rated @ 1,000 watts output at a delta t of 50c and room temperature of 20c

That means if the mean (average) flow temperature was 70c (typical of many gas or oil boilers) and the room was 20c that radiator would deliver 1,000 watts of energy to the room, it could consume 1,000 watts from the heat source.

At a delta t of 30c, so a mean flow temperature of 50c and the same room temperature of 20c that same radiator can only deliver about 500 watts of heat to the room. This is a typical design point of many heat pump installations. That radiator can now only absorb 500 watts of heat from the heat pump.

At a delta t of 10c, so a mean flow temperature of 30c and the same room temperature of 20c that same radiator can now only deliver about 135 watts of heat to the room and it follows that it can now only absorb 135 watts of the heat produced by the heat pump.

So radiators need to be really, really big to run at low flow temperatures, not just to be able to heat the house but also to absorb the minimum amount of heat delivered by the heat pump.

If half your radiators are `closed' by the trvs then there is not enough capacity to deliver the heat produced and the heat pump has to turn off.

If the heat pump is now only flowing water through your downstairs radiators the heat pump will try and put in enough heat to achieve your desired room temperature. Depending on your settings this might involve the flow temperature increasing but you could find that you never get the temperature you want, the heat pump cycles on or off quickly costing you a fortune or that you end up consuming more electricity to heat just your downstairs than you would have done heating the whole house.

I know I’m coming across as really stupid, but I am struggling to understand this.

The downstairs temperature is easily reached and stays on target all day and night at the constant setting. 

When you (and others) use the term ‘cycling’ do you simply mean switching on and off? Isn’t that what the pump is meant to do? Surely the alternative is to stay on or off, neither of which will maintain the constant temperature. 

I do ( I hope) understand what you say about the upstairs radiators not using up the produced heat and therefore causing the pump to switch off. I think I might experiment with putting the upstairs thermostat the same as downstairs ( except in our own bedroom). That will ‘switch on’ an extra 6 radiators. 

You are not stupid. Some of this is very counter intuitive. It's catching me out and I am working through the process I am describing to you below. It is a bit overwhelming how much there is to take in and understand to get the best from a heat pump.

 Generally speaking, heat pumps consume the most electricity compared to the heat output in the first few minutes at start up.

If your heat pump runs for 5 minutes, switches off for a few minutes and then starts up again for another 5 minutes  and so on and so on then all your electricity will be consumed in the most power hungry part of the heat pumps heating cycle. This is called short cycling. It will cost you the most amount of money to run.
The heat pump will become more efficient the longer it runs as it is all up to temperature and it is only adding relatively small amounts of heat to the water returning from the radiators.

Of course, you don't want the heating pump running for longer than necessary as consuming more electricity than you need and producing more heat than you need is just a waste. It is a balance between efficiency and overall electricity consumption.

Your zones, trvs and room thermostats are masking what is going on with your heat pump.

What should happen is that your radiators are sized to match the heat loss of each room, none too big, and none too small, so whatever the water temperature flowing through them is gives the room temperatures desired. 

So to start with you try to run with no influence from any of these items, turn the room states right up and the trvs wide open.

Then set up a weather dependent curve on your heat pump controller where the temperature of the water flowing around you radiators is controlled by the outside temperature.

You adjust the curve as necessary so that whatever the outside temperature is the house is a warm as you want. You will then be running with the lowest and probably most efficient flow temperatures for the weather conditions.

Once that is set up you can introduce some influence as necessary from room thermostats and trvs.

Having significant influence from room stats and trvs will make it very difficult to get the heat pump running at it's most efficient.

This process takes time and may throw up areas of the design that are not perfect.

Try and look on other forums where people talk and discuss your particular heat pump.

As always, this is my view and I got this from people more knowledgeable on the subject than me.

Other people will have their own views.

Also look at Heat Geek videos on youtube, lots of useful information. There is a video about zoning and why you should avoid it if possible.

Thank you so much! 

I have registered with another forum and can now interact with others who have a Grant ASHP. 

The important lesson of today, was the realisation that our hefty pump was put in to accommodate the new - yet to be converted- bedroom that is still a garage. The pipework was installed but, as yet, no rads. So from me switching off half the house using the zones I’d forgotten this additional under use. In effect we were only running about a third of the proposed rads. No wonder the HP was switching on and off so much! 

I have read today that ‘cycling’ 3 -4 times an hour is a acceptable. However 5-6 is too much. I like having precise numbers to compare with. 

I am reading that there is still research ongoing as to whether 24/7 on or setting back is more economical/efficient. It seems the jury is out. Even with future data it seems likely the answer will be individual depending on location, house (size,type,construction), weather, desired level of comfort etc. 

As a result of todays lessons I have boldly made changes  . I have increased the target temperature to 22 degrees throughout ( except leaving the TRV off in our bedroom). I have added a schedule to setback to 18 degrees for 5 hours overnight. I have taken the water off constant and scheduled it to heat for 2 hours from 2 pm. 

I am hoping to leave it at that and see what happens. Hubby is happier as he likes it to be toasty. I’m not sure that it won’t be too hot for me, but we will see how it goes. 

Reed_Richards

matt_drummer said:

Heat pumps do not work anything like a gas or oil boiler and some of the concepts are difficult to get your head around.

I completely disagree.  People often overcomplicate heat pumps and I fear this is what @matt_drummer has done.  

Oil boilers, gas boilers and ASHPs (air-to-water) all heat water for radiators or Underfloor Heating.  Oil boilers don't modulate.  They are either full on or off.  Gas boilers can typically modulate over a wide range of powers.  Heat pumps also modulate their power output but typically not over as wide a range of outputs as a gas boiler.

The most fundamental control mechanism that applies to gas boilers, oil boilers and heat pumps is that they run until the temperature of the water returning to the boiler gets too high and then they turn themselves off for a while.  In the case of a gas boiler or a heat pump when the return water temperature starts to approach the turn-off temperature they will modulate down their power outputs and, particularly in colder weather, there may be a point where the heat being lost from the house matches the heat input from the boiler/heat pump and you reach a steady state.  This won't happen with an oil boiler, which will always cycle on and off.

Your oil boiler, your gas boiler and your heat pump will all cycle on and off at certain times, particularly in milder weather.  If the cycle time is short it's called "short cycling".  This is recognised to be a bad thing for gas and oil boilers and I think this mostly because the initial phase of combustion will be at a lower than optimum temperature and less efficient.  People tend to assume that running in short cycles is also bad for heat pumps but you'll struggle to find much justification for this  

Anything that reduces the average temperature of the house will reduce the rate at which it loses heat and so will reduce the fuel/electricity consumption of the boiler/heat pump.  But if you turn off a radiator in a room or a zone you will save some money but that room/zone will still take in heat from the rest of the house so the saving may not be as great as you might imagine.

cannugec5 said:

I think I am perhaps overthinking the whole thing and just perhaps need to focus on comfort rather than so much on economy or I will be a wreck by March! 

Absolutely!  First get the comfort you want then you can take your time to think about whether that comfort can be maintained more economically.

Reed_Richards

cannugec5 said:

I am reading that there is still research ongoing as to whether 24/7 on or setting back is more economical/efficient. It seems the jury is out. 

I think this is rubbish.  If a house is at 21 C 24/7 it will lose more heat than if it is at 21 C for 16 hours and at 18 C (minimum) for 8 hours overnight.  If it loses more heat then that cost you more.  That's pure physics, no need for ongoing research.

But there are two complicating factors:

1. If your gas boiler or heat pump goes all-out trying to raise the temperature after the night time setback then it may be operating less economically whilst it is doing this.  This is particularly likely if you have a feature called "Load Compensation", which is actually much more commonly found on gas boilers than with heat pumps.  Anyway, the way to avoid this is to do what I do and program the temperature to rise in small steps after the night-time setback.  Those people doing "ongoing research" haven't thought of this.
2. If you have an electricity tariff that gives you cheaper electricity at certain times, typically overnight, it makes good economic sense to try to run your heat pump "hard" whilst your electricity is cheap and let it relax at other times.     

Reed_Richards

cannugec5 said:

I have read today that ‘cycling’ 3 -4 times an hour is a acceptable. However 5-6 is too much. I like having precise numbers to compare with. 

Your heat pump or boiler wants to protect itself from overheating and it does this by shutting itself off when the return water temperature gets too high.  Your controller should do better.  I use a Drayton Wiser controller and buried amongst the many settings is an option to tell it what type of heating system you have.  Don't choose the "Electric" option because then it is assumed you have an electric boiler which can cycle very fast without harm.  If I choose "Oil" then the number of cycles per hour is restricted to 3.  So when the boiler is switched on it is not allowed to be activated again for another 20 minutes, even if it only runs for a short time.  So you get fewer longer cycles.  There is also a "Heat Pump" option but actually it does the same thing as "Oil", it restricts the number of cycles to 3 per hour.

I would imagine that your Hive controller, if you decided to use it, can be programmed to do the same thing.    

matt_drummer

I am also running my heat pump in the same way as @Reed_Richards

I build up the temperature slowly after the night time set back.

However, I am using the heat pumps own room temperature controller to do this rather than one from a third party which I find preferable.

One of the big differences between a heat pump and a gas or oil boiler is that the unit cost of fuel is not necessarily fixed, it depends on your electricity tariff which could be at different rates during the day. The cost of oil or gas does not normally vary during each day.

It is important to be comfortable in your home and for that reason you can have any temperatures you like in each room and at different times of the day.

It is not in dispute that lower average internal temperatures will require less heat to be produced.

With a gas or oil boiler you would assume that a lower heat requirement for a day will use less fuel, and that is usually true.

But that is not always the case with a heat pump.

With a heat pump there are many situations where you can consume more electricity to produce less heat.

Here is an example.

If heating the whole house for a day @20c requires 50kWh of heat and you do that at an efficiency of 400% then you will consume 12.5kWh of electricity.

If you turned off some of the radiators and now only needed 40kWh of heat but your efficiency drooped to 250% then you would have consumed 16kWh of electricity.

In my made up example you have consumed more electricity to produce less heat.

Zoning, aggressive set backs, short cycling and high flow temperatures can all result in outcomes like this.

It would be interesting to know how @cannugec5 heat pump is performing.

We do know that they don't have all the radiators fitted that the system is designed to run and turning off more is likely to be a problem for their heat pump at the moment.

None of this matters though as long as the house is warm enough.

And that is how I am running mine.

I need to make some changes as I too have a heat pump that is putting out more heat than my radiators can handle and the heat pumps minimum output and flow temperatures of around 30c.

My system was poorly designed, as I suspected and I need to correct it to get the best efficiency.

But it does heat our house and it's costing me less than gas so I am not entirely unhappy.



If you are worried about costs though the correct way to tackle it is to not heat the house more than you need to but with an eye on efficiency as the savings are not always from the actions you may think.

matt_drummer

Here is a good example of how differently heat pumps behave to gas and oil boilers.

This relates to my hot water production for the last three days, everything is the same apart from the temperature of the tank when the heating cycle started. Saturday the tank was warmer. The tank is heated to 50c at 14.00 each day.

On Friday we used 0.981kWh of electricity to produce 2.450kWh of heat - a COP of 2.50
On Saturday we used 1.232kWh of electricity to produce 2.076kWh of heat - a COP of 1.69
Today we used 0.942kWh of electricity to produce 2.776kWh of heat - a COP of 2.95

So on Saturday even though the tank needed less heat to get to 50c it actually used more electricity than either Friday or today.

It was more expensive to heat our hot water tank yesterday. It is not what you would expect or predict, you would think that not having to heat the tank so much would be cheaper.

Originally we were heating the tank to 45c at 14.00 each day with reheat mode enabled if the tank dropped to 35c.

Sounds good, you would think that would be more efficient as the water is at a lower temperature and less energy would be required, but no.

Some days the tank would reheat mid morning and then cool to around 42c and then perform the scheduled heat to 45c at 14.00.

This used even more electricity and the COP was worse.

It is cheaper and more efficient to heat the tank to 50c once a day. That gives us enough hot water just as heating to 45c + reheat did, but the cost is less, a lot less.