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Air Source Heat Pumps (ASHPs)
Last Post by Kev M 4 years ago
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 Peacenes
(@peacenes)
Active Member Member
Joined: 5 years ago
Posts: 8
Topic starter 17/11/2021 6:44 am  
Posted by: @kev-m

Is there a sensitivity setting on your room thermostat will stop it reacting to small variations in room temperature

Morning,

so this wouldn't let me go, I googled some more about the DIP switches on the back...and finally had what feels for me like a major breakthrough. I blame these poor and outdated little manuals that come with the package and can be found on the manufacturer's website for not getting it sooner, terms like TIP, CHP, hysteresis make sense now 🙂
So I found out that this two switches aren't just simply turning one or two settings on the RT on or off, like changing the sensitivity from +/- 0.5  to   +/- 0.25, putting them in the correct up or down position enables one of four protocols or algorithms on the RT, which are controlling the RTs behavior.
By default both switches are down, and in this state the "TPI 6 CPH" protocol is active, this protocol has basically nothing to do with the +/- 0.5  or  +/- 0.25 sensitivity change. 
TPI (Time proportional and integral protocol) is a self-learning algorithm which calculates the furnace run time, it makes measurements a few times per hour (6cph or 9cph) to predict how long it takes for the furnace to achieve the desired room temperature and according to the measurements turns the furnace on or off. This definitely didn't work for me, and caused the ASHP sometimes to cycle every 10-15 minutes.
So by just pushing one switch on the RT in the other direction I enabled the "Hysteresis +/- 0.5" protocol, this is the simplest two-position algorithm meaning if the room temperature is like 21.7C and I set the desired room temperature on the RT to 22.0C, it won't start yet but wait until room temp drops to 21.5C and than start and work till it achieves 22.5C.
I tested this and think this suites me the best, in the ASHP log for the last 6 hours i can see that running times were from around 60 to 80 minutes, with 40 to 60 minutes off time...so no short cycles.
When my RT wasn't correctly wired to the ASHP, I used the schedule method with similar run times.
 
So after figuring this out, the only question open would be number 2, related to correctly setting and enabling weather temp set/compensation. 
 


   
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 Kev M
(@kev-m)
Noble Member Member
Joined: 5 years ago
Posts: 1276
17/11/2021 10:13 am  

@peacenes 

Mirnes, good progress. You now have a slightly more clever thermostat, which will switch the ASHP on and off less often, running at a constant flow temp.  The next step (I think) is to do the same at a weather compensated flow temp and see how that goes. 

What I, @batalto and others are trying, is to bypass the thermostats by turning them up high (so that they are always calling for heat) and let the weather compensation control the temperature of the house by varying the flow temp.  Currently, doing this, my house has stayed at 21-22C when it's been between 6C and 12C outside.  So far so good but I need some colder weather to test it more. 

      



   
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 Peacenes
(@peacenes)
Active Member Member
Joined: 5 years ago
Posts: 8
Topic starter 17/11/2021 10:47 am  
Posted by: @kev-m

is to bypass the thermostats by turning them up high (so that they are always calling for heat)

I understand, but wouldn't that mean that the ASHP is always working, because weather compensation just lowers or raises the waterflow temperature, but doesn't actually turn the unit OFF? 

I know when the ASHP is always on, that it runs consistently with less effort, smoother and at lower compressor current which reduces energy consumption, but shouldn't it "rest" now and there (wear and tear)? 
You guys surely use the ASHP for heating sanitary water as well, maybe this is one reason  to keep it always working.
My ASHP is only used for heating the radiators/rooms (not sanitary water), I have a buffer tank which stores the heated water...so when the unit is OFF for let's say an hour and starts again, it takes just 10-15 minutes for the radiators to get warm again and room temperature rises because it's pushing the stored warm water from the buffer tank. 
In these first 20 to 30 minutes, the ASHP is working a little harder (more noisy, higher compressor current) but soon reaches target flow temperature and then goes back to running smoother.
 
My October bill was surprisingly low using mainly the schedule option or manually shutting it down, things the RT will do now instead of me


   
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 Derek M
(@derek-m)
Illustrious Member Member
Joined: 5 years ago
Posts: 4425
17/11/2021 12:17 pm  

@peacenes 

Hi Mirnes,

Your ASHP will still start and stop when on weather compensation, but should operate more efficiently. There are two main factors which affect the efficiency of your ASHP, one is the outside air temperature, which as it gets colder means that your ASHP has to work harder to extract heat energy from the surrounding air, and the other factor is the water flow temperature. The higher the required water flow temperature, the longer and harder your ASHP has to work to achieve the required water temperature. When your ASHP is operating on weather compensation, the required water temperature is set dependent upon the outside air temperature, but in the same way as your thermostat stops your ASHP when the room is up to temperature, the weather compensation will stop your ASHP when the water is up to temperature. During milder weather conditions the required water flow temperature will be lowered, so your ASHP will not have to work so hard and will be more efficient.

Because you have to select a weather compensation curve from those available, it is unlikely that it will perfectly match the requirement for your home, so you will still need to operate your system with the thermostat controlling the indoor temperature. But by setting the water flow temperature automatically using weather compensation, should improve the overall efficiency of your system.

Do you have any reading of Coefficient of Performance (COP) on your controller?

Just like a car engine, your ASHP is designed to operate for many years, and prefers to operate for longer periods, rather than be starting and stopping.

To keep the room at a more constant temperature you could set the hysteresis on your thermostat to +/- 0.25C, which again could slightly improve the efficiency.  



   
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 Kev M
(@kev-m)
Noble Member Member
Joined: 5 years ago
Posts: 1276
17/11/2021 12:48 pm  

@peacenes,

Mirnes,

Here are a couple of graphs that show my ASHP working your way and then my current way. The first is 24 hrs, the second is 6-7 hours.  Take care looking at the y-axes; the scales are different although the units are the same.

Here, the ASHP is responding to a thermostat with the flow temp controlled by weather compensation.  It's set to 21C morning and evening and 18C at other times.  From midnight, it's coming on periodically because the ambient temp is 5C or below - that's automatic.  I assume it's to stop watrer pipes freezing.  In the morning and evening the ASHP responds to the thermostat and comes on.  In between the house stays at 18C or above so no heat required. The peak midday is hot water. In the evening you can also see the thermostat stop calling for heat, making the ASHP turn off and then calling and starting up again. The energy peaks are high, although the ASHP is off a lot of the time.  

 

IMG 20211107 085823~2

 

Below is with the thermostats turned up.  As @derek-m says, the ASHP isn't heating all the time if it doesn't have to.  Most of the time it runs for a bit then stops.  These cycles are every 30-40 mins.  When it gets a bit colder (look at the blue line), it can run at its minimum level constantly.  

151121Graph

As Derek also says, these units are built to last for years.  They are the same technology as your freezer and fridge and these don't break very often. My freezer compressor turns on and off all the time.

Which way is better?  Well currently my house is warm all the time, which is nice.  The ASHP is running more efficiently (better COP), which is good.  But, it's running for more time.  Is it cheaper?  I suspect it is, but I don't know yet.

Hope this helps.    



   
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