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Low temperature heating - does it feel like heat is more evenly distributed?

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TechnoGeek
(@technogeek)
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@toodles Yes I have noticed a more even heat distribution since installing my heat pump (12Kw Samsung Gen6). I have a particular problematic room (also identified on the heat loss survey) which is heated using a vertical radiator. With the heat pump heating characteristics of low temperature and longer run times per cycle, this vertical radiator performs better, giving off more heat and the room does not get so cold. It now settles at approx  1 deg cooler than the remainder of the house, however with the oil boiler it was 3+ deg cooler.

As I have been fine tuning my system this Winter, I have now acquired a deep understanding of my smart thermostats behaviour (Honeywell T6R+HW). When the thermostat learns its controling the heat pump, the thermostat demands heat in each cycle ( 3 cycles per hour, 1 cycle is max 20mins)  for approximately 13 minutes. When the thermostat learns the oil boiler is running (I have a bivalent system), the heat demand per cycle is reduced to about 5 - 6 minutes.

I am not a thermodynamics expert but I think because of the longer run times of the heatpump the heat output of the rads gets more chance to circulate the room before cooling. With the short 5 - 6 minute blast from the oil boiler, I am thinking the rad output cools down before it has chance to circulate the room properly?

With my vertical radiator mentioned above, due to the oil boiler not running long enough, the rad does not get chance to heat properly and hence the room is a lot cooler.

As far as heating rates, I have found the thermostat heats at approx 0.5 degree per hour regardless of heat source. The advantage I am finding with the heatpump is the lower longer heat output does not give you the chance to feel any sudden temperature cooling like the high short bursts of the oil (or gas) boiler and the rads are warmer longer so the heat distribution is better?


   
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(@guthrie)
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Posted by: @technogeek

@toodles Yes I have noticed a more even heat distribution since installing my heat pump (12Kw Samsung Gen6). I have a particular problematic room (also identified on the heat loss survey) which is heated using a vertical radiator. With the heat pump heating characteristics of low temperature and longer run times per cycle, this vertical radiator performs better, giving off more heat and the room does not get so cold. It now settles at approx  1 deg cooler than the remainder of the house, however with the oil boiler it was 3+ deg cooler.

As I have been fine tuning my system this Winter, I have now acquired a deep understanding of my smart thermostats behaviour (Honeywell T6R+HW). When the thermostat learns its controling the heat pump, the thermostat demands heat in each cycle ( 3 cycles per hour, 1 cycle is max 20mins)  for approximately 13 minutes. When the thermostat learns the oil boiler is running (I have a bivalent system), the heat demand per cycle is reduced to about 5 - 6 minutes.

I am not a thermodynamics expert but I think because of the longer run times of the heatpump the heat output of the rads gets more chance to circulate the room before cooling. With the short 5 - 6 minute blast from the oil boiler, I am thinking the rad output cools down before it has chance to circulate the room properly?

With my vertical radiator mentioned above, due to the oil boiler not running long enough, the rad does not get chance to heat properly and hence the room is a lot cooler.

As far as heating rates, I have found the thermostat heats at approx 0.5 degree per hour regardless of heat source. The advantage I am finding with the heatpump is the lower longer heat output does not give you the chance to feel any sudden temperature cooling like the high short bursts of the oil (or gas) boiler and the rads are warmer longer so the heat distribution is better?

It makes sense that longer heating at a lower degree will give time for the air to circulate more which would lead to heat percolating through more of the room.  The only thing that worries me is that your system is turning on and off as often as 20 mins; I have a dumb Honeywell and the installers left it set with 5 cyclers per hour, so for the first 2 or 3 weeks it was turning on for 6 min off for 6 mins, wasting large amounts of energy.  Once I worked it out I turned it to 1 cycle per hour and it comes on for half an hour, much more efficiently and also easier on the system.  I have adjusted the weather control to try and lengthen the on period.  Again, it was set quite high, something like 34 at 15C and45 at +2, I have it at 31 or 32 at +15 and 42 or so at -2. 

Radiators surely also make a difference, the installers got 2 things right, one of which was upgrading the radiators to larger double fine high output ones, meaning there is more air circulating out the top of them.  I have been trying to get everyone else in the house to stop using radiators for drying things on, because that interrupts the air flow, but they refuse to listen to technical stuff.

(The other thing they got right was doing a simple clear install without a buffer tank, the thing they got wrong was the dumb control system is a problem)

 


   
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TechnoGeek
(@technogeek)
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@guthrie 

Posted by: @guthrie

The only thing that worries me is that your system is turning on and off as often as 20 mins; I have a dumb Honeywell and the installers left it set with 5 cyclers per hour, so for the first 2 or 3 weeks it was turning on for 6 min off for 6 mins, wasting large amounts of energy.  Once I worked it out I turned it to 1 cycle per hour and it comes on for half an hour, much more efficiently and also easier on the system.  I have adjusted the weather control to try and lengthen the on period.  Again, it was set quite high, something like 34 at 15C and45 at +2, I have it at 31 or 32 at +15 and 42 or so at -2.

I have a balancing act with my system because it is a bivalent set up. Oil boilers like a duty cycle of 3 cycles per hour unlike gas which is 6 cycles per hour. Also each cycle run time can be varied by the thermostat between 4 and 20 mins depending on the heat demand of the house. In addition, my thermostat has a minimum setting of 3 cycles per hour.

All the HVAC websites I have found suggest the ideal duty cycle for a heat pump is 2 to 3 cycles per hour, each cycle duration being between 10 and 20 mins long. So this fits in nicely with the way my oil boiler likes to be controlled and keeps the run time of the heatpump to a minimum, so extending the compressor lifespan :-). The biggest loss of energy I know of is the startup surge current of the compressor but the compressor can cycle on and off during the heat pump run time, when it achieves its water flow temperature. I do not think there is much to be saved regards power when running a heat pump with a 50% duty cycle and running the heat pump for extended periods shortens the life of an expensive compressor :-), however I could be incorrect in my assessment 🙂


   
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(@guthrie)
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@technogeek That's a funny sort of system, I have not heard of, but if it works for you that is fine. 

I find it odd the HVAC websites say 10 to 20 mins long is best, that's different from what I have read on here and elsewhere on heat pump focused places.  As for compressor lifespan it is not something that people generally bring up, where did that concern come from?  My understanding is that turning them on and off a lot isn't great, 5 times an hour is definitely bad, and running it for a while is good because everything gets up to temperature. 


   
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TechnoGeek
(@technogeek)
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@guthrie my friend, I feel we need to conclude this conversation as I fear we are diverting from the original topic of this thread 🙂
I am happy to continue discussing this topic in a thread of your choosing but to conclude:

Posted by: @guthrie

I find it odd the HVAC websites say 10 to 20 mins long is best, that's different from what I have read on here and elsewhere on heat pump focused places.

Regards suitable duty cycles I simply put the phrase "Heat Pump Duty Cycles" in google search engine and it came back with a number of results saying the same thing, "cycle 2 to 3 times an hour, each cycle 10 to 20 minutes long". This seems to fit with the abilities of my thermostat which has 3, 6, 9 or 12 cycles per hour. Currently set to 3 to satisfy both the heat pump and oil boiler.

Posted by: @guthrie

My understanding is that turning them on and off a lot isn't great, 5 times an hour is definitely bad,

Not sure why 5 times an hour is mentioned but for clarity my system is as follows:

1 Cycle = 20 minutes

Therefore, 60 mins (1hour) / 1 cycle (20 mins) = 3 cycles per hour.

My thermostat, during a single cycle, can demand heat from the pump for approximately 13 minutes of that 20 minute cycle, that is the pump is on for 13 minutes and off for 7 minutes.
With 3 cycles per hour this means the heat pump will have 3 starts per hour and a total run time of 3 x average 13 minutes = average 39 minutes per hour.

During cold weather this heat pump on period in a cycle can be extended by the thermostat to between 15 and 18 minutes making the off time only 2 minutes. Likewise, in very mild weather the cycle on period can be reduced down to approximately 6 to 8 minutes and an off time of 12 to 14 minutes.
The number of heat pump starts remains constant at 3 times per hour. I have noted during very mild weather the system can switch off the compressor during a cycle "on" period but mainly it varies the compressor speed to maintain the LWT when reached.

To be fair this is similar behaviour to your humble fridge freezer.

For your information, for an outside temperature range of +5 to +14 degrees C, I am getting an average COP of 4.33. This could be improved by replacing all my old 1970's /80's oversized single panel radiators with modern ones however I fear that would be a step too far with "She who must be obeyed"! lol

Also a bivalent system is simply a configuration that allows you to set an outside temperature threshold (in my case +5 degrees) and when the temperature is greater than the threshold, the heat pump operates or if the temperature is less than the threshold, the oil boiler operates.

Normally this type of system is used to help keep the building warm in very low outside temperatures when the heat pump can no longer efficiently supply the required heat. I am using it as a cost saver because the electric tariff I am on, I find the cost of heating the house below +5 deg outside temperature, significantly cheaper with oil sadly.
However over the next year the situation may change, oil may get more expensive and electricity (hopefully) cheaper in which case the temperature threshold will be moved down and the oil boiler used less and less.

 


   
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(@guthrie)
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@technogeek Yes, we are going far off topic, some drift is tolerated here so we should be fine.  Your COP sounds good, but with the old radiators you will struggle to get lots of heat into the house, it is a pain having them replaced.

I am now wondering why I haven't read much about lifespan of the air pump systems, I may start a thread on that after more research.

The on and off 5 times an hour seems to be a widely agreed upon number of cycles per hour that would be bad for your heat pump, but I haven't seen exactly where that number came from, it might be that one of the long term members has an idea.


   
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