Volume again, part 2
Graham Hendra
Heat Pump Subject Matter Expert, Refrigeration and Air conditioning lecturer
Last week I discussed system volume to stop short cycling. today I want to talk about volume for defrost.
When it gets cold outside the coil of your heat pump freezes up,
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This is unavoidable, it’s just what happens, to grab heat out of the air heat pumps have to run the coil in the outdoor unit about 8 degrees below the air temperature. So, if its colder than 8 degrees C the coil is below freezing, and it will start to ice up. Its literally freezing the water out of the air.
If you run the coil below zero degrees C. Eventually the coil gets blocked with ice so it needs to be defrosted. This is done automatically.
To defrost the coil all we do is reverse the refrigeration process, if in doubt read this. https://www.dhirubhai.net/pulse/how-does-lump-brass-make-your-fridge-heat-pump-graham-hendra/?trackingId=veY%2BOPfyQuelyeEmMDVESQ%3D%3D this makes the coil outside warm, but sucks heat out of the water running through the house. Defrost is just your heat pump running in cooling mode, its cooling your house and warming itself.
Defrost typically takes a maximum of 6 minutes and happens a maximum of once an hour. So you get toasty rads for 54 minutes and they cool off a bit for 6 minutes, trust me you shouldn't notice this.
When operating in heating mode your heat pump is a machine that adds 5 degrees to the water every time it goes through the unit. The radiators remove 5 degrees from the water and send it back colder to the heat pump, so it can heat it again. ?
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In defrost both the heat pump and the radiators subtract 5 degrees from the water every time the water passes through.
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Let’s assume we have a 14kW unit which moves 40 litres of water through the machine every minute. If defrost takes 6 minutes you need to find 40 x 6 = 240 litres of water if you only want it to go round the system once in 6 minutes. It is of course possible to run the water through the system twice or more which means you only need 120 litres. You would assume if you ran it through the heat pump 2 times the temperature of the water would drop 10 degrees in defrost and you would be right, but you must consider the radiators too.
In defrost you have 2 heat leaks, the first is the heat is still leaving the system via the radiators, (we lose 5 degrees C) they are still warmer than the rooms so they are still warming up the rooms, added to this the heat pump itself is sucking heat out of the water, the heat pump removes a another 5 degrees C) so in one cycle of the system the water drops 10 degrees C.*
Note how the radiators are dropping in temperature but are still dumping the heat as defrost continues. This obviously has a dramatic effect on the temperature of the water in the pipework. Especially if the water goes through the system more than once.
Note on my system here below:
Just before it goes into defrost the outdoor coil temp is -9.2C, the outlet water is 49.7C and its -1.6C outside.
But at the end of defrost the coil temp is 6.6 C; the outlet water is 24.4C and its -1.6C outside.
My water (this is my old heat pump) looses 25 degrees off the water temperature in defrost, the water goes through the whole system 2.5 times to do this.
Once defrost is done, the unit goes back to heating and everything is groovy, but is it?.
?Fridges (a heat pump is a fridge) rely on a pressure / temperature difference between the cold bit (evaporator) and the hot bit (condenser) to make the refrigerant move. After a defrost, when the hot bit is the water in the house and the cold bit is the coil in the garden, if the defrost has over cooled the water there isn’t a very big temperature difference to work with.
If you come out of defrost with a low water temperature, it’s hard to build up a healthy pressure and temperature difference in the system. ?So, we try to avoid the system water getting too cold in a defrost cycle. If the water gets very cold you can get stuck in a spiral where the unit just won’t get going again.? I’m going to discuss defrost in more detail next week.
?As I discussed last week, its hard to work out the actual water circulating volume without making some measurements. Anyone can measure how much water they put in the system, but you can’t tell how fast it’s all moving and how much of it doesn’t move at all. The volume in the system tells you nothing about how much heat you are loosing
The best way to see if you are going to have defrost problems, is run the system on a cold day, let it defrost and see how quickly the water temperature falls in defrost mode.
You can assume that there will be a 10 degree fall in the water returning to the heat pump if the water goes through the system once, 20 degrees if it goes through twice etc.
I would suggest if the water returns to the heat pump at anything under 15 degrees C you are looking at a unit which is on the edge of causing problems.? If the water is coming back at 5 C to put it bluntly, you are looking at trouble. You need to look at getting access to more water by opening up valves or installing a heater or buffer.
It would be nice if your heat pump tested this and let you know, a heads up would be amazing.
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Tune in next week for more defrost shenanigans. and a few ways to solve the issues above without a buffer.
45 years creating refrigeration & heat pump solutions.
2 个月There are three big losses here. 1. The reduced rad output, no heat output when it’s often still needed (even though the rads are slightly warmer than a room as you say) 2. A minimum of 10 minutes per hour running a heat pump without offering any heat. With a 16kW unit it’ll be close to 4kW of energy. 3. The extra running of a heat pump to get water back up to the 5C or more its lost in the defrost, more if the home wasn’t already to temperature.
Technical Compliance Manager
2 个月Brilliant article Graham. Thanks for sharing sir ????
HVAC Consultant and Trainer | Consulting and educating busy HVAC professionals with limited resources to understand system design and control ?? Technical and Service Engineer at Panasonic ?? Ultramarathon Runner ????
2 个月You can have only few degrees temperature drop in well designed and balanced system. Here is the diagram of 300 m2 underfloor (approx. 250 L system volime) with 12 kW heat pump. Very low flow temp. and only 3 C drop.
Managing Director Rowlandson Heating and Plumbing Ltd
2 个月Interesting……. ‘Defrosting happens a maximum of once per hour’ I disagree with this point, after observing data on hundreds of heat pumps, in certain conditions defrosting happens multiple times per hour. Your description of defrosting and system volume is great, It just concerns me when I hear installers saying things like ‘minimum system volume’ like it’s something to aim for. Minimum system volume is just that……the bare minimum not the ideal..! The problem we see out in the field is system’s with low volume and or flow rate causing the death spiral at worst or at best cycling causing premature inverter or compressor failure. If a heat pump doesn’t defrost in a timely manner and as I sate above defrost multiple times per hour it isn’t doing its job (heating the home) and as more installers are going down the no volumiser/buffer route and not guaranteeing (correct system volume) when the going gets tough their systems will suffer or fail prematurely cancelling out their Scops of 9.5 ?? Thoughts welcome ?? Side note, I agree open loop no buffer is the most efficient, when done correctly if the installer has Correct system volume, and the homeowners lifestyle and property suits!
Design and Innovation Manager | Principal Designer @ Ecogenica.
2 个月Hi Graham, What has been your experience with heat injection de-icing? Capillaries can be useful as a means of slowing down reverse cycle de-icing cycles. Whilst you loose some efficiency it can lead to a more stable heating cycle. It’s an old idea, however I was wondering if you’d experienced it with modern system optimisation technology.