Defrost, in heat pumps. Explained.

Every heat pump needs to allow for defrost. Its completely natural, your heat pump runs its outdoor coil about 8 degrees below the air temperature.

If you run the outdoor coil below 0 degrees it will freeze the water in the air and get blocked up with ice. Defrost peaks in wet, cold weather, when its foggy and 3 degrees C outside you get the most amount of freezing. Bizarrely when the weather is very cold, below -2C defrost happen much less, this is because there is much less water in the air, so ice build up is slower.

As discussed in last weeks article, we defrost the coil by reversing the system and sucking heat out of the water in the house, if there is insufficient heat in the water the unit struggles to defrost. If this gets really bad your unit never fully defrosts and it slowly turn itself into an ice cube. This is a combination of bad heating system design and horrible software, so we are all to blame. The key is the unit wants to see lots of heat to do a nice quick defrost. The hotter the water in the rads the more effective and quicker the defrost is.

So what actually happens in a defrost?,

The algorithm works something like this, the unit asks, have I been running for more than 47 minutes since last defrost?, is the coil temperature below 0 degrees C? and is it falling?, Is the water temperature reaching the set point or is it slowly falling?. If yes to all 4, its defrost time. see picture above.

At this point the unit either stops or severely slows the compressor, the outdoor fan is stopped and the reversing valve reverses the direction of the refrigerant in the system. Now our cold frozen coil gets the warmth and the heat exchanger which was heating the water for the house starts to cool. Note the fan stops outside, they do this to get the coil hot quickly and to allow the refrigerant pressure to rise quickly as the compressor ramps up. We need this pressure to move the valve and the refrigerant, the valve does not move mechanically, its moved by pressure difference.

After a few minutes the coil gets warm, the ice melts and dumps the water out the unit on the ground, the unit watches the coil temperature, once its a few degrees above freezing defrost is done. The unit slows, reverses the valve again and off we go back in heating. Fan on, compressor ramps up, coil gets cold and the freezing starts again.

The water in the system never stops moving. If you defrost and the water ends up at a very low temperature the unit struggles to return to normal heating operation. Two things are happening here, we need a pressure differential to move the valve back to heating again, and the system relies on a pressure difference to get the refrigerant moving so heating (and cooling) can get going again.

So its really important that we come out of defrost with the water in the heating circuit not being too cold. We need the heat from the water to give us our pressure and temperature difference..

If you've ever been to a unit which doesn't defrost fully its because there was not enough heat available to defrost the coil in time and the unit got stuck in a loop. To avoid this the manufacturers recommend a minimum water volume. If you put a massive buffer vessel in the system the idea is the water temperature doesn’t fall too much in defrost, there is so much water available it only goes through the machine once in the 6 minutes the system is in defrost. This is a simple but not very imaginative solution. Its just fixing the system with a massive ugly bucket. There has to be an easier and cleverer way.

We could send a signal from the heat pump into our heating circuit and make every zone and underfloor loop open up and run all the pumps. This would give you volume without needing a buffer. Its cheaper, simpler and smaller.

Some manufacturers put an electric heater in the heat pump to boost the water temperature so it doesn’t fall too much during defrost. Basically when defrost starts the electric heater is turned on, this heats the water in the circuit making sure it doesn't fall to low in temperature. I dont think this is a great solution, it works for sure, but its expensive to run and it sends a message that heat pumps cant work without heaters.??

You could try to make an artificial pressure difference, instead of worrying about the water temperature you could drop the cold coil pressure, to do this you do two actions, firstly rev the crap out of the compressor so it sucks like mad, dropping the evaporator temperature (the cold coil in the garden). you could also assist this by running the fan slowly so the coil plummets in temperature, this gives the pressure and temperature differential required, so the unit gets going. Its crude but it works. The side effect is its energy intensive so it costs money and it also encourages quicker freezing of the coil because the coil is colder than it needs to be, so more defrosts are needed.

One of the more popular solutions on big commercial machines (30kW or so) is to split the refrigeration circuit in 2. This means that you can defrost one half of the system first while the other half is heating, meaning you dont need to rob any heat from the water circulating in the building. Once the first half has finished the second half of the unit can defrost. Its a bit more hardware in the machine and a bit of clever control but it could easily be used in a domestic heat pump. It would just be a bit more expensive to buy.

And then there is what I think is a beautiful solution, so beautiful no one does it. Remember I said that when the unit goes into defrost it stops the fan , this allows the coil to get quickly up to temperature. We call this head pressure control. Basically adjusting the pressure with the fan. We could do the same with the water pump.

Once defrost is finished we could stop the water pump, or run it very slowly. This would allow the heat exchanger to very quickly get up to temperature and pressure allowing the unit to get the valve switched over and off and running quickly. All modern heat pumps can see the flow rate, the water temperature and the good ones can see the refrigerant pressure. The water pump could modulate to hold the pressure up and get the unit away every time.

But this is just me dreaming, so for now we will continue sticking buffers in the system just in case. its using a sledge hammer to crack a nut. I loathe it. There is no allowance for system temperature and in many cases they recommend the same size buffer no matter what size unit you buy.

Whatever you do I would recommend looking at the operation of your units, check they dont cycle too much and check when they defrost you are not over cooling the water. IF your system has this feature you can look at this in the app.

If not I would put some monitoring kit on the system. I recommend Planet Devices it outputs all the data you need and very shortly it will advise you about system volume / defrost performance too.

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