Burst heat exchangers in heat pumps

Over xmas and new year its been pretty cold in the UK, below freezing a fair bit so we have been busy on tech support. One call I had was for a heat pump with a cracked heat exchanger and water pouring out of the unit.

All heat pumps use a plate heat exchanger to transfer heat from the refrigerant into the water. Water and refrigerant do not mix so a plate is ideal for this job, for those of you who have never had the pleasure of dismantling a heat exchanger , they look like corrugated cardboard inside, hundreds of tiny tubes next to each other each one holding either refrigerant or water.

Burst heat exchangers are very rare, over the last 11 years I have seen 4 of them in the 8000 heat pumps we have sold. 2 of them were caused by people messing around and 2 happened legitametly in the field, So what happens?

Heat pump manufacturers measure the water temperature going into and out of the heat exchanger, they do not measure the temperature of the water inside the plate itself. So you would assume that they check the temperature of the water as it enters and leaves and make sure it doesn't get below 0 degrees. In Cold weather your heat pumps spends most of its time heating but they need to defrost the air coil every hour or so, defrost is just a case of Cooling the water and warming the coil a few degrees to get rid of the ice. this photo shows a part frozen coil.

Manufactures shit their pants about defrost, they are concerned that if we cool the water it could freeze. But think about it, why if the unit knows the water temperature would it ever let itself freeze up? So lets look at what happens without all the thermodynamics.

A 16kW heat pump can add 5 degrees to the water in heating if the water is flowing at 30 litres a minute. In defrost it reverses so it removes 5 degrees from the water as it passes through the unit each time. Lets assume we have been heating and the water in the heating system is at 35 degrees C. In defrost the first time the water goes through it enters at 35C and leaves at 30C. Defrost typically takes 6 minutes so unless you have more than 30 X 6 = 180 Litres of water in the system the water will go through the unit more than once.

If the system has a tiny volume of 30 Litres then the water will go through the unit 6 times in the 6 minute defrost cycle, so it will drop from 35-30-35-30-15-5 degrees C in that time. Now if you have a ridiculously small volume of water you can see that its possible that the water would drop below 0 and the heat exchanger will freeze. You would hope that the onboard software would be able to see this was happening well before it froze up. Its possible that the software engineers (if that's not an oxymoron what is?) dont think this will happen and allow the unit to freeze up but in time they would learn . But they dont need to because that's not the problem.

Heat exchangers have tiny channels in them for the water to flow through, typically they are only 3 or 4mm diameter, this is done to increase efficiency. The problem is if your friendly plumber doesn't get all the crap out the system or doesn't install a filter, the crap blocks up these channels reducing flow in part of the exchanger. So when we defrost you get localised cold spots. To help you are encouraged to put anti freeze in the water, even tough the water in and out of the exchanger is not anything like freezing temperature. As the water freezes it expands and it cracks the exchanger. the refrigerant leaks into the water, the water gets sucked into the refrigerant circuit and the unit is a write off. Only an idiot changes the exchanger if it bursts.

So back to my unit with the burst exchanger, who's at fault?, its possible the software is crap but if it was you would get lots of split exchangers in cold weather, likewise if the exchangers were poorly made. Im afraid the culprit is crap in the heat exchanger and or not enough antifreeze was used.

So what now? the homeowner claims on the house insurance for a new heat pump, just like they would do if their boiler heat exchanger was damaged. My hot tip is ask your friendly plumber if your system has a strainer and when did they last back purge water through the heat pump to flush out all the crap, I recommend this is done every couple of years as part of the maintenance. I just did mine, it took me nearly 5 minutes and a hose pipe to do it.