Expansion valves in heat pumps, discuss.

Every heat pump and air conditioning unit has a valve in it that controls the speed and quantity of refrigerant moving around the circuit. We call them EEVs or LEVs

If you are a heating engineer think of it as a thermostatic radiator valve TRV but controlling refrigerant, not water. Years ago we also used mechanical valves to do this job but we called them ?TEVs or thermostatic expansion valves like this.

When I was at college in the early 90s my mentor Hugh Hawkins used to give me loads of grief because I had never worked on the mechanical valves, in aircon they were obsolete by the mid 80s. The beauty with a refrigerant electronic expansion valve is its really cheap, reliable and very accurate in its control.

All heat pumps also use these valves, they very rarely go wrong but its nice to know how they work so here goes:

There are two major types of these valves in existence, Lets start at the top with the good ones (I call them Daikin valves) but you see them in Samsung heat pumps too.

These expansion valves are made of two parts, a head (the silver and black bit) and a body, the brass bit, if you carefully unbolt the big brass nut the head comes off the body, no refrigerant will leak out, there is a bellows to keep the refrigerant away from the head. A nice feature of these valves is if they are stuck you can loosen the head ? turn and it frees them up. note how the nut and body have a line to mark the correct tightness.

Daikin published a book in the 80s explaining how air-conditioning worked, its got a catchy title Si14c. Its completely unobtainable now but luckily I kept mine. Inside it shows a great diagram of what's inside these valves.

Its just a motor driving a gearbox which pushes on a driven section (pin/ plunger) which operates the valve. If you take the head off the body the valve is fully open, its fairly obvious but NEVER run the unit with the head off.

The cheaper valves look like this

They also have a body and a head but in these valves case you can only remove the motor windings (black part), they just pull off. The problem is you cant get to the mechanism to free them up, so if they are stuck you cant fix them.

All expansion valves are electrically auto resetting they DO NOT have springs in them, they use a motor to move them so if you unplug the valve it will not move at all. It is very poor practice to run any refrigerant system with the valve unplugged.

If you turn off the power to the unit then turn it on again while ?holding the body of the valve you can actually feel it moving, it fizzes in your hand as it moves. Once its closed it starts chugging and knocking like a wood pecker, this is a good sign, if it doesn’t do this either the valve head or board sending the signals to the valve is faulty.

There are a few myths about valves which are un-true, removing the motor windings does not open the valve, putting a magnet on the valve wont open it and hitting it with a hammer does nothing, but it does make you feel good.

Apart from the physical differences there is an electrical difference, the valves have 5 or 6 wires, they are wired slightly differently inside so are not interchangeable but…………………..

If you ever come across a valve which is stuck and you know what you are doing you can use the electronics to force the valves open. I told you that when any unit powers up it automatically drives the valves close. If you carefully swap the blue and yellow wires over in the plug, (its fiddly) plug the valve in and then reset the power it now forces to full open when you turn on the power, clever huh? We use this feature if we want to purge refrigerant systems through or to open stuck valves. Once the valve is free reverse the wires again and reset the power.

The golden rule is this though: once you disturb the valve, the wiring or the pcb you must always reset the power . the pcb only knows where the valve is when its fully closed after power failure, resetting the power puts the valve back to a safe position.

This is an extract from my new how to fix old Samsung heat pumps course starting in August.