Alright writing this from the top of my head but heat pumps are neat and something I'm hoping to work on one day with the rest of the HVAC systems out there. First an overview on some stuff before I get to the why heat pumps are neat.
Refrigerants people know them as freon sometimes but that's a brand refrigerants are the stuff in fridges that make it all work. They are categorized by toxicity and flammability and the reason they're heavily regulated is because of the damage they do to the environment. The main part is the free chlorine that can go on to destroy the ozone layer but after a while that got phased out and now the main concern is their global warming potential. Still despite it all it's their ability to remove heat from a system.
It starts in the compressor which can vary by type but the refrigerant enters it from the suction line as a cooled vapor (idealy only as vapor since liquid refrigerant can damage the compressor though through the use of crankcase heaters and accumulators this can be prevented). The compressor does it's part and compress the low pressure vapor into a high pressure high temperature vapor and leaves out through the discharge line. Out it goes through the discharge line at an ideal higher tempature (not exceeding 225 F) towards the condenser. The condenser is usually known as the heat rejecting part of the refrigerant cycle. Subcooling occurs and turns the hot vapor into a hot liquid an important part for the next part. It leaves through the liquid line towards a metering device (these can vary from fixed to ones that regulate based on tempurature or other means. These regulate and help transfer the refrigerant towards the evaporator where it enters it in a state of vapor mixed with liquid. Here air from the supply side blows over it to cool it down transferring the heat from the air to the refrigerant. The refrigerant is then superheated and converted back into a low pressure low temperature vapor and goes through the suction line back towards the compressor to do it all over again.
Short of is fridge is cool inside because the hot air that leaves it at the bottom but what if we could use that hot air or make frige hot instead? Heat pumps can reverse the refrigerant making the heat rejecting part and heat absorbing parts reversed through the means of a reversing valve. This valve has one line in from the discharge line and three valves on the other side with the middle one always used for the suction line plus the other two outs depending on if it's in heating or cooling mode. The condenser and evaporators no long have to be just outside and inside (another metering device maybe needed based on which unit is now acting as the heat absorbing part).
Heat pumps are ideal for saving energy heating a home since it takes heat outside and brings it in, even in colder climates since their is so much more outside than in it just keeps taking until it can't. Heat pumps start to not work around 0 F so they are paired with other heat sources such as gas or electric in order to meet a homes needs. The balance point is the spot where the heat pump can't meet the needs and switches over to a different source of heating.
The other thing that makes a heat pump different from AC is that the outside door unit is equipped with a defrost mode. Since these units can go below freezing point they do build up frost on the coils so a defrost is needed. These can be sensed from either a thermometer on the coils, be time set, or both. The out door fan will shut down, the compressor will go off and the temperature will go up until the frost is cleared.
On troubleshooting them there are a number of things that can go wrong from leaks to dirty indoor or outdoor units (condensers/evaporators for heating, evaporators/condesors for cooling), air supply issues (dirty filters or something blocking the returning air from going back) low charge to too much charge (refrigerant), electrical issues (contactors, tranformers, relays, capacitors, faulty thermostats) and compressor issues. It's best to figure out what went wrong instead of just assuming you need a new unit or just more charge.
Yeah there's more and this is what I want to make a career of, with a little hope and a lot of luck I'll make it into the hvac system. Everything in hvac is just the refrigeration cycle over and over again so by studying this I hope I can make it and if not at least amused some of you.
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