Indeed. You do need to describe your system better.
This is a 'Heat Pump'
Is this what you have? Your comment that it has thermistors would also fit this scenario. But then when you talk about it being connected to radiators and plugging it in and it blowing air, makes me think you are referring to a 'Pump' of the type used in central heating systems, something like this.
If you are referring to a simple pump as above, (in reality, pump is a misnomer, because they are actually circulators, because they move water around a sealed circuit) Sometimes as they get weak, they are still able to move free air, but are not strong enough to move gallons of water, sludge, crud etc., around a heating system. However, if this is what you have and you say yours has been replaced and the new one still isn't working, then it needs someone who can trace electrical circuits to check if the pump is getting a live supply, and if not, why not. Changing lots of components in an 'ad hoc' fashion and hoping to find the problem by chance can be an expensive way to fault find, and is not often successful.
The problem is, if the above is what you do have, then I have never seen thermistors associated with this type of system. Do you mean thermostat?