I have a wood stove with a wet back. I'm using a Grundfos UPS 25-40 pump to circulate the water. Some of the radiators are not getting hot, particularly the ones far down the line. I had a heating guy look at it and he straight away said that my pump wasn't powerful enough and recommended I upgrade the pump.
The pump is on the hot side, it pulls hot water from the stove and pushes it around the house. All the radiators are in parallel.
I have an identical spare pump lying around. Can I just put this pump in series on the cold side (the input to the wet back) and have it push water through the stove as well? Would this not work like two locomotives and essentially double the pumping power?
There's a second reason I want to do this. Sometimes when air gets caught in the boiler (the wet back), it starts to boil and if enough air is in there, I get steam pushing all the way into the pump and the pump goes into vapor lock--it can't pump vapor, it's designed to pump liquid. The system goes into high pressure, the safety valves start blowing steam and the whole system needs to be cooled down before it explodes!
If I have a second pump on the cold side, this pump would never get steam and hence I should never get into a state where the circulation could stop. Hence, I'm viewing it as a safety measure.
The question is, is there anything wrong with putting in a second pump in series like this? Or should I just get a larger pump and not use the identical second one I have? Is the existing pump on output (hot) side in the correct place? Or should the circulator pump be on the input (cold/return) side?
If it is ok to use two pumps in series like this, any reason why one on the hot side and one on the cold side might cause a problem? Hot water being less dense than cold water and the stove heating the water means there's more volume going out than in. Is this going to create a situation where one pump is always ahead or behind the other and create some sort of imbalance?