Hi there, we have recently had a multifuel stove fitted, it has a 40,000 BTU boiler fitted and is not heating the radiators, the manufacturer says it will run 16 rads, but we only have 13, 7 rads but 6 are double, we have a roaring fire and turn the pump on the and they are lukewarm and don't get any hotter. Our central heating engineer who fitted the system says there is no air blockage.

It almost has to be a vapor lock or bad pump. What is the temperature at the supply and return. Are there any purge and balance valves, or other type of valve that is not fully opened?

It almost has to be a vapor lock or bad pump. What is the temperature at the supply and return. Are there any purge and ballance valves, or other type of valve that is not fully opened?
It gets hot in 2 of the pipes but when you put the pump on they immediately go cold and stay cold, not sure about the valves thanks

Something sounds strange. How experienced is your engineer? There may be a mistake Northern Heat never imagined anybody would make. If the pipes immediately go from hot to cold, they have cold water flowing in them. There may be another answer, but I wonder if the make up water line is connected to the pump intake? Check your water meter to see if it moves when the pumps run.

Study the pump. There should be a line running from the boiler to the center of the pump. Then another line should come off the outside edge, perhaps a snail shell looking thing, and connecting to the lines to the boilers. Any valves should be wide open. Modern ball valves will have the handle parallel to the line when open. With an old gate or globe valve, you just have to try turning the handle back and forth. Post back with what you find.

Something is goofy in Denmark. I don't think its taking on too much make-up water. The reason is a boiler will run about 12 psi. cold and 18 psi hot. It's a sealed system and if it starts to take on make up water the pressure should go above 30 psi when the pop-of/relief valve will let the pressure go. Unless you have a leak in the system it's not taking on fresh water. Also, of coarse when you turn on the pump cold water from the radiators flood the boiler and it turns cold. There are 2 pipes in and out of the boiler that go to the radiators. The pump should be on the return side. Let the boiler run with the pump-on (never when it is off) after 1 hour record the water temp' at the pump pipe and the other one of the same size (the supply) lets se what you've got.

Thanks for your help, but we have had another engineer in and he thinks it's the chimney that needs lining as he says that the fire is not drawing enough, I'm still very confused but will give it a try and see if it helps.

Does your roaring fire have a blue flame? If so, I doubt the problem is the draft. Give Northern Heat the info he asked for and try to work with him.

ive have same problem, 25kw stove and it won't even heat 8 rads upstairs with the downstairs rads switched off. Had plumber out today and i was losing heat going to the oil burner outside!! Have closed the valve and seems to work. But need a good roaring fire to get it going!!