What is the expected temperature differential?

ScottPowell

I have an outdoor wood boiler. The water-to-air heat exchanger is in an oil furnace. If my boiler gets the water up to 180 degrees, and the return air is, say, 65 degrees, what temperature should I expect after the air has passed through the exchanger? Yes, I know it depends on the exchanger, but what should I be shooting for?

EBEBRATT-Ed

I would think 110 would be minimum. and maybe 130 on the high end

ScottPowell

Sounds about right, but I'm only getting 91. I do not know what I got when it was installed many years ago, but I believe it was much better. Can it get clogged? I have not noticed any issue with my water quality/clarity.

EBEBRATT-Ed

91 will feel like cold air when it is blowing. Follow the piping between the boiler and the coil and see if they are hot. I am assuming this is an open system so it could get clogged. Check to see that the circulator pump is running and turning. Also check the water level.

If the pump is running and not air bound and you suspect the coil is plugged you could cut two drain valves in the supply and return near the coil and connect a washing machine hose to those valves, open these valves, and see if you get flow by bypassing the coil through the hose

ScottPowell

The air doesn't feel warm which is why I was asking. The piping is hot to the touch, so I know there is flow, but not sure how fast. Water level is correct.

I assume I can just replace the NX (heat exchanger). That sounds easier than messing with the lines just to test.

One other question about the heat exchanger: My heat exchanger is on the air inlet of the oil furnace. The furnace fan is pulling air right through the heat exchanger, then blowing it through the furnace. I have been told that this is unusual. Normally, the HX is added to the top of the furnace. However, I think I understand why it was done this way. Since the oil is my backup heat source, only kicking on when the temperature in my water drops to a certain level, why would I want to cool the air and heat the water is the situation when the oil is running because water loop is cold (fire when out) ?? However, I think would like to add a second HX at the top of the furnace so that the air gets heated twice before it goes into the house. Is that crazy?

ScottPowell

For anyone who finds later, the real issue is that the heat exchanger is caked with dust. Getting to it and cleaning it is going to be difficult, but I'm sure that's the biggest part of my issue. Yes, the filter was missing.

DZoro

That usually is the problem, just a little dust will reduce efficiency greatly.
D