psssssssssst BOOM
right?

what specifically was replaced?

Mind you I'm not disagreeing that modines suck... I find they rust prematurly, but the problems you describe are gas related, not brand related

generally speaking, flame rollover/flash back/hangfire (which is what I assume is happening) is caused by improperly adjusted gas pressure.

modines should run on around 13inch water column for gas pressure. (i'd double check tho) Buy or build a U-tube manometer. I built one out of a few plumbing fittings, a piece of vinyl tubing and a wood board.

You have to remember how these work, propane in pure form isn't going to burn properly if at all. Only when mixed with the correct air mixture does it burn as it should.
If there is a crack or obstruction in the mixer tubes thats a problem too.

so a checklist:
check gas pressure
check shutters for proper adjustment
check mixer tubes for cracks/rusted spots etc and while your at it, blow the fins out with compressed air.

later,
Drew

I stuck a modine in the first g'house I ever erected,sixteen years ago. I just replaced it last year and had I stuck in the s.s. heat exchanger, it would still be cranking. That original furnace outlasted the one in the next g.h. two years newer and a different brand. I replaced that one with a modine. All the rest of the heaters are modines as well. I have not had problems with them and the other units are now at least thirteen years old. Is it flashback you are hearing? You can tell if you just stand up there and watch as it cycles on.

The heater starts up with a very loud humming sound, then a bit of a pause, then back to the loud humming. Once the heat comes out it is fairly quiet. There is no boom.

I always thought they blew...
No problems here.

Suck is just an expression.

One of our greenhouses is located next to the home we have just completed building. We were so happy to finally live within walking distance of the greenhouse. Two nights ago we started the greenhouse for the season and had the same exact problem. My daughter thought it sounded like a howling ghosts. We all wore earplugs and lost sleep. The next day I contacted the installer and with a slight air adjustment on the propane burner the problem was completely fixed. Try it out as it took only two minutes to fix a problem that could have created real stress the entire season.

I notice this tendency more with the high efficiency units. Is yours one of those MD?

Take a look at this troubleshooting guide. It should help you determine what the problem is. Good luck.

Here is a link that might be useful: Modine Heaters Troubleshooting Guide

A couple of years later I still have the same modine heater. It's not as loud now. I was ranting in the first place because a half dozen so called experts told me that the heater was probably defective. One guy was even looking at the unit while smoking a cigarette. Finally the right person came out, did some minor tweaking to lesson the noise. He said the heater was placed too close to the end wall and I need a higher flue to quiet it down some more. I paid out several hundred dollars for installation flaws, including wiring, and lots of bad advice, too.
My apologies to Modine.

kellyj , thank you a million times for your post. The noise in the greenhouse was unbearable. Very loud hum(roar) when the heater came on and ran. A simple adjustment of the air was all it took. Noise is completely gone. And saved me expensive service call.