Using a wood boiler to heat Modines in an open system

tbart16

I have a customer who wants to install an outdoor wood boiler to heat 3 Modine heaters in his garage.(sounds easy enough) Problem is he does not want a heat exchanger he wants to keep the system open (unpressurized). For what reasons I do not know and he doesn't want to explain. the heaters will be mounted about 12' high. any advice on how I can make this happen thanks

Don_197

Well.....if I am correctly understanding your issue........remember it takes 2.31 pounds per sq inch to lift a column of water one foot vertically..........so if he wants a truly open system, he will need an open tank, holding at least 3.34 gallons of water (more is better), elevated to a height a little taller (higher is better) than the modines to maintain enough pressure on the system to keep constant circulation without cavitation through the system, and a small circulator to keep it moving. The piping has to be large enough to carry whatever gpm rate at whatever temperature the modines are sized for (I assume these are hydronic fan coils?) while keeping the velocity between 2 and 4 feet per second. Open systems are widely used on wood fired boilers BECAUSE they are not "certified pressure vessels"......keeps em out of code limbo.

wogpa67

Will glycol be in the system?If not he's going to have to have some kind of freeze protection.

EBEBRATT-Ed

@Don I agree with everything you said, however where does the 3.43 gallon tank size come from? I am guessing you calculated the the temperature rise of the water and this is the size of the open expansion tank??

Harvey Ramer

@Don I think you made a slight typo. Roughly 28" wc equals 1 psi.

I have done Modines on an open wood boiler. I put the pump on the supply side and use a delta-t pump on constant circulation. That keeps the air bubbles scrubbed out of the lines. Operate the fan with a thermostat. The pump should be located right at the boiler and as close as possible to the bottom of the water vessel. This prevents low suction pressure and cavitation. The pump must be appropriatly sized for to overcome both the vertical lift and the head loss of the piping. To figure the flow, use the universal hydronic formula. GPM=BTU/h / (delta-t x 500). Add the btu load of all the Modines, figure about a 15deg delta-t, and plug in the formula to get your flow.

Most will tell you to use a stainless steel circ. Personally, I have quit doing that on wood boilers that have a steel constructed HX. Reason is, I have seen more pump failures from debris in the impellers or seizing within the rotor, than from anything else on these systems. A stainless volute does not prevent these types of failures. And the pump will fail in time!

The best thing you can do is some water treatment and keep maintaining it regularly.

It is not the best situation, but it sounds like the customer has made up his mind and if you don't do it, someone else will. Keep the system as simple as possible because many hydronic components will also fail when exposed to this environment.

Good luck!

hot_rod

A couple things to consider with open systems, under some conditions you can pull sub atmospheric conditions in the system and flash to steam at the high points.

Also circs need some NPSH to prevent potential cavitation. The pressure required go up as temperature goes up. I think Grundfos suggests 4-5 psi at 190f, you may not have enough head above the pump to provide that assurance.

An isolation HX eliminates a bunch of potential issues and fluid quality problems.

Good examples of these potential concerns here.

http://www.caleffi.com/sites/default/files/coll_attach_file/idronics_10_0.pdf

tbart16

Thanks a lot guys I really appreciate the help.

Don_197

@Harvey Ramer ..........yep..........got it bassackwards........you need one psi to push a column of water up 28" (roughly 2.31ft) (not accounting for friction loss) Thank You 8-)

Don_197

That's what happens when you leave hydronics for a season in favor of working with Hearth Products