Hello Time to change out my sawdust burning forced air furnace. Have a question about modine 93% gas hanging heater. I would like to use a conventional condensing furnace so I could use in place ducting. But return air temps. are to low and I do not need the shop to be heated to 55-60 all the time. Does anyone here know why the modine 93% hanging heater can tolerate air temps of 40 degrees. Or does someone made a conventional 95% furnace made for low return temps. Venting a 80% furnace (power vent or 25'of vent) is a problem. Sales people are no help. Thanks for any advice.
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93% modine hanging heater
- Thread starter cos
- Start date
There are a couple of issues with any heat exchanger and excessively cold inlet air - condensation is not something you want to occurring in the primary heat exchanger (the "tubes") on an 80% or a 90%+ furnace. they're typically made of steel, and steel rusts. factor in the fact that NG condensate is acidic, and you have a problem. you can make any 80% inducer furnace more efficient by increasing airflow and delta T beyond design specs, at the expense of the heat exchanger.
now on to the 90%+ condensing furnaces. they have a downflow primary HX, and a secondary heat exchanger (made of stainless steel), where the condensing happens. obviously you wouldn't want this to freeze, or it might get blocked, then your furnace shuts down on high limit or rollout.
even if you don't have issues with freezing the secondary HX, you may not get enough temp rise out of the secondary HX to avoid condensation issues with the primary HX. this is less of an issue with an 80% unit as the hottest part of the HX is where the coldest air is (and likely too hot to condense flue gas).
you could "solve" these issues with a selectable restriction or a bypass duct (like one would have with a bypass humidifier) to let a certain amount of warm air back into the return. you'd probably want a 24v low temp thermostat and damper to turn it off once the room gets up to the minimum inlet temp to avoid hurting efficiency or leaving the operating envelope (or tripping out high limits).
Modine probably designed their unit with low temp operation in mind - most houses don't get down to 40F unless the power has been out for a long time, so it's not needed. Might be best to go with the right tool for the right job here.
now on to the 90%+ condensing furnaces. they have a downflow primary HX, and a secondary heat exchanger (made of stainless steel), where the condensing happens. obviously you wouldn't want this to freeze, or it might get blocked, then your furnace shuts down on high limit or rollout.
even if you don't have issues with freezing the secondary HX, you may not get enough temp rise out of the secondary HX to avoid condensation issues with the primary HX. this is less of an issue with an 80% unit as the hottest part of the HX is where the coldest air is (and likely too hot to condense flue gas).
you could "solve" these issues with a selectable restriction or a bypass duct (like one would have with a bypass humidifier) to let a certain amount of warm air back into the return. you'd probably want a 24v low temp thermostat and damper to turn it off once the room gets up to the minimum inlet temp to avoid hurting efficiency or leaving the operating envelope (or tripping out high limits).
Modine probably designed their unit with low temp operation in mind - most houses don't get down to 40F unless the power has been out for a long time, so it's not needed. Might be best to go with the right tool for the right job here.
Thanks for reply. I have thought about the bypass duct idea. Some of the local HVAC guys are unaware of maybe a problem and some say it will not be a problem. Makes me wonder, factory cautions about it. Looks like furnace companies would make a unit with primary SS HX and header for this. I think that how Modine gets around low return temps.
You could always install a wall unit (hot water) and add a fan coil in the existing ductwork .... this would allow for keeping the other heat source.
The boiler would produce high heat for the coil .. you may not get the full 95% -- but it would allow for PVC vent piping. They are more than a hot water heater and less then a full boiler.
The boiler would produce high heat for the coil .. you may not get the full 95% -- but it would allow for PVC vent piping. They are more than a hot water heater and less then a full boiler.