Looking for advice on condensing unit heaters

Solid_Fuel_Man

I've got a job of heating a large agricultural building, ~30,000 square feet. There will be product in the building (potatoes) which will generate some heat. As the winter goes o , trucks are loaded and the storage will have less and less potatoes in it. The ideal temp is 40F. The building is not that tight with many large opening doors etc. There was an 80% 250K gas unit heater which kept up I am told. But has long rusted out. 

The ideal location of the heater is near the center of the building and I do not want to make a flat roof penetration. 
The flue would be approx 50' horizontally, and sealed combustion is a must due to chemical gassing of the produce. 

I'd really like to think this would be the ideal place for a plastic vented condensing unit heater. But I have heard negative things about the Modine Effinity 93. 

I was looking at the 180K model 93. 

Anyone have any thoughts? A pair of 100K condensing furnaces with minimal ductwork? 

lkstdl

> A pair of 100K condensing furnaces with minimal ductwork?
We use this approach to keep about 60,000 sq ft of warehouse space at 55F. We typically use Goodman 2-stage 120K furnaces (GMEC961205DNA) . They are (fairly) easy to set up horizontally, so you can even get by without any ductwork if you like (just a filter rack). In the manual they spec how to hang it from the ceiling using threaded rod and angle iron (we use strut channel) With 3" vent pipes, your 50' length is no problem.

On the Goodmans at least, you will be slightly out of spec at 40F return air temperatures. From the manual: "When the furnace is heating, the temperature of the return air entering the furnace must be between 55°F and 100°F" My assumption is that at lower temperatures, condensation could occur earlier in the heat exchanger, which could potentially cause corrosion? You can could lower the fan speed to get a greater delta T if needed.

Luke

motoguy128

All furnaces need to stay above 50F return temp.

I’d still be concerned that a condensing unit heater might have the trap freeze. It would need to have some sot of a trap less design. Plus you usually don’t have drains in out building so now you have a big pile of ice somewhere outdoors.

A tube heater might be the best. The radiant heat keep surface warmer even if the space temp is lower. Hard to heat tall storage buildings normally.

Solid_Fuel_Man

We dont want radiant heat as it will cook (literally) the potatoes. 

There is constant air circulation with several 10HP fans pushing air up through the potatoes through in floor ductwork. 

I may just do a sealed combustion non condensing heater. The idea of a long horizontal sheetmetal flue in a cool space didnt sit well with me. 

I wish someone made an entirely stainless heat exchanger condensing furnace/unit heater. 

This is a cool, wet, and dark place. Everything steel rusts in this environment, painted or otherwise. 

pecmsg

Solid_Fuel_Man said:

We dont want radiant heat as it will cook (literally) the potatoes. 

There is constant air circulation with several 10HP fans pushing air up through the potatoes through in floor ductwork. 

I may just do a sealed combustion non condensing heater. The idea of a long horizontal sheetmetal flue in a cool space didnt sit well with me. 

I wish someone made an entirely stainless heat exchanger condensing furnace/unit heater. 

This is a cool, wet, and dark place. Everything steel rusts in this environment, painted or otherwise. 

https://www.modinehvac.com/products/unit-heaters/commercial-gas-fired-heaters/low-profile-stainless-steel-power-vented-ptp-btp/

EBEBRATT-Ed

Anything you install inside won't last long. How about a direct gas fired MU air unit outside on a pad discharging into the building. Some models take 100% OA but there are units (Rupp I think) that can take 20% OA/80% RA.

No flue to contend with

mattmia2

What about hydronic unit heaters? They'd be cheap to replace when they rot out or you could go stainless and pex wouldn't care about damp and cold. Could find a place away from the potatoes for the boiler, might even not be a big deal for a farmer to dig a trench to another building for it.

Solid_Fuel_Man

I'd actually love to put a boiler in and a few hydronic unit heaters. That is on the radar. 

I've been in touch with Modine, I can get a 100% stainless HX condensing heater which would love the 38F building. We are looking into lead time right now. 

My gut tells me that lead time will be like next summer......