Furnace replacement - options and sizing

[weston]

Greetings all -

This summer I acquired a 3600 sf shop built in the 1950's, that, until 2 days ago, was heated by a forced air fuel oil burner which is ducted under the slab with 4 outlets spaced around the perimeter of the floor. The furnace motor burned out and I am told that replacement parts are unobtainable. I am weighing my options and would welcome any comments and opinions.

The current burner is/was a 375K BTU unit dating from the 1950's.

I could:

1. Try to replace the gun with a universal beckett but no guarantees how much longer the original fire box and blower fan might hold out

2. Replace the whole thing with a new forced air oil burner

3. Convert/replace to a forced air natural gas burner

4. Convert to nat gas, install one of those hanging units, and abandon the existing ducting

5. Something even better I don't even know about?

Then comes the question of sizing - the building is cinderblock with recently installed exterior elasotmeric coating which I imagine helps with infiltration to some degree. The ceiling is 16' high, 30 year old R19 fiberglass batts installed in something less than a diligent fashion and covered with OSB. Overhead doors are new 2" insulated Clopay 3200 with a claimed R value of 9. I am in upstate NY, climate zone 5. Outdoor temps were in the low teens the other night and I was able to hold the space at 55 degrees with a 45K BTU salamander.

Thanks in advance for any input. Budget is of course a consideration but I'm the sort who is willing to pay for the value of having it done properly.

-weston

 

[Jim greengo]

Is the building one wide open bay,or is it sectioned off?

 

[weston]

Mostly open...along the back wall there is an enclosed 15x15 office in the back corner (with its own separate electric baseboard), and then there is an adjacent 15'x45' mezzanine, also along the back wall. There are no sections or walls between bays. Building is 60x60.

 

[6768rogues]

I have two heated buildings with 14 foot ceilings, one 1800 square feet and one 2400 square feet. The smaller is a pole barn with vinyl siding, the other is a metal building. Both are moderately insulated, nothing super energy efficient, no floor insulation, and I am near Rochester, NY. Each one has a Reznor 100K BTU NG hanging heater. They are set at about 45 degrees all the time and only run occasionally. If I turn it up to 65, it is warm in 20 to 30 minutes. They are pretty bullet proof reliable and are up out of the way, taking no floor space. I would go that route, perhaps with a larger unit or two units, allowing you to run one in shoulder peak heating times. They cost about $750 each.
I worked at a public school that had under slab ducts. They rotted out and had water infiltration, making the wing of the building like a rain forest. We abandoned them and put unit ventilators in the classrooms. My point is that if you invest in an expensive oil burner and the ducts fail, you will have to run ducts in your building.
Upstate could be near me or further upstate where it gets a lot colder. If it were my building, I would install two 100K Reznor hanging heaters, letting you run one in moderate weather and two in extreme weather, and providing redundancy if one fails. Note that I have not had a failure in 25 years with one of mine. Or you could use one larger unit and it would be less money. Or put in one now and see if it works well enough, perhaps adding another on the other side later if needed. NG is more affordable than oil.

 

[brewchief]

Before even considering replacement and reusing the existing under slab ductwork I would want to know what size the ductwork is and what the condition of it is.

If natural gas is available then I would go that way no matter what type of heater I used.

Sent from my SM-G965U using Tapatalk

 

[danski0224]

Before even considering replacement and reusing the existing under slab ductwork I would want to know what size the ductwork is and what the condition of it is.

+1 on duct size.

For proper airflow and not high velocity, those ducts must be relatively large, I'd guesstimate at least 14 inch round (or equivalent) for a standard efficiency system. Equal size in Btu but induced draft (80% AFUE) will require approximately 20% more airflow.

OP needs to do a load calculation to determine equipment size, and will most likely need to
add ducting for proper airflow and equipment life.

 

[yeldogt]

It's possible the old unit was only running in the 60% range -- unfortunately, you did not have any history with it to tell how much it needed to run at various outdoor temps.

One of my projects was an old building converted into a house that had ductwork buried in the slab -- also oil. When the heaters (building had two) started it took a while for the ducts to heat the surrounding concrete and finally fill the far flung areas with heat. Oil burns hotter -- so the hotter exit air for the furnace was a plus. The units were reasonably sized for the space so they had long run times ... oversized under these conditions would have been less successful and more costly to operate.

I think you should get some estimates -- having a few eyes on the situation may give some good thoughts on moving forward. Obviously, if the ductwork is in good shape and correct -- that would give proper distribution.

You also need to give thought to how you want to use the space and what temps will be maintained -- large resets need thoughts.

 

[weston]

Thank you all for your insights and comments.

I am in the southern tier of NY, so I'm probably colder than Rochester, but not regularly subject to the brutality of the North Country.

The existing ductwork is a 23"x30" plenum feeding 4 @ 8" galvanized ducts. I don't know what shape they're in but I could call a friend with a camera...although I suspect they're not in very good shape simply due to their age. They all move air but no telling how much was getting pumped into the ground - the blower on the old oil burner is a beast.

I'm not too concerned about having perfectly consistent temps throughout the space, but I can't afford any freeze ups.

 

[danski0224]

Four each 8 inch ducts is nowhere near enough.