Shop heaters: ventilation first, BTUs second

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Research. No heater in our shop yet and no winter worked out there. What follows is how the fuels differ, what it takes to heat an uninsulated building, and the carbon monoxide question. When we have been through a cold season, this page gets updated with the date.

In an uninsulated barn the heater is the small decision and the building is the big one.

The three fuels

Electric. No flame, no exhaust, no water vapor, no fresh-air requirement — and a hard ceiling. A 120 V outlet gives 1,500 W, about 5,100 BTU/hr, and 12.5 amps is already past what a 15-amp circuit should carry continuously. Real shop heat means 240 V on its own circuit: 5,000 W, about 17,000 BTU/hr. A heater plus an electrician. Most expensive per BTU, and the only one you can walk away from.

Propane. Forced-air torpedoes, 30,000 to 150,000 BTU/hr. Cheap and quick, and unvented — everything the flame makes stays in the room, including most of a gallon of water per gallon burned. That is what condenses on cold steel and rusts your tools while the shop feels warm.

Kerosene. More heat per gallon, same unvented problem plus smell and soot, and loud. Burn only the fuel the manual names.

Vented. A hung unit heater with a flue, or sealed combustion. Several times the price, and it needs installing. The only one here you can run for six hours with the doors shut.

The tank cannot keep up

A 20 lb grill cylinder holds roughly 430,000 BTU. Under a 100,000 BTU/hr torpedo that is about four hours. Liquid propane also has to boil off to feed the burner, and does that more slowly as the weather cools and the bottle empties. On a cold morning the cylinder frosts, output drops, and it looks like a fault in the heater.

Big unvented heaters want a 100 lb cylinder, or a pair of 40s manifolded. Add the tank, hose, regulator and somebody to fill them: the hidden cost of the cheapest heater in the aisle.

Carbon monoxide, and the alarm that will not help

Every unvented heater makes carbon monoxide: very little when clean, a great deal when dirty, tipped or fuel-starved. Nothing to smell.

Manuals specify a fresh-air opening, commonly 2 to 3 square inches per 1,000 BTU/hr. Read yours. At 100,000 BTU that is a couple of square feet of open door — precisely the thing you were trying to shut.

Then the part that catches people. A household CO alarm is built not to nuisance-trip: the residential standard lets it sit silent below 30 ppm and allows four hours at 70 ppm. In a shop where you burn fuel three feet away all afternoon, that means it stays quiet through exactly the exposure you wanted warning of. A low-level monitor reading from single digits up is the one worth having — and check its rating; many stop at 40 F.

A CO monitor is the cheapest thing in the building. Same argument as the gate latch.

What an uninsulated barn costs to heat

Heat loss is area times temperature difference times how bad the shell is. Take a 30 by 40 shop with 12 ft walls: about 1,700 sq ft of wall, 1,200 of roof. Bare metal panel on open framing loses roughly 1 BTU per hour per square foot per degree. Hold 60 F inside against 20 F outside and the shell alone asks for over 100,000 BTU/hr, before the gap under the door.

So the 125,000 BTU torpedo is not oversized. It is barely adequate, it empties a cylinder proving it, and the minute you shut it off the building is cold again. Not a heater problem. An unpaid insulation bill.

Insulation first, and heat the person

Forced air heats air, and in a leaky building the air leaves. Radiant heat warms surfaces and people, so it works in a draft: a panel over the bench at 30,000 BTU beats a torpedo at 100,000 warming the rafters. On a small place, framing and insulating one room inside the shop and heating that is the honest answer.

• Ceiling first. Heat goes up: a 12 ft ceiling means warming four feet of dead air overhead before any reaches you.
• Air-seal the obvious. Door gaps, the ridge, penetrations.
• Then size the heater, against the building you now have.

Insulation is bought once and still works in an outage. Buy the heater first and you buy it twice.

What we are buying

The building comes first, and that part is decided. Framing and insulating one room inside the shop, then sizing heat against what that leaves, is the order this article argues for and the order the money goes in here. Vented against unvented is the decision still to make, and the article is right that everything else follows from it. Our own building is the other blank: dimensions, construction, what insulation it has, which is what turns the heat-loss arithmetic above from an example into a figure. Buying puts the heater, its fuel and rating, what it cost and the date in this section. The Research line at the top of the page is waiting on a cold season worked out there, because the number that decides whether it was the right heater — what fuel or power came to across a winter — cannot be read off a receipt.

Before you buy

• Decide vented or unvented first. Everything else follows from that one.
• If it burns anything, buy a low-level CO monitor and read the fresh-air opening in the manual before the first burn.
• Price insulation and the propane tank in the same conversation as the heater. Both are larger than they look.

— Chris D., McEwen, Tennessee