Hot Dawg air supply solution? Comments?

[Rick Schubert]

As I wrote a few days ago I'm installing the 45,000 BTU model. Since I have worked hard to seal the cracks and insulate my shop, I'm concerned about possible negative pressure and starving the heater for air, possibly creating a CO problem.

I hate to just cut a 7-8" opening in the wall that would allow cold air in all the time. (And we do have cold air here in the winter!) The instructions call for a 1" square opening for every 1,000 BTU's.

So I wonder if this would work: Cut a 4" hole in the wall under the heater and install 4" metal ducting entering the shop for 1-2 feet. Then a 90 degree elbow and continue 4" duct up close to the heater. (A "L" shape) The heater is close to the wall. In the long vertical section I would install a 4" duct booster fan. I would get a switch, that I can't remember what it is called. I have one from Sears that I can plug tools and my Fein vacuum into so the Fein comes on when the tool is started and they both go off together.

I would wire the heater with a plug and plug it into the switch and then the fan so the fan only works when the heater is running, demanding air.

I would also put a flapper in top of the 4" duct, kind of like the flapper on top of the vertical mufflers on old farm tractors. This would prevent air from entering all the time, but would move up when the duct fan pushes air out. I would insulate the ducting.

The switch is about $25, the duct booster is $22 at HD and some ducting. Probably a total cost of $60 or $70.

I'm planning on maintaining the shop at 40 degrees during the winter, then when I'm working I'll fire up my wood stove to bring the temp up to 60-65 degrees. So I'm figuring the Hot Dawg won't be running a lot.

What do you think? Or am I being paranoid??

Thanks, Rick

[Don Baer]

Rick,
I work in the workers health and safetyt field. Carbon Monoxide is produced from incomplete combustion typically from improper air flow. If the heater requires 1sq.in. per 1000 BTU then you need 45 sq.in.
according to my calulations you need a pipe diameter of 7.57 in. so I wouldn't use anything less then an 8" pipe.

Area = pixRadius squared

So working backwards

((45 sq.in/3.14)^.5)x2=7.571317

Sorry, but thats what I'd use.

[Jim Becker]

Don, could he put an automatic/swinging damper on that inlet to conserve heat when the unit isn't running?

[Bob Marino]

Rick,
I work in the workers health and safetyt field. Carbon Monoxide is produced from incomplete combustion typically from improper air flow. If the heater requires 1sq.in. per 1000 BTU then you need 45 sq.in.
according to my calulations you need a pipe diameter of 7.57 in. so I wouldn't use anything less then an 8" pipe.

Area = pixRadius squared

So working backwards

((45 sq.in/3.14)^.5)x2=7.571317

Sorry, but thats what I'd use.

Don,
Not meaning to hijack a thread, but what is the minimum height- floor to ceiling, that's allowed for this heater? I have 8' ceilings - am I in trouble?

Bob

[Frank Hagan]

As I wrote a few days ago I'm installing the 45,000 BTU model. Since I have worked hard to seal the cracks and insulate my shop, I'm concerned about possible negative pressure and starving the heater for air, possibly creating a CO problem.

I hate to just cut a 7-8" opening in the wall that would allow cold air in all the time. (And we do have cold air here in the winter!) The instructions call for a 1" square opening for every 1,000 BTU's.

So I wonder if this would work: Cut a 4" hole in the wall under the heater and install 4" metal ducting entering the shop for 1-2 feet. Then a 90 degree elbow and continue 4" duct up close to the heater. (A "L" shape) The heater is close to the wall. In the long vertical section I would install a 4" duct booster fan. I would get a switch, that I can't remember what it is called. I have one from Sears that I can plug tools and my Fein vacuum into so the Fein comes on when the tool is started and they both go off together.

I would wire the heater with a plug and plug it into the switch and then the fan so the fan only works when the heater is running, demanding air.

I would also put a flapper in top of the 4" duct, kind of like the flapper on top of the vertical mufflers on old farm tractors. This would prevent air from entering all the time, but would move up when the duct fan pushes air out. I would insulate the ducting.

The switch is about $25, the duct booster is $22 at HD and some ducting. Probably a total cost of $60 or $70.

I'm planning on maintaining the shop at 40 degrees during the winter, then when I'm working I'll fire up my wood stove to bring the temp up to 60-65 degrees. So I'm figuring the Hot Dawg won't be running a lot.

What do you think? Or am I being paranoid??

Thanks, Rick

Interesting ... the 1" per 1,000 btu/hr of air is often the quoted amount for combustion air being supplied by a duct that communicates directly with the outside. My concern is that the damper in the duct will cause a problem ... it is restrictive ... and the heater will expose you to low levels of CO without you knowing it. Besides that, your system is fairly complicated with the duct fan, venting with ells, etc.

There's a much better solution ... use an opening with motorized louvers interlocked with the heater.

You have to oversize the louvers in this case to get the "net free area" of 45", but a 10 x 10 motorized louver would be enough (they usually deduct 50% for the louvers). When the heater is going to come on, the signal first goes to the louver motor, it opens the louvers and when they are fully open, the heater comes on. At the end of the heating cycle, the louvers close. If the louvers don't open, the heater doesn't come on (unlike your duct fan, which might not come on, and you might not notice it until you're looking down from Heaven watching your wife find your body.)

There are also combustion air fans that provide the air only when needed .... see the "fan in a can" at http://www.fieldcontrols.com/cas3-4.html This turns on the fan, and supplies the air only when the heater is operating. But these are more expensive than the motorized louvers.

The motorized louvers are probably the best bet. The greenhouse supply places all carry them, and the wiring between the heater and the louvers is simple enough.

[Rick Schubert]

Don,
Not meaning to hijack a thread, but what is the minimum height- floor to ceiling, that's allowed for this heater? I have 8' ceilings - am I in trouble?

Bob
Bob,

I'm pretty sure the instructions said 10 feet maximum.

Rick

[Bob Marino]

Bob,

I'm pretty sure the instructions said 10 feet maximum.

Rick

Is that minimum?

Bob

[Rick Schubert]

Sorry, I misread your message. I looked again in the instructions and didn't see any reference to minimum height. Only the clearances to combustables which are 1 inch top and bottom.

Although I obviously could have misread or missed it entirely.

Rick

[Rick Schubert]

Rick,
I work in the workers health and safetyt field. Carbon Monoxide is produced from incomplete combustion typically from improper air flow. If the heater requires 1sq.in. per 1000 BTU then you need 45 sq.in.
according to my calulations you need a pipe diameter of 7.57 in. so I wouldn't use anything less then an 8" pipe.

Area = pixRadius squared

So working backwards

((45 sq.in/3.14)^.5)x2=7.571317

Sorry, but thats what I'd use.
Don,

I assumed the 1 inch square was for an opening in the wall. With the booster duct fan I figured it would be pulling more air in than a hole in the wall. So I thought I could go with a smaller opening.

From what I read between the lines in previous posts on these heaters, I assume most applications are not using any air replacement. I think most are relying on leaks and air seepage. So I thought this might be a lot better than nothing. Probably incorrect thinking.

Rick

[Bill Turpin]

What is the flow of the booster fan with 4" duct and the back pressure of the elbow and flap damper? To be safe you need a combustion air pressure switch to prove that the fan is truly on.

Bill in WNC mountains

[Randy Carver]

So what provides the fresh air intake that the woodstove needs? I'd be much more worried about that than the Hot Dawg. The Hot Dawg has a powered vent to make sure the combustion gasses leave the shop. The stove only has convection.

[Randy Carver]

Bob, as for minimum height, I have a similar restriction, and it's posed no problem to my installation. As long as you observe min clearances, you'll be fine.

I suspect the minimum height formulas are for maximum distribution of the heat, and not safety. You can leave your hand on the case of the unit while running. The distribution fan in the unit makes sure the hot air moves around the shop.

[Randy French]

Rick,

I have a 4' x 4' window on one wall in my shop and was planning on leaving it cracked an inch or so while in the shop and using the heater. Or I could leave my main shop door cracked an inch or so since it is a 6' door.

Was your original thought with your design to have more of an automatic way of having air available when the heater was running?

Randy

[Randy French]

Bob,

I read that the unit should not be installed below 7' from the bottom of the unit for commercial applications and 5' from the bottom of the unit for residential applications. So you should be okay with the 8' ceilings.

Randy

[Bob Marino]

Rick, Randy and Randy,

Thanks - off to the Town office for a plumbing permit!

Bob

[Frank Hagan]

So what provides the fresh air intake that the woodstove needs? I'd be much more worried about that than the Hot Dawg. The Hot Dawg has a powered vent to make sure the combustion gasses leave the shop. The stove only has convection.

This is not correct. A powered vent does not eliminate the need for adequate combustion air to prevent you from dying.

There are many scenarios. Here is one:

1. The fan in the heater is pushing an amount of air out of your shop equal to that which can come through an opening of 45 square inches. But there is no such opening in your shop. The room "goes negative" and air is compressing and trying to infiltrate through whatever space it can. A high pressure air system exists in this negative pressure room ... the vent pipe. Flue gasses will spill out into the room through tiny unsealed portions of the vent pipe. I have knowledge of a family dying this past winter because of this very scenario.

You can do what you want, of course. But don't fool yourself into thinking you know more than the American National Standards Institute, the National Fire Protection Association, the American Gas Association (now CSA) and the engineers at Modine, the manufacturer.