Hot Dawg Heater Venting - Fresh Air

[Rick Schubert]

I am also (Randy French's post) going to be hooking up the 45,000 Hot Dawg heater in my garage/workshop. I have worked hard to insulate the shop well and seal all the gaps with caulk. Now I'm concerned that running the heater may create a negative pressure and could possibly be dangerous (CO gas).

Is that a concern for anyone else? And has anyone rigged up some sort of an outside air supply with flaps that open when air is needed or some arrangement. I could just cut a hole in the wall, but that would defeat all my caulking!

Thanks, Rick

Thanks, Rick

[Russ Massery]

That's why I bought a direct vent unit. You might try to bring a pipe right to the combustion camber, so the air consumed by the flame won't cause a draft.

[Frank Hagan]

That's why I bought a direct vent unit. You might try to bring a pipe right to the combustion camber, so the air consumed by the flame won't cause a draft.

The National Fuel Gas Code requires combustion and ventilation air to be provided ... one vent within 12" of the cieling and another within 12" of the floor. The "open free area" of each vent has to be 1" per every 4,000 BTU of the appliance (for a 54,000 BTU appliance, each vent has to have "free open area" of at least 13.5 square inches). If those vents are connected to ducts that lead outside, you need to double the "free open area" of each one.

BUT, check with the manufacturer. They may be able to avoid the more stringent requirements with their design.

There are even some combustion air delivery systems designed to help with this (see http://www.fieldcontrols.com/cas3-4.html). That's an "engineered solution", and will work fine if you get the company making the air delivery system to sign off on it.

[Bill Turpin]

Your heater at 100% output will require 450 cubic feet/hour of fresh combustion air. That is over 58 gallons per minute. Do not depend on even light-weight doors to open to prevent negative pressure and CO. The heater will consume 45 cu foot of natural gas/hour. One thousand cu ft of natural gas is a decatherm or about $14.50. This amount will be your daily cost if run constantly.

Bill in WNC mountains
who burns ± 20 decatherms/hour at work

[Mike Deschler]

I have been running my 45K BTU propane Hot Dawg for three years now with no problems. Modine, the manufacturer gives detailed instructions for both vertical and horizontal venting. I chose the latter and am running a 3" double walled pipe per their instructions. I do not recall their comments, if any, regarding additional venting. Besides the unit has a closed combustion chamber. If you have any concerns, buy a couple of detectors and hang them in your shop and contact Modine in Racine, Wisconsin for applications advice.

[Frank Hagan]

I'll reiterate: follow the manufacturer's instructions. If they can't be found, then follow the National Fuel Gas Code. Carbon monoxide poisoning can be cumulative, and low level, and the victim often thinks they have chronic headaches, or a series of flu like symptoms from some virus, rather than CO poisoning. Mental retardation can result from low level exposure, as well as paving the way for faster death when exposed to higher levels.

The terms get confusing fast ... "sealed combustion" and "direct vent" have a specific meaning in the code. Hot Dawg heaters, at least the ones I have seen, are not "sealed combustion". "Sealed combustion" appliances draw air from the outside through a fresh air duct, and do not take air from the room.

There are cases where you can take "room air" for the heater's use and be safe, but not in a space where you've taken care to seal everything up with caulking, weatherstripping, etc. In the code, such a space is called "unusually tight construction" (the word "unusually" should be stricken, but remains ... most people try to seal up the drafts in their homes nowadays!)

So you need to provide some air for the heater to use ... it burns air and gas to produce the heat. Yes, you will let the cold in doing that, and that seems like a waste, but without enough combustion air you won't be burning all the gas you are using anyway. Its really a waste to send a lot of the fuel up into the air unburned.

[Frank Hagan]

I have been running my 45K BTU propane Hot Dawg for three years now with no problems. Modine, the manufacturer gives detailed instructions for both vertical and horizontal venting. I chose the latter and am running a 3" double walled pipe per their instructions. I do not recall their comments, if any, regarding additional venting. Besides the unit has a closed combustion chamber. If you have any concerns, buy a couple of detectors and hang them in your shop and contact Modine in Racine, Wisconsin for applications advice.

I had only seen what the business calls the "atmospheric" kind of heater, but I did find in their brochure that they do have a direct vent / sealed combustion model called the "Hot Dawg Model HDS" which they call a "separated-combustion gas-fired unit." The HDS model does indeed use ducted combustion air, drawing all combustion air from the outside through a vent. The brochure is available at http://www1.modine.com/publications/litnav.php?c1=CHVAC%26R&c2=Modine+Hot+Dawg&c3=Consumer+Literature

I couldn't get to the installation instructions for the standard, "Hot Dawg Model HD" to find their combustion air requirements. But using the HDS model avoids having to worry about it ... you just run a duct of the right size to the heater and it supplies all the air the heater needs.

To those of you who have one ... have you had to replace the hot surface ignitor (HSI)? The reason I ask is that usually HSIs are susceptible to wear due to fumes or dust, so I would expect the HD model to have shortened ignitor life in a shop environment. The HDS model would not have that problem unless the air supplied through the duct is bad.

[Randy Carver]

I have the non-HDS 45K version in my shop. I've never had any issue with ventilation at all, but I'd be hard pressed to call my shop "unusually tightly sealed". I have double-hung windows, and I open the top of the window a couple of inches. The unit itself is power-vented to the outside, and the flame is enclosed (not sealed). I consider this to be a very safe installation. The unit hangs from the rafters, and as such is not in the path of most dust debri. A simple shot of air from the compressor once in a while cleans it up nicely. Note that I do not spray finishes at this time, I'd be careful with that.

I've not replaced the HSI (or anything) in the 3 years I've had the heater. Spare parts are available online from greenhouse suppliers. Shouldn't be any problems there. These really are excellent units, and up here in NH it gets a workout. I keep the shop at 40 degrees all the time, and bump it up when I'm out there working. So far, this is plenty affordable for me, and prevents me from having to evacuate all latex products.

[Frank Hagan]

I have the non-HDS 45K version in my shop. I've never had any issue with ventilation at all, but I'd be hard pressed to call my shop "unusually tightly sealed". I have double-hung windows, and I open the top of the window a couple of inches. The unit itself is power-vented to the outside, and the flame is enclosed (not sealed). I consider this to be a very safe installation. The unit hangs from the rafters, and as such is not in the path of most dust debri. A simple shot of air from the compressor once in a while cleans it up nicely. Note that I do not spray finishes at this time, I'd be careful with that.


You are probably getting about 60 square inches from your open window, right? 2" in a 30" window would provide that. That would provide enough air for the unit, as a vent communicating directly with the outside only needs to be 1" per 1,000 BTU (or 45 square inches) if there is only one vent. The Hot Dawg might even have a requirement that is less than the ANSI standard (National Fuel Gas Code). I can't tell because I don't have the instructions ... they don't post them on their website.

The problem with this type of installation is the next homeowner might not open the window, or you could forget. So, while I might do the same thing in my shop, I couldn't recommend it when talking to a customer from a liability standpoint.

The problem with CO is that you simply don't know if you are getting exposed or not. The symptoms for cumulative low level exposure are so subtle that many, many people are exposed for years without connecting their "flu" or "headaches" to the heating system. You might buy a CO detector for the house, and take it into the shop after you've had the heater running for a while just to check. Most of the CO detectors measure the rise in CO rather than the actual level, so moving one from the house to the shop will tell you if your shop heater is producing CO.



I've not replaced the HSI (or anything) in the 3 years I've had the heater. Spare parts are available online from greenhouse suppliers. Shouldn't be any problems there. These really are excellent units, and up here in NH it gets a workout. I keep the shop at 40 degrees all the time, and bump it up when I'm out there working. So far, this is plenty affordable for me, and prevents me from having to evacuate all latex products.

I'll have to look at their design and the ignitor they are using. The average life for ignitors in the HVAC industry is about two years. The HVAC guys tell me its much shorter in wood shops because of the dust and fumes from finishes. Our heating boilers sometimes get 3 - 5 years from ignitors, but rarely longer than that.