On Saturday the ASHRAE committee that has been trying to consider a standard affecting unvented gas space heaters for over 10 years was again prevented from issuing proposed standard language for public review.

Just so we are all on the same page, unvented gas space heaters dump 100% of their combustion products into the house. These products include NO2, CO2, CO, H2O, etc. None of these pollutants are good for human health when concentrated within a home. NO2 for example is a carcinogen that the ASHRAE committee is potentially using to determine the “acceptable” size of unvented space heaters within a home.

To be clear, it is the opinion of many experts that unvented gas equipment is a hazard to human health. Just Google: “gas combustion products and cancer”. For example the Canadians suggest that you replace a space heater with a vented one.  https://www.healthlinkbc.ca/healthlinkbc-files/combustion-products

In order to get past the product manufacturers’ objections to limiting the size of these indoor polluters, the committee made a clean calculation. Based on the amount of NO2 generated per BTU capacity, how big must a room be to keep below the 1 hour limit for exposure to NO2? The calculation is based on the legally allowable limit of NO2 generation for this equipment.

When the proposal to limit the size of these unvented heaters based on the size of the room, the committee again was unable to garner sufficient votes for the standard action to go out for public review. The manufacturers’ delaying tactics again won.

I hope some of you will submit comments to me about specific experiences with unvented gas space heaters.

Tags: CO, Heaters, NO2, Safety, Space, Unvented

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I put a "air advice" in a small house with gas turned off - lack of payment. CO over a 24 hr time ave 39PPM with window open 1". She thinks she has the flue for 3 mo. This was just 11,000 BTU's for 430 SF.

Teck with frozen pipes job used a 40,000 BTU temp heater got sick and up checked his lunch

Thank you Eric

So was this device a propane or natural gas heater. Do you have any specifics about the make and age?

I presume the Tech that got sick was using a propane heater. Am I right?


Both were LP gas.  The more you test the more you find.   This after noon I saw a large office with a open flame of Nat Gas for looks - after it sooted up in flue was 2560 PPM CO but in office was 44 PPM CO with doors open plus dirt - particles was red lined and left a layer of soot on paper and desks.  When you walk in its smells like gas till you get used to smell.  He spent $23K on the center piece,  with glass and tile but vent system was not large CFM to over take air returns in house.  When install was done gas line was to small and dropped the presser with high gas usage,  drops under 2" of water col and then makes soot.  There are 9 ways to make high CO on this job 4 where there.  

I just do not see why open flame is allowed in a building, OK for cooking  with venting to suck out gases.   Just like having a fire in middle of house your house is now a flue.  

Following to learn more.

In 2009 I sat in on a ACI conference session in Kansas City with Paul Franscisco who had just finished a study on un-vented gas heaters. It was a 3 year study conducted by Univ of IL Urbana/Champaign about the indoor air quality effects of unvented heaters in actual home environments.

I wrote a blog article about  the conference session: Here is a link to the blog on ventless gas logs.

Unfortunately the location of the study summary moved, so the ink on my blog is not good anymore (Ill have to fix that), but here is a link to the PDF: http://www.indoorclimate.istc.illinois.edu/images/LO-09-087.pdf

It is a dry read, unless you are a data-wonk. But what it says about NO2 levels in particular is enough to grab your attention.

John,  of the byproducts listed,  the CO, NOx and H2O I suspect are the most important to consider.   For the unvented gas space heaters used in a cold dry climate during the winter time -- probably adds just a portion of the H2O that would be needed to bring up the RH in the residence.   That leaves the CO and NOx.... 

What is the target market of the unvented heaters --- as alternatives to fire places?  Or secondary heat by older homes that are lacking adequately sized heating systems?

Do any of the heaters list the CO/hr or the NOx/hr?   

At previous ASHRAE/IAQ conferences (many) there have been a number of good papers that looked at the impacts and they've quantified the health risks for family members at risk and young children.   Is the committee looking at coordinating with the health care industries?   Number of insurance claims?  Or worked with NFPA - to help them with standards they're willing to push?  NFPA includes the residential building insurances... and probably the insurers for some of the manufacturers.... 

I am not a fan of the unvented heaters - I really dislike the idea of any NG/Propane device that isn't using outdoor combustion air and venting the combustion products outside....  but I do know they are commonly and frequently used --- infrared heating at ceiling height for the check out lines of some warehouse stores...


This is anecdotal, but the people I talk to on the tech support line that are using unvented heaters and gas logs are using it, or installing it, because they were sold on the "100% efficient heating" pitch. Starting a bonfire in your living room is 100% efficient at heating as well, but it doesn't mean it is good for you.

You can try to reason with them that a direct vent gas fireplace insert is 80% or more efficient, and it wont try to slowly poison you. But these are price sensitive folks that like the low price tag of the non vented unit, and love the "100% efficient" pitch.

Dennis, combustion appliances do not put out any specific amount or range of CO (or CO/Hr). A very small amount (in the single digits) of CO may be given off with a new appliance, but it is often after they have operated for a while that the CO starts to rise. The orifices start to rust/degrade, or the air supply gets compromised or the gas supply pressure goes below what the appliance requires (which is often a result of undersized gas supply piping).
The air supply to the gas fireplaces and ventless "stoves" is often reduced to give the yellow flames that simulate a wood fire. This can easily lead to higher CO levels.
What many people are not aware of is that CO alarms will never go off, or display anything other than a zero, unless the CO level is above around 35 PPM.
I have seen ventless fireplaces installed in brand new homes! Some people not only love the heat, but also the "ambiance".
One homeowner told me: Hey, if Home Depot sells it, it MUST be safe, right?

I like the up word push of air in most warehouse stores, with infrared heating.  Most gases push up with heat and go out the holes in roofs & big doors.   I have tested box stores for CO CO2 dust VOC RH% temp selling HVAC - there air flow a lot of air flow exchange,  Most box store use LP gas for fork lift - I have seen 1000 PPM 4' behind tail pipe with a untune up fork lift, the gas just follows the air stream.  If you do not test you do not know


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