We may have a fix for you tomd63, As orchiddude said, ""Radiant heats up things, while the blue flame heats the air."" I would have beaten him to it but my internet has been down all day! Why two different kinds of heater you ask? Well, the blue flame type heat up the air, you would put one in your dining room where the dinner table is the focus, or in a basement where the pool table etc is being used. The whole area needs to feel warm. A blue flame heater makes the air warm. Infra red/radient, well that is in the living room, TV room. The place where everyone is facing the same way and the heater makes everything that it is radiating heat onto be warm. Both types of heater warm up the room in different ways. The blue flame warms the air. The radient warms up objects (chairs, walls, the cat, your orchids) that then warm up the air. Then the thermostat shuts off when the AIR gets to the temperature you set. Your radient heater may be warming your benches, your cat, your orchid buds to 120F and beyond, before they in turn heat up the air that gets back to the heater enough to turn it off. OK think toasting a marshmallow... Try using a 1500w fan heater, then try a 60w light bulb. Because your greenhouse is small, (and do not take that the wrong way) and your heater is large, a fix may not be easy. What I sugesst is you make sure none of your plants can "see" your heater. This means there is no direct path for IR from the heater to the plants. Also try to set things up so that your heater shines (for the want of a better word, um, ok a better word is RADIATES) the heat onto something that will not catch fire but will heat up then heat the air. A rusty 55gal drum springs to mind. (fill it with rusty old water and you keep the thermal mass team happy too!) Not withstanding the vented/unvented issue I think the ones using unvented should advise on the blue flame/radient issue. It is -14F here again.... Glad I don't have a heater!!!

Mike, Do some calculations just for grins and see if a tankless would work for you. A Rheem 74 will do 4.7 gpm at a 70 degree temp rise (35 degree inlet to 105 for a shower). There are some heaters that have a higher BTU input that will do a little more gpm if you need it. The Rheem is about $1200 with the vent kit and the valve kit. It would save you about 41% in fuel costs over a tank and you would get a tax credit. The only tankless units that had problems that I'm aware of were the Bosch units that used the water to power the ignition. Very lame. I think there were other people, particularly with electric units, that worked properly but weren't sized properly. If it were me, the choice would be between the normally aspirated sidewall vented (commonly called direct vent) tank heater or the tankless. The largest direct vent tank heater I normally see is 50 gal and they have crappy energy factors (in the high 50's). I think you will find them for around $700. The powervent costs about $1200 bucks for a 50 gal and are about as complicated as the tankless and about as noisy. I think the early models were more noisy that later ones. If you can purchase a tankless that will serve your needs for between $1200-1400, including the vent kit, and you have adequate gas service to the home, you will pay back the additional cost of the tankless over the direct vent tank heater in less than 5 years assuming you save $120 a year and take advantage of the $300 tax credit. The Rheem tankless has an energy factor of .82 while the Rheem direct vent 50 gal has an energy factor of .58 That's a 41% improvement and would easily net $10 a month difference in energy savings. If you go with the direct vent tank, it will pull air from the outside. If you go with a tankless, I would get the direct vent option so that you don't have to worry about venting the utility room (and letting the noise into the rest of the house). BTW: Page 12 and 13 of the link below do a good job of explaining the restrictions on side vents. Of course check with your inspector, but if you are looking for guidelines, these are pretty good. Here is a link that might be useful: Installation manual for tankless side venting

Powervent water heaters do not have a dedicated air inlet . They still require free air for combustion. The difference is, they dont have a draft hood. If your house is tight or in a negative, a conventional vent water heater will not vent properly, and the products of combustion could spill out the draft hood. A way to combat this is by bringing fresh air in from outside .ether through an hrv or a duct. The other option is direct vent (2 pipe, brings air from outside .) And the price sounds high, but navien is a very cheap brand and they also come with a lot of different options. My navien has a 6 L tank with an internal recirc pump. I also needed to run a new gas line as well as upsize my water supplies to accomodate. I dont quote retro fits, but navien installs usually come out to between 2800 - 3000. They are time consuming . Also consider hidden costs . Mark up to cover warrenty . Valve kits. Copper. T and p. Gas line. Set up. 636 vent . Drilling through the house. Mounting it. And following every step of a 75 page install manual. As not to voide the warranty. The time and fuel to get the unit and bring it to your house. Also the first $30 of my hourly rate goes to operating costs and government agencies. You get what you pay for. Make sure your installer is tssa certified. Has a gas licence. Plumbing licence and insurance. There is an insane amount of misinformation on this forum. I am a licensed plumber, gas fitter and back flow tester . As well as a small business owner, if you told me to do a job for x dollars , i would tell you to have a nice day. Be more respectful of professional trades people. Not everyone is a crook . And if someone quotes a job low, they will cut every corner they can. Hence why i would never buy a new house.

ionized_gw: I think that gas tankless has become more mainstream. In addition, the incentives have started to go away. Much of the initial complaints with gas tankless were around bad installation and/or bad retrofitting. I don't think they are sliding in popularity as much as they simply aren't the new kid on the block any longer. On new installs, gas tankless doesn't cost significantly more than a power vent heater and you see them installed on new upscale homes here because of the unlimited hot water. Rheem has addressed the "cold water sandwich" by delaying a "post purge" cycle when the heater is turning off and on frequently. This means that the heat exchanger doesn't rapidly cool off -- eliminating the "cold" part of the sandwich. I insulate my hot water lines also to reduce heat loss between uses. I know I'd never go back to a standard tank water heater. As for electrical requirements on the hybrids - it's the same as a standard electric water heater. The reason is because they sometimes have to fall back to electric resistance heat if the ambient temp is too low or quick recovery is needed. There was some reliability issues on the early hybrids that has since been worked out, I believe. As a side note: Hybrids are being linked up with PV -- meaning that while you would never use solar PV to power electric resistance water heater, it's a good match up for a hybrid water heater. Oil isn't much of "a thing" where I am, so I can't really comment on that. SaltiDawg: The op was seen screaming and running from the building