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My house currently is heated by a hot-water boiler, with radiators in the main rooms. The bedrooms have no radiators (it's an old house and I guess when it was built they figured the central radiators were enough, which they are if you leave the bedroom doors open a bit).

The boiler is nearing the end of its life and I'm wondering if there is any reason why I couldn't replace it with wall-mounted ventless gas heaters (like this, as an example only: https://www.homedepot.com/p/Dyna-Glo-30-000-BTU-Natural-Gas-Infrared-Natural-Gas-Wall-Heater-IR30NMDG-1/206556628). I'd make sure the total BTUs met the requirements of the space. (I'd also make sure my carbon monoxide detectors work, just in case, given that these are ventless.)

Would this create any code violations or other problems? It's common in my area for folks to install one or two of these in their houses as auxiliary heat, but I've never heard of anyone using them as their only heat source, so I'm curious.

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  • Investigate the cost difference between running a gasline to each heater and a hot/return pair waterline. Also that boiler is going to be feeding other hot water users so you'll still need a new one. Apr 12, 2019 at 14:59
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    "ventless" heaters require adequate room air ventilation - there's nothing magic about them them makes ventilation optional, it's just omitted from the appliance itself. Replacing the entire heating load with ventless heaters will very likely cause problems with adequate ventilation, unless the house leaks like a sieve - in which case, vented or sealed combustion heaters and tightening up the house will save a lot of heat you are/will be wasting on heated air leaks.
    – Ecnerwal
    Apr 12, 2019 at 15:27
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    Why are you so keen on wall-mounting ventless heaters, instead of sticking something in an exterior wall that vents to the outside? Also, yes, can you explain why you think that the boiler is reaching EOL, and why you don't want to invest in a replacement boiler, which is likely going to be more efficient than throwing wall heaters at the problem in this day and age? Apr 12, 2019 at 22:17

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If it were me I would investigate the ability to use the existing boiler to heat the entire house. You said that the boiler is reaching the end of it's useful life; please explain why you think so. Hot water boilers can last as many as 100 years. At that age they are not usually very efficient but they will still heat a home or building. With a useable boiler you have many options to extend the heating system into the unheated rooms. Using unvented heaters as you mentioned is not a good option. You will have the smell, the excess humidity, running new gas lines to each one, the depletion of breathable oxygen and other things to consider. I would never use one of those heaters in a confined space or room. They are cheap for a reason. Go find a good boiler guy and investigate your options using the old boiler. my 2 cents

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Natural gas ventless heat is very efficient in that none of the heat is lost to the outdoor exhaust. They are considered quite safe with use of a catalytic converter to minimize unburned fuel (also known as soot, volatile organic chemicals, and carbon monoxide). The burning of the fuel will produce humidity, which may be bad or good depending on how well humidity is controlled by other means.

If you have access to an exterior wall for vented heaters a pipe can go through the wall for combustion air and exhaust. These operate like a draft style water heater and so have efficiency like them, something like 85%.

There are forced vent styles that are more efficient, operating like a 90%+ efficiency furnace. These save on fuel costs but have more complex install by needing electricity. Because they need electricity they won't run in a power outage.

In my opinion they'd work great as primary heat. You'll need multiples of them to cover the corners of the house and get enough heating power to stay warm. I like them as they can add humidity to the space, and no worries on trying to run a vent out or electricity in. If by chance one such heater fails then the rest should be able to still keep up with perhaps one room a bit chilled.

I don't know how much humidity they produce so I'd suggest investigating that. Then to mitigate the humidity put in vented units that exhaust outdoors. The humidity can be controlled by varying output from vented units and ventless units. Of course a dehumidifier is an option.

For an extra off the grid and/or retro look consider gas lights. These are still sold and are in a way a kind of ventless heater too.

I'll repeat the point on humidity since it seems other answers consider this might be a problem. Consider using vented heaters where you can, on an exterior wall with nothing outside that might interfere. Nothing wrong with mixing vented and ventless heaters in the home. I prefer the ones that don't require electricity to run as it avoids the cost of running wires and the issue of them not working if the power goes out.

How much work is being planned? Is duct work being considered for central air conditioning? That gives options for a heat pump, an often minor expense above an air conditioner. Air source heat pumps are little more in cost than an air conditioner but do not provide heat if the outside air is too cold. Ground source heat pumps have limits too but much lower. Again humidity may be a concern so consider a whole house dehumidifier. An air conditioner does dehumidify but does so by dumping heat outside. This may be acceptable for small humidity problems but big problems should have a proper dehumidifier. An air conditioner removes heat and in the process removes some humidity. A dehumidifier removes humidity and in the process adds some heat.

An air sourced heat pump is a common air conditioner run "backwards". They can provide heat in mildly cold temperatures and so can serve as a backup. If things get real cold these kinds of heat pumps can be forced into augmenting the heat in the home, it's often not much heat and if care is not taken it can do damage to the heat pump. I mention this because this has crossed my mind after a furnace failure. If I had to I could set the heat pump to run in temperatures that are "not recommended" to stay warm. In your case you'd likely just run the heat pump a week or so a year to keep from having to turn the gas heaters on and off in that transition from fall to winter. After that the central air will be for filtering the air, stirring up the air to prevent hot and cold spots, and maybe manage the humidity.

You can go with an outdoor venting heaters and then use a central humidifier for humidity control. Dehumidifier, air conditioner, heat pump, or whatever can be added later so long as there is sufficient space in the mechanical area.

That was long so I'll try to summarize. Ventless heat should work fine for primary heat. I'd suggest ducted central HVAC for air conditioning in summer, and in winter to filter, circulate, and remove or add humidity, perhaps also to introduce fresh air. Consider also vented heaters instead or as a mix with ventless to mitigate issues of humidity and such. I would trust ventless heaters in my home, and I may consider it in the future.

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