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Is it ok to reduce a heater vent from 4″ to 3″

user-389891 | Posted in Energy, Heating & Insulation on January 17, 2021 05:41pm

I am buying a Williams 50,000 btu floor propane heater (side vent)’ Is it ok to reduce the vent size from 4″ to 3″ primarily because the 3″ size has more options at the big boxes.

2nd question. This is a 50,000 btu heater with a 36,000 btu output. I’m a little confused about what btu really means. I had bought a 30,000 btu with twin heater elements for my 20 gallon propane tank. It wasn’t hot enough. I built a cabin (I paid for it) with all electric heat. I didn’t want to pay $5000 for a natural gas line. They work but slowly so I am in the cold for an hour or so. I am captiulating and installing a propane tank (heater and tank will be about $2,ooo), My question is: can I compare the 30,000 btu output with the project 36,000? Or do I just install and try?

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  1. User avater
    unclemike42 | Jan 17, 2021 07:48pm | #1

    from the heater instructions:

    Use vent pipe of the same size as the outlet on back of heater. In no case should a different size vent be used.

    the 50,000 BTU rating is the energy that you put into the heater (total heat value of the fuel when burned)

    36,000 BTU output is the result of a 72% efficiency rating. The rest of the heat is going to go up that chimney pipe.

  2. User avater
    unclemike42 | Jan 17, 2021 08:01pm | #2

    If you want to compare how the new heater will help warm up the cabin, you might want to understand how many BTU the installed electric heating puts out.

    You should be able to find labels on the heaters and convert watts to BTU.

    compared to a 30,000 btu output non-vented propane heater, your new heater will provide 20% more heat, and not put all the combustion gasses into your cabin.

  3. firedudec56 | Jan 17, 2021 10:30pm | #3

    to back up Uncle Mike - NO! IT IS NOT OK TO REDUCE THE VENT SIZE. You need to go to a plumbing or mechanical supply house and get the proper venting material.
    You'll "choke" the exhaust and end up with Carbon Monoxide in the cabin.
    I'm going to assume there is a thermostat and the hour you're being cold is when you first arrive at the cabin. One way to solve this would be to install a wi-fi enabled thermostat and use it to raise the cabin temperature before you arrive.
    hopefully you already have a CO detector installed on every living level - if not, please get them
    other quick thought is to investigate the cost of solar panel installation to assist with the electric heat
    good luck

    1. user-389891 | Jan 19, 2021 01:06pm | #5

      Thanks all, 4" it is. I didn't know about the wireless thermostat. That will solve a lot of probelms.

  4. User avater
    chrisjacksonbest | Jan 19, 2021 09:13am | #4

    For most installations, a 40–gallon water heater with a 3–inch outlet on the draft hood is required to have a vent connector that is a minimum of 4 inches in diameter. That means the vent connector has to increase in size by 1 inch.

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