We need to replace our propane furnace. One contractor said he needs to replace our existing 2 inch external PVC pipe with a new 3 inch one (requiring drywall work). Another says they can use a non-direct vent setup using inside air. I've read other posts about pros and cons but one thing that wasn't mentioned is whether the non-direct will suck all the warm air from upstairs down into the basement. My house is a mix of 100+ year old and modern and the heating is very uneven from room to room. Will non-direct make that better or worse?
Will a furnace drawing combustion air from the basement improve circulation throughout the house
hvac
Related Solutions
You'll have to find a unit that can match the required intake air flow of the furnace.
Estimating required air flow.
To estimate the required amount of air the furnace needs, you'll have to know what type of furnace you have, and how many BTUs the furnace is rated for. Once you have this information, you'll divide the furnaces rated BTU value by 10,000.
100,000 BTU / 10,000 = 10
Next you'll multiply that value by the CFM/10,000 BTU value for your type of furnace.
- Natural Draft Furnace = 100 CFM/10,000 BTU.
- Induced Draft Furnace = 130 CFM/10,000 BTU.
- Condensing Furnace = 150 CFM/10,000 BTU.
130 * 10 = 1300 CFM (Cubit feet per minute)
So if you had a 100,000 BTU induced draft furnace, you'd need a filter with at least an output of 1300 CFM.
As the filter gets dirty, air flow will naturally be restricted. Because of this, you'll want to choose a filter system with a higher CFM rating than you need. In our example above we wouldn't want to get a unit that is rated at 1300 CFM, we'd want one rated somewhere closer to 1400 or 1500 CFM.
Measuring Air Flow
You can determine the actual air flow using a handheld Anemometer, like this one
Start by measuring the area of the duct (Length * Width). Next measure the air speed using the anemometer (ft/min). Finally multiply the area of the duct by the air speed.
(20" * 25" = 500 sqin.) / 144) = 3.47 sqft.
3.47 sqft. * 375 ft/min = 1301.25 CFM
To determine if the air filter is restricting air flow, calculate the CFM both with and without the filter connected.
Watching for signs of reduced flow.
If you notice any of the following problems after installation, you likely have restricted the air flow too much.
- Furnace has trouble producing a flame, or flame is small.
- Furnace constantly overheats.
- Furnace struggles to keep up with heating demand.
- Output air flow is weak and/or cold.
I have a couple of long runs too and I used a fan until I upgraded the HVAC unit. Before you get a booster you need to measure the air flow out of the two vents and compare them to other vents in the house.
How to measure air flow? Well pick up an Air Flow Meter. However I have a suggestion before you spend $$$ on this. Get a large trash bag, configure it to open about the size of your duct opening (take off cover), hold it tight on the duct opening, measure how long it takes to fill... compare several vents. Note the air should be running when starting this - since how long it takes air to come out factors very little into the cooling equation. If you are confident with the bag results and they are obvious then skip the pricy tool. ##Side Note## (A friend of mine uses one of those garden wind spinning things and measures the rotations over a given time to compare air flow. You have to have the angles/distance the same from the vent and he has to have his iphone slow wdown the frame rate to count. But damn that mcgyver-bastard, pretty sure its more accurate than my empty trash bag! But it does involve slow-motion so my solution is more practical)
I would also take temperature readings inside each duct while on after 5-10 minutes. How cold is the air you are getting?
If you are getting comparable air flow and temp you can add another duct or bigger. Really up to you.
If you aren't and I am guessing you aren't then you need to first get rid of the flex lines. They squash air flow, they are affected by outside temperatures more, and they leak. Put rigid metal ducts in. If the ducts are in the attic then you need to insulate the crap out of them.
Then if you still have issues you need to measure air flow and temp again. If air flow is poor you are looking at a booster. If it is relatively the same, add another duct.
Then you are talking about adding a return to the room and over-insulating the attic above the room.
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Best Answer
First of all, warmer air always tends to rise, and colder air tens to descend. So inside a room you will find warmer air near the ceiling and colder air near the floor. (anyway, the difference is not huge - maybe 5 °C ). My opinion is that your furnace will never suck the air from the upper half of your living room, yet it may suck the air from the lower half - or may not - depending on how well your windows and doors are insulated. I shall explain that briefly.
As shown in the diagram, your furnace is by no means a vacuum pump; every bit of air that rises is immediately replace by an equal amount of air. This mass of replacement air can come either from above (from the flue) or from front (from the ventilation holes in the furnace's door)
Situation 1: your house is hermetically insulated - all your windows and doors are perfectly air proof (this rarely happens in practice) This means that the replacement air from the furnace's door will have to be sucked from the basement, which in turn will suck it from the rest of the house, meaning that some amount of air will leave the house without being replaced, thus your furnace would work like a vacuum pump. BUT PHYSICS DON'T WORK THIS WAY!!! Because the air pressure inside the house will tend to be equal to the air pressure outside the house, all the replacement air will be sucked from above, via the flue. So, in this situation, NO AIR CIRCULATION inside the house.
Situation 2: your windows and door are not perfectly air proof Still, the largest amount of replacement air will be taken from the above, via the flue, but some air will be also taken from the furnace's door vents. This air will be sucked from the basement, which will suck it from the rest of the house, which will suck it from the outside of the house, via your non-hermetic windows and doors. But, the air flow WILL ALWAYS FOLLOW THE SHORTEST PATH POSSIBLE. As shown in the picture, you will get some air circulation, but only in the lower half of your room. This means that the air in the lowest half of the room will get replaced by outside air, which is colder; the upper half of the room will not get disturbed, but since air masses will exchange heat until thermal equilibrium, this means that OVERALL, YOU WILL END UP LOOSING SOME HEAT.