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HVAC ducting - Climate Zone 7a

The house is 40 year old bungalow with heated basement. It has an older 80% efficiency forced air natural gas furnace with an input BTU/hr 88,000 and output BTU/hr 71,000 with 1400 CFM.

The living room is 268 sqft and is ~24% of the total volume of the main floor, which is open plan so it is connected to the dining room (268 srft) and kitchen (137 sqft), each being ~12% of tot vol, respectively. The living room is at grade level with 9.3' high ceilings, compared to the rest of the main floor that has 8' height ceilings and is ~2' above grade.

It currently has 1 heat register under a new energy-efficient 78"x80" patio door from a long 5" heating duct that is ~24 feet long from the main supply duct. It has another two other heat registers under the 96"x58" living room window, each supplied by their own 6" ducts that are ~32' long from the main supply duct. The airflow pressure to these registers doesn't feel as forceful as the others in the house. It also has an air return in the room.

The rest of the main floor has a master bedroom + ensuite, 2 bedrooms, and 1 bathroom. This is a 1400 sqft bungalow but I measured ~1120 sqft in terms of 'living space'. Each room has a heat register, and there is also one by the front door and another by the back door. These are all 5" ducts.

I still have to complete the insulation of the basement, the living room is at grade level, it is the furthest room from the furnace, has the longest ducting, and it has 82 sqft of windows so there are a few reasons why this is the coldest room in the house.

I have adjusted the dampers throughout the house but just wanted to know if anyone had any thoughts and/or recommendations on the heating of the living room and maybe how the ducting could be changed/ improved?

Thanks in advance.

Asked by user-6759891
Posted Mar 19, 2017 10:08 PM ET
Edited Mar 20, 2017 12:50 PM ET


7 Answers

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You are asking basic questions about designing a forced-air heating system. Briefly, here are the steps:

1. Someone has to perform a room-by-room load calculation.

2. Someone has to choose a heating appliance that is correctly sized.

3. Someone has to determine how many cfm are required for each room.

4. Someone has to design and install a duct system capable of delivering the required cfm to each room, and capable of collecting and delivering the return air to the furnace.

5. Someone has to commission the system by measuring the air flow at each register and grille.

Most of these steps are described in this article: All About Furnaces and Duct Systems.

The proof is in the pudding: If your heating system had been designed properly, you wouldn't have any cold rooms. If some rooms are noticeably colder than others during the winter, it's a good bet that the ducts are delivering enough cfm to the cold rooms. (Your hunch is right: in Climate Zone 7, you don't want any 5-inch diameter ducts that are 24 feet long.)

-- Martin Holladay

Answered by Martin Holladay
Posted Mar 20, 2017 7:41 AM ET
Edited Mar 20, 2017 7:42 AM ET.


Thx, Martin. Yes, I had already read that article and others, and have downloaded several things giving more detailed info, as well as trying to get the various manuals. I had done all of the room and window measurements, but not yet an in-depth a room-by-room load calculation, etc, which I'm working on.

Admittedly, I was trying to cheat a little... anticipating there were people here that knew enough to immediately understand and recognize where I could have some potential issues based on the info provided, and hopefully have them throw out a few suggestions that I could could then look more into.


Answered by user-6759891
Posted Mar 21, 2017 4:16 PM ET


Hard piped ducts, or flex?

Is it an absolutely straight run, or does it twist & turn?

Have you caulked all the duct boots to the subfloor, and sealed every seam & joint with duct mastic?

Every bend induces turbulence and impedes flow, and any sharp throated ells are WAY worse than radiused bends. Flex duct can be very high impedance too, unless it's fully stretched tight & straight, and not a good idea if you're trying to recommission & tweak the existing ducts while hacking in an extension. If you have to rip it all out and start from scratch, design it per ACCA Manual-D, don't just hack & hope.

An un-sealed un-insulated basement in a 1-story house can easily account for as much as 25% of the total heat load for the house, and fixing that will lower the air flow requirements for each room. But isn't likely to affect the room to room temperature balance issues by very much. A 71,000 BTU/hr furnace would likely be somewhat overkill for any insulated 1400' house in zone 7A, and is could be approaching 3x oversize factor for the 99% load after you've tightened it up and insulated the basement. The heat load isn't proportional to the volume of the room, but is roughly proportional to the exterior wall & ceiling areas.

Answered by Dana Dorsett
Posted Mar 21, 2017 6:12 PM ET


Thanks, Dana

All ducting is galvanized and is either 26 or 30 ga, so no flex. It's a typical trunk and 2-way branch design. These ducts measure 12" x 8" and each branch is ~12' and 23', respectively.

All round duct pipes are 5" and run straight off their branch. The only exceptions to this are (1) the 5" round duct that's ~24' long and goes to the register under the patio door in the living room, and (2) the two 6" round pipes that are ~32' long each going to the registers under the living room window.

These 3 all run straight off the main branch, then go down 45* and the lengths between this angle joint and the next 45* are approx 2-3'... they then run straight to their registers. This is not a bi-level home, but the living room is 'sunken' so ~1.5' lower than the main floor. As such, it's at ground level. The ceiling space under it is still 6.2'. Fyi, I included all angle joints and everything when calculating total round pipe lengths.

I have sealed everything and that has very much solved the "recurring dust" issue I was having... so learned here that my air return was obviously sucking insulation dust from the attic. I've fixed that, but still plan to go up in the attic soon to properly seal everything that might not be.

This house, being ~40 yrs old, has typical wall assembly of stucco, mesh, construction/tar paper, 1/2" particle (buffalo board), 2x4 wood stud @ 16" oc, yellow fiberglass batt (~R12), poly vapor barrier, 2 layers of 3/8" thick drywall. The basement's not yet finished, but as per my earlier posts, I will be doing this with 2" rigid foam and R14 roxul in 2x4 wood stud @ 24" oc.

The living room, which is the room I was specifically referring to, has 9.3' heigh ceilings, and 3 external walls with a total combined area of 496 sqft, of which 82 sqft are double glazed windows; the floor/ ceiling area is 268 sqft. Attic has been insulated with blown in (pre my time here... I'm guess ~R40 or so.

Fyi, I only ever put the heat up to 67*F during the day and down to 64*F at night and the house feels relatively warm at this level, even considering its construction and unfinished basement. It's just the livingroom that is the coldest.

So this isn't the original furnace and the previous owner upgraded so I've inherited whatever they've done. I'm now renovating the entire place and seeing as the basement is unfinished I have some flexibility in what I can do... but seeing as I'm doing a lot of things I have to be mindful of costs.


Answered by user-6759891
Posted Mar 21, 2017 7:07 PM ET
Edited Mar 21, 2017 7:17 PM ET.


PS I can't do a "hack & hope" with anything I do; hence, all the bloody research, etc I do... sometimes I wish I could be a half-asser... I'd then get things done faster and cheaper! :)

Answered by user-6759891
Posted Mar 21, 2017 7:14 PM ET


I removed the fresh air intake duct for the furnace and have installed all new 4" 30 gauge galv pipe but I'm not going to use the standard garbage insulation wrap one can buy. My plan was to wrap the pipe in Roxul instead of fiberglass...

I was going to then wrap everything up with 6 mil poly but started wondering if using Membrain would be better, considering the level of insulation the Roxul will provide won't technically prevent condensation around the pipe.

The original duct pipe I removed was completely rusted out, and its insulation was wet, proving that small amount of normal duct pipe insulation wrapped in poly isn't sufficient in this climate. So at least the Membrain will 'breath' and allow any condensation on the pipe to dry out to the exterior. Thoughts/ recommendations?


Answered by user-6759891
Posted Mar 23, 2017 3:28 AM ET


First of all, you don't want to use 4-inch-diameter duct for delivering air from a furnace to a register. The absolute minimum diameter is 6 inches -- and a 6-inch duct will only work for a short distance (with few elbows) from a trunk duct to a register. There are many circumstances that require an 8-inch duct or a 10-inch duct -- or rectangular duct that has a large cross-section.

Don't use the above paragraph to design your duct system. Do the math.

You don't want to wrap ducts in polyethylene.

The way to seal duct seams is with mastic.

The way to insulate ducts is with duct insulation. Duct insulation has a polyethylene jacket on the room side of the insulation, not near the duct.

-- Martin Holladay

Answered by Martin Holladay
Posted Mar 23, 2017 6:10 AM ET
Edited Mar 23, 2017 6:11 AM ET.

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