Ductwork issues, balance and basement renovation

[interalian]

We have a 2-storey 2600' house with an unfinished 1500' basement. There are two furnaces: one for top floor (70,000BTU with 3ton AC) for ~1100'. The other for main floor/basement (120,000BTU, no AC), ~3000' combined. All supply lines 5". I'll explain the ducting layout and issues by floor and room - doors usually left closed on top floor.

Top floor:
-Master/ensuite/WIC: Five supply registers and one 8x16" return grille. Two exterior walls. Thermostat for upstairs furance is in this room and this room is always comfortable.
-Bedroom 2: One supply register and one 8x16" return*. Two exterior walls. This room is cold in winter.
-Bedroom 3: One supply register and one 8x16" return. One exterior wall. This room is warm to hot in winter so we keep the register mostly closed.
-Bonus room/nook. Three supply registers and one 8x16" return. Three exterior walls. This room is open to the main hallway and is cool in winter.
-Bathroom: One supply register, no return. This room is warm in winter.
As far as I can tell, three of the returns share a single 12x16" joist cavity and a 3.5x16" wall space duct to run down to the furnace in the basement. Bedroom 2 has its own wall duct to the basement.

Main floor:
-Front hall: One supply register, no return. Three exterior walls (one is garage). This area is cool in winter.
-Front room: One supply register and one 8x16" return. Two exterior walls and a cantilever bump-out. This room is cold in winter.
-Half bath: One supply register, no return. One exterior wall (garage). This room is hot in winter. There's a damper in the supply duct that's in the closed position. We keep the register mostly closed as well.
-Laundry room: No supply register, no return. No exterior walls. We have a standup freezer as well as the laundry pair, so this room is always warm/hot in winter.
-Back hall/walk-through pantry: Two supply registers, no return. One exterior wall. This room is warm in winter. One supply register is blocked off, the other has a damper that's in the closed position. We also keep the register mostly closed.
-Kitchen/dining/living room (open area): **EDIT** Four (yes, only four) registers for the entire space. One 8x16" return (yes one) for the entire space. Living room ceiling is 20' and adjoins two-flight stairs to upstairs. Three exterior walls and cantilever off dining room.

Basement:
-One open area, three registers. One return air grill in ceiling that appears to be connected to BOTH furnaces cold trunk. 2x4 framed and R10 insulated concrete walls, concrete floor.
This area will be finished and have one bedroom and a family area plus a full bath.

Based on the above, what would need to change to help balance the system? I have access to the main floor ducts now, and will be looking at the required supply/return for the finished basement.

I really don't think one return for almost all the space on the main floor is anywhere large enough. It would be easy enough to add another return in the kitchen/dining open area by installing in the kitchen island which divides the space. I could also add a supply duct/register under the kitchen cabinets.

Any/all thoughts welcome. I can provide photos if they'd help. The house is a drafty nightmare.

 

[Dana]

The design heat load of a typical 2600' house at Calgary's -27C/-17F 99% outside design temperature is 50-65,000BTU/hr. Typical cooling loads for a house that size in that location would be about 1.5-2 tons. Your heating system(s) is(are) at least 3x oversized for the actual load, which is an oversize factor big enough to have comfort issued based on the low duty cycle alone. ASHRAE recommends holding the line at 1.4x oversize factor, which is enough to cover the load during arctic blasts from Polar Vortex disturbances and to allow effective use of overnight setbacks, and still have a duty cycle high enough to provide the full comfort you're paying for during normal cold weather.

Situations where there are three to five supply registers to one return in one area like the MBR/ensuite or bonus room/nook usually results in pressurizing the room to make "the great outdoors" part of the return path. That's an efficiency problem as much as a comfort problem. That can be mitigated with duct sealing and sealing the envelope of the house, but balancing the supply & return air volumes to the extent possible is worth it. An Energy Star duct system would have no greater than 3 pascals (0.012" water column) pressure difference between adjacent rooms under all operating conditions, doors open/closed, all air handler speeds. This may require creating bigger return paths, or installing balancing vanes in the excessive supply ducts.

If any of the ducts penetrate the pressure and thermal boundary of the house, say an air handler & ducts in a vented attic, above the insulation, the parasitic load of any pressure imbalances are amplified- it's more than just the conductive losses/gains to the ducts. Any duct leakage (supply or return) by definition is going via the outdoors.

Bonus rooms usually have somewhat different heat loss characteristics than the rest of the house when over an unconditioned garage, and often run cool. This can't be fixed with simple duct tweaks when the furnace is grotesquely oversized for the loads and running a low duty cycle. Giving the bonus room more air flow will increase the average temperature, but will in most cased increase the temperature swings- a hot-flash followed by the long chill in an oversized system. Were the system right-sized for the load it would be running more than a 50% duty cycle during cooler weather, which helps even out the temperatures, not overheating during the on cycles, with less chill time between cycles. (With a 3x oversized system it's running less than a 35% duty cycle even when it's -27C outdoors).

The inexpensive dual port hand held manometers used for commissioning duct systems don't have the resolution or sensitivity for doing a full-on Energy Star assessment of room to room pressure differences, but at the low end of their range are good enough to spot the worst-offenders. A dual port manometer with 0.01" resolution can be useful for finding the crazy pressurized & crazy depressurized . For the already warm or overheated rooms that are being pressurized, dialing back the supply flow with balancing vanes would be the first step toward dealing with the room to room balance issues on pressurized rooms, which can also help the drafty house feel elsewhere. Pressurizing one room always creates a depressurized room elsewhere, sucking in outdoor air.

To get a handle on your actual oversize factor, run a fuel-use load calculation on some wintertime gas bills tracking fuel use against weather data. The uses the furnace as the measuring instrument, and it measures the entire load, including any parasitic distribution losses, etc. With an unbalanced duct system the parasitic loads are well into double-digits as a percentage of the total, so don't be surprised if the fuel use load number drops after extensive rebalancing.

 

[Sponsor]

 

[interalian]

Thank you for the detailed response! That's a lot to chew on.

On review of my original post, I found an error - there are four, not three supply registers in the kitchen/dining/living area. 2002 build, not particularly well sealed envelope. There are no ducts outside the building envelope (attic), and no HRV. Each furnace has a cold air 'make-up' to outside (code).

I have a cheap PYLE manometer that says it has 0.01 resolution on in/H2O.

Furnace sizing is what I recall from when an HVAC guy came to inspect. I'll take a look at the model numbers and see what they actually are as I can't believe the builder would have put such an oversize in place. Yes, duty cycle on coldest days is maybe 30 on/70 off. Cold air is always spilling down the staircase into the livingroom.

The back hall/walk-through pantry are above the furnaces, so the tile floor in there is always warm due to the heat from below. I could probably close off the other supply register.

I've seen some reno shows (Holmes et al) where builders pump heat in the floor cavity under bonus rooms in an attempt to stave off cold floors. I don't know if our house has that. For what it's worth, our garage is fully insulated and heated. I have a 50,000BTU unit heater in there as I use it as a shop.

There'd be no way to seal any of the ductwork above the main floor level (basement access) without tearing up drywall, but I had planned to do duct sealing (goop, not tape) as part of the basement work.

More to follow. Thanks again.

 

[Dana]

Unfortunately 3x oversizing is FAR more common than right-sizing. In fact, 3x oversizing would be the typical number for new construction in my area, not the exception. The common, rampant oversizing is part of what gives hot-air heating a bad reputation for comfort compared to hydronic heating.

As long as all ducts are fully inside the pressure boundary of the house the duct leakage issues are much subdued, and can be mitigated with balancing.

For pressurized rooms that are running cold its usually possible to increase flow by improving the return air path by using a partition wall stud cavity as a jump-duct, with a grille near the floor on the cold-room side of the partion, and another grille near the ceiling on common-area side of the room. That allows at least some light & privacy isolation while reducing the pressure, improving the supply flow into that room.

Go ahead and measure the pressure differences across doors with the Pyle when the air handler is running. When operated at the very low end of it's range it's sometimes useful to change directions to ensure that you're not just reading some inherent offset in the instrument. Any number of 0.02 or greater would clearly fail an Energy Star certification, but when you're at 0.03"+ it's usually worth making what modifications you can to reduce that.

Sealing the register boots to the gyprock or floor is usually possible without ripping open walls, as is sealing the seams of the boots, and worth doing, even if it's not going to magically fix the problems. The less air that gets diverted behind wallboard or subfloors, the better overall control you have.

 

[interalian]

Correction: Main furnace is 100,000BTU, top floor is 75,000BTU.

I like the idea of sealing all the register boots to the subfloor, along with the boot joint as accessible. Note in this region, supply registers are floor mounted, returns wall mounted at floor level.

I'm considering a zone control for the main floor and basement.

Not 100% on the manometer use. Do you mean a differential between the two sides of a door by running a hose under the door to read both sides? Should the hoses be the same length/dia?

 

[Dana]

Yes, measure the pressure differential across the door with the door closed, air handler running.

The length & diameter of the hoses don't matter much, as long as they are long enough to extend under the door far enough that it isn't fluctuating so much on the air currents that you can't get a stable reading.

 

[interalian]

I wasn't able to get any readings higher than 0.02" across any door, which is so close to the Pyle's stated resolution that I'm going to call it noise.

I had a well respected HVAC company out to look at the system on Friday and quote for my renovation work, and his first thought was my main floor return system is woefully inadequate (surprise- 9 supply registers and 2 small returns). In addition to the existing 8x16" grille (he actually corrected me that my grilles are 6x14" not 8x16") on the main floor, he suggested I install one more that size in the island as I'd noted above, and a 6x30" (!) in the hallway. As luck would have it, the area under the proposed location for the 30" matches to an adjacent joist space in the basement to the existing one, and has a free run to the return plenum. This, he said, would balance supply and return for the main floor. I've also confirmed that the one return in the basement is actually connected to both furnace return plenums, which he said is very wrong.

I'm now looking at an air handler now to have separate zones for the main floor and basement.

But in addition to that, I think I need a way to run the blowers continuously, but not at full speed (except when heating) in order to keep the cold rooms warm and elecricity costs low(er) - at least during cold snaps like we're having now. As suspected, my system duty cycles are 30 on/70 off, maybe even worse. Cold rooms are cold due to the duty cycle mostly.

So: is there a way to put a speed control on furnace motors? Or a replacement blower motor that's designed with multiple speeds that could be substituted?

 

[Dana]

But in addition to that, I think I need a way to run the blowers continuously, but not at full speed (except when heating) in order to keep the cold rooms warm and elecricity costs low(er) - at least during cold snaps like we're having now. As suspected, my system duty cycles are 30 on/70 off, maybe even worse. Cold rooms are cold due to the duty cycle mostly.

So: is there a way to put a speed control on furnace motors? Or a replacement blower motor that's designed with multiple speeds that could be substituted?

Seriously, the efficiency of heating those far rooms with tepid room temperature air running an air handler at low speed is ATROCIOUS! Your electricity use will go up, while barely moving the needle on comfort. In most cases you would be better off heating those rooms with small resistance heaters than running the air handler.

Air handlers can move quite of heat at low electricity overhead when it's deliving 45-50C air into a 15-20C room, (a 30C temperature difference), but it's VERY expensive way to move heat by feeding 20C air into a 15C room (a 5C difference). The air handler is using the same amount of power either way, but it's moving more than 5x as much heat when the furnace is burning than when it isn't.

Worse yet, the wind-chill/draft of even low air handler speeds is going to make the humans less comfortable, even though the room temperatures might be higher.