Balancing my HVAC system?

[Wade Lippman]

I bought a 5 year old house 4 years ago. It is about 1700 sf 1st, 800sf 2nd 1700sf basement. NG forced air and A/C.
When I got it, the 4 ducts going to the basement were closed. That was fine in the summer when the basement stayed cool, but it is pretty cold there in the winter.
The big problem is that the 1st floor master suite is cold the winter and hot in the summer.

Yesterday I tried to "fix" it by closing the ducts going to the 1st floor, except those going to the master suit and one other, and opening one in the basement. I have 20 ducts, and 9 (7 on the 1st floor, and 2 in the basement) shut off. When they are shut off, the "feel" like the air is cut back to about 25% of open.

At 25 degrees outside, the whole house is a nice 72! A neighbor told me that was great, but I risked damaging the heat exchanger if it doesn't get enough air. I called the guy who installed it and does my yearly maintenance. I told him what I had done, but was concerned about damaging it, and asked if he should come out to check it over. He said the that dampers leak enough air that there wouldn't be a problem.

1) Does that seem like proper advice?
2) What will happen with A/C if I leave the ducts like this. Any problem there?

thanks

 

[Reach4]

Your pipes in the basement may (or may not) have dampers for equalization purposes. Those allows you to use the top side controls for changing the flow on a temporary basis, and the dampers in the pipes would compensate for long-term conditions, like being closer to the furnace.

 

[Sponsor]

 

[Wade Lippman]

Right, when I said I closed 9 of the 20 ducts, I meant with the dampers you showed.

So my question really is if closing 9 of the 20 ducts is safe for the heat exchanger.

 

[Reach4]

So my question really is if closing 9 of the 20 ducts is safe for the heat exchanger.

I am not a pro, but if closing those ducts was a problem, I am confident I would have heard about that.

I would not have worried about that if closing 15 out of 20, myself.

Some blowers have a way to vary the speed. If you are blowing to less space, you may want to blow less air. Or not. Certainly reducing blower speed would increase heat exchanger temperature even more.

 

[Wade Lippman]

Actually I have read a recommendation to increase the blower speed to cool off the heat exchanger.

 

[Dana]

It's virtually impossible to balance a basement & first floor operated as a single zone, since the heat loss characteristics are so radically different. If you managed to dial it in reasonbly when the outdoor temps are in the 40s, it'll be way off when it drops into the 20s, and conversely.

Even if you manage to balance it all well for the heating season, it will again be way off for the cooling season.

Closing off ducts is in general bad for efficiency, since it increases pressure differences between rooms, increasing the amount of return air using " the great outdoors " as the return path. Closing off ducts will increase the temp at the heat exchanger , but furnaces typically have overtemp cut outs to keep it from reaching dangerous levels.

The bigger risk is to the air conditioner. Without adequate flow on the air conditioner coil the coil can ice up &/or some of the refrigerant returns to the compressor as liquid, destroying the compressor. You DEFINITELY want to open most of them up before running the AC, and rebalance for cooling only judiciously.

A cold basement is usually a symptom of air leakage into the basement, or no foundation insulation, the combination of both. A basement insulated to IRC 2012 code minimums usually doesn't have enough window area to represent a very big heat loss, and would coast along in the mid to high 60s F unless it's leaking a fair amount of air.

For the record, what furnace model/size, and what AC model/size?

 

[Wade Lippman]

The furnace is a Trane XR90 TUS080C942D. A/C is a 4TTX404B10000AA.

I called the guy who installed it and services it. He says the dampers leak so badly that closing half won't matter. I asked if he wanted to come out and test it, but he didn't think it was necessary.
I found on youtube how to test the delta T. The air out was 125. I didn't measure the return air, but it pretty much has to be between 70 and 75; I don't see how it could get cooler. That gives me a delta of 50 to 55. The manual says it should be 35 to 65. So I am good?

I will make sure to open the ducts before turning the A/C on. The service sheet says he twice tested the delta T on A/C at 20. I don't see in the instructions what it should be.

In my case, I think the master bedroom is cold because the run is too long, and it shouldn't also have the dining room on it. If every room that gets air also has a return, why should anything go outside? (actually I have never seen a return in the basement, but there might be one somewhere.
Is there anything else that will increase air flow the master bedroom? I actually bought a duct fan, thinking that might help, but they are awfully noisy.

I appreciate everyone's help on this, and will look forward to seeing how well I am balanced on Thursday when it reaches 2.

 

[Dana]

Return registers are usually near the floor and run 3-5F cooler than the average room temp. If the thermostat 50" off the floor is set to 70, the return air could be as cool as 65F, putting you at the high end of the delta-T. Hopefully you were measuring at the plenum, and not at some register further down a duct, right? At the high end of the delta-T range at your measured temps you might be giving up 0.5% in raw combustion efficiency, but you're not in danger of burning the thing up.

Having a properly sized return for every supply duct lowers the outdoor air infiltration drive, but if you throttle the supply duct in that room it lowers the pressure of that room relative to other rooms, and often relative to the outdoors too, since the return register is still drawing at a lower pressure.

The air flow problem to the MBr could be due to inadequate return duct size, inadequate supply, or duct leakage. If the system is hard-piped rather than flex, it's worth air-sealing every seam & joint with duct mastic on both the supplies and returns. If it hasn't already been done it's also worth caulking the register boots to the subfloors or ceiling/wall gypsum with polyurethane caulk (or expanding can foam if the gap is large), and taping the seams of the air handler with a decent quality foil tape. Fixing all the air duct & boot leaks can make real difference in the amount of noise too.

A typical IRC 2009 code-min 1700' + 800' above grade with a full 1700' of basement would come in around 30,000 BTU/hr @ 0F, under 25K at +15F (NYC & Long Island's 99% design temp), which means the ~74,000 BTU/hr of specified output is likely more than 2x oversized for the whole house load. That's not an efficiency problem but it's sometimes a comfort problem (too much noise & induced draft), but not nearly as bad as the 4-5x oversized stuff that's out there. If you're curious you can get a pretty accurate handle on it by measuring fuel use against heating degree-days. For a 2x6 framed house, use a HDD base temp about 10F lower than your average thermostat setting.

I couldn't find a data sheet on it, but I'm guessing that it's a 4 ton coil in the AC, with a similarly sized condenser/compressor unit. That too is almost certain to be 2-3x oversized. It'll cool the place, but the latent cooling (moisture removal) may not always be up to snuff, particularly on the muggy but not super hot days. A delta-T of 20F indicates decent air flow &/or a compressor sized slightly smaller than the coil in the air handler was designed for, with low risk of icing-up (unless you normally air condition to below 60F? ). But with less air flow the delta-T (and icing risk) will grow.

Are the foundation walls insulated? Under IRC 2009 for NY in the climate zone 4 & 5 counties code min is R10 continuous insulation (1.5-2" of rigid polyiso foam, or 2.5" of EPS, if all foam) and in the zone 6 locations it's R15 c.i. (3" of polyiso, 4" of EPS.) . Insulating foundations with batts & non-structural studwalls carries mold risks, unless there is at least some amount of foam between the studwall & foundation. At code-min R an air tight basement doesn't have a lot of heat load, and just the leakage at the closed registers will probably keep it warm. If there are no returns in the basement that will drive air infiltration, since it pressurizes the basement, driving air out the exterior walls, and up any open flues such as an atmospheric drafted water heater, or out the clothes dryer vents, etc.

 

[Wade Lippman]

Attached is a terrible photograph of my thermometer in the furnace. (the dark round thing just below the duct). Is that metal box the plenum, or at least close enough to it? I took advantage of an existing hole.

My returns are at the top of the walls; so I figured they might be a few degrees warmer than the thermostat. I "presume" that is done to maximize air conditioning effectiveness at the expense of heating. But I put a thermometer a foot from the ceiling and a foot from the floor, and they both measure the same as the thermostat. Maybe they shouldn't, but they do.

I have a humidifier that puts hot air back into the return. When I turned it to "Summer", cutting back on the flow, it lowered the air temperature from 125 to 120 degrees. I figure that makes sense because if you put hot air into the return and the heat rise remains the same, then the air out should go up. Does that make sense?

 

[Dana]

For better accuracy on the return temperature measurement, put the thermometer in a high flow return register, not the room.

Your mental model of the feedback from the humidifer is close enough to reality. The specific heat (thermal mass) of the air changes slightly with the added humidity, and the temperature drops on the feedback loop due to the heat of vaporization of the water injected, but those factors are really second-order effects.

Humidifiers are (almost) universally a bad idea, particularly in wood framed wood sheathed houses. Most people set them to much higher levels than is good for the long term health of the house. In any NY climate keeping it north of 40% RH @ 70F during the coldest 12 weeks of winter results in moisture levels in the sheathing high enough to support mold/rot when spring temperatures arrive. It can also result in high mold spore counts in the indoor air in the early-mid spring. (The wicking elements in some models of humidifier are themselves capable of supporting & distributing mold, and need to be swapped out regularly.)

If the house has dry air (defined as under 30%RH @70F, the lower end of optimal human health & comfort) in winter it's an indication that the ventilation rate or (more likely) outdoor air infiltration rates are too high. The better solution is to tighten up the house and/or back off on the ventilation rates. In some case exfiltration paths can experience enough condensation during the cold weeks of winter to actually drip or develop ice/frost if the indoor humidity is in the 45%+ range or higher, which isn't good for the house or the human occupants.

If you want to manage the moisture content by the ventilation rates that usually works. A crude rule of thumb that works for both humans & the house is once the heating season has begun in earnest (daily highs in the 50s) to keep the %RH roughly at or below the mean outdoor temp for the day or week (take the daily high & low temp and average them), at least until the you're looking at 35% RH, then adjust it to keep it between 30-35% until warmer temperatures arrive. The dew point of 70F/35% RH air is about 41F, so when the wood sheathing is below 41F there will be a vapor pressure across the wall, and moisture will migrate via diffusion & air leaks into the walls, and is adsorbed into chilled wood. The wood won't release that moisture until temperatures rise, though it can slowly dry to the exterior through the wood. (OSB & CDX plywood are both vapor retarders, about 0.5-1 perm when dry.)

 

[Wade Lippman]

I never like the humidifier. The filter clogs yearly and has to be replaced, as does the panel. I have an automatic humidistat that sets the humidity according to the outside temperature. I have a chart that shows the recommended humidity by outside temperature, and it seems to be working, according to my hygrometer.

However I have just now turned it off to see what will happen. Maybe you are right and it isn't necessary! That would be nice.

 

[Dana]

This is Aprilaire's human health & comfort chart, which is comparable to similar charts from the health community, though many of the latter put 50% at the upper bound since that's knee in the curve where dust mite populations go exponential (drawn linearly in their ramp-wedges):

In most of the eastern half of the US keeping it at or below 50% RH requires mechanical dehumidification in summer, but it can be adjusted by ventilation rate at least 6 months of the year in most NY locations. The dew point of 50%RH/70F air is about 55F, so any time it's below 55F outside ventilating with outdoor air would lower the RH to 50% or less, but most of the time even when it's warmer than that out dew point is below 55F. This is Weatherspark's graph of NYC's historical dew point averages over a year:

You'll notice that between late November and April Fools day the outdoor dew points are pretty reliably below the 37-40F dew point of 30-35% indoor air, which means ventilation would be able to lower the indoor humidity during the cold months if your house was so tight that it needed it. Houses that test at <3ACH/50 (air exchanges per hour at 50 pascals) , which is required for meeting code under IRC 2012 & 2015 would normally need to ventilate in winter to keep it under 35% RH unless it's a large house with few occupants or a very low occupancy rate. That isn't always the case for houses build in 2011, but it's usually achievable with blower-door directed air sealing in houses that vintage, if it turns out that your air is too dry. Check to see whether your HVAC system is set up to intentionally draw in some ventilation air whenever the air handler is running, and back it off (to zero, if need be) if your air is too dry.

But from half past June to Labor Day it's pretty reliably above the 55F dew point mark of 50% RH indoor air, which means dehumidification (or air conditioning) would be necessary most days to stay below 50%. Many of those days don't have much of a sensible (temperature) cooling load, so folks with dust mite allergies really need to run dehumidifiers at times.

Buy a few cheap (and not super-accurate, but good enough) AccuRite or similar humidity & temperature monitors and put them in a few open common areas & the master bedroom (but not bathrooms), to get a sense of where the RH levels track. In colder areas (say, a cooler than you like 60F basement) it's fine if it runs at 40% (the dew point of 40%RH/60F air is about 41F), but where it's normally 65-72F it should be under 35% during the extended cold weather. (A 2-day Polar Vortex event followed by springtime temps doesn't really count.)

 

[Wade Lippman]

I have one of the devices on the left.

Yesterday it read 38% at 34 degrees outside. I turned the humidifier off. This morning it was 28% at 14 degrees. I turned the humidifier back on. After a couple hours it is up to 30% at 16 degrees outside. According to my chart, the proper humidity is 34%

 

[Dana]

The indoor air temp was... (65-70F?) I assume the 34, 14, and 16 degrees cited were all outdoor temperatures.

Assuming fairly stable indoor temps at which the readings were taken, the fact that it dropped from 38% to 28% in less than a day is an indication of massive infiltration/ventilation flows. In my (not super-tight but tight enough) wood framed 1920s bungalow it takes a few days for the indoor humidity to move a full 10%, even when outdoor temps drop 40-50F. It's not strayed out of the 35-38% range @ 70F for the past week or so (or least not that I've noticed) despite outdoor temps that ranged from 15F to 40 F over the course of the week.

At my over-ventilated office the indoor dew point tracks 10-20F above the outdoor dew point. As of this minute the humidity monitor in the office reads 16% RH @ 66F indoors, which is a dew point of ~20F. The local Wunderground.com weather station is currently reading 21F with an outdoor dew point of -1F, which is 21F below the indoor dew point indicated by the cheapo AccuRite. The outdoor temperature & dew point have been falling all day as the Polar Vortex front approaches, and I expect by mid-day tomorrow the indoor humidity will have fallen into it's normal tracking range relative to the outdoor dew point. That doesn't happen at my house. It takes a week of artic weather to get the indoor RH under 30% unless I crank up the exhaust fans for hours.

Is the chart you are looking at available online, in a PDF of the manual or something?

 

[Wade Lippman]

I am using a table like:
http://www.rmsab.com/articles/proper-indoor-humidity/

The house is 10 years old. Andersen doors and windows. Code enforcement is pretty loose out here, but the guy who owned the house acted as his own master contractor and knew what he was doing, so I don't think he seriously cut corners. The one time I was in the attic I saw it was well insulated and ventilated.

So yeah, I was shocked at the drop in humidity. The only exhaust is a induced draft water heater. There is a radon fan, but that shouldn't have a significant effect should it?

We are experiencing 30mph winds; perhaps that matters.

I appreciate your help; it is all so confusing.

 

[Dana]

A radon fan in combination with an air-leaky basement slab can be a substantial air & moisture mover. Air leaks in to the basement make it worse, if the slab is leaky enough that the radon fan depressurizes the basement. Any drains or sumps really need to be sealed to prevent this, but cracks too. If the fan is way oversized for what's needed that makes it worse too. Ideally the basement slab would be poured on a heavy vapor barrier, air sealed at all plumbing penetrations, with no unsealed sumps. But even those often leak air at the perimeter where the slab meets the foundation wall. The perimeter edge crack can be quite substantial in cross section- 200' of 1/8" gap is 300 square inches, the equivalent of a ~19" diameter round hole.

High performance builders often extend the vapor barrier over the footing, under the foundation wall to aid in air sealing the slab, and goop the crack with sealant for good measure:

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The tighter the slab, the less radon leakage there is into the house, but also the more effective a slab-depressurization strategy with a radon fan becomes if elevated radon levels are detected.

When the slab is relatively air tight, leaks from the upper floor ceiling into the attic or air-leaky flue, plumbing, or electrical chases that run from basement to attic & beyond can still suffer from stack effect infiltration drives that move a lot of air.

The most commonly found LARGE air leak in basements of wood-framed homes is the seam between the concrete or CMU foundation and the foundation sill. The most common foam sill gaskets are really LOUSY at air sealing (expensive EPDM sill gaskets are much better), and in most homes that one long skinny crack ends up leaking more than all the window & door crackage in the house combined, as verified by blower door testing. A bead of expanding can-foam under the foundation sill when the sill is first placed usually seals well, as does retrofit air sealing the foundation sill and band joists with can-foam. Wood-to-wood seams such as the foundation sill to band joist, and band joist to subfloor are readily air sealed with polyurethane caulk (not cheap painters caulk, which will fail far too soon, even if it seals well initially.)

Most clothes dryer vents leak badly, and are also a common basement air leak, though smaller than the foundation sill & band joist leaks.

Aside from basement leaks, fireplaces (even those with pretty good dampers) and the associates chimney chases can drive a lot of passive air leakage. Recessed lights in upper floor ceilings, even so-called "air tight, insulation contact rated" fixtures will often leak copious air. There are 1001 places in a typical house that can leak air, and it takes some sleuthing to figure out most of them.

When it's cold outside you can detect many air leaks into the basement with a pistol-grip infra-red thermometer (especially easy if a radon fan is depressurizing the basement, and not just the slab. ) But to find the leaks in the upper floor ceiling or utility chases often requires blower door depressurization of the whole house, and an infra-red imaging camera. If you get serious about chasing air leaks, fix all of the obvious stuff starting with the bigger leaks first. If that hasn't helped the dry air situation it may be worth hiring a pro with a blower door & camera to find & fix the rest of the big leaks.

That RMS page with the humidity vs. outdoor air temp chart doesn't really give a rationale why the humidity "should" be the stated values at the stated out door temps, and it promotes values that are both human-unhealthy as well as uncomfortable for outdoor temps below +10F.

Their statement that "Heated air dries out wood framing around doors and windows, too", is utter crap. DRY air dries out wood, sure, but heating air the does not remove (or add) moisture. Replacing humid interior air with much drier outdoor air at a rate faster than the indoor activities such as breathing bathing & cooking are adding moisture to the air is what dries out the air. Too dry or too humid air is a ventilation rate issue, not a heating issue, and over-ventilation along with outdoor are leaks are the prime culprits.