Pull fresh air for Gas HVAC

[RMAR10]

Are there any guidelines on pulling fresh air for an LP gas HVAC unit (40K BTU)? Interested in how far and what diameter pipe could be used. Need to pull it about 20 feet. Would like to pull the fresh air from the end of the house so I don't have to go thru the roof. HVAC is in the middle of the attic halfway from both ends. Will exhaust the output thru an existing attic ventilator.

Thanks

 

[Dana]

The installation manual for the unit would specify that sort of thing.

If it's an HVAC unit that normally pulls it's combustion air from the equipment room, the cross sectional area of ventilation openings from the rest of the house is usually prescribed in state & local codes, which vary from a square inch per 1000 BTU of burner to a square inch per 2000, and the square inches double if the air is being ducted to the room.

The IRC calls out a square inch per 2000 if pulling outdoor air through a horizontal straight duct to a gable in the attic:

-------------

(2) Where communicating with the outdoors through horizontal ducts, each
opening shall have a free area of not less than 1 square inch per 2000
Btu/h (0.001 m2/kW) of total input rating of appliances in the enclosure.
[See Figure 701.6.1(3)] [NFPA 54:9.3.3.1]

--------------

So for a 40K unit you need at LEAST 20 square inches a 6" round hard piped duct would do it (28 square inches) , but 8" round (50 square inches) would be better, especially if it has to make any turns.

Read the whole section of the IRC to see which variation is more directly appropriate to your application.

 

[Sponsor]

 

[Jadnashua]

The flue/exhaust must meet the manufacturer's specifications, and a roof ventilator generally would not qualify. Not following those instructions is a path to carbon monoxide poisoning, roof deck damage, and probably other issues.

 

[RMAR10]

Thanks for the input, I will check out the code.

 

[RMAR10]

Is it ok to pull combustion air from the house being heated? I have a good size gap in my attic stairs door that would provide enough air for combustion.

 

[Dana]

Yes, but the requirements are different, and it puts a big hole in the pressure boundary of the house that will convect 24/365, adding to both the heating and cooling loads. It's much better for efficiency as a system if the whole thing (furnace, ducts and all) are fully inside the pressure & insulation boundary of the house, and ducting combustion air from outdoors.

 

[Jadnashua]

First off, you've already paid to heat that air. The combustion air has to come from somewhere, so will come in through any cracks or openings in the house. That will pull in potential dust, pollen, humidity, smoke, etc. that may be outside, cooling the house, requiring more heat to offset it, and creating drafts that will make things more uncomfortable, requiring a higher temperature to be comfortable. It's much better for efficiency and comfort to get that combustion air from outside. The downside is that pulling that air in through cracks does dilute any indoor pollution you may have created (cooking odors, perfumes, cleaning products, etc.). There are better ways to manage that, though. It's best to do that in a measured manner than haphazardly.

 

[RMAR10]

Thank you for the inputs. I was not clear with my question. I have two electric furnaces, one in my upper attic and one in a side attic (2 units/2 story house) (heater/evap coil/ducts all in the attic spaces). I'm replacing both with 96% afue LP gas furnaces and new evap coils/outside condensors. My a/c guy specs 60kbtu upper attic /100kbtu side attic. And 3" concentric pipe vents for both, for combustion/exhaust air. These would go thru the roof. Is 3" large enough and is additional fresh air needed for the units?

 

[Jadnashua]

3" is probably enough, but the only way you'd be able to tell is to look at the installation manuals. Generally, these things have two pipes. They can terminate into a concentric cap when exiting the structure. They generally use either 3" or 4", some of the determination is based on how far it has to go.

On a side note, though, it would be highly unlikely that you'd need 160K BTU to heat anything in Texas. You might have a huge cooling load, but your heating load is almost certainly much less than that. To reach that level of efficiency, the unit has to be sized properly, and it probably is way oversized.

You can get an idea of your heating load if you analyze your electrical bill. 1Kw = 3412BTU. Trying to separate the other electrical loads may be tough, but you should be able to make a decent guess.

 

[RMAR10]

That's what I thought about the sizing also. but they are high efficiency furnaces which are two stage (high fire and low fire). So the high fire stage only comes on when extremely cold or upon startup for a short time. For the 60Kbtu unit the high fire is 56k and the low fire is 39.2K. For the 100K btu unit the high fire is 98kbtu and the low fire is 68.6k. So most of the time they would be running at 39K and 68k. If a furnace is oversized, is that as much of an issue (other than upfront install cost) as it is with an A/C (short cycling and humidity)?

 

[Dana]

From the initial question I presumed this was a single 40K propane furnace, which might be oversized for the average house in Irving, but not ridiculously oversized. But 160K of burner is ridiculously oversized for a5000 square foot house in Fairbanks Alaska (where the outdoor design temp is -41F.)

Just the low fire 39.2K of the smaller furnace could keep my 2400' sub-code 2x4 framed house +1600' of insulated basement warm down to about 0F outdoors. I know houses are bigger and generally leak more air in Texas than in my neighborhood, but seriously!?! Are you expecting a cold snap to take it down to -150F or something? Is this a 10,000 square foot barn with no wall insulation and single pane windows?

There isn't much of an efficiency hit with grotesque oversizing of furnaces, but there's a comfort hit. At this level of oversizing you get the warm-flash followed by the chill, repeat, when it's actually cold outside. With a right-sized 2 stage furnace it runs about a 70% or higher duty cycle at the 99% outside design temperature, which in Irving is about +25F. A furnace running a 70% duty cycle at 25% outdoors would be enough furnace to cover the heat load at 0F outdoors or a few degrees below without giving up even 1F on indoor temperature.

A right sized furnace would run nearly continuously when it dropped to 20F and cooler (rather than hot flash then wait), which is much more comfortable. Folks accustomed to furnaces so oversized the thermostat gets satisfied in less than 15 minutes under all conditions it might seem like it's "struggling", but it's really just doing the right thing- providing actual comfort when conditions are more severe. (Satisfying the humans is more important than satisfying the thermostat.)

A third party calculated Manual-J cooling & heating load is called for. That is not an HVAC contractor's calculations, but rather an engineer or RESNET rater' s numbers- somebody whose bread & reputation is made on the quality of their numbers rather than on installing & maintaining HVAC equipment. In your neighborhood in almost all cases a right sized 2 stage heat pump reasonably sized for the cooling load will have sufficient capacity to cover the heating load, and will usually be cheaper to run than a condensing propane furnace. A right sized modulating heat pump would be even better, but is a lot more expensive up front.

 

[RMAR10]

Wow, seems crazy they would oversize the system by that much.

Thanks for the heads up!!

 

[Dana]

How big is this house, and what vintage, type of construction, etc?

 

[RMAR10]

2002 two story box, 2400sf, frame siding, lots of cheap windows, not great insulation, all electric, bills avg $400/month @10 cnts/kwhr

 

[Dana]

Even a fairly air-leaky version of a 2400' slab on grade 2x4/R13 two story with more than the typical square footage of U0.5 clear glass (not low-E) double-panes would have a heat load in the 30,000 BTU/hr range @ 25F. A 1.4x oversizing factor would bump that to the 40K range.

A $400 @ $0.10/kwh is 4000 kwh/month. A typical December-January monthly heating degree days in the Dallas area is 550-600 HDD (base 65F.) Call it 4000 kwh over 550HDD- that's 7.3 kwh per degree-day, or (/24=) 0.30 kwh per degree-hour. 1kwh =3412 BTU, so for a linear estimate for every degree below the 65F presumptive balance point the load increases by (0.30 x 3412=) 1024 BTU/hr (which is really high for a house that size).

The 99% design temperature is about 25F, which is (65F - 25F =) 40F heating degrees under the presumptive heating/cooling balance point. So the implied load is then

1024BTU/degree-hour x 40F = 41,000 BTU/hr

Odds are pretty good that at least 10% of that power use was heating hot water that went down the drain instead of heating the house, so call it 37,000 BTU/hr. The ASHRAE recommended max 1.4x factor would put the furnace sizing at 1.4 x 37KBTU/hr = 52KBTU/hr.

The AFUE-tested 1.7x oversize factor would be 1.7 x 37K = 63K, which would enough furnace to keep the place warm well into negative double digits F!

So a 50-60 condensing furnace (or 3 to 3.5 ton heat pump w/resistance strip heat backup) would be the most furnace this place would ever need.

Having lots of extra glass (especially west-facing) can increase the peak cooling load, but being multi-story usually reduces it relative to a single story ranch house, due to fewer square feet hot roof overhead. Most homes of that vintage and size would have a cooling tonnage per floor area ratio on the order of ton per 1000' of conditioned space, but it varies quite a bit, which would put your house in the 2.5 ton range, but possibly higher. Putting insulated ducts & air handlers in the attic above he insulation adds about a half ton or so of load, so a 3.5 ton compressor wouldn't be ridiculously oversized, but a 5 tonner almost surely would be. An outfit in the Atlanta area that does HVAC design and careful Manual-J load calculations all over the US (but mostly in the gulf coast states) compiled this graph of square feet per ton, against house size, based on the actual Manual-J cooling load calculations of their clients, with houses of all types & vintage:

If you look at the cluster in the 2400' house size range the worst of them is about a ton per 800', which if that were your house would be a 3 ton load. The middle of the cluster is about a ton per 1200-1400', which would be about a 2 ton load.

I suspect a 2-stage (or modulating) 3 ton heat pump with heat strips to cover the Polar Vortex cold snaps depths is really the right solution here, but getting better load numbers could fine tune it. That company Energy Vanguard runs those numbers and specs the equipment for a fee. There are others in that business too, probably some more local to you. Just be sure to use a company that makes their living off of accurate numbers, not an HVAC company whose bread & butter is installing and maintaining equipment. Odds are pretty good that most of the fee would be recouped in lower cost smaller equipment now, and higher as-used efficiency (and higher comfort levels) down the road.

 

[WorthFlorida]

2002 two story box, 2400sf, frame siding, lots of cheap windows, not great insulation, all electric, bills avg $400/month @10 cnts/kwhr

Before you invest in a major change to your heating and cooling system, you may want to consider buying two Honeywell WiFi thermostats model TH9320WF5003. Can be bought at HD. Not only they work great, convenient and WiFi, Honeywell sends a monthly report via email on what the the system has done in the last month. Below is my report. Central Florida has had a very cool winter. The outdoor temperature report is from a weather service. The thermostat has four time periods but I set all four to the same temperature and this report shows a "home" and a "away" time period. This is a mute point for me since I usually try to keep the same temperature the same all day. But this will give you some idea where your needs are and this thermostat is a two stage heat/cool unit so it most likely will work any system you replace.

I have a two story house 2400 square ft in the Orlando area of Florida and my electric bill has not every been near $200/m. Usually not more than $160 in the summer months. A SEER 15.9 heat pump 3.5 ton with electric heat strips installed in 2014. No children and no pool. The thermostat has a "circulate" setting for the fan control and it runs about 30 minutes per hour when there is no call for heat or cool. It allows the hot air from the second floor to mix with the entire house and provides a more comfortable environment. For 27 years I had another 2 story home with two AC units and to keep the whole house comfortable, the second floor thermostat had to be set 2-3 degrees higher than the down stairs. When both were set to the same temp, both unit cooled the whole house but the downstairs thermostat would be satisfied and shut off the unit. But the second floor unit now would run nearly 24/7 since it was enough to cool the first floor but not the second floor. Only occasionally would the first floor unit turn on. I keep the second floor usually at 77 and the first floor at 75, this cycled both units. I had to teach this to the people who bought the home from me since they complained that the second floor AC was not working properly. It was replaced before closing since they were expecting the cool the entire house. You may have similar situation.

Anther is if your AC units are original to the house they are probably a SEER 12. Today it requires a minimum of SEER 14. Have you ever had the gas pressures checked? I had a leak once on the low pressure side of the lines and after replacing the tubing, my bill drooped about $40 a month.

Since you mentioned crapy windows and poor insulation, you may want to invest in getting the home more energy efficient. Switching to propane my not save you money if your home is leaking too much air. Propane costs can really raise much faster a than electric rates. If there is one thing where my home built in 2007 make it easy to cool is the use of of insulated foil in the attic. Such as https://www.homedepot.com/p/Reflectix-48-in-x-100-ft-Double-Reflective-Insulation-BP48100/202092205. This product is stappled to the roof raffers and it reflects radiant enery back toward the roof. In the summer I can go into the attic and it is cool, not blazzing hot.

Before replacing your heating system, replace the AC unit with a more efficient heat pump unit. I would upgrade the windows or have then filmed, upgrade insulation where you can, and perhaps watch the thermostat more closely.

Lots of luck on your decision.

 

[Dana]

Better thermostats are worth it no matter what gets installed, but they won't overcome the sub-1 COP of a resistance electric furnace with ducts in the attic, and won't fix the comfort hit of grotesque oversizing. Investment in major changes to the heating & cooling system ARE called for here, ahead of windows & thermostats.

Any time the HVAC equipment is being replaced is an opportunity moment for right-sizing it, an opportunity that won't come along again for anothe 15-25 years. The heating equipment proposed by the contractor would be a mistake. Whether the 160K of propane burner is being married to similarly oversized pre-existing AC or not or whether there is a plan to replace the AC isn't known, but it's not insane to retire the AC after 15 years and replace it with a heat pump more appropriately sized for the heating & cooling loads to cut the heating bill by more than half, even if it's only cutting the cooling bill by a little.

The average residential retail price of electricity in Texas is about 11 cents/kwh, while residential propane is running $2.40/gallon. At 95% efficiency for the propane burners and an HSPF of 8.5 for the heat pump, a million BTU (MMBTU) of heat costs:

The resistance electric furnace: 1 kwh= 3412 BTU, 1,000,000/3412= 293 kwh/MMBTU, which at 11 cents costs $32.23/MMBTU.

Propane: A gallon of propane has 91,600 BTU of source fuel energy x 0.95 efficiency= 87,020 BTU/gallon heat delivered to the ducts, or 1,000,000/87,020= 11.5 gallons /MMBTU, which at $2.40/gallon costs $27.60/MMBTU.

That's about a 17% reduction in heating costs from the resistance electric furnace (this year, anyway- propane pricing is volatile.)

Heat pump: HSPF of 8.5 is 8500 BTU/kwh, 1,000,000/8500 = 118 kwh/MMBTU, which at 11 cents/kwh costs $12.98/MMBTU.

That's less than half the cost of heating with propane, and only a bit more than a third the cost of heating with the resistance electric furnace. An HSPF of 8.5 is really just a middle of the road, nowhere near best in class heat pump.

At 3-4x better heating season savings alone it's worth the additional investment of a right-sized heat pump compared to a cheap propane furnace, and it fixes the venting & combustion air problems that were only adding complexity & cost to installing a fossil burner solution.

At the average grid-mix in Texas a heat pump even has lower PM2.5, NOx and CO2 emissions than a condensing propane furnace. At the rate they keep building out wind power in that region the emissions will continue to shrink over the lifecycle of the heat pump.

 

[WorthFlorida]

I made the suggestion for smart thermostats so RMAR10 may get a better idea were his dollars are going, heating or cooling. What I got out of his predicament was trying do something to reduce his monthly electric bill. His mentioned electric rates and are about the same as in Florida. Summer temps, humidity and durations are probably about the same. I'm not sure what his winter time heating needs are. I've had a neighbor that had huge electric bills and one problem was teenage boys. When it is 90 degrees outside they had the AC set to 70 while sleeping under blankets. Sometimes to reduce your utility bills requires a change in your living habits.

Dana, you're probably the best there is on the forum, your knowledge of HVAC is very impressive and I love reading your comments. I never realized that there was that much into designing HVAC systems. I'm not saying he doesn't need to upgrade but if you have holes in your environment and your replace it with another system, it will still take the same amount of energy. So wouldn't plugging up the holes and maybe a slight change in your living habits first make sense? When you put a new engine in a rusted out 53 Oldsmobile, you still have a rusted out Oldsmobile.

 

[Dana]

Dude, you're making my head swell so much I'm having a hard reaching around to scratch my ears!

Irving is in US climate zone 3A, near the Dallas Ft. Worth International Airport, which according to ASHRAE's datasets logs 2275 base 65 F heating degree-days (HDD) , to 2719 cooling degree days (CDD). That's only about 20% more CDD than HDD, a fairly balanced heating to cooling average annual load.

Orlando Florida is zone 2A, and heavily cooling-dominated, averaging 544 HDD to 3379 CDD (at the Orlando International Airport).

The Irving's peak cooling load would be measurably higher than Orlando's , with a 1% outside design temp of 98.4 to Orlando's 92.6 with but with similar wet bulb /humidity temps. Despite the lower peak load Orlando logs a lot more cooling hours per year adding up to quite a few more CDD, but Irving/Dallas logs more than 4x as many heating degree days, which adds up to a lot more heating energy used.

Even with a pretty good heat pump, a house in Irving they would use more power for heating than heating than dropping the same house in Orlando and heating with an electric furnace.

But it's still true that better thermostats provide far more comfort & efficiency than the most basic units.

I'm usually all-over the building envelope upgrades as the first and best place to spend the money, with mechanical systems as only a secondary factor. But heating a 3412 BTU/kwh with an electric furnace instead of 8500 BTU/kwh with a heat pump is a performance upgrade impossible to match at anywhere near the same upfront cost.

Air sealing (both the ducts and the house) and improving the insulation where it makes sense is definitely worth doing. The payback on radiant barrier isn't as good in Irving as it is in Orlando, since it slightly increases the heating energy needs, whereas spending that money adding to the insulation depths in the attic with a cellulose overblow (after air sealing) improves performance year-round.

If air sealing and insulation improvements are anticipated, only the "after improvements" conditions should be entered into the load calculations, to yields Even if the new HVAC is installed before the house improvements are done, the smaller HVAC unit appropriately sized for "after" picture will usually cover the 1% and 99% loads of the "before" picture, or nearly so. I suspect the peak cooling load will be something like 1/2 ton to 1 ton lower in this instance, and the peak cooling load would likely drive the sizing of the heat pump more than the heating load. If it's the other way around, sizing for the cooling load and using heat strips to cover the heating shortfall difference is probably the better choice.

As a general rule radiant barrier only pays back in a big way when the attic insulation is thin (R19 or less), and the ducts & air handler are uninsulated, located above the insulation level.