Hello all! I'm thinking about converting from oil to natural gas. First and for most the furnace I have now is roughly 15+ years old. Anyway I have a few questions. Firstly, I have been quoted $6500 for a 98.5% Efficent Carrier Furnace with Variable speed. $5900 for a 96% efficent Carrier 2 stage. And $5300 for a 95% efficent Carrier Single Stage. All prices include install. Would it be better to spend the $1200 and go for the Variable speed? Also about vents. Because high efficency use draw air from outside the home, what will happen with the cold air return cents in my home currently? Will they be reversed to blow warm air? Thank you!!

The outside air goes to the combustion chamber: it has no effect on the airflow in the ducting. It's better than low efficiency equipment because it's not drawing conditioned air from your house which is then replaced by sucking in outdoor air via cracks and leaks in the home.

If you're going to live in this house for awhile, I'd go for the variable speed air handler. It should be quieter and more efficient and should recoup the cost differential in some reasonable-ish timeframe, depending on your fuel usage/rates and electrical rates.

It's worth doing the analysis to right-size the furnace to the actual loads too, even with variable speed units. There is no point to a variable speed or 2-speed unit if it is so oversized that it can deliver the entire heat load at the lowest possible firing rate.

Most existing oil-burners out there are 2-5x oversized for the heat loads of the house they are heating, so just swapping the oil-burner out for a gas-burner of the same size is usually a mistake. If you right-size it it'll run quieter and use less power, have less wind-chill, steadier room temps and overall better comfort.

A fuel-use to heating degree-day calculation using the existing furnace is a good way to ball-park the actual heat load of your house. You never want to size it dead-nuts on the heat load at the 99% outdoor design temp, but even 1.25x oversizing will carry you through Polar Vortex events, and 1.4x is sufficient for using overnight setback strategies. AFUE testing presumes 1.7x oversizing, but if oversized 1.7x with most 2-stage furnaces even the low-fire stage would be oversized at the 99% heat load.

With a ZIP code (for weather data and 99% design temp purposes), the input BTU & D.O.E output numbers on the nameplate of the existing boiler, and the amount of oil used between two recent exact fill-up dates it's fairly simple arithmetic to ball-park the heat load with sufficient accuracy. (Unless you spent 2 weeks in Cancun with the thermostat backed off to 55F during that period, or ran a wood stove or mini-split as supplemental heat, etc.)

A fuel-use to heating degree-day calculation using the existing furnace is a good way to ball-park the actual heat load of your house.

You can also get close with "what percentage of the time did the existing furnace run on the coldest night" and adjusting accordingly.

Consider having an energy audit done to see if more insulation and air sealing makes sense. They can also help with the analysis of how much it it worth paying to get greater furnace efficiency.

I think I'd shop around before paying what they are asking. I also would not limit my search to Carrier which no longer may claim to be the biggest in the biz. Parts are expensive once warranty expires.
At the end of the day however your dealer will have more to do with your satisfaction than the brand you buy.

Posted By joe.ami on 04 Apr 2015 09:06 AM
/> At the end of the day however your dealer will have more to do with your satisfaction than the brand you buy.

No truer words were ever spoken. See Rheem, Bryant or American Standard... And the guy that produces a proper ACCA Manual 'J' heat load gets extra credit!

“A fuel-use to heating degree-day calculation using the existing furnace is a good way to ball-park the actual heat load of your house.”

Borst Existing Building Energy Usage Analysis Software

I'm most partial to Amana furnaces (since purchased by Goodman who is now the largest resi equipment manufacturer in the US).
Carrier , Bryant, Payne, Day/Night, Weathermaker, BDP all the same though last I looked Payne had 10 year part warranty standard while Bryant was 5.
Colman, York, Borg Warner Luxaire all the same.
Comfortmaker, Arco Aire, Heil same.
American Standard, Rheem, Ruud the same.
ETC. There are not as many manufacturers as you might think.

We heated with wood for 30 years. At retirement the wife wanted to be able to get away so we had 4 professionals come out to size and replace the old propane furnace. I made sure to give them all the R-values and U-values since we had just remodeled. Every professional over sized the unit and wanted $4000 to $5000 to replace it. I did my home work and asked if a 45,000 Btu furnace would work. Everyone to a person were extremely reluctant to go with a 45,000 Btu unit. Dismayed with the professionals I purchased a 2 stage, high efficiency Goodman and installed it myself. I fired it up on January 9th. The furnace always kicks in on high stage then goes to low stage (35,000 Btu). It will go to high stage if it can't keep up. Not ever during even the coldest days did it ever need to go to high stage. The moral of the story is; educate yourself and be wary of professionals after all they are salesmen first and foremost. If you are unsure about sizing the equipment hire an engineer. They have nothing to sell.

what size unit were they quoting?

One professional quoted and 80,000 Btu unit while the other three were at 60,000 Btu. I really questioned their sizing but they stuck to their guns. The old furnace was a 60,000 Btu unit. It heated the house adequately when the home had ZERO wall insulation, single pane windows and an R-5 ceiling level of insulation. These guys were in the business lots of years and were familiar with Manuel J and said they had proprietary software to size it with.

We bought a "fixer upper" in Florida and am finding the same level of knowledge or lack there of down here. During my working career I always sought to keep myself up to date with technology even in times of extreme time pressure. And I always took pride in giving people solutions to their problem based on the best the science available at the time. My dad who would have been 101 this year always took pride in his work. Is pride of workmanship eroding away to be replace with the "making sale"?

I'm not an HVAC expert, but some HVAC contractors have the reputation for using "rule of thumb" (X btu/SF) estimating even when it is not supported by manual J calculations. My understanding is that it's (RoT) always worked for them so why change? "worked for them" meaning no one has complained about being cold. So if they install a furnace and you don't complain about being cold, they installed the right unit (In their opinion). Their biggest fear is that they will undersize the unit & they will have to replace it with a larger unit, losing money.

With the majority of houses poorly air sealed and poorly insulated, and no good accurate way to test houses, this method worked; sometimes well, sometimes not well. Now that we know how to build - and are building - houses that work, we can actually use the calculated btu loads & be warm.

Let the companies that quoted the work know that you used that size unit & why; maybe it'll help their education.

AFUE testing presumes 1.7x oversizing

So with a 96% efficient furnace, reducing a 1.7x oversizing down to a riskier 1.25x can't possibly save more than 4% (I'll guess more like 1%). Compare that to say ~10% that thermostat setback will save in a typical house. A manual J performed by a typical (vs careful) contractor isn't very accurate. Usually over-estimating, but you never know. Add in Bob's point and the fact that the next size larger gas furnace might be only $100 more and some over-sizing is pretty logical.

Bob's point about educating the companies is a worthy notion.

As for Jonr's point then why put any emphasis at all on doing a Manuel J? I've looked at many entries on this site about sizing HVAC equipment and find in variable the phrase "do a complete room by room heat loss analysis". If there is that much slop in the home system I guess flying by the seat of your pants is okay. Being somewhat facetious, I guess my balloon has been burst and all this mambo jumbo of ACCA and Manuel J is just another money making ploy.

Posted By jonr on 07 Apr 2015 01:15 PM

AFUE testing presumes 1.7x oversizing

So with a 96% efficient furnace, reducing a 1.7x oversizing down to a riskier 1.25x can't possibly save more than 4% (I'll guess more like 1%). Compare that to say ~10% that thermostat setback will save in a typical house. A manual J performed by a typical contractor isn't very accurate. Usually over-estimating, but you never know. Add in Bob's point and the fact that the next size larger gas furnace might be only $100 more and some over-sizing is pretty logical.

While 1.25x oversizing won't leave you cold, it's a bit tight for using deep overnight setback strategies, but it would have to be a sufficiently crummy house that it would reliably reach those deep setback temps to save as much as 10% even with a single-speed.

The difference in just the raw combustion efficiency between high-fire and low-fire is about 5% for most 2-stage furnaces, so you're eating up some (or maybe even all)  of the fuel savings you'd get from the setback in lower combustion efficiency during the recovery ramp.   The AFUE test assumes 1.7x oversizing, so it means that nearly all of the time the thermostat is satisifed on the low-fire part of the burn, but that won't be the case if you are setting back 10F at night and spending over an hour on the recovery ramp. (Most code-min houses shouldn't lose 10F overnight during an average winter night.)  Using shallower setbacks sometimes uses less fuel than deeper setbacks.

ACCA recommends 1.4x oversizing to be able to reasonably manage setback strategies and have margin against record cold, but it doesn't buy you anything in either efficiency or comfort to go any higher than that (or even that high.)

If you oversize the hell out of it (the way most contractors do) you end up with more duct noise, higher wind chill/draft, and temperature over/under shoots.

If you have a fuel use history on it you can KNOW the actual heat load and size accordingly, since that's a measurement, not an estimate based on construction assumptions.  Using a fuel-use derived heat load it's wise (but not at all risky) to upsize by at least 1.25x, but anything over 1.4x is a waste.

So.

If a contractor uses a "rule of thumb" he is ignorant and if he uses Manual 'J' he is just a salesman...

Just another money making ploy...I love it!

ACCA Manual 'J' is the residential standard. If you are radiating a room, or just pushing warm air, no one you will meet in the HVAC industry can do a room-by-room heat load and properly size equipment to control each room to its potential long hand, as Dana does here regularly. And if they could they would not do it for free.

The layman should be involved and weary but appreciate the contractor's challenges. Some clients think they want over-sized. Most will complain shrilly if the house drops off one degree or the price is $100 over their competitor's. In the end the homeowner dictates the outcome, be individually or collectively.

Smart people get what they pay for, the rest, well.

If a room-by-room analysis is just an academic exercise, why do I see it suggested at this forum time after time? I'm just trying to understand and learn from those who are in the know. Or perhaps I'm just viewing the clash of two different world views of HVAC. Or maybe more appropriately, a clash of science vs. the art of HVAC design? Help me out.

Reality is complex.

Manual J analysis is very helpful when you have some idea of whether what you're calculating meets reality. Think, modern construction from a known source, or new construction you can look at while it's going up, or a house that recently had walls opened up and/or insulation/sealing updates... things that make the insulation type more or less "known". Lots of things can affect that. Sounds like you're in that situation (recent upgrades, might make fuel use analysis less accurate unless you've had them through a recent heating season) and an MJ might be in order. Maybe. There is more to the story though!

Here in my area, we have a ton of houses that are very old, and while most of them have insulation in them, not all of them do, and the ones that do could have 70's settled cellulose, crappy 80's batts in half the rooms, etc. So those are some cases where a true manual J may not reflect reality because realistic inputs are simply not available, and the contractor has no choice but to estimate either the load directly or the inputs for the load calcs.. either way, the contractor has to be conservative here or risk a big money rectification on his dollar if he comes up short. No one has ever had to replace a boiler or furnace for some short cycling.

Fuel use analysis is good too but it's very, very hard to be very accurate with it for some subsect of people: it's strongly affected by your thermostat settings and if you have multiple thermostats at different temperatures or you change thermostat settings a lot this is little better than a SWAG. If you keep a fairly consistent temp in the house or have during a complete fuel billing period it gets a lot better.

In MANY cases, houses are small enough that manual J is unnecessary for heat source sizing because it would not change the heat source selection, the client is already below the lowest output in a given size range. That 45k furnace is good for most heated areas 1500-2000 sq ft in size in modern construction, not counting cathedral glasswall greatrooms. In BoilerVille, 60k is about the minimum in most product lines with a few at 50k input. If all I'm doing is sizing a boiler a room by room or even block load calc on a 1500 sq ft house isn't going to tell me anything new: you'll get a 60k boiler or 50k boiler depending on the product line chosen (and it's usually chosen for other reasons, like service familiarity). With boiler's 5 to 1 turndown the difference isn't big there (for oil though it's a lot bigger and the minimum there is usually 80k.. kinda sucks).

For furnaces with their lower turndowns (on modulating equipment) and faster cycling it's probably a bigger issue. But still, the contractor is probably starting with a product line he or she is familiar with so he or she can service it and install it economically, and you were offerred the smallest ones in those ranges. Most likely. Is it the best? Maybe, maybe not... all depends on how easy to service the unit you have installed is for the local talent base. Even the best equipment needs to be fixed eventually. Of course, maybe the guys you've worked with didn't give you the due diligence you need, that happens too, all too often.

Room by room loads are for emitter/duct sizing first and heat source selection second. Block (building) loads are for cases where there is some cause to believe an existing heat source is oversized for the current conditions, and/or when you're in a size range that isn't automatically the bottom end of the range. There are still plenty of cases where a formal load will not be significantly more helpful than a ballpark though.

FWIW I think you'd have been better with a modulating burner than a two stage one.

Good explanation for one reason mini splits are taking off; starting at 9000 btus.

...and a 9000BTU mini-split can deliver ~11,000 BTU/hr @ +5F, ridiculously oversized for individual room loads where they are often installed. (And that is despite having a pretty good turn-down ratio. )

As better/higher-efficiency mini-duct cassettes are becoming available, splitting the output between rooms is often a better option than overheating the space with the ductless head to limit the use of resistance heating in rooms doored off from the space with the mini-split head. Fujitsu's ARU-9RLFCD 3/4 ton mini-duct cassette tests at HSPF 12.2 / SEER 21.5, which is better than many 3/4 ton ductless mini-splits, and fully is specified at operating temps down to -5F.

At Rob points out, reality is complicated and thermostat settings affect the numbers on fuel-use calculations, but at any thermostat setting or setback profile up-sizing by 25% won't leave you cold, and upsizing by 40% would not impede comfort. Fuel use calculations are a VERY good stake to put in the ground for evaluating the sizing of replacement equipment, and takes a miniscule amount of time compared to a Manual-J or I=B=R load calculation, all of which have inherent error issues as well. A fuel use calculation is a measurement, with fewer places to err badly.

Of course you don't want to use the fuel-use/HDD numbers for a period where you headed to Cancun for two weeks with the thermostat set to 55F, nor do you want to use it for the mid-winter weeks when the windows were left open to vent the painting fumes, etc. While it's easy to skew a Manual-J by over 25% with error factors, it's much harder to end up with an error that large with a fuel use calculation.