How to choose a replacement for dead furnace in record time

A heat load calc is a must. Then whoever is your installing company must also design the duct system to match the heat load, not just use your existing ducting. The load calc., a comprehensive load not a block load will allow the installing company to ensure each room is receiving the proper amount of air for THAT room. The duct must be insulated to a minumum value of R-6 and R-8 is even better, and sealed with mastic duct sealant and or Ul listed all weather tape (for longevity). Uninsulated poorly sealed ducts can loose 20% to 35% of the air flowing through them. This means the house will experience outside air infiltration due to the loss of conditioned air into the basement via leaky ducting. The old ducting was most likely sized for heat only, not heat with AC. AC requires larger ducts be installed, this not only is important for comfort, proper dehumidification but also for mechanical reasons. Too little air flow across the indoor cooling (evaporator) coil will cause the system compressor to fail early, cause inefficiencies which relate as more $$ spent on utility bills and as mentioned will cause higher humidity in the home. I would insist the contractor show you a room by room heat load and the ensuing duct design. The duct design BTW will be for the supply ducting and the return air ducting. If this contractor says anything like "I've done lots of installs and we don't need to do a heat load calculation", tell him/her thanks but goodbye. Spending money on an HVAC system is a costly venture and like buying a car you would not spend lots of money on something that is not running right and will only cost you more down the line. You may want to visit ACCA.org and look up a contractor on their site. They have lots of information for home owners too so you can make the right choices with the contractor regarding your budget, system size, and ducting. Good luck, there are some really great contractors out there that want and can do the right thing, you must be educated on what to look for and the ACCA website is a very good resource for information.

Curious why a high efficiency forced air option was not offered.

As stated above heat loss comes first. Send them packing if they don't want to do it.

I don't know how long you have lived in this house. Sounds older, but may have had insulation or window upgrades since the 50 year old system was installed. That's why it's important to do a heat load calc. Chances are the old furnace is way oversized.

Switching to gas is it natural, or LP?

Switching to natural gas, and 80% efficiency will save tons, but may as well go in the 90% range efficiency while doing it.

We don't know accessibility of the duct work for rework so that could be a hurdle for the installer. Your quoted price does not reflect much duct work resizing if new AC is in that proposal.

Use the find a contractor option . You may get lucky .

Eastman said:

A mod/con based hydro air system might be a good option if you like the idea of some kind of radiant heat but can't commit to scrapping the forced air system just yet.

That's where I was headed -- but then I noticed that she mentioned only oil. Depending on her location (and assuming that NG is not available) a modern ASHP could be her best option.

Ack, missed that! Too many posts, too late at night.

No reply from the homeowner yet.

To the OP Rich probably has the best advice in using the find a contractor. Hopefully there is someone in your area from here willing to come out. This is a hands on observation from beginning as to options to the installation. Guidance of what type of system, seeming urgency,and budget constraints. We can suggest all sorts of ideas, but in the end sight seen is the best.

icesailor said:

""" The old ducting was most likely sized for heat only, not heat with AC. AC requires larger ducts be installed, this not only is important for comfort, proper dehumidification but also for,,,, """

I'm confused as usual.

I understood from long ago that cooler air in a specific volume will take up a bigger space when heated. That cool air in an AC system, because of the BTU heating/cooling content of the air. That AC air carries the BTU's of 70 degrees returning but 50 degrees leaving the compressor. The ductwork will be some temperature above 70 degrees returning and above 50 degrees leaving. There because of a 25 degree +/- differential, so AC duct can be sized smaller, But heating ducts deal with hot air, 70 degrees or lower returning to the HX with 140 degrees +/- leaving. With the same heated air expanded and contracted to the tune of 70 degree +/- air. Therefore, you needed larger duct sizes to carry the same BTU's for heating into a space than you do for removing BTU's (AC) for the same space.

A lot of Cape Cod houses had WA heat alone. No AC. With massive ducting for the supply and return with returns in each room. Then, AC came along. I saw dinky, tiny ducts, nowhere as large as the heating ducts. Then, they started using the same dinky ducts to push the WA around with one big return over the furnace in a hallway, by the stairs.

What changed?

I looked this up with ACCA. They have mention of heating and cooling with Duct Sizes. But no mention of any difference in duct sizing. Just in heat gain with AC during the day, but no mention of heat loss during the night.

ACCA heat gain and ACCA heat loss seemed to be in a different universe.

I'm confused.

It can be due to geographic location when you size cooling ducting. In the south we require more cooling than in the North and less heating. So we size our ducts based on cooling (or heat pump) cfm values, which are usually larger than heat only cfm for our area. So in a northern climate where a structure may need 100,000 btuh input and require a duct system sized to carry the specified cfm, which would be large compared to the same structure located in the south, the southern structure may only require a btuh input of 50,000. The heat load/loss calc done room by room will tell u the size of each branch duct for each room and for the trunk size or sizes if transitions are needed to keep the velocity up. The most important thing is to look at manufacturers specs for cfm and how it relates to the duct static pressure values in the manufacturers literature. Since most manuf. with PSC or even X-13 blower motors suggest an external static of .50 " wc ( up to .80 if a true variable speed blower motor is used) the static pressure measurement is a must if you want to prove the Cfm value meets manuf. specs. The duct design is done using ACCA's Manual D, which is often incorporated in various heat load software programs like Wrightsoft. Keep in mind too that no matter where the geographic location, heating systems were often installed without a load calc and yesteryears equipment did not have the variety of btuh options like todays equipment has.

The ACCA Manual J stipulates we size systems based on the local design temps. Which is usually worst case scenarios. Here in Norfolk they are 22 F for winter and 92 F for summer. We rarely see 22F for more than a few days. Where as in part of Vermont for example the design temps are 82F and -15F. The outcome of the different calcs will determine equipment and duct size. Most of what we see here is equip sized on 400 to 500 sq ft per ton of cooling ( which is wrong of course) and the ducting seems to be installed with whatever the contractor had on hand!

If you haven't read Manual J or D yet it may be worth the while even if u have a load/duct design software program. WARNING: It is mighty dry reading and can cause sudden and uncontrollable sleepiness ;0) but is good info.

don said:

Judy are you in norflok va?

Yes Don I am, are u in Norfolk?

don said:

Yes.lowes mechanical in va beach.Welcome to the wall Judy.

Thanks Don. This site has been a godsend, esp. For a steam boiler we are installing! Do you have much experience with residential steam Don?

I absorbed Manual J and Manual D. Along with all the old IBR manuals.

They aren't even in the same room.

That said, I once heard a guy complaining about the quality of his help. "How can anyone expect me to soar with Eagles when all I have to work with are turkeys.

For many Turkeys, reading is not a requirement of membership in that species.