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Q&A Spotlight

Our Ancient Furnace is a Goner: Now What?

A furnace replacement is an opportunity to ensure high quality indoor air as well as efficient heat

This furnace is toast, but before replacing it, our expert recommends a number of other steps aimed at tightening up the building enclosure. Photo courtesy of Jennifer McEachern.

Jennifer McEachern has finally said goodbye to the original oil-fired furnace in her 1953 Connecticut ranch, and is leaping at the opportunity improve indoor air quality as she ponders a new source of heat.

There are other issues as well, as she explains in this Q&A post, such as lots of air leaks in her home’s building envelope. A blower-door test shows 15.7 air changes per hour at a pressure difference of 50 pascals, 10 times what a conscientious builder might shoot for today.

McEachern says her first choice for a new heating system is a boiler that runs on liquid propane (LP) coupled with radiators. That makes some sense because the house already uses LP for other appliances, including a generator. At the same time, McEachern also would like to add a heat-recovery ventilator or a HEPA ventilation system with its own ductwork. In addition, someone has recommended she consider at AtmosAir ionization air purification system.

“Does anyone have any advice about this equipment and/or plan?” McEachern asks. “The heating contractors we have had look at the system just provide us with quotes for whatever we ask about and don’t provide advice (that we desperately need).”

While all of this is still up in the air , McEachern and her family are staying warm with a wood-burning fireplace insert that was installed a few years ago, along with a few heaters in the bedrooms.

Winter may have settled over New England, but McEachern is in no rush.

“We are prepared to take some time to get this right,” she says. So, what is the right answer? That’s the topic for this Q&A Spotlight.

Consider a heat pump instead

Propane is an expensive fuel for heat, says Keith Gustafson, who suggests that McEachern consider heat pumps instead. They would almost certainly be cheaper to run.

Drawing on personal experience, Duane recommends using minisplit heat pumps as backup heat. They can provide cooling as well as heat, and dehumidify the air during shoulder months.

“I’m heading in nearly an opposite direction,” Duane writes. “I’m looking to replace my older LP boilder (and radiators) to minisplits as our backup and supplemental heat next year after we get our leaky house tightened up.”

McEachern says she has considered just that, installing a heat pump and using an LP boiler as backup heat. But, she adds, indoor air quality is a “super important” part of the equation.

The problem, adds Akos, is that the house “leaks like a sieve.” Assuming the outdoor air is of good quality (that is, not near a source of pollution), “adding in fancy filtration and HRV will do nothing.” The amount of air leaking into the house would overwhelm what an HRV would provide. Even if the HRV air were filtered, it would be quickly diluted by air leaking in from the outside.

“Switching to radiant heat will also not do much for IAQ,” Akos adds. “Typical older furnace with leaky ducting in the attic is actually pretty good for improving IAQ. They tend to depressurize the house and suck in even more ‘fresh’ outside air. Properly sized and installed radiant tends to be more comfortable in older, leaky homes but that is a lot of $$ to spend for not much benefit.”

The current system is probably too big

Most HVAC contractors propose heating and cooling systems that are much too big, writes Dana Dorsett, so a good place to start is to calculate the heating load based on how much fuel is used (an article describing that process can be found here.)

“Even at CT electricity rates a right sized air-source heat pump will be less expensive to operate than an LP boiler (and a lot cheaper to install than a bunch of new radiation, be it flat panels or radiant floor),” Dorsett says.

Further, it may not be necessary to replace the existing air ducts. Some adjustments for better balanced air flow may be needed, but a wholesale replacement isn’t necessarily going to be required.

Because the old furnace was probably grossly over-sized, he says, it was running only one-third of the time even on the coldest day of the year—”not a recipe for comfort.”

“A right sized heat pump will be far more comfortable than the oil-burner,” Dorsett says. “The overall duty cycles will be much longer, and grow longer as outdoor temperatures drop. With the oil burner you were likely getting the hot-flash followed by the extended chill between cycles, whereas a properly sized heat pump will be yielding a nearly continuous summer-breeze flow at lower cfm with warm but not as hot as the fossil-burner’s output at the registers. It’s the high duty cycle that keeps it comfortable.”

Dorsett suggests McEachern read parts of a Nate Adams “House Whisperer” blog on this topic.

A variety of fixes have already been made

McEachern has already taken a number of steps to make the house more comfortable: new siding, doors, and roof, and excavation to solve a mold problem in the crawlspace.

“And,” she adds, “we plan to replace windows and attic insulation, along with insulation in the rim joists. We had a HERS rater come out and his recommendations should come shortly.”

But, she adds, two heating contractors have told her that an air-source heat pump wouldn’t be enough to heat the house without a supplemental heat source for the coldest days.

“I think it will, especially with electric strip backup and our very efficient (and warm!) wood-burning insert,” she says. “All of the articles and Q&As I have read on this site state exactly that, along with the fact that we are living comfortably with little heat right now.”

That’s not a surprise, Dorsett adds. “I’ve usually had to spec the equipment myself and push back on contractors when they say it’s not enough or won’t work. They’re all too nervous about undersizing (human nature), when it’s in your interest to be more concerned about oversizing.”

Heat pump will mean more even heat

BFW577, who also lives in Connecticut, says a minisplit heat pump system will provide much more even heat—and he offers his own house as proof.

“I switched from my minisplits to oil heat around 8 a.m.,” he says in reference to the graph below. “Notice the minisplit held the house at exactly 67 overnight and the large temp swings of my ridiculously oversized oil burner. It’s amazing how precise the minisplits can regulate temperature.”

An oil burner showed spikes of heat followed by sharp drops in temperature.

BFW577 says the minisplits have been much less expensive to run than an oil-burning appliance. “Propane in CT is more expensive than oil and has far less Btus,” he adds.

Our expert weighs in

Peter Yost, GBA’s technical director, adds this:

In typical GBA fashion, knowledgeable folks have shared a ton of great information for Jennifer McEachern. I am simply going to sum up:

Use the Database of State Incentives for Renewables & Efficiency (DSIRE). This excellent and continuously updated online resource will guide you to the financial support available at both the federal and state levels. When I typed in my own zip code, 54 different programs came up for me to consider.

Prioritize your work. If you can muddle through with your current heating conglomeration for a little bit longer, or do some additional temporary heating along with heavier sweaters, follow this approach to upgrading your building enclosure before tackling your heating system:

  • Manage bulk water: You need to manage moisture and energy with equal intensity or you will end up with moisture problems.
  • Manage air (in this order): top (attic), bottom (especially above-grade portions of your foundation, including the rim joist), shafts and chases, and walls. This last one surprises some people, especially as it relates to windows. Replacing windows involves a lot of options (interior or exterior storms, sash replacement, full replacements) and this is never the place to start unless it’s about a bulk water management issue. For more on this, see this blog by Martin Holladay.
  • Vapor: As you work on your enclosure, ask yourself whether you are maintaining drying potential and whether you evaluated household sources of moisture for management. For more on this topic, see this article.
  • Thermal: It also may be surprising to some that insulation is at the end, not the beginning, of this list of performance improvements.

Now that you are down at the end of this list of improvements to your enclosure, you can go about selecting and sizing the heating system suited for the performance of your building. Dana and crew have nailed this for you in the Q&A.

Find the right contractor. Jennifer already seems to know that this may be the hardest part. While we qualify and certify energy and building performance auditors and raters, we don’t have a similar national system for those who do the actual work. I wrote a draft of “how to interview your builder” and attached it to this GBA blog: How (Older) Homes Work . I hope it helps.

-Scott Gibson is a contributing writer at Green Building Advisor and Fine Homebuilding magazine.


  1. Doug McEvers | | #1

    At 15.7 ACH50 the first priority has to be air sealing of this house. I do not know about mini splits and the heat distribution but it seems there will have to be heat in every room. The highest ACH50 I remember was about 8.5 in a 1948 or so 1 1/2 story in St. Paul. 15.7 is a very leaky house, this must be addressed first before all else.

  2. Jennifer M | | #2

    Peter Yost, thank you! I will follow all of your additional advice.

    In just the week or so since my original question, I have ordered SmartBaffles for the attic and insulating "cups" for the can lights, and I have already installed an attic door "box" with insulation that instantly warmed up the hallway. I will keep moving forward!

  3. Nick DeFabrizio | | #3

    I had a similar old boiler. I did some air sealing and installed an Energy Kinetics (EK) boiler and two mini splits. God boiler and mini splits supplement each other. The EK boiler is pretty efficient as are the mini splits. Still the old boiler- that performed without trouble for 50 years- was an amazing piece of equipment. And built locally ( as is the new EK boiler). I doubt the mini splits will last as long of bd as trouble free. In s way Ghar boiler epitomizes why fossil fuels are hard to get rid of purely on economic principals. But we have to do it ax best we can

  4. Nick DeFabrizio | | #4

    Sorry for the typos, this phone is small!

  5. David Herbert | | #5

    A couple of years back I decided to have all of my R20(ish) insulation vacuumed out of my attic and then I personally air sealed it (not sure I trusted the local contractors to do it well)... then I put R60 back in... I was shocked at the impact it had on heating bills. Cant recommend too much if you have a leaky environment.

    1. Doug McEvers | | #6

      I talked to a builder at my continuing ed class 2 years ago and he did about the same thing. Cleared the attic of existing insulation and then had a 2" layer of closed cell foam placed over the ceiling. He then added blown insulation to code and said the increased comfort was transformative. Start at the top of the building envelope with air sealing and added insulation, this will give the greatest results for efficiency and comfort. I did this in my own home in 2006 and the payback has since occured, now it is free money.

  6. Jud Aley | | #7

    I’m also in CT and we do a lot of energy upgrades on existing homes. We don’t build new homes.

    A few questions about your existing HVAC system.
    Where is your duct work located, is it all in the basement, all in the attic or some in both?

    If your duct work is all in the attic, then I highly recommend you Air seal & insulate the roof of the attic and not just the floor which is what it sounds like you have done so far. Some people might tell you with an insulated attic floor to then insulate the duct work but that is only going to get you to R-6 on the duct work at best. If you insulate the roof of your attic you could conceivable insulate it to R-49 or more. If you do air seal and insulate the attic roof and your duct work is leaky now at least its leaking into the insulated envelope of your home and you’re not losing all the energy. If the roof of the attic is insulated you dont have to worry about 20 year old leaky recessed lights or bath exhaust fans or any number of penetrations typical to a 1950's house

    If your duct work is in the basement then make sure you don’t have any open crawl space vents, still very common here even on new construction additions. If you do have crawl or even basement vents then at the least close the vents, but even better remove them and have a mason fill the openings with concrete or block.

    Do you have a basement bulkhead door (A Bilco Door)? The majority of homes around here have them and the only thing between the basement and the outdoor temperature is the steel door itself! What’s the R-value of 1/8" steel, maybe R-.25? If you do have a Bilco then you need to have an insulated weather-stripped door at the bottom of the stairs. My Senior carpenter loves to build these doors in the winter. He will bring a thermometer with him when he does the job and likes to report back to me the basement temperature before and after, often a 5-degree increase or more. That heat now trapped in your basement will rise up under your first floor instead of leaking out the Bilco and the vents, the first floor of your house will be that much warmer and less drafty. Here again if you do this then no need to insulate your duct work and if the ducts leak a bit its not the end of the world because the heat is staying inside the house.

    I deal with lots of existing oil burners, furnaces, boilers and water heaters and most of them are poorly maintained and running at a very low efficiency rate, 50 or 60% efficient but if you can find the right service technician who is willing to do a full service tune up that includes brushing & vacuum out the heat exchanger, cleaning out the fire box (Re-lining the fire box if needed, run a combustion test and install the right nozzle your existing burning could very well run at 80% efficiency or more. One property we manage and has a 65-year-old oil fired boiler and runs at 80% efficient, we looked at replacing it with a new unit but could not justify it given the cost of a new unit and that it would run at 87% efficient at best.

    So before you go and spend $1,000’s replacing your existing oil burner with heat pumps think about the payback time and think about the embodied energy of the existing system and the new system, what’s the environmental damage done by the manufacturing, transportation, installation, removal and disposal of all your existing equipment and duct work? Not to mention all the energy used to manufacture it in the first place 50 years ago?

    We install 2 or 3 heat pump systems every year, I think they are great and have one in my own home along with a propane fired boiler and a solar hot water system, but if you are on a limited budget the things I suggest above can go along way to improve your comfort and reduce your utility bills. Additionally, the work I’m suggesting here is low tech and can be done incrementally if need be for budget reasons and with material available from most lumber yards.

    Attached is an article I wrote for Building Energy Magazine that goes into a bit more detail.

    1. Jennifer M | | #12

      Jud, I am just seeing your comment. Thank you for the great information and the link to your article. It is all very helpful!

      1. Jud Aley | | #13

        Glad to hear the information is helpful. Lets us know how things turn out. - Jud

  7. MKCF | | #8

    Wouldn't it be better to seal and insulate the attic ducts rather than try and insulate the whole attic? No one says you have to stop at R-5. A properly vented attic has a lot of advantages (see Joe Lstiburek's great article). If you mean no venting, it seems like there are only two really good ways to prevent condensation issues: 1. Spray foam, which is super expensive and made from yucky chemicals, and 2. Exterior rigid foam, also yucky chemicals and also means you are replacing your roof and adding new trim. Besides, nice tight well-insulated ducts increase efficiency, so you should do it anyway. Ranch houses don't have very useable attics in terms of increased living space, so not much advantage to insulating the rafter area anyway.
    Once you've got a well-sealed and insulated attic floor, I would look at the rim joist and any above-grade foundation area next. Then the walls, and finally, seals around doors and windows.
    I also agree that it is certainly cheaper to heat with fossil fuels than to buy a fancy new heat pump. Yes there is a great deal of embodied energy in new heat pump manufacturing, but probably about the same as a new propane furnace. Is it a good idea to replace old tech with more old tech? Even a new furnace will have to be thrown out eventually. Besides, heat pumps give you AC as well as heat, so that's actually two pieces of old tech replaced for the price of one.
    Finally, this is the Green Building Advisor. Cost is not the only consideration here. We've got to stop burning fossil fuels at some point- if a heating device needs to be replaced, putting in one that can be powered by solar panels is a relatively painless way to move the needle forward, as they say.

  8. User avater
    Dana Dorsett | | #9

    >"Wouldn't it be better to seal and insulate the attic ducts rather than try and insulate the whole attic? No one says you have to stop at R-5. "

    Depends on what you mean by "...better...".

    In an air leaky older home it's more than just about duct losses- it's about controlling stack effect infiltration, which is necessary for both comfort and efficiency. Air sealing and insulating the attic keeps the first floor warmer and more comfortable while barely moving the comfort level needle on upper floors, since it lowers the amount of air getting sucked in to the lower levels by the warm air escaping out the top.

    In a slab on grade ranch house those effects are much lower than on a 2-story colonial with a full basement- there is some amount of judgement call, but it's generally "better" to bring the ducts fully inside the pressure and thermal boundary of the house, and often the cheapest reliable way to do that is to insulate (vented or unvented) at the roof.

  9. Jud Aley | | #10

    I agree with Dana about leaky houses and controlling the stack effect.

    When insulating an attic roof we typically do a fully vented roof, if you look at the article I attached to my original comment you can see some photos of the rafter vents we typically install, plus many of the older houses we work on, from the 1700 or 1800's once had wood shingle roofs with skip sheathing, this allows additional horizontal air flow and venting, though on a 1950's ranch here in CT 1x6 diagonal T&G sheathing would be more likely.

    Our insulated attic roofs are pretty air tight giving the foam board over the rafters with all joints taped, does a very good job of reducing stack effect.

    I'm skeptical about closed cell foam hot roofs and condensation, try to avoid un-vented roofs as much as possible but it has become very difficult to do with the current R49 code, so as much as I hate spray foam we have done several Hot roofs on new additions to get to R49. I know I can fir down the rafters to 14" and use fiberglass and vent but so far the homeowners have preferred the extra ceiling height sp spray foam it has been.

    And any time we are replacing a furnace or water heater I always advocate for heat pumps over propane or oil.

    1. User avater
      Dana Dorsett | | #11

      >"And any time we are replacing a furnace or water heater I always advocate for heat pumps over propane or oil."

      I'm at the point where I'm advocating cold climate heat pumps even over natural gas in a southern New England climate. Air conditioning loads are going up with climate change, and the cost of natural gas, while at near historic lows right now, is volatile, and if New England is going to meet even it's current RGGI commitments heating homes by burning fossil carbon needs to taper off.

      So far the regional state policies haven't been pushing back on natural gas burning very strongly (opting to promote higher efficiency burners instead), but there is reasonable (if still controversial )case to be made for retiring natural gas infrastructure rather than expanding it. The pure financial case for heat pumps vs. #2 oil or propane is a fairly easy one, but at New England type residential gas & electricity rates it doesn't always pencil out in favor of heat pumps for houses already on the gas grid.

      >"I know I can fir down the rafters to 14" and use fiberglass and vent but so far the homeowners have preferred the extra ceiling height sp spray foam it has been."

      That's a tough one. The carbon emissions "payback" on closed cell spray foam roofing is pretty much "never" for homes in New England heated with heat pumps, even with HFO or water blown foam. The same may soon be true for fiberglass. But cellulose is "sequestered carbon", with a negative carbon footprint, and at R49 has enough thermal mass effect (or "thermal diffusivity") to provide a modest energy use (and peak cooling load) advantage over fiberglass or foam in a roof assembly.

      Rather than furring out the rafters to be able to accomodate R49, converting the rafter to a truss with thin gussets to accommodate R35-R38 cellulose would hit nearly the same energy use and peak load performance points of R49 fiberglass in furred out rafters (due in part to the lower thermal bridging.) But that too is a tough sell in a high labor cost market.

      Exterior foam using RECLAIMED roofing foam can be almost as green as cellulose. It isn't a carbon sink, but it's CO2e footprint is miniscule, since there is no new polymer or blowing agent hit being taken.

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