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Community and Q&A

Gas furnace replacement during large renovation

SouthDakotaRoof | Posted in Mechanicals on

I have a 10 year old 60,000 BTU Trane forced air unit located in a freezing vented tiny attic in Zone 6a. It was a diy install by previous owners. Trunk & branch system with every single duct is 6″ flex. No mastic in sight and crap job as a whole. Minimal insulation. This does a surprisingly ok-ish job at heating the main floor but doesnt do much for the basement.  I want a new unit installed in the finished basement and having new indoor ductwork installed inside the warmth. (Mom is taking old unit)

Not one company in this town will do a manual j or the other manuals. One said they would and they faked it, took them 5 min to ‘calculate the J’ 😖 gas is cheap here, electricity is high. And the heating the leaky old hardly insulated 1953 house is beyond insane. A large renovation is poised to take place. Gut to the studs, about R10 continious exterior insulation, r13 cavity fluffy insulation with zip sheathing air sealed & housewrap. (There is currently 0 insulation in the walls) Metal roof with a layer of zip on sheathing to kill truss thermal bridge attic insulated to r49 except at my 2″ of space at exterior wall top plates will be r20 🥺. Attic airsealed and emptied of mechanicals plus booting out the 3 can lights in favor of track lighting. Replacing five of the thirteen 10 year old leaky double glazed vinyl windows with three solid picture windows, 1 glass block, And one window reverting back into solid wall. Replacing 2 of the 3 doors.

I’d say even if the builders do a poor job at my requested extra attention to airsealing and such, this house will still be in a whole new realm of warmth & comfort. 

HVAC crews are all giving me estimates to replace & move the 10 year old 60,000 btu furnace with a new energy efficent 60,000 btu furnace. That has to be wrong. My reading says I should be downsizing it because of the renovations. What do you say? The main floor + finished basement is 1,750 sq ft together. But that # is as a whole, including inside walls and everything. 8ft ceilings on main floor. 7 1/2 ceiling in basement. My gut says buy a 40,000 btu? (But I really dont know anything) idc about the ac, the heat is way more important!! Photo of sticker in existing unit included

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  1. SouthDakotaRoof | | #1

    Sorry the photo is upside down!!

  2. user-723121 | | #2

    I installed a 95% Lennox 2 stage with ECM fan in 2006. It is 66k btu high fire, 45k btu low fire. This house has a design heat load of about 30k btu. I would not have it any other way, this furnace has been fabulous, very quiet and dependable. I did replace the inducer fan a couple of years ago as it was getting noisy, otherwise no repairs. Clean the flame sensor every 3 years or so and change the furnace filter regularly. The sizing of this furnace allows for a nightly setback with a quick recovery in the morning.

  3. tommay | | #3

    Well if you do all those upgrades maybe you won't need a furnace. I see nothing wrong with replacing the furnace with one that has the same output especially if it is supposedly more efficient. How much cheaper would a smaller unit be? Is the smaller one readily available or special order? Realize no matter what size unit you install its still a time/ temperature relationship. eg. It's either run the 60 for 30 min or the 40 for 50 min. to deliver the amount of BTU's needed to replace or raise the temperature of a room. And what if you or future owners decide to expand.....

  4. Expert Member
    Dana Dorsett | | #4

    Start by running a fuel use based load calculation of the house in the "before renovations" condition:

    That will reflect both the heat load of the house in the where-is-as-is condition, including the parasitic load of the air handler and ducts being above the insulation.

    If it is a 2x4/R0 type house with 6" of fluff in the attic and ducts & air handler above the insulation, a more air-tight R13-R15 + R10 continuous insulation would cut the wall losses by about 2/3 - 3/4 , maybe more.

    Oversizing a hot air furnace by more than 1.4x (the ASHRAE recommended factor) results in comfort issues. It is NOT simply "....a time/ temperature relationship..." as Tom would have it. The lower duty cycle of a 2-3x oversized furnace will very often fail to fully heat the rooms at the far end of the duct runs, with much bigger room to room temperature differences. Gross oversizing also results in temperature overshoots of the setpoint, followed by an extended off cycle that lets drafts build. At 1.4x oversizing the duty cycle will be (1/1.4 =) ~70% duty cycle at the 99% outside design temp, delivering the more comfortable "warm summer breeze" effect at lower cfm (= less wind-chill), less noise, etc. As it gets even colder the duty cycle increases further.

    Take the time to read Nate Adams' freebie book chapters and view the short videos on the topic. He has made a business out of fixing comfort issues in older houses, and a big part of it is right sizing the equipment. Too often the only way comfort can be achieved requires getting rid of (even brand new & efficient) oversized equipment:

    Using a computer spreadsheet tool, start an I=B=R type load calculation of the "after upgrades" picture. If you need assistance figuring out the U-factor of the new-improved assemblies, post a question about it here. Manual-J is just a superset /refinement of the classic I=B=R methodology. If working from an I=B=R type calculation the oversize factor from the calculated load should usually be no more than 1.2x. The basic methodology lives here: (generic infiltration rules are usually WAY beyond reality in a tightened up house, so be extra-aggressive on this part.)

    1. tommay | | #5

      "It is NOT simply "....a time/ temperature relationship..." as Tom would have it.".......well then perhaps you can explain why or how it isn't.....and what are these comfort issues of which you speak?
      All your numbers and assumptions are useless until you have the duct design figured out......can you do that?

      1. Expert Member
        Dana Dorsett | | #6

        >""It is NOT simply "....a time/ temperature relationship..." as Tom would have it.".......well then perhaps you can explain why or how it isn't.....and what are these comfort issues of which you speak?"

        Comfort is more than simply delivering an average temperature or a requisite number of BTUs every hour. Duty cycle and air speed both matter (as partially described in my prior post). A ducted air heating system is most comfortable when it's delivering warm air at a moderate or low cfm, not the intervals between on cycles. This becomes even more important as the outdoor temperature drops. A low duty cycle/high BTU/hr high cfm system sees much bigger temperature swings than a system that's running a 70% duty cycle at the 99% outside design temp.

        While the low duty cycle/high BTU rate system will deliver the same AVERAGE temperature it's a bit like standing next to an open fire when its sub-zero F outside. You can easily find the distance from the fire where the average temp is right, but the average is meaningless if your face is frying while your ass is freezing (or conversely.) Meeting the average temp with a much lower differential is more comfortable.

        Do take the time to look at Nate Adams' stuff, which goes in to specific examples and explains it in more detail. Lately he has become a fan of right-sized modulating air source heat pumps, which deliver nearly continuous on-cycles and superb comfort when right sized.

        >"All your numbers and assumptions are useless until you have the duct design figured out......can you do that?"

        Sure I can, but the starting point for that is the room by room load numbers. Pieces of the existing ductwork may or may not be re-useable, but there is no down side to low duct velocity from "oversized" ducts as long as those ducts are inside the thermal envelope of the house.

        1. tommay | | #7

          Well comfort is relative, so any detailed explanation is moot. If you are saying to use low velocities with warmer air..... does that not represent a time / temperature relationship....?
          And yes a room by room load calculation is necessary to determine the flow and therefore what the duct size should be, but as always it is not until the system is in place that you can make a better assessments and make adjustments accordingly. A lot easier to slightly close a damper than to realize the furnace isn't putting out enough BTU's.
          Another factor is how the home owner is going to use the house. Are they ones that use timer t-stats and want the house to heat fast on their way home or do they shut off other parts of the house and need to heat them fast if they decide to use the space. Are they stay at home people and want constant temps. There can be many personal factors involved that can also contribute to system operation and therefore the type of system and controls that apply. As always, more complicated....more problems.

  5. Expert Member
    Akos | | #8

    In older houses like this the basement is cold not because of furnace issues but because of air leaks. Stack effect tends to move cold air into the basement through the multitude of gaps older houses have. Throw in there a couple of leaky windows and you have a cold basement. Generally once the house is air sealed and the basement is insulated, it will need very little heat to stay comfortable even in the coldest winter days.

    A small house with a deep energy retrofit like you are proposing will be well bellow the smallest standard furnace you can find which is 40000 btu around me.

    If you want a "right sized" unit, my guess is you have to look at a hydro air coil supplied by either a high efficiency water tank or a tankless water heater.

    Something like a Rheem RHWB 40000btu hydronic furnace paired with a condensing tankless unit would work.

    The unit is rated at 40kBTU but unlike a regular furnace, you can reduce the water flow and air flow through it and get lower BTU to better match your house.

    These tend to be very compact, you can get away with a much smaller utility room than a standard water heater and furnace combo.

    P.S. These work very well if you don't have very hard water. If you have hard water, I would not use this setup.

    1. SouthDakotaRoof | | #9

      That was impressive. Thank you so much for taking the time to help me. 🥰

      1. Expert Member
        Dana Dorsett | | #10

        There are hydro-air solutions suitable for almost arbitrarily low heat loads. FirstCo has literally dozens of models suitable for use with condensing water heaters.

        There are also a few low output condensing furnaces that are easy to use. The tiny Detton Chinook series fits in a medium sized cabinet, and uses tiny flexible ducts small enough to be routed through non-structural 2x4 walls:

        If the place is also going to be air conditioned, there are cold-climate heat pumps that can be suitable too.

        How big is this house?

        Are the basement/foundation walls currently (or soon to be) insulated?

        1. SouthDakotaRoof | | #11

          Main floor plus basement is about 1560 sq feet. The Basement is insulated the old/wrong way. Fiberglass directly on concrete at probably R 10-R15 zone, covered with poly. I'm aware this is what you do if want to start farming mold. As I have no interest in cultivating lush fields I will be taking down walls and replacing with foam board. This project I intend to do myself though instead of hiring out l. So I'm sure it will be a extended slow affair.

          1. Expert Member
            Dana Dorsett | | #12

            As lousy as rules of thumb are, but a 2x4/R13 + R10 continuous insulation reasonably tight house with new code-min windows house and a code-min R15 c.i. basement typically comes in with a heat load around 10-12 BTU/hr per square foot @ 0Fcounting just the above-grade floors, to maybe 14-15 BTU/hr per square foot @ -10F.

            Assuming a design temp of -10F and ~800 square feet (about half the 1560 square feet total) is above grade you're probably looking at a heat load of about 12,000 BTU/hr, give or take. Going with ASHRAE's recommended 1.4x oversize factor you'd be looking at a furnace with an output of about 12,ooo x 1.4= 16,800 BTU/hr to have enough capacity to cover cold snaps and to be able to use overnight setback strategies.

            A Manual-J or I=B=R load calculation at your real 99% outside design temp would be more accurate & useful. Online tools such as or coolcalc.come can be useful, but be super-aggressive on what you put in for R-values/U-factors and especially air tightness or those tools will oversize by quite a bit. (loadcalc doesn't have as many window or wall assembly options, making it less useful than coolcalc in this instance.)

            Using any computer spreadsheet to do an I=B=R type and inserting your own U-factors for wall, window, door & ceiling U-factors may be easier and better than less-flexible online Manual-J tools. A primer on how that is done is found here:


   (In a tight don't use more than 100 cfm for the air leakage/ventilation part unless you are actively ventilating at higher rates than that for some reason. A 50cfm number is more likely.)

            Use 68F or 70F as the heating design temp, and find a nearby location on this list for the 99% outside design temp:


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