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Installation of furnace in a basement with a sub-floor

Greenpeace | Posted in Energy Efficiency and Durability on

Thanks to the help of this forum, I have decided to install a high-efficiency furnace (2-stage 60000 btu with multi-speed motor) this weekend.  I can foresee potential risks of water leaking from the pump and the furnace based on the questions discussed in this forum and my research. In order to reduce future headaches, may I ask for your advice on proper installation.

My basement has a sub-floor . Existing furnace, water heater tank and the filter rack are sitting 2.5″ below the sub-floor; making it a little bit inconvenient to change the filter. (have to twist the new filter before putting into the rack). An one-inch filter rack will be installed and the return duct will be changed from 8″ x 24″ to 10 ” x 24″ x 7′.

1. Do you see any advantages or disadvantages in the long-run if I raise the new furnace up by 2″? e.g. I need to use a condensing pump and there may be risks of water leaking from the furnace. would it be a good idea to separate the furnace from the ground?
2. should I use 8″ x 8″ x 2.3″ interlocking bricks or a big patio stone underneath?
3. the sub-floor was created by the previous owner? Could someone tell me what might be the reasons that a sub-floor was put in for the basement? keep the space warmer?
4. The acidic solution will go to the laundry drain and I saw posts about the need to neutralize the solution that would certainly “eat” metal pipes over time if not neutralized. could I put limestone on a soap dispenser to filter the acidic water before it goes to the laundry drain? 
5. could you tell me what I should do to neutralize the acidic water? how often should I do it?
6. If the existing conventional furnace and the old gas water heater tank are using B vent, there is no need to worry about installing a liner in the B vent. Is that right? FYI, I will do a retrofit to replace my steel-framed basement windows. there will be less fresh air going to the basement. please advise if new risks will be created.
7. do you think it is beneficial to change the old gas turnoff valve near the furnace? some contractors offered it but the one coming said it is not necessary.

I really hope that the HE furnace will not create new risks to the safety and structure of the house.

Thank you very much.

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Replies

  1. User avatar
    Dana Dorsett | | #1

    Are you sure a 60K furnace is the right size for this house? That's more than 1.5x the heat load of my 2400' sub-code 2x4 framed antique (+ 1600' of insulated basement), and 2x as big as what most newer 2500' houses would need.

    Before breaking open the crate on the furnace, if you have a heating history on the place run these numbers:

    http://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/out-old-new

    It doesn't hurt to raise the furnace or boiler off the floor, though not usually necessary. If it would make you sleep better, go for it. Patio pavers are fine. Installing a couple inches of EPS and a half-inch or thicker subfloor under the furnace & water heater to bring it up to the rest of the room's floor (a bit higher is OK) would be better.

    Before putting anything on top of the pre-existing subfloor that's 2.5" above a slab, first find out whats under that subfloor. (Insulation? Sleepers? Vapor barrier?)

    1. Alan B | | #2
  2. Alan B | | #3

    "Thanks to the help of this forum, I have decided to install a high-efficiency furnace (2-stage 60000 btu with multi-speed motor) this weekend. I can foresee potential risks of water leaking from the pump and the furnace based on the questions discussed in this forum and my research. In order to reduce future headaches, may I ask for your advice on proper installation."
    Glad we were able to help make a good decision

    "My basement has a sub-floor . Existing furnace, water heater tank and the filter rack are sitting 2.5″ below the sub-floor; making it a little bit inconvenient to change the filter. (have to twist the new filter before putting into the rack). An one-inch filter rack will be installed and the return duct will be changed from 8″ x 24″ to 10 ” x 24″ x 7′.

    1. Do you see any advantages or disadvantages in the long-run if I raise the new furnace up by 2″? e.g. I need to use a condensing pump and there may be risks of water leaking from the furnace. would it be a good idea to separate the furnace from the ground?

    2. should I use 8″ x 8″ x 2.3″ interlocking bricks or a big patio stone underneath?"

    I assume the basement was finished during the furnace's lifetime and instead of raising the furnace and rewiring, repiping and reducting it they just worked around it.
    The advantages of raising it is to get it off damp concrete and make your filter installation easier. I would even add a vapour barrier between the raised floor and the new furnace, plexiglass, vinyl or something durable (and not 6 mil vapour barrier).
    Also raising it helps prevent it frame getting rusted if there were a water leak, i have an unfinished basement and once a pipe broke and the furnace got wet. If it had been raised an inch off the ground it would have stayed dry.

    "3. the sub-floor was created by the previous owner? Could someone tell me what might be the reasons that a sub-floor was put in for the basement? keep the space warmer?"
    Possibly or to support flooring besides concrete. Try to determine non destructively determine what the layers are.

    "4. The acidic solution will go to the laundry drain and I saw posts about the need to neutralize the solution that would certainly “eat” metal pipes over time if not neutralized. could I put limestone on a soap dispenser to filter the acidic water before it goes to the laundry drain? "
    What are your drains made of copper, iron, cast iron, ABS, clay something else?

    "5. could you tell me what I should do to neutralize the acidic water? how often should I do it?"
    That is the focus of another GBA thread

    "6. If the existing conventional furnace and the old gas water heater tank are using B vent, there is no need to worry about installing a liner in the B vent. Is that right? FYI, I will do a retrofit to replace my steel-framed basement windows. there will be less fresh air going to the basement. please advise if new risks will be created."
    Someone else will have to answer this one

    "7. do you think it is beneficial to change the old gas turnoff valve near the furnace? some contractors offered it but the one coming said it is not necessary."
    I would

    "I really hope that the HE furnace will not create new risks to the safety and structure of the house."
    Like what?

  3. User avatar GBA Editor
    Martin Holladay | | #4

    Greenpeace,
    Q. "There may be risks of water leaking from the furnace."

    A. Why? Plumbing fittings shouldn't leak.

    Q. "Could I put limestone on a soap dispenser to filter the acidic water?"

    A. You don't have to jury-rig your own invention. You can buy a condensate neutralizer. For more information, see this Q&A thread: "Where should I drain condensate from a high-efficiency furnace?"

    Q. "If the existing conventional furnace and the old gas water heater tank are using B vent, there is no need to worry about installing a liner in the B vent. Is that right?"

    A. No, that's not right. Appliances have very specific venting requirements. Most high-efficiency furnaces require either a PVC vent or a stainless-steel vent, depending on the appliance. Moreover, if you remove an older furnace from a B vent shared with a water heater, you may discover that the B vent is now oversized for the water heater, resulting in venting problems for the water heater. Get a qualified professional to analyze the venting requirements of both appliances.

  4. User avatar
    Dana Dorsett | | #5

    What HE said!

    An "orphaned water heater" (google it) is a well known backdrafting and flue condensation risk in winter when removing the furnace from the stack.

  5. User avatar
    Dana Dorsett | | #6

    BTW: The fuel-use based heat load on the prior thread posted on 27 September was a bit screwy. With the 18% discount for water use the heating system (apparently) went through 1,839.3 M^3 of gas over 2175 HDD.

    That's 1,839.3/2175 = 0.8456 M3/HDD, or (/24=) 0.0352 M^3/degree-hour. A cubic meter of typical natural gas has 35,494 BTU, so that's ~1250 BTU per degree-hour.

    At 80% efficiency that would be 1000 BTU per degree-hour

    At 70% efficiency it would be 875 BTU per degree-hour

    Toronto's 99% outside design temp is +1F/-17C, which is 64 heating-degrees below the presumptive 65F/18C balance point. Without explanation the spreadsheet attachment to the prior thread used 53 degree (53HDD/day= 53 degrees).

    So at 80% efficiency the implied load at +1F is 1000 BTU per degree-hour x 64 degrees= 65,000 BTU/hr.

    If the old furnace is really 70% efficiency the implied load would be 875 BTU per degree-hour x 64 degrees= 56,000 BTU/hr

    If the 18% estimated deduction for hot water use was low there's still some margin with a 60K furnace, but if it's an overestimate it may not keep up at 1F/-17C

    Similarly with the 70% combustion efficiency estimate- is that the nameplate efficiency, or a derating for age?

    An ASHRAE upsizing-factor of 1.4x would call for an 80K furnace at the 70% efficiency estimate, or a 90K furnace at the 80% efficiency estimate.

    If it turns out it's coming up short don't panic- that's a pretty hefty heat load @ +1F for an average residence, which implies there may be quite a bit of low-hanging fruit left to pluck.

    1. Alan B | | #7

      Thats a very interesting analysis, i am curious what a similar analysis would come up with for my place though i don't want to squat on the OP's thread, if i start a new thread can i ask you to scrutinize my numbers?

    2. Greenpeace | | #8

      HI Dana,

      Thanks for replies. I really appreciate you taking your valuable time to review my heat load calculation. Frankly speaking, I do not really understand these scientific things. Someone posted his heat load calculation on another forum and I just followed his example. I did not really understand the formula and some terminology and numbers.

      Yes, my existing furnace is 115,000 btu at 70% efficiency on the name plate according to contractors. I definitely will miss my 45 year old furnace after installing the HE furnace. It sounds like HE furnace will not give me peace of mind but troubles in future - chance of water leak from the pump or furnace over its life time, backdrafting issues, pipe damage by acidic condensate, costly repairs for blower. Most importantly, I am concerned if it will lead to fire or cost human life?

      The following numbers are what I just blindly copied from the others without knowing what they are and should be.
      1. heat loss consumption at 53 HDD/day?
      2. heating consumption by degree day
      3. Gas energy content 37.5 MJ/m3
      4. energy content of electricity 3.6MJ/hr = 1 KW

      Are there numbers applicable for everybody?

      1.4x upsizing factor - is it recommended for everyone?

      Since the supplier originally advised me to order 80K btu, now I ordered 60K btu based on my heat load calculation. They may easily deny responsibilities if the new furnace does not work properly. Do I need to change my order now? The installation date is scheduled for this Saturday. Hot water use is about 18-20% as laundry is done frequently as well as hot water showers! Does it sound normal for a big household?

      There is no insulation/sleepers between the concrete ground and the sub floor now.
      what vapour barrier should I buy?

      any issues of putting bricks at four corners of the furnace?

      Thank you.

      1. Alan B | | #9

        "any issues of putting bricks at four corners of the furnace?"
        I wouldn't, i would support the entire furnace with a more robust base.

        What foams are good for underslab use, would a thick base of it with plywood on top then furnace on top of that be enough to support a furnace?

      2. User avatar
        Dana Dorsett | | #11

        1: Heating degree-days per day is arithmetically heating degrees, the difference between your 1% outside design temperature (only 1% of all hours over the past 25 years are colder than that), and the presumptive heating/cooling balance point temperature, which is typically ~65F for older, less insulated structures, but ~60F for 2x6/R20, low-E window type construction.

        The 1% outside design temperature isn't the same everywhere. It's +25F in Seattle WA, -41F in Fairbanks AK, and +1F in Toronto ON. At a 65F balance point that would be 40F heating degrees in Seattle, 106F heating degrees om , and 64F heating degrees in Toronto. If the house is in Toronto and the indoor design temp is 68F/20C (about right for a 65F balance point), you'll need to size for 64F heating degrees, not 53F heating degrees. So no, it's not the same for everybody.

        2: The heating consumption per degree day divided by 24 becomes the linear constant. When the difference between the outdoor temp and the balance point temp doubles, so does the heat loss, if it's 10F colder the heat loss is 10x colder than at 1F below the balance point (mostly -kinda). A linear approximation like this isn't perfect, but it's fine for sizing a heating appliance.

        3: Most people use 37 MJ/cubic meter, but the exact energy content of natural gas varies a bit. Using 37.5 MJ isn't "wrong", it's still the right range. Converging cubic meters of gas to MJ is an intermediate step, since heating appliances aren't rated in MJ/hr.

        4:The conversion to MJ/hr to kw only makes sense if your heating appliance is rated in kw (as it usually is in Europe). Since we're talking about BTU/hr, I used the direct conversion from cubic meters to BTUs. (35,494 BTU per M^3, per one source, though as stated earlier, the actual energy content of natural gas varies a bit.

        The methodology behind fuel use load calculations is spelled out in some detail here:

        http://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/out-old-new

        The only thing different in your case is the unit conversions from cubic meters of gas to BTUs.

        " It sounds like HE furnace will not give me peace of mind but troubles in future - chance of water leak from the pump or furnace over its life time, backdrafting issues, pipe damage by acidic condensate, costly repairs for blower. Most importantly, I am concerned if it will lead to fire or cost human life?"

        These are all unnecessary worries. Direct vented heating appliances simply CAN'T backdraft, the venting pipes are made of acid tolerant plastic, the total amount of condensate disposal volumes are quite small and easy to manage, and it's acidity isn't really very high, typically pH 3- 4, about the same acidity range of wine, not nearly as acidic as vinegar.

        A 60K condensing furnace is usually good for 57,000 BTU/hr at high fire, so it WILL cover the calculated 56K load, but not with a lot of margin. It has to stay below +1F for at least a few hours before it would noticeably lose ground. Perhaps ironically, an undersized furnace is actually MORE comfortable than a right-sized furnace, right up to the point where the load is more than what the furnace covers, and it starts to lose ground. If this furnace is undersized, it's not undersized by much.

        There is nothing holy about ASHRAE's 1.4x oversizing factor, which is a compromise between comfort and efficiency when using setback strategies for single-stage heating appliances. For a 2-stage furnace it doesn't need to be that high, nor does it need to be that high if you're not using setback strategies. But since the water use percentage was really a WAG it's probably worth going one step higher. An 80K furnace is good for ~76K of output, which is about a 1.35x upsizing multiplier. That's good enough to cover the error-bars on the hot water use estimate.

        So if it's not too late, go with the 80K furnace. If it IS too late, don't worry about it- it'll still probably cover the load with the house as-is, and with load as high as 56K @ +1F there are usually some cost-effective building improvements still on the table.

        1. Alan B | | #12

          But why did you recommended 60K for this house in the other thread?

          1. Greenpeace | | #13

            Thank you Dana for the explanation although it is still too technical for me. Again, my house is about 1800 to 2000 sq ft in Toronto, 2 storey detached house. House facing NE. The backyard is facing NW - an open area where the public can play soccer. can be windy in winter or spring;. windows are at least 18 years old but will reinstall the basement windows and the patio door soon. have elderly people at home.

            What is the assumed outside temperature of Toronto in this heat load analysis? At what outside temperature 60K furnace will find it hard to cope that will make the people in the house feel cold? please use celsisus as I am not familiar with Fahrenheit.

            I am wondering whether it will violate the code to raise the furnace and not installing it on the concrete floor.

            Thank you.

  6. User avatar GBA Editor
    Martin Holladay | | #10

    Greenpeace,
    Don't panic. You've explained that "I do not really understand these scientific things," and that's OK. I don't think it's a useful exercise for you to try to understand all of the concepts behind heat load calculations and heating system design in the short number of days between now (after your furnace has already been ordered) and the day your furnace is installed. At this point, you'll just have to develop a good relationship with your contractor.

    If you install a new high-efficiency furnace, there is no reason to believe that the condensate pump will leak. There is no reason to believe that you'll have backdrafting issues. Take a deep breath and trust your contractor.

    1. Greenpeace | | #14

      Thank you Marin for your kind words. it is not easy to make a decision for a lay person. Hope in future the manufacturers will have more guidelines for the consumers.

  7. User avatar
    Dana Dorsett | | #15

    >" Again, my house is about 1800 to 2000 sq ft in Toronto, 2 storey detached house. House facing NE. The backyard is facing NW - an open area where the public can play soccer. can be windy in winter or spring;. windows are at least 18 years old but will reinstall the basement windows and the patio door soon. have elderly people at home."

    A load of 56K @ +1F for the described house is an indication that it may have no foundation insulation (?) and leaks excessive air, or perhaps a higher than average amount of window area. Most tight, insulated 2000' houses would run about 40,000 BTU/hr or less.

    >"What is the assumed outside temperature of Toronto in this heat load analysis?"

    I used the 99th percentile temperature bin for Toronto, which is +1F (= -17C ) according to the ACCA's Manual-J tables:

    https://articles.extension.org/sites/default/files/7.%20Outdoor_Design_Conditions_508.pdf

    ^^ See page 24 (p 30 in PDF pagination)^^

    >" At what outside temperature 60K furnace will find it hard to cope that will make the people in the house feel cold? please use celsisus as I am not familiar with Fahrenheit."

    Some people feel cold even when it's 24C in doors, most people are fine with 20C or even 18C (most of those people are from the UK), so that's really not an answerable question. I believe some Canadian codes require being able to maintain 22C at the 99% outside design temperature.

    Assuming the 60K furnace has an output of 57K, according to the analysis it will keep up at a temperature difference of 57,000/875 BTU/F-hr= 65F degrees (36C) below the heating/cooling balance point. The balance point is usually a couple of degrees celsius below the room temperature so it'll be about a 38C difference before it even begins to lose ground. So...

    ...if the indoor temperature is set to 22C, it'll still keep up at (22C - 38C=) -16C...

    ... if the indoor temperature is set to 20C, it'll still keep up at (20C -38C=) -18C.

    >"I am wondering whether it will violate the code to raise the furnace and not installing it on the concrete floor."

    That is NOT going to be a code problem. Furnaces get installed on wooden floors and other platforms all the time.

    1. Greenpeace | | #16

      Thank you Dana for your response. This is really helpful.

      May I know if a furnace will lose 1% efficiency each year?

      The dealer said they will put anti-vibration pads for furnace. Is it good idea?

      Thank you.

  8. User avatar
    Dana Dorsett | | #17

    Degradation by 1% per year would be a pretty extreme case, highly unlikely. It might lose 1% of capacity & efficiency per DECADE, but not much more than that.

    Anti-vibe pads aren't a bad idea though not absolutely essential. Quiet is good.

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