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Replacing 170k BTU 1989 Furnace

Horsefly
Horsefly Member Posts: 6
Greetings from a noob!

We live in the Denver suburbs (~5800 ft elevation) in a 3600 sq ft home, built in 1989. It's single zone, and we still have the original 170K BTU furnace. The gas valve has gotten stuck a couple of times, and I'm thinking we are probably due to replace the furnace. I'm working at getting some proposals, but have one so far. The guy that came out to look at the stuck valve gave me a bid for a 135K BTU 80% efficient Lennox. I've looked on-line (including a cursory search here) and now have some questions:
  1. I'm not completely sure we have room in our furnace room, but if we do does it make sense to consider two dual furnaces (smaller, but maybe more efficient) instead of a single one? What are the tradeoffs?
  2. The guy guessed that my current furnace is something like 60% efficiency, and I have no reason to doubt that. If that is correct, then a new 135K BTU at 80% is pretty close to the 170K BTU at 60%. Does 60% sound like what a 1988/1989 furnace might be?
  3. This contractor also said that we probably needed a DC motor blower to move more air in such a large house. I'm an electrical engineer, but I deal mostly with computer electronics. Still, I'm not sure I get why a DC motor would be better, and I can't see that the lennox he is bidding has a DC motor. Any thoughts?
  4. I know we don't talk price here, but I sure have lots of those, and I will have more once I get more proposals!
Thanks in advance!
Steve

Comments

  • SuperTech
    SuperTech Member Posts: 1,792
    edited March 2018
    He guessed the efficiency? They make tools that real professional would use to measure efficiency. Where did he come up with 135k BTUs? Another guess? Real pros do a room by room heat loss calculation to confirm the size of the equipment needed.

    Sounds like you need a new contractor. This guy sounds like a clown.
  • Ironman
    Ironman Member Posts: 6,869
    edited March 2018
    Do an ACCURATE, scientific heat loss calculation on your house and size the furnace to that. The numbers won't lie if you provide the correct info. SlantFin has a free app that you can down load. I doubt that you would need more than 100k btus output from a furnace.

    The DC motor that the contractor mentioned is actually an electronically commutated motor (ECM). It will save a good bit of energy over the standard PSC motor but it's not necessarily gonna move any more air.

    I'm curious why he didn't offer a 95% + efficient furnace with an ECM that has two stage firing?

    I'd definitely get some more bids.

    Try the blue "Find a Contractor" tab at the top of this page.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    SuperTech
  • Horsefly
    Horsefly Member Posts: 6
    Fair enough. Thanks @SuperTech and @Ironman. I will definitely get more quotes.

    I believe - but I'm not certain - that he said I needed a large enough furnace that it is larger than they make in high efficiency models. I think that's how he got to an 80% efficient Lennox model.

    I know the current furnace has to work pretty hard to heat this place. Two of the 5 bedrooms (unfortunately including our master bedroom) stay quite a bit colder than the rest of the house when it is cold outside, and when the outside temp gets below zero the furnace basically runs all the time. I guess I figured that if the current one is 170K and 60% efficient then a 135K at 80% made some sense.

    Any thoughts about my question on the dual / tandem?
  • Jack
    Jack Member Posts: 1,046
    Actually, the efficiency of pre-92 gas furnaces was, as a standard, about 60% day one and then it went down from there. The codes changes in '92 required a minimum efficiency of 78% on gas furnaces. You could cover up a lot of system mistakes with a high stack temp;) I do not know what your vent (chimney) system is, but the increase in efficiency, in many cases, required that the new furnace required a re-line of the flue. You need to look at the whole system and not only the appliance. That system analysis includes your hot water heaters vent system as well.
    I agree that a proper heat loss should be done done on the home. It does sound like the contractor is sizing the new furnace off the rating plate on the old unit. I would question why the contractor is not offering a high efficiency unit. The non-condensing Lennox is a 78% appliance and the condensing unit will be in the 95% range. I think that a substantial difference. Using the contractors method you can do the math on sizing. I think the upcharge on the DC drive is beneficial. Look a bit further for a contractor. You have plenty of options in that area. History says that in this business, what you buy is not as important as whom you buy it from. Check the after install service history of the contractor/company.
  • SuperTech
    SuperTech Member Posts: 1,792
    edited March 2018
    As far as the dual furnaces go, in many houses it works better and offers better temperature control and balancing. I like to see a two story house with a furnace for each floor. Most of the time it was originally done with one furnace as a way to cut costs. Do you have zoning on your existing furnace? The problem you describe sounds like an air flow issue.

    High efficiency equipment is always an option. Contractors will try to sell you 80% equipment because they think it's easier to install. Don't hire anyone who doesn't perform a heat loss calculation as part of the estimate. Do your homework on who you hire. Make sure they perform combustion analysis as part of the start up procedure.
  • Horsefly
    Horsefly Member Posts: 6
    SuperTech said:

    As far as the dual furnaces go, in many houses it works better and offers better temperature control and balancing. I like to see a two story house with a furnace for each floor. Most of the time it was originally done with one furnace as a way to cut costs. Do you have zoning on your existing furnace? The problem you describe sounds like an air flow issue.

    No we don't have zoning. That's why I was asking about using two units in tandem, not two separate furnaces for two separate zones. I was thinking we need more BTUs than maybe is practical for a single unit.

    You may be right that it may be an air flow problem. Like I said, it is a large house (3600 sq ft main and 2nd story, plus another 1300 sq ft finished in the basement). It's been colder in the master bedroom and one of the other bedrooms (the two most distant rooms from the furnace) for the 20 years we've been here. Not sure what I can do about it.

    In fairness to the contractor that gave me the quote, I emailed him and asked him for a quote, telling him I was going to be out of the country but would look at it when I got back. He didn't really have an opportunity to walk through the house or anything. Maybe I should give him another shot, and still get other proposals.

    Thanks everyone for the comments. I really appreciate it.
  • John Mills_5
    John Mills_5 Member Posts: 935
    Most variable speed ECMs will maintain desired airflow up to about a 1" static pressure which is a pretty lousy duct system. The standard motors (PSC) fall off on airflow pretty fast as the duct system gets more restrictive. That's his thinking on the DC or ECM.

    An existing house can be zoned with a damper system or 2 furnaces but pretty hard to do right. You can look into a 2 stage or 2 levels of heat on a high end 80 or 95% efficient furnace. Good comfort with a good 2 stage thermostat. No way to install a 95% furnace - no way to vent it?
  • Horsefly
    Horsefly Member Posts: 6
    edited March 2018
    Our friends across the street have a similar house (3400 sq ft + 1600 finished basement, and only one furnace), and they got theirs replaced with a 2 stage, 40K-120K 96% efficient Goodman. They say it heats about the same as the old furnace (not sure how big it was), and is quieter. They believe the gas bills were lower since they replaced it in the summer of 2016, but note we also had a couple of milder winters.
  • Horsefly
    Horsefly Member Posts: 6

    Most variable speed ECMs will maintain desired airflow up to about a 1" static pressure which is a pretty lousy duct system. The standard motors (PSC) fall off on airflow pretty fast as the duct system gets more restrictive. That's his thinking on the DC or ECM.

    An existing house can be zoned with a damper system or 2 furnaces but pretty hard to do right. You can look into a 2 stage or 2 levels of heat on a high end 80 or 95% efficient furnace. Good comfort with a good 2 stage thermostat. No way to install a 95% furnace - no way to vent it?

    Thanks for the explanation about the DC motor fans. I had to look up ECM to understand. Sounds like in my situation (bad / long distance air flow) it is good to go for the ECM or DC motors.

    I'll press the estimators that I bring in to look at my flue and see if it supports the newer high efficiency units. We just replaced the water heater, and I seem to remember they said something about it being less demanding on the exhaust than the old one was? I dunno. The water heater was also original from 1989, and when it failed it was a little more obvious than the furnace.
  • Ironman
    Ironman Member Posts: 6,869
    90%+ gas furnaces vent with PVC pipe, usually through the nearest side wall. They cannot be connected to the old chimney. That means the water heater would be "orphaned" to the old chimney which usually presents a problem because the chimney is way too large for it. The solution for that is to replace the water heater with one that can be vented through the wall also. That means a power vented tank water heater or an instantaneous heater. I'd recommend you stay with a tank type - they have far less issues and maintenance.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    SuperTech
  • captainco
    captainco Member Posts: 600
    Actually old furnaces could operate as high as 69% efficiency. The so called 80% induced draft has a maximum efficiency of 72.5%. This requires both to be set up properly with a combustion analyzer. The 90% is close to 90% and is the best deal.
    Water heaters often vent better when furnaces are removed because there is no more dilution air from the idle furnace cooling the flue gases. In most cases furnaces and water heaters operate independently not at the same time. If a water heater isn't venting after a furnace is removed then is wasn't venting when it was there!!
    Dan Foley
  • Ironman
    Ironman Member Posts: 6,869
    captainco said:


    Water heaters often vent better when furnaces are removed because there is no more dilution air from the idle furnace cooling the flue gases. In most cases furnaces and water heaters operate independently not at the same time. If a water heater isn't venting after a furnace is removed then is wasn't venting when it was there!!


    That's just not so. Leaving a water heater orphaned on an over-sized masonry chimney will cause cold stacking and flue gas condensation which will rot the mortar joints and eventually cause the chimney to collapse. The 3" round flue of gas water heater goining into an 8 X 9" or larger clay liner is not up to the fuel gas code.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    HVACNUT
  • JUGHNE
    JUGHNE Member Posts: 9,765
    Unless there is separate ductwork in the basement from each floor, it seems that a single 2 stage condensing furnace would be my choice.

    IIRC twinning 2 furnaces together (parallel ducting) might not work with ECM blowers as they arm wrestle by changing speeds as one affects the other air flow.

    Isn't it always amazing that in a 3600 foot home that the furnace room is so small.

    The cool bedrooms are probably at the end of the duct line, are they over an unheated or cold garage?

    Often second floors are shortchanged on return air openings/ductwork. Do the rooms heat better if the doors are left open to the hallway and then open to the first floor for better return circulation?

    If side wall venting/intake is not feasible for new furnace,
    would it be possible to use existing chimney as a chase for furnace exhaust and combustion air piping and also PVC venting for new power vented water heater?
  • Horsefly
    Horsefly Member Posts: 6
    Yes, the coldest bedroom (our master, as luck would have it) is over the garage. The garage is insulated but not heated.

    I spoke to the contractor this a.m. He is coming by to do a full survey this afternoon. I'll also get at least one more proposal.

    Thanks for all the comments. I do feel like I'm learning.
  • captainco
    captainco Member Posts: 600
    That's just not so. Leaving a water heater orphaned on an over-sized masonry chimney will cause cold stacking and flue gas condensation which will rot the mortar joints and eventually cause the chimney to collapse. The 3" round flue of gas water heater goining into an 8 X 9" or larger clay liner is not up to the fuel gas code.

    If you check the temperature of a water heater above where the idle furnace and water heater connect you will find the temperature is reduced. Remove the furnace and the water heater flue temperature increases and vents better. Water heaters operate most of the year in that great big scary flue all by themselves. Furnaces have never helped them vent better. If the water heater don't vent it is the water heaters problem not the flue. Having worked with contractors in Minnesota, Wisconsin, Michigan etc., they have never had a water heater not vent in a flue by itself when the furnace was removed.

    What test verifies venting? It is not draft or smoke!!

    I have seen hundreds of three story apartments and condos with 3 furnaces and 3 water heaters common vented in the same flue. The main flue is 12-14 times larger than any single water heater vented into it and they do run independently.
  • HVACNUT
    HVACNUT Member Posts: 4,594
    edited March 2018
    Are you able to zone the system?
    Is there only one or two central returns, or does each room have a return?
    If the set up is right, get the heat loss done and look into a 2 stage furnace with an ECM motor and an Arzel zoning system. Obviously the contractor must be familiar with Arzel.