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  1. #1

    How reliable is HVAC-Calc Residential?

    Kind of following up on an earlier thread with a slightly different question.

    I could not find a contractor willing to do a load calc for me, nor do I have the patience to screen them, so I did my own using HVAC-Calc, even accounting for kneewalls, insulating air pockets, kitchen cabinets, and future work. I figured nobody knows my house as well as I do, and with the cost of energy, I don't want my system supersized.

    The final number surprised me: just under 60K BTU/H for an uninsulated 90 year old, 2000 square foot stick-and-shingle home in the Seattle area. It's surprising because the 22 year old locomotive in the basement says 105K @ a measured 78% efficiency (~80K). On the other hand, at nearly 30 BTU/sq. ft., it's almost 3X the published target for well-built new houses here.

    Can I trust the software to be right, assuming my inputs are correct? I'm considering a new 75K unit @ 92%, giving me a 15% margin before insulating.

    Thanks.

  2. #2
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    Its as good as the data entered into it.

    My brand doesn't have a 75,000, it comes in 60,000, then 80,000.
    Got some old homes same size and a bit bigger with 80,000BTU furnaces. They heat fine.
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  3. #3
    I've run HVCALC on two houses that I've owned, and my Mom's condo.

    For my 1950's split level in New Jersey HVCALC actually came in on the
    high side. This encouraged me to buy a 90% 120,000 BTU input furnace,
    where a 100,000 would have been alright. I think the reason was there
    was actually much better insulation than was apparent and the multiple
    layers of siding, which couldn't be put into HVCALC really helped. This
    house was about 1800 ft^2.

    For my initially totally uninsulated glass 1960's home here in San Jose it
    came out pretty close. This time I assume zero insulation and was right.
    And I put in a 80% 100,000 btu in a house of 1700 ft^2.

    For my Mom's 1978 originally all electric condo ( actually just one common
    wall ) which is pretty well insulated HVCALC came out light. It was fortunate
    the contractor refused to install a unit as it projected. Still the 80,000 BTU
    80% unit for 1400 ft^2 is too big.

    It sounds to me like you are on the light side. If you turn your thermostat
    down at night, despite all you read on this board, you will need a bigger
    unit to recover on cold mornings. The reality is in mild climates like San
    Jose and Seattle that if you want to be able to recover quickly you still
    need a pretty big furnace.

    The other thing to point out is that both the units I bought here in San Jose are two stage. That helps quite a bit if the unit is too large.

    So my personal belief is oversizing on a furnace is not a diaster. While
    undersizing is. Now I'll be bombarded.

    I do concede in humid climates ( which San Jose is not ) proper sizing
    of A/C units is important for dehumidification.

  4. #4
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    Quote Originally Posted by caterpillar View Post
    The final number surprised me: just under 60K BTU/H for an uninsulated 90 year old, 2000 square foot stick-and-shingle home in the Seattle area. It's surprising because the 22 year old locomotive in the basement says 105K @ a measured 78% efficiency (~80K). On the other hand, at nearly 30 BTU/sq. ft., it's almost 3X the published target for well-built new houses here.

    Can I trust the software to be right, assuming my inputs are correct? I'm considering a new 75K unit @ 92%, giving me a 15% margin before insulating.

    Thanks.
    I suspect a right-sized unit will run continuously, but do the job, at the low design temp for Seattle. Old oil furnaces were typically oversized.

    Try running some what-if versions of your house, lowering the R values and seeing if the answer changes much.

    -HF

  5. #5
    Join Date
    Sep 2006
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    1,253
    caterpiller,

    Unless you have had a blower door test done on the home, the figure you used for infiltration is a total guesstimate. Infiltration has a significant impact on your heat load. Play with that number a bit to see what it does to the load. You may even want to have a test done before you finalize your decision.

  6. #6
    Join Date
    Sep 2005
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    Texas
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    2,793
    HVAC-CALC is not ACCA approved, most municipalities require approved software.....and yes I own HVAC-CALC also

  7. #7
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    Location
    PA
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    Quote Originally Posted by bobRitchie View Post
    I've run HVCALC on two houses that I've owned, and my Mom's condo.

    For my 1950's split level in New Jersey HVCALC actually came in on the
    high side. This encouraged me to buy a 90% 120,000 BTU input furnace,
    where a 100,000 would have been alright. I think the reason was there
    was actually much better insulation than was apparent and the multiple
    layers of siding, which couldn't be put into HVCALC really helped. This
    house was about 1800 ft^2.

    A 1950s 1800 sq ft shoudn't have needed 50 BTU per sq ft to be alright.

    For my initially totally uninsulated glass 1960's home here in San Jose it
    came out pretty close. This time I assume zero insulation and was right.
    And I put in a 80% 100,000 btu in a house of 1700 ft^2.

    For my Mom's 1978 originally all electric condo ( actually just one common
    wall ) which is pretty well insulated HVCALC came out light. It was fortunate
    the contractor refused to install a unit as it projected. Still the 80,000 BTU
    80% unit for 1400 ft^2 is too big.

    What size did HVAC calc say it needed. Houses with electric baseboard are easy to double check the calc on.

    The reality is in mild climates like San
    Jose and Seattle that if you want to be able to recover quickly you still
    need a pretty big furnace.

    And how quick is quick. At 0F outside temp, your furnace isn't really suppose to recover 10 in 30 minutes. An oversized furnace will have a few short cycles after recovering quickly, since the walls, and contects don't heat up as quick as the air.

    The other thing to point out is that both the units I bought here in San Jose are two stage. That helps quite a bit if the unit is too large.

    You are aware that many 2 stage furnaces are not as efficient in first stage as they are in second, aren't you? So over sized 2 stage furnaces can cost more to heat with.

    So my personal belief is oversizing on a furnace is not a diaster. While
    undersizing is. Now I'll be bombarded.

    Over sizing like you are suggesting tends to make people turn their stat higher to be comfortable, making them use more fuel, costing them more money to heat their house.

    I do concede in humid climates ( which San Jose is not ) proper sizing
    of A/C units is important for dehumidification.
    Check the temp rise on your replacement units.
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  8. #8
    Originally Posted by bobRitchie
    I've run HVCALC on two houses that I've owned, and my Mom's condo.

    For my 1950's split level in New Jersey HVCALC actually came in on the
    high side. This encouraged me to buy a 90% 120,000 BTU input furnace,
    where a 100,000 would have been alright. I think the reason was there
    was actually much better insulation than was apparent and the multiple
    layers of siding, which couldn't be put into HVCALC really helped. This
    house was about 1800 ft^2.[/COLOR]

    A 1950s 1800 sq ft shoudn't have needed 50 BTU per sq ft to be alright.

    As I said it was definately way oversized. However I was not eaten alive
    by gas bills. However it was probably even more difficult to maintain
    even temperatures in the lower level of the split than it would have
    been with a correctly sized unit. Its funny my 1950's split in Poughkeepsie
    NY had next to nothing for insulation, but my similiar house in Berkeley Heights apparently had good insulation in the walls, and under the attic
    floorboards. ( Something I found out when I did some tearing up of things
    after the furnace was installed. )


    For my initially totally uninsulated glass 1960's home here in San Jose it
    came out pretty close. This time I assume zero insulation and was right.
    And I put in a 80% 100,000 btu in a house of 1700 ft^2.

    For my Mom's 1978 originally all electric condo ( actually just one common
    wall ) which is pretty well insulated HVCALC came out light. It was fortunate
    the contractor refused to install a unit as it projected. Still the 80,000 BTU
    80% unit for 1400 ft^2 is too big.

    What size did HVAC calc say it needed. Houses with electric baseboard are easy to double check the calc on.

    It had a heat pump originally. I can't find the one I did for her, its probably
    at her place. For my 1700 ft^2 home in San Jose with design temps
    of winter 30 and summer 96 with the original case of only a tiny bit
    of attic insulation, tons of single pane glass, an uninsulated Living room
    ceiling I got 50,000 BTU AC 66,000 BTU Heat.

    I put in a 4 ton SEER 12 Trane A/C and a 80% 100,000 BTU input
    furnace. I have since added attic insulation and double pane windows.
    I think HVCALC was pretty close. But even with that size furnace it
    takes more than an hour to recover a 7 degree setback on the coldest
    mornings, which here are in the low 30's. The A/C will hold 75 degrees
    when it is over 100. An event that normally happens once a year, but
    has happened several times this year already.





    The reality is in mild climates like San
    Jose and Seattle that if you want to be able to recover quickly you still
    need a pretty big furnace.

    And how quick is quick. At 0F outside temp, your furnace isn't really suppose to recover 10 in 30 minutes. An oversized furnace will have a few short cycles after recovering quickly, since the walls, and contects don't heat up as quick as the air.

    The other thing to point out is that both the units I bought here in San Jose are two stage. That helps quite a bit if the unit is too large.

    You are aware that many 2 stage furnaces are not as efficient in first stage as they are in second, aren't you? So over sized 2 stage furnaces can cost more to heat with.

    The specs on my 80% Trane say it has the same efficiency on both
    stages. I would think the first stage would inherently be more efficient
    since you have more heat exhanger area per BTU. The trick with an
    80% is to prevent condensation in the flue on low stage.


    So my personal belief is oversizing on a furnace is not a diaster. While
    undersizing is. Now I'll be bombarded.

    Over sizing like you are suggesting tends to make people turn their stat higher to be comfortable, making them use more fuel, costing them more money to heat their house.

    If you have a poorly insulated house, you save quite a bit from turning it
    down at night and when you are not there. When you turn it up it runs
    for a good solid period which is quite efficient. If you have a super-insulated
    modern house it takes a long time for the temperature to drop when you
    set it back, and there is probably little point in setting it back when you
    leave for several hours. But if you are going to set back, you want a unit
    that can recover pretty fast. In dry climates like this one setting A/C
    up when you leave makes sense, and turning it down when you get home.
    This argues for a bigger A/C. In humid climates you want a small A/C running
    a lot for the dehumidification.



    I do concede in humid climates ( which San Jose is not ) proper sizing
    of A/C units is important for dehumidification.

  9. #9
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    2 stage units do get the same AFUE rating in both stages.(Its based on a few unreal parameters.)
    That is not a combustion efficiency though.
    The flame is much cooler, so heat transfer rate from flame to HX is reduced.

    In first stage, they lose a few percent. Some more then just a few percent.
    And some people have found that their oversized 2 stage cost less to use when it is set to work as a single stage by going to second stage right away.(thses are usually oversized by 40 plus percent)

    If you think of it sometime. When your at your mothers place. See if you can fnd that load calc, and then if you can find out how much aux heat the HP had.
    See how close the calc and the HP were. Many contractors are afraid of complaints that the furnace doesn't heat any quicker then the HP did, so they size the furnace bigger.

    A furnace that takes an hour to recover 7° at OD design temp, is sized about right. And uses less fuel then one that does it in 30 minutes.

    I have an oversized oil furnace. If I fired it at its full rating, I could recover 10 in 30 minutes. But I have it down fired, and use 30% less oil.

    I start it a little sooner in the morning to get the house up to temp.
    Simple and economical.
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  10. #10
    Quote Originally Posted by beenthere View Post
    2 stage units do get the same AFUE rating in both stages.(Its based on a few unreal parameters.)
    That is not a combustion efficiency though.
    The flame is much cooler, so heat transfer rate from flame to HX is reduced.

    In first stage, they lose a few percent. Some more then just a few percent.
    And some people have found that their oversized 2 stage cost less to use when it is set to work as a single stage by going to second stage right away.(thses are usually oversized by 40 plus percent)


    I didn't know that the first stage was less efficient. I guess I
    just believed the AFUE ratings.

    But if what you say is true, that getting rid of the first stage:

    Is saying you are better off with a massively oversized, but slightly
    more efficient unit than with a smaller unit that is less efficient.

    Which would say unit efficiency is everything, and oversizing doesn't
    hurt much.

    Because by deleting the first stage use, you are effectively going to
    a more oversized, but you say more efficient unit

    If this is true the only purpose of two stage units is to get more
    even heat??


    If you think of it sometime. When your at your mothers place. See if you can fnd that load calc, and then if you can find out how much aux heat the HP had.
    See how close the calc and the HP were. Many contractors are afraid of complaints that the furnace doesn't heat any quicker then the HP did, so they size the furnace bigger.

    The HP was actually converted into an A/C only unit sometime in the mid
    80's when the original gas furnace was brought in. What the unit had when
    my Mom bought it was a 30 year old HP being run as A/C and a 20 year old
    gas furnace. The HP auxillary heat was discarded in the mid 80's.
    Most of my Mom's neighbors still have HPs. The old folks hate them because
    they blow cool air in the defrost mode. My Mom with her gas furnace is
    the envy of the neighborhood.

    xxxx

    A furnace that takes an hour to recover 7 at OD design temp, is sized about right. And uses less fuel then one that does it in 30 minutes.

    I have an oversized oil furnace. If I fired it at its full rating, I could recover 10 in 30 minutes. But I have it down fired, and use 30% less oil.

    I start it a little sooner in the morning to get the house up to temp.
    Simple and economical.
    junk to make accept ....

  11. #11
    Quote Originally Posted by mchild View Post
    caterpiller,

    Unless you have had a blower door test done on the home, the figure you used for infiltration is a total guesstimate. Infiltration has a significant impact on your heat load. Play with that number a bit to see what it does to the load. You may even want to have a test done before you finalize your decision.
    I have not had a blower test done, nor do I think I will, but I selected the leakiest setting that HVAC-Calc would allow. IIRC, that's 1.2 ACH.

    Quote Originally Posted by beenthere View Post
    I start it a little sooner in the morning to get the house up to temp.
    Simple and economical.
    This is exactly what I assumed one would need to do with a furnace sized closer to actual loss than what I have now. Last winter, the locomotive would kick on at 5:30, and have the house from 57 to 67 in about an hour on cold days (35-40F). I figured I might have to now start at 4:30 or 5, but use less energy despite that. Or, just raise the overnight low a few degrees.

  12. #12
    Join Date
    Sep 2006
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    Quote Originally Posted by caterpillar View Post
    I have not had a blower test done, nor do I think I will, but I selected the leakiest setting that HVAC-Calc would allow. IIRC, that's 1.2 ACH.
    I used one of the default settings (I thought it was the leakiest at 1.0ACH) and found it to be way more than I actually had (more like .25ACH) so the calc was significantly overstated.

  13. #13
    Quote Originally Posted by caterpillar View Post
    I have not had a blower test done, nor do I think I will, but I selected the leakiest setting that HVAC-Calc would allow. IIRC, that's 1.2 ACH.



    This is exactly what I assumed one would need to do with a furnace sized closer to actual loss than what I have now. Last winter, the locomotive would kick on at 5:30, and have the house from 57 to 67 in about an hour on cold days (35-40F). I figured I might have to now start at 4:30 or 5, but use less energy despite that. Or, just raise the overnight low a few degrees.
    I would say for my taste your locomotive is about the right size.
    I think you are going to be suffering to save pennies. And if you
    end up reducing the setback since the new unit can't recover fast
    enough you end up paying more for gas than you need.

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