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.

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.

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.

Quote:

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Originally Posted by caterpillar

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

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.

HVAC-CALC is not ACCA approved, most municipalities require approved software.....and yes I own HVAC-CALC also

Quote:

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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.

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 0°F 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.

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 0°F 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.

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.

Quote:

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Originally Posted by beenthere

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 ....

Quote:

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Originally Posted by mchild

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:

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Originally Posted by beenthere

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.

Quote:

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Originally Posted by caterpillar

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.

Quote:

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Originally Posted by caterpillar

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.

2 stage is for comfort, not savings. So over sizing a 2 stage effectly means you paid for what your not getting.
So it becomes a waste of money.

If you use a massive furnace for quick recovery. You just putting more money out the chimney/flue.

A larger then needed furnace, requires larger duct. More money up front.
If the old duct is left inplace. Then your heat rise across the HX is higher then it should be, and you lose efficiency from that. Your flue temp goes up, meaning more money wasted.

Its a balancing act between size and comfort.

The earlier start, will help the walls and furniture to warm up to room air temp, instead of remaining cooler.

Just because the room air gets to temp quickly, doesn't mean the furniture warmed up that quick also.

Quote:

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Originally Posted by beenthere

2 stage is for comfort, not savings. So over sizing a 2 stage effectly means you paid for what your not getting.
So it becomes a waste of money.

Gee you busted my bubble, I was happy that I never got out of
first stage, except in recovery mode. However it makes me even
less happy with my thermostat, which by trying to avoid overshoot
insures that a lot of recovery is done by stage 1.

If you use a massive furnace for quick recovery. You just putting more money out the chimney/flue.

A larger then needed furnace, requires larger duct. More money up front.
If the old duct is left inplace. Then your heat rise across the HX is higher then it should be, and you lose efficiency from that. Your flue temp goes up, meaning more money wasted.

Isn't the duct size pretty much governed by the A/C. If you have
adequate duct for A/C you will have adequate duct for a massive furnace?

Its a balancing act between size and comfort.

The earlier start, will help the walls and furniture to warm up to room air temp, instead of remaining cooler.

Just because the room air gets to temp quickly, doesn't mean the furniture warmed up that quick also.

---

...

Duct size is determined by the operation that requires the most CFM.

An 80,000BTU 90% furnace at 50* rise requires 1212 CFM, if it had a 2.5 ton A/C, the A/C may only need 950 CFM. to meet sensible and latent load requirements.

So the duct in that instance should be sized for the heat.
But many forget that, and size for the A/C. And would end up with a heating temp rise of 70°F, which is the extreme limit of many furnaces.
So as the filter becomes dirtry, it over heats, shortening the furnaces life span.

And yes. Over sized furnaces, tend to over shoot stat set point.

Quote:

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Originally Posted by bobRitchie

I would say for my taste your locomotive is about the right size.

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How about if we look at it this way:

The locomotive is delivering roughly an effective maximum 82K BTUs, which is just about the same as a new 90K @ 92%. I have 7 runs in the house, and installing that bigger unit will be pushing 1200-2500 CFM through them in order to keep the HX rise in check (I know this has been a problem with the locomotive for the house's previous owners, because the blower motor was downsized from 3/4hp to 1/3hp to reduce noise. Come to think of it, the BTU output would have been derated to match, no?)

Anyway, my experience is that the locomotive cycles about 2-3x an hour on most average winter days, and 1-2X on the cold ones, at whatever rate it's actually burning at. Right away, that suggests it's over, doesn't it?

Second, I have a relatively modest sized house. At 2000 square feet including the basement, it's half the size of an average McMansion, and furnace manufacturers must be offering sizes roughly in proportion to homes. So, a match between a house in the lower end of the scale (in a mild climate) to a furnace in the upper end of the scale, with the largest blower offered, just doesn't feel right.

I think you need to keep at least the BTU's you've got unless
you are going to immeadiately massively insulate.

I live in a tiny house here in San Jose now. But in Texas I could
afford, but did not buy a McMansion.

I never saw a McMansion with fewer than three furnace A/C units.

How much sq ft is basement.

Quote:

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Originally Posted by beenthere

How much sq ft is basement.

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~400. Currently air heated partially finished, on it's way to being fully finished and insulated. Two 220/30A circuits for electric heat available there, and perhaps preferable since that space is not used as often and can be zoned separately.

So your really only talking about conditioning 1600 sq ft then.

Maybe you have real cheap electricity there because of hydro.

But generally electric resistance heat can eat you alive.

Remember heat pumps are at least 2X as efficient as electric
resistance.

So if you are going to heat the basement more than occaisonally,
you want to do it with your main furnace.

In your area you might need to heat the basement just to
keep it from getting mouldy in the winter.

Quote:

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Originally Posted by beenthere

So your really only talking about conditioning 1600 sq ft then.

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Perhaps. The basement has a laundry, TV room, and guest suite. It certainly does not need a morning recovery from the overnight low unless someone is staying down there - otherwise the whole space only gets used in the evenings. It's the best-insulated space in the house, but its also tempting to put it on its own zone for the reason above.

Winter electric here is 3.76 cents/kWh for the first 16kW, twice that afterwards. Is that cheap? We're taking an electric HW heater off line, and half the basement is already electric, so I think it will roughly even out. OTOH, I like the simplicity of heating from a single source.

Would this make sense?: Run a couple of ducts in basement but keep existing electric in place. If more BTUs are needed upstairs, pinch off basement ducts, reducing the effective volume of the space for the furnace to heat (by some percentage that's probably not as simple as 2000 feet minus 400 feet :confused:). If this occurs regularly, install electric in rest of basement. Yes, the runs down there will be wasted, but we're talking maybe 50 feet of duct.

A supply or 2 to that 400 sq ft won't hurt.
It don't take much to heat a basement.

Just curious, how did you list your first floors floor.
Over conditioned space?
Or over unconditioned space?

Quote:

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Originally Posted by beenthere

A supply or 2 to that 400 sq ft won't hurt.
It don't take much to heat a basement.

Just curious, how did you list your first floors floor.
Over conditioned space?
Or over unconditioned space?

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Both. We have an unheated, insulated and vented crawl space that's under ~30% of the first floor - the heated basement is under the remaining 70%, and much of its ceiling is insulated. I made sure to factor this into the load calc, since that part of the first floor gets much colder.

Ok.

Wow I envy your electric rates.

Here we start at 11 cents / kilowatt for the first tiny quantity
and then grow to 36 cents for heavy useage.

With electricity that cheap you should do a heat pump!
Forget gas. I assume gas is not cheaper than the rest
of the country?

I guess it really is cool enough that you don't need A/C
there, but you do get A/C for free when you do a heat
pump.

The truly deluxe solution is a heat pump with a little
gas back up for recovery and when its real cold.

I average 10.5 cents per KWH. :)

Quote:

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Originally Posted by bobRitchie

With electricity that cheap you should do a heat pump!

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1) In order to get the line put in for free, which is now done, I had to commit to 2 gas large appliances.
2) Definitely don't need A/C
3) Don't want the giant wart in my yard.

>>Winter electric here is 3.76 cents/kWh for the first 16kW, twice that afterwards. Is that cheap?

That is surreal. These days the average in the US is about 10-12 cents/kwh I believe, and I am paying right around 17 cents/kwh in Texas. Texas is a problem state IMO as is California, and the cheaper rates tend to be in low growth Midwest states which have not had deregulation of any sort, nor a large building program.

Do you pay the lower rate for the 1st 16,000 kilowatt-hours in a month? And some houses use *more* than that? Quite a large usage there, my max is around 2,000 kwh for a summer month.

Count yourself lucky, but from your level I would only expect the rates to go up.

Best wishes -- Pstu

I didn't realize that your locomotive was running on oil.

Everywhere I have ever lived gas company's charge dearly for
new hookups because they provide the most economical source
of heat.

It sounds like in your area, electric, at least if used in a heat pump
might be substantially cheaper.

The down sides of heat pumps are slow recovery times, and blasts
of cool if not cold air when it goes into a defrost cycle. I had one
in Dallas TX.

I remember from when I lived in the Northeast, a radio commercial
for oil heat that had two woman talking. The upshot is they would
not allow gas into their home because of the danger. I guess a
neighborhood blows up now and then from gas, but the risk is pretty
low.

You don't want the wart of a unit in your yard, but are O.K. with the
mess that will result from digging for a new gas line.

I've been very chatty today since our computers here at work
have been hosed...

Quote:

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Originally Posted by bobRitchie

I didn't realize that your locomotive was running on oil.

Everywhere I have ever lived gas company's charge dearly for
new hookups because they provide the most economical source
of heat.

It sounds like in your area, electric, at least if used in a heat pump
might be substantially cheaper.

The down sides of heat pumps are slow recovery times, and blasts
of cool if not cold air when it goes into a defrost cycle. I had one
in Dallas TX.

I remember from when I lived in the Northeast, a radio commercial
for oil heat that had two woman talking. The upshot is they would
not allow gas into their home because of the danger. I guess a
neighborhood blows up now and then from gas, but the risk is pretty
low.

You don't want the wart of a unit in your yard, but are O.K. with the
mess that will result from digging for a new gas line.

I've been very chatty today since our computers here at work
have been hosed...

---

The gas contractor came and went in 3 hours and used a mechanical mole to tunnel the 30' line. Total damage to my yard was 3 square feet of displaced sod, which they replaced. Cost me $80 in fees and permits, plus the useage commitment. Couldn't be happier with the work. They even agreed to let me hide the meter in some shrubs since it gets read remotely.

Heat pumps are as rare as insulation in old houses here. The lots are small, and many people care about every square foot. For the 2 weeks a year that you need A/C, a window unit does just fine. I can run mine 12 hours a day, and it barely registers on my bill.

The City owns the power utility here, and they're vertically integrated right back to the hydro dams, which were paid for 20 years ago. Now, ask me about our property taxes, and we can get REALLY off-topic.

Quote:

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Originally Posted by bobRitchie

I remember from when I lived in the Northeast, a radio commercial
for oil heat that had two woman talking. The upshot is they would
not allow gas into their home because of the danger. I guess a
neighborhood blows up now and then from gas, but the risk is pretty
low.

---

This marketing tactic is called FUD. Fear, uncertainty, and doubt. It is used by established markets to defend against disruptive technology. It is seldom more than a holding tactic against losing market share too quickly, unless the old market can truly reinvent itself in some way and become disruptive technology itself.

Quote:

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Originally Posted by bobRitchie

You don't want the wart of a unit in your yard, but are O.K. with the
mess that will result from digging for a new gas line.

---

Grass grows back. Warts just rust.

Quote:

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Originally Posted by bobRitchie

I've been very chatty today since our computers here at work
have been hosed...

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That would get me canned where I work.

-HF