How reliable is HVAC-Calc Residential?

[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.

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.

[bobRitchie]

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.

...

[beenthere]

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.

[caterpillar]

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

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.

[bobRitchie]

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.

[beenthere]

How much sq ft is basement.

[caterpillar]

How much sq ft is basement.

~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.

[beenthere]

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

[bobRitchie]

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.

[caterpillar]

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

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 ). 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.

[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?

[caterpillar]

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?

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.

[beenthere]

Ok.