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View Full Version : Heat load calculations: is my furnace oversized?

serik
07-22-2009, 11:54 AM
Hello!
Upon results of a heat load analysis, we came up with 30,000 BTU per hour heat loss for the 2400 sf house, design temperature 70F, outdoor 3F, summer heat gain is 20,000 BTU.

Original house was 1800 sf and we are adding 600sf by converting attic to the living space. BTU values sound a little bit too small for me. House is well insulated though, R13 wall cavity+3/4" foam board, roof deck is sealed with 3.5" of closed cell foam, 2" of foam on gable ends, energy efficient windows and doors.

I have an 80,000 BTU furnace. with 75,000 BTU output in high heat / 49,000 BTU in low heat. 2.5 ton AC

If calculations are correct even low heat mode would be sufficient all the time. Note, this excludes duct loss; I excluded it since we decided to place the ducts inside of a building envelope. Original numbers were 35,000 heat/26,000 cool

Still it sound too low to me and why original builder put such a huge furnace then? But i think numbers match, on a calm winter day with relatively constant temperature, i took gas meter reading and they matched up precisely.

I am a little bit confused.

Kevin O'Neill
07-22-2009, 12:03 PM
Many contractors and builders size equipment by Rules of Thumb, unfortunately.

Oversized equipment is far too common.

motoguy128
07-22-2009, 03:33 PM
If the numbers are correct, then at 3F outside, your existing sytem should be cycling on about 60% of the time.

Did you account for infiltration? How well sealed is the home?

serik
07-22-2009, 07:20 PM
Yeah everything is accounted for including infiltration, slab perimeter, windows, doors... I have a J1 form with all the numbers.

What bothers me, furnace does kick into high heat mode when its freezing cold and windy. But it does not stay on for too long, keeps cycling on and off. If the actual heat loss was 35,000 btu at 3F, why does it need to output 75,000? Why not run at 49,000 with longer cycles?

I noticed wind does make a difference and I can feel cold air coming out of electrical outlets. House supposed to be very tightly built as was advertised by the builder

This winter will be the 1st winter after the attic conversion, adding a spray foam insulation and moving ducts into conditioned space. Last winter gas bills were \$230 with t-stat set to 66F !!!

amd
07-22-2009, 08:24 PM
What bothers me, furnace does kick into high heat mode when its freezing cold and windy. But it does not stay on for too long, keeps cycling on and off. If the actual heat loss was 35,000 btu at 3F, why does it need to output 75,000? Why not run at 49,000 with longer cycles?

Is your furnace being controlled by a single stage t-stat?

mayguy
07-22-2009, 09:28 PM
Where are you from?

Is your furnace being controlled by a single stage t-stat?

I was just going to ask what they have for t-stat.

serik
07-22-2009, 09:56 PM
I am from central Indiana, t-stat is Honeywell, its a 2 stage programmable unit. FocusPRO TH6220D

wildbill99
07-22-2009, 10:46 PM
Define "tightly built". Exactly how many air changes per hour is that.

You have already defined one place that was not "tight".

The only way that you will know for sure is to get an energy audit with a blower test and maybe an infra-red scan.

"If the actual heat loss was 35,000 btu at 3F, why does it need to output 75,000? Why not run at 49,000 with longer cycles?"

What determines the furnace going into high fire? The stat, the furnace?

Maybe those settings need to be adjusted.

serik
07-22-2009, 11:33 PM
What determines the furnace going into high fire? The stat, the furnace?
Maybe those settings need to be adjusted.

Stat tells furnace to go into high fire. This setting is not adjustable on my stat. All I can do is disable high fire altogether. I am also considering to replace it with a Carrier Infinity stat to match the furnace.

mayguy
07-23-2009, 08:06 AM
I am from central Indiana, t-stat is Honeywell, its a 2 stage programmable unit. FocusPRO TH6220D

I find it odd that the stat would allow the furnace to go into high fire, then cycle off..

I am wondering if the t-stat is not wired up right, and the furnace is running on a timer.

DanW13
07-23-2009, 01:36 PM
I would bet the current stat being used is allowing the furnace to run in med fire and then switches up to high fire and when it does that's when your getting the short run time when in high fire. I would switch out the stat and get the Infinity controller especially if you already have the Infinity furnace I don't understand why you wouldn't have purchased the stat at the sametime ?

WhoIsThat?
07-23-2009, 02:38 PM
Define "tightly built". Exactly how many air changes per hour is that.

"In typical modern U.S. residences, about one-third of the HVAC energy consumption is due to infiltration. Another third is to ground-contact, and the remainder is to heat losses and gains through windows, walls, and other thermal loads. As such, reducing infiltration can yield significant energy savings, with rapid payback. In cold climates, with a 15 MPH wind, residences often have air exchange rates of 1.0 to 1.5 ACHs, far in excess of the ventilation air needs and are thus called loose construction. While it is possible to build a house too tightly, e.g., 0.25 ACH, it is very easy to reduce infiltration rates to less than 1.0 ACH. Smoke candles and blower-door tests can help identify less-than-obvious leaks. The Weatherization article describes methods for energy savings further."

serik
07-23-2009, 05:48 PM
Manual J pros, how do you account for the heat loss thru the ceiling in case of a sealed attic? Its not exactly a cathedral ceiling, since it goes room -> drywall ceiling->attic air->insulated roof deck?