I have some questions about the sizing of the HVAC system our installer is planning to install in about 10 days.
We have a 2200 sq.ft., 2 story bungalow in the S.F.Bay Area, the lower half is new & under construction, no existing ductwork.
I ran the HVAC-Calc program & with these results:
Sensible Heat Gain: 28,631
Latent Heat Gain: 959
Total Heat Gain: 25,590
Total Heat Loss: 23,551 [28,261 with 20% safety factor]
Summer temp. 72
Winter temp. 72
Relative Humidity 50
Summer temp. 82
Winter temp. 36
Summer grains of moisture 59
Daily temp. range: Medium
Were looking at a Carrier Infinity 96 system. The installer wants to put in a 60,000BTU furnace, with a blower capable of about 1,050CFM, claiming that with the 2 stage burner & variable speed blower, it will be fine.
This looks like double the furnace the HVAC-Calc. would suggest, no? Wouldn't that give us short cycle times, & decreased efficiency & comfort? I really want to maximize efficeny, not to mention comfort.
The installer, using his rule-of-thumb sizing [never heard of Manual J], does not inspire confidence, but, due to a heat wave here 2 weeks ago, all the good HVAC pros are booked for over a month. [Our contractor needs to sheetrock over the ducts in ~2 weeks.]
So, I'm depending on you Pros to help us avoid installing an expensive & poorly designed system.
As far as AC [which we'll need only 20-30 days/year], he's planning a 2 ton system.
I mentioned my HVAC-Calc results to "Mr. No-J", and he said that the smaller 40,000BTU sytem has a larger cabinet, which may not fit in our available space. Does it come in a 30,000BTU size?
So, thanks in advance for any help you can offer!
The 40,000 BTU unit was built to win an efficiency trophy (that's the particular model that hits 96.6% efficiency... the other sizes are 94%). But to achieve that efficiency, yes, they put it in a HUGE cabinet (24.5" wide versus 17.5"). That is already an unusually small capacity furnace for the market segment; there is nothing smaller. It's a nice unit and a about the best fit available to your small demand for heating, given your desire for top efficiency. If you can cram it into the space available, you aren't going to do much better for matching the capacity to your need for heat. Even at that it will be somewhat oversized for your actual load. I'd avoid the 60,000 BTU unit if at all possible. You're going to be somewhat oversized with the smallest furnace available, and stepping up a size would be excessive in my book.
While I am an Infinity fan, I should point out that York and Amana make two-stage high efficiency furnaces with variable speed blowers in the 40-45,000 BTU size range that won't require an oversized location to fit. York also makes a modulating furnace that, while it is technically a 60,000 rated furnace, can modulate down as low as a 20,000 BTU output (lower than the aformentioned smaller two-stage units can do). Rheem's modulating furnace is conceptually similar and also very nice but won't go quite that low.
Your HVAC-Calc results don't look right to me on the heat gain side. Sensible heat gain plus latent heat gain should equal total heat gain, and they clearly do not in the numbers you've posted. It looks like you may have a typo in your sensible heat gain- 24,631 instead of 28,641. That would work out so that 24,631 + 959 = 25,590.
And are you really going to leave the AC at 72 degrees routinely? If not, that will bias your heat gain numbers way up, and end up with the cooling equipment being oversized for how you really use the system. I would suggest re-running with a more common design temperature (75 is most common but in a dry climate 78 is often fine too) unless you're hell-bent on 72. Then we can re-check to make sure that two tons really is right. And if you ARE hell-bent on cooling the place to 72 (I'd turn blue), you will need every bit of 2.5 tons to get there based on this load calc.
[Edited by wyounger on 08-11-2006 at 06:20 PM]
Have you considered a Heat Pump with a variable speed air handler unit to handle the situation?
Thanks for catching that goof. Yes, the Total Heat Gain was 29,590.
I've heard good things about the Rheem modulationg furnace. Do you how low the BTU ratings go?
As to the 72 deg. set point on the AC, I'm inclined to say yes, it's essential. It's usually very cool in the Bay Area, so even though I grew up in St. Louis, my body has re-adjusted to local climate. At 77 deg. outside, we start complaining about the heat!
Our office is set to 72, and it feels just right to most of my coworkers.
I'd like to consider a heat pump, but electric rates are $0.12-.21 in our area, and I'd heard that to be efficient, it would have to run in cooling mode most of the summer, instead of 1-3 days every 3 weeks.
I'm probably mistaken. Can you tell me why you suggested a heat pump?
For sizing and to satisfy your loads, your best choice is a 2 stg heat pump with var speed blower. But anyone familiar with Cal extreme electric rates know this is simply not reasonable.
I would go with Carrier's Infinity system, a 3 ton 2 stg AC with the smallest 3 ton blower rated Infinity gas furnace. Of course you will never need the high stg of the gas furnace. But this is one of the problems with systems like this-you need the cooling capacity with the blower rating of a 2 stg furnace. This is the same situation in the Southeast. It is really not a problem other than paying for something you don't need and will never use.
How about it Carrier and Trane-when can the HVAC residential market expect true var speed both in cooling and heating and forget this 2 stg nonsense(and I don't mean modulating systems either)?
I didn't realize that electric rates were that high out in Cali. I am in PA and our rate is about $0.08, which is currently a much better deal than gas or oil.
Scrap the heat pump.
From what you posted you need a new contractor. If the load calcs you have are accurate you need a 40k BTU furnace. But he needs to be able to go through your calcs and verfify them, or do his own. Is the house comfortable now? What size is the current equipment?
Do you [or anyone] know if there is a 30,000BTU Infinity furnace?
If so, would you know the dimensions of the cabinet?
[Love the name. Given that I'm a little too concerned with comfort ;-)--do you offer comfort-addiction therapy?]
Anyway, we don't have any HVAC currently [living in an apt. while a new floor is being built on the old house]. The old heater was an ancient gravity furnace in the living room floor, which warmed up about 60% of the house [built in 1918].
And, yes, I am trying to find a new HVAC contractor, who can squeeze us in within our 2 week window.
I'm guessing the 2 weeks is due to construction timeframe. Perhaps you can find someone that can do it in pieces. Ductwork and roughin portions now, then set units in a few weeks when they can schedule it. I've never been to SF but looking at your summer design temp (82**°) it doesn't seem like you should need the complete system installed right away. Just a thought based on the info provided. I really think it would be a mistake having a system (especially Infinity) installed by someone that doesn't know ManJ.
With 72 degrees inside temperature, remember to derate the equipment - that reduces the "nominal" size of your unit by about half a ton in this case. Infinity only comes in full ton sizes, and nominal tons on those systems are a little less than 12000Btuh anyway. From those numbers it looks like a 3 ton Infinity air conditioner would be appropriate, probably with a "4 ton" matched coil for the extra bit of SEER.
The 40K Infinity 90%+ furnace can handle 3 tons of AC just fine. You would probably not see high stage often, but it wouldn't be too badly oversized, and the Infinity Control will do a lot to keep you comfortable regardless.
One of the advantages of two-stage systems, and especially the Infinity with its controls package, is that they're more tolerant of slight 'oversizing' than single-stage equipment. You will not have humidity problems being slightly "oversized" for AC, for example. That's not to say that an Infinity system should be intentionally oversized, just that it's fine to round up when sizing an Infinity system.
Also, keep in mind that Manual J results are required in order to get a certificate of occupancy in many jurisdictions.
Perel: I'm assuming, since you're talking about the 40K Infinity, that there is no 30K model(?). Or is it that the 30K cabinet would not handle the 3 ton AC? Since our potential installer claims that the 40K model "probably won't fit" in our available space, I was wondering if the 30K cabinet might have better [smaller] dimensions.
I was unaware of the Manual J requirement. I'll have to check into it. Do you think my HVAC-Calc would count as a Manual J?
Comfort Doc: We are looking at dividing the project into 2-3 stages. We'll install the ducts in ~10 days, along with the evaporator coil, before the sheer-wall & sheetrock. Then the furnace install in about 6 weeks, and, since we don't know how effective our ceiling fans & insulation will be, we will delay installing the outdoor AC unit until next summer, if it's even necessary in our "mild climate".
HVAC-Calc is ManJ. it is still a good idea for the contractor to do his own or at least verify yours. He is the one that will have to provide the warranty on your system.
You should consider a "hydro-air" unit that gets heat from a gas water heater. Installed one in our last house and worked great