Sorry, I must have looked at someone elses profile by mistake. "Great White North" is clear enough for me.
This has been an interesting topic to learn from. A little confusing, but interesting.
I call it oversized because your 74k load calculation is based on unrealistic design conditions. The calc you have now is such that you have enough capacity to keep the house at 74 degrees F, even if it stays -40 forever, no matter what.
***How can I have a sizing problem? The next smaller furnace in the mid-efficient range is a 88K btu input which will only out put 63k btu. Way below what we'd need (73,581, to be exact) on a -40 night. As I mentioned previously, if you have more btus going out than coming in, the thermostat would never be satisfied. Currently, we are experiencing adequate run times...about 20 minutes at slightly below freezing outside. We can't be that oversized if at all
But you don't heat the house to 74 degrees, for one. Second, you don't do a load calculation based on the coldest temperature anyone can remember having seen; Manual J has specific guidelines on how to choose the temperature (no, I don't know them offhand!). That's not to say that 0 F is right, either, like you mentioned. It may be that the correct number is -35 and not -40.
The unit I suggested in my earlier post is what I would consider *ideal* sizing, from what we know based on the 74 degrees at -40 ambient case. Rerunning the Manual J with slightly different numbers would allow for more confidence in making an exact decision. Of course, I suggested my idea having forgotten the high altitude issue (leave it to a sea level kinda guy to forget that part).
We don't know what city (or even province) you're in, so weather data is hazy. To give my Sun Belt mind a reality check, though, I pulled up weather data for Yellowknife, in the NWT. Average January low there is -26 F (-32 C). If your weather runs along those lines, -40 sounds like a reasonable design temperature. Given that 79k output as installed, you're right... it doesn't sound oversized. Even at 70 instead of 74 inside, the heat loss is probably in the high 60k range.
As for zoning, two stages should be fine, especially in cases where the zones are of reasonably similar sizes (ie half the house, the other half of the house, and the basement) and the ductwork isn't horribly small. You're already halfway to the best zoning system available. Infinity/Evolution zoning (in conjunction with an Infinity/Evolution furnace or air handler), not only does not require a bypass, but doesn't allow them. It manages by using variable position zone dampers; instead of all-or-nothing, it controls exactly how much air goes where. It also knows that while a zone may not demand heat at this moment, it may in a few minutes, so it watches trends and balances these things out on the fly. For example, on my two-zone system, if one zone isn't really calling for heat but will need some eventually, it may send 150 CFM to that zone and 800 CFM to another. Or if it was really just one zone that needed heat, and zone couldn't handle all 1050 cfm by itself because of duct restriction, it would select a one or more dump zones, based on conditions in the other zones, to make best use of the excess air.
Traditional zoning would try to send all of the airflow to the zone that was calling, use a dump zone or bypass damper blindly to handle the excess pressure, and then shut down when the zone stopped calling. But ten seconds later, another zone could call for heat, and it would have to start up again. With an Infinity/Evolution system, it knows the conditions in each zone all the time, so would have just divided up the airflow as appropriate and satisfied all of the zones together. It's been in for a year and it still fascinates me to watch it do its thing.
Actually, as nifty as the modulating furnace is, nobody has a zoning setup that can handle true modulation. The best we can do (as far as I know) is to set up the modulating furnace to act like a three stage furnace in conjunction with traditional zoning.
wyounger: Good stuff. I appreciate your comments. I looked up our January temps and the minimum normal is -19 deg C (0 deg F) and the max normal is -8 dec C. Many times things around here aren't normal with the mountains only four hours away and the Pacific not far on the other side. We've seen -40 in November and above zero deg C temps in January. I am in Edmonton, btw.
With the rising cost of natural gas, I've been giving myself a kick in the butt for not opting for the high efficiency furnace. But like I said, at the time, gas was almost free. Contractors in this area are still installing 80% units for the most part.
I have also perused the Manual J results that the tech had supplied and can see where if we get a bit more realistic about inside temps and take the Manual J recommendations for design, I see the h/e furnace with 80k btu probably fitting the bill. I see the resulting heat loss is 73,581 and a h/e 80k unit will put out 67,680 or so. That's only about 6,000 btu difference. I bet we can find that if we redo Manual J with the correct parameters for the area...or even plug off one fireplace (lol). It would also allow us to opt to the smaller blower which since day one I have hated the big blower on this 110K unit. On efficiency setting, it is trying to push the air way to hard and way too fast. In past discussions with DASH, I mentioned the Evolution control indicates static that exceeds 1.0 when it's on high fire. Way too high and I know this.
Anyways, it's never too late to make things right. If I can find a market for a 1.5 year old furnace for the right $$$, then at least the $$ to change won't be a big deal. If I can't get anything back for my 110k unit, then I really have to convince my wife that we need to do this. The new Bryant has the same dimensions so the fit will be without any mods. And when someone built this house 23 years ago, he ran 2" white pvc up to the roof like maybe he thought he would install a high efficiency furnace one day. So, that would even make the retrofit a half day job since there is so little that needs to be fabbed. Actually, next to nothing, really. A condensate drain and a new drain for the humidifier...not much more.
So, let's leave it at that and move on to the next person's q & a. Thanks all for the input and thank god you don't have potential to have $2600 gas bills!
Your airflow comments got me thinking. I have similar issues with duct static in my own home when my Infinity furnace (Carrier's twin to the Evolution line) runs on high heat, so I have stewed on this issue a good bit in the past, too.
Your current 110k input 80% Evolution furnace is set for:
Comfort 1110 CFM low/1330 high
Efficiency 1320 CFM low/1475 high
Based on the fan data charts this one will apparently hold its intended CFM right up to high static, as is normal for a variable speed blower.
If you see in excess of 1.0" ESP on high heat in Efficiency it's no wonder you have some duct noise. Your ductwork can't handle 1475 CFM. Switching may not help your noise and duct static concerns, though. The 80k input 94% efficient Evolution furnace comes in two blower sizes- a three ton and a five ton. Both are set for (these numbers are Efficiency, can't find Comfort) 705-720 on low, but 1500 CFM on high. The difference I see in my data sheet is that the three ton unit can only move 1170 CFM at 1.0" ESP.... so at higher duct statics the airflow will drop off. The five ton blower model can hold 1500 CFM even at 1.0" ESP.
I don't know why Carrier/Bryant insists on so much airflow in heating. The Rheem modulating furnace (75k input) calls for a maximum of 995 cfm. Why does Carrier want so much more?
What's caught my interest, though, is derating output for altitude on variable speed units. The furnace control board is set for certain CFM rates to achieve the temperature rise the factory intended. Normally that's great. But at your altitude, though, you're starting off with 10% less heat input than they expected, and the furnace doesn't know about that. Thus you're going to have less than normal temperature rise. It would be good to measure the actual temperature rise to see, but it sounds to me like at altitude you will probably want to set either model furnace we're talking about at the lowest airflow it's willing to run- meaning Comfort, not efficiency. If the heat rise is low enough, you may even want to leave the Low Heat Rise mode off even
though you do have a bypass humidifier, as I remember.
Your current unit is rated for 25-55 degree F temperature rise on low heat, and 40-70 degrees on high.
It does sound like some ductwork improvements are in order to reduce your ESP- or at least a different filter design. It'd be better for the equipment, take less electricity, and make less noise.
What sort of filter are you running?
What is your return ductwork like?
And why didn't the installer correct the high ESP to start with? In my book their work isn't done if the system is hitting 1.0".
[Edited by wyounger on 11-09-2005 at 01:42 PM]
This is some interesting discussion.
First off, are your sure about the CFM rating for the 80k 94% on high? I see 1400 cfm at 0.5" wc but have a hard time believing that they would actually run the blower at 100% of rated capacity for heating. Especially hard to believe that with the output difference from the 110k and 88k...why would they need that kind of blower speed (cfm) for a furnace putting out 20k less heat? My father in law has a Carrier 88K, 80% and his unit runs very quietly in his 35 year old 1200 sq ft bungalow. Not even close to the sound my machine makes. Mind you, he might be set on comfort but still, it's very quiet. Just want to double check the blower info with you.
I have checked the heat rise on low and high, comfort and efficiency. All fell into the "mid range" per the nameplate spec info. I don't have the numbers but do recall that the numbers were well within spec...not even close to the upper or lower limits.
I am running a Aprilaire 2400 filter cabinet.
The return ductwork... I have six returns on the upper level (1700 sq ft) and two returns for the basement. All rectangular tin, no flex. I don't have any other details like size of runs, etc.
BTW, if we run these units on comfort, are we losing more heat up the chimney due to lower air flow across the exchangers?
It does seem odd to me that the airflow is so high. That's what's on the product data sheets currently on Carrier's web site, though, and that data agrees perfectly with the 80% Carrier Infinity 66kBTU ("070") furnace I own. My ductwork doesn't like the 1050 cfm that this furnace produces on high heat (comfort).
Keep in mind that the sound the blower makes is in direct relationship to how hard it's working against the static pressure. I would expect that your furnace, if it into a situation where the ducts flowed easily and static pressure were under 0.5" or so on high heat, would be almost inaudible. When mine isn't working too hard (under 0.5" or so) it's almost inaudible, even when you're right next to it. But if to achieve the same airflow your static is at 0.8" or more, you are going to hear the motor humming, and you're probably going to hear air velocity noise in the ductwork.
It's just like in a car with an automatic transmission... if you're going a steady 80 km/h, the engine is turning maybe 2000 RPM, and the air intake and exhaust from the engine don't have too much velocity. Everything is pretty quiet. Now floor it. The engine goes to 5000 RPM, so you hear lots of mechanical noise, and there's lots more noise from the air moving through the intake and exhaust because the velocity has gone way, way up. At 1500 CFM and 1" ESP, your blower is probably turning as fast as the motor will allow (1300 RPM) and the air is going through the ductwork so fast as to be noisy. If your father in law's ductwork isn't so restrictive, the same 1500 CFM could result in half the fan RPM and slow, quiet airflow through the ductwork. Depending on exactly which model he has, though, the actual flow could be a lot less on that model- 1090 CFM for an Infinity/Evolution unit, less on a non-variable-speed unit.
From what you've said so far the return system and filter sound reasonable. Is that a 16x25 or 20x25 filter? It may be something simple and relatively easy to correct, like a hard turn in the supply or return trunk. Is the noise more apparent at the supplies, at the returns, at the furnace itself, or all over?
The amount of difference in heating efficiency between Comfort and Efficiency should be trivial, especially given that you're in the middle of the temperature rise range. Precise control of airflow is more important with respect to air conditioning and heat pump heating than to furnace heating (as evidenced by those wide temperature rise ranges allowed on the furnace).
[Edited by wyounger on 11-09-2005 at 04:14 PM]
I have a 20 x 25 cabinet.
The noise is most apparrent at the supplies. Returns are quiet. The closer to the furnace, the louder the supplies. The furnace itself is quite noisy, too. Kind of to the point where it almost seems to be huffing and puffing with all its might. Some vibration is apparent. But air is flowing from the supplies...lots of air.
Father in law has an 80%, 88k btu variable Carrier. He has a smaller home with way less supply and return ducts. There's no way he's pushing 1500 cfm in his house being as quiet as he is. No way.
On high comfort, I'm pushing 1169 rpm with 1311 cfm with 1.08 static. On high efficiency, I'm pushing 1250 rpm, 1460 cfm with 1.21 static.
On low comfort, I'm pusing 970 rpm with 1108 cfm with 0.74 static. On high comfort, I'm pushing 1170 rpm with 1320 cfm at 1.08 static.
As you can see, on high-efficiency, I am running the blower for the full 1 horsepower or darn close to it.
Those static pressures are unacceptable- out of spec- and it's the installer's responsibility to correct it, period. That blower won't last long working that hard! No wonder it's noisy. You hit it on the head; it's huffing and puffing with all its might.
Is there any data anywhere (fan curve) that shows the rpm for a given cfm at various static levels for this blower?