Drawbacks to zoned forced-air systems?
They no doubt improve comfort, but can anyone with long-term experience on zoned HVAC forced-air systems talk about how they might affect longevity of the equipment?
When only one zone is open, that portion of the duct system will not be sized to handle the airflow. The usual answer is to install a bypass damper to relieve backpressure on the fan, but it seems to me that the bypassed air will simply short-circuit without actually changing temperture much, so would there be risk of overheating the heat exchanger in heat mode, or, in cooling mode, creating problems with superheat at the condender?
Replies
I don't have "long term"
I don't have "long term" experience, but we've had a 2-zone system for, I think, three seasons now. When only one zone is calling for heat AND the fan is on high, you do begin to hear a hint of a whistle in the ductwork. But this scenario is a rarity, since it's a variable speed fan and modulating burner -- we only get full fan speed in really cold weather, and then both zones are apt to be calling for heat for much of the time.
In my mind an ideal zoned system ought to be married to a "smart" variable speed/modulating burner system, where the system would be made aware of the CFM capacity of the currently active zones. I think there are a few systems like this, but not most.
(Silly thing is, there's a computer in each thermostat, the zone controller, the furnace, and, in our case, the humidifier, but they communicate with each other at a data rate little better than smoke signals.)
Your setup sounds smarter than many I've seen. Variable speed fans and modulating burners are a big improvement, even on a single-zone system.
I guess it's like most things--anything's possible, all it takes is money.
I'm not familiar with modulating gas furnaces, but have done extensive work on multi-zone commercial HVAC systems.
Like you said, given enough money, it is possible to design and build an efficient multi-zone system. A smart system would use duct static air pressure to control the variable speed fan, and if I understand the modulating gas burner concept correctly, the hi-lo fire function of the burner. Damper position of any or all zones would then be integrated into the program by individual zone stats.
Now finding a residential contractor that understands how they are suppose to work is another issue entirely.
Just a small comment on terminology. FYI A multi zone system is a horribly energy inefficient system .. which is why they are generally banned in most energy codes. A multi-zone system is much like a dual duct system. Multi-zone systems are different than multiple zone systems ... they both have multiple zones, but a multi-zone system is a specific type of system. This is common terminology in the industry, but some people refer to multiple zone systems as "multi-zone" systems. A bit of symantics, but it's important to know the lingo of the trade for good communication.
So what is the technical difference between "multi-zone" and "multiple zone"?
Multiple zone system is a term not usually used as the specific system type is usually referenced by name, but a multiple zone system is simply any system w/ 2 or more zone control capability. It includes e.g. constant volume reheat, variable air volume, dual duct, multi-zone, VVT, etc. type systems. All with 2+ zones served by one air handling unit.
A multi-zone system is historically refered to as a system w/ a hot deck and a seperate cold deck that both supply air to each zone. Dampers mix the two for the right temp for that particular zone. Essentially a multi-zone system is a dual duct system except the mixing boxes are attached/located right at the air handling unit.
ASHRAE differentiates these as I've stated above.
Your better high-efficiency furnaces all have variable speed fans and modulating burners -- they need it to wring the last 1-2% out of the thing, plus it produces a more pleasant experience for the occupant (most of the time the fan is on low and you can hardly hear it).
What's missing is purely the software to lash everything together.
My experience is with my current house, built in 97, owned by me since Dec 2003. Bryant 383KAV 80%, 154KBTU/hr input, 2 zone (pressure side only dampered, no bypass).
I have just recently realized that when I run only on one zone and the system has to run for a while to reach thermostat setpoint (like on a cold morning, when I wake up and turn on/crank up the 2nd floor zone only) then the limit switch in the heat exchanger will trip prior to setpoint being reached. This creates a 3-3 code. The burners shut down, the blower will keep runnning as it should to cool the heat exchanger (so theres non heated air blowing out of the registers). The temp of the heat exchanger drops and then the burners relight and eventually the setpoint is reached or the process repeats until the setpoint is reached. Because the process will repeat until the setpoint is reached, it is not readily apparent there's a problem.
I've done some investigating to arrive at this and I believe the root cause is an inadequate duct system for running this furnace on only one zone.
I've taken air temp measurements in the ducts just upstream of the dampers. i also checked out 2 neighbors houses. neighbor A has same furnace, different larger house, different duct work. He has no problem and air at damper is only ~130F. I checked out neighbor B's house. i told neighbor B of my problem and he said he hadn't experienced it. Anyway neighbor B has identical furnace and house. I measured temp at closed damper and I saw it reach 160F just like on mine, and then whaddya know, the burners shutdown, blower kept blowing and same 3-3 code. he has same issue and didn't even know it.
If I run the furnace temporarily with the door removed to the blower compartment (unrestricted air supply, door safety bypassed) then it runs fine and temps are fine.
The only other thing I'm thinking it could be (besides too restrictive ductwork) is if motor is running slower than when it was new. I don't think that's the case since air at the register doesn't feel too slow (subjective, yes). If I had a manometer I could measure pressure difference from inlet to outlet of blower, to be less subjective. Perhaps someone could comment on liklihood of a blower motor running significantly slower after aging, with the same load. I replaced the motor run capacitor (not a start capacitor, this is a PSC motor) to eliminate the cap as a possibility. New motor is like $400+. Won't replace that unless I'm 98% sure and I'm inclined to think it's fine.
I should mention that I've replaced the filter and its very non restrictive and none of the outlets are blocked. The blower speed for heat is the 2nd highest of 4 available. 1st reserved for AC use.
If anyone has any suggestions/thoughts please speak up.
Anyways that has been my experience with a zoned Furnace. Sorry if I've gone on a tangent.
Regards,
Tim
Not a tangent--yours is the kind of experience I'm interested in.
The supply temp of 160 means a rise of about 90, and most brands of furnace want the rise to be in the range of 35-70. This, of course, explains why the furnace is bumping the hi limit and causing the unit to cycle on the hi limit instead of the t'stat. This can result in premature failure of the heat exchanger.
I tend to agree that the blower motor is not slowed--but it's seeing very high static pressure, indicated by the high temp rise.
154K, 80% means about 123K output. Knowing this and the 90 degree rise lets you estimate the CFM that's being delivered: about 1245 CFM, which seems low for a furnace that big. If you have access to Bryant's blower performance data, you can check for ~1245 CFM in the med-hi fan speed column and see what the static pressure is, approximately. Guessing up near or above 1 IWC if the blower is a 5-ton rated model. That's also way out of range. (0.5 IWC is where most brands want you to be.)
It may be that performance could be helped by giving the blower more return air, as you have done when removing the blower compartment door. (Be careful about that if there's a gas water heater with a draft hood in the furnace room--the room could go negative and backdraft the WH.)
If you have A/C on the system, inadequate airflow can really reduce the output from what it's rated for. A 5-ton unit would be starved for airflow in the 1300 CFM range, and would perform more like a 3- or 3-1/2-ton unit, depending on your humidity levels.
The odds of an induction motor running slow is vanishingly small. They do necessarily "slip" a little from the theoretical 3600 rpm because of the way induction motors work, but that only amounts to 100 RPM or so.
I don't think
any discussion was about variable flow induction motors ... just variable flow supply fan motors.
Well, if you'd read the posts you'd see that in the referenced post nov141992 was asking if it was likely that his problem was due to the motor running too slow.
But you do raise a good point: Some induction motors have multiple speeds, set with jumpers or via a switching mechanism in the control logic. It is vaguely possible that his motor IS running slow due to an incorrect setting. Just not likely due to a motor failure -- that's generally all or nothing.
First, thank you everyone for your input so far.
rdesigns how did you calculate 1245 cfm? Was this q=m dot *C*deltat T, solve for m dot, figure CFM from M dot
? If you've got an equation that gives CFM directly I'd be interested to benefit from knowing/seeing it.
Per http://www.xpedio.carrier.com/idc/groups/public/documents/techlit/pds383kav.35.8.pdf (manual for my series furnace), pg 5 unit size 060155 .2" / .5" static pressure for heat / cool; pg 6 bottom two rows, one side only inlet = 2045 CFM for med/high heat and .2" and 2000 CFM for high cool. Looking towards the right to find 1245 as you suggest, takes me off the chart which I'm sure is above 1", just as you said.
more information on my system:
~2800 sq ft house in Massachusetts
design max outlet air temp 170F per label on furnace
temp rise 50-80F per label on furnace
limit switch designed to trip at 160F
I do also have A/C, I think it's a 5 Ton unit,
Power Vent on outside of house, used for furnace and gas Water Heater, only mention this because I think given this it is unlikely I'd have a back draft
Danh, my blower motor does have 4 speeds, two of which can be connected at one time, The blower speed for heat is the 2nd highest of 4 available. 1st reserved for AC use.
I did take air temp measurement with both zones calling for heat, both dampers open. Air Temp appeared to level off around 154F after 28 minutes. Seems pretty high given the 160 limit trip and temp rise 50-80F per label on furnace.
At one point I removed blower and looked up through heat exchanger to verify there wasn't a lot of dust on A coil (fear was dirty A coil could cause excessive restriction). Looked clean from what I could see.
I have thought of paying someone to look at it but haven't because I don't know that whoever will come out will know much or not, based on a lot of horror stories. I suppose if I do hire someone I could ask them to reduce gas pressure. Or I could try it myself (I know I'm going to hear it for this paragraph).
I will be refinishing my basement and am planning on heating it using furnace. I would think that tapping off of 1st floor trunk will increase total CFM, which should help.
Any other thoughts or ideas?
Thanks again
CFM formula
CFM = BTU output/temp rise/1.1
Your example: 123,000 BTU output; 90 degree temp rise. (1.1 is rounded up from 1.08, which is btu per cf)
This formula will give you a fair estimate of CFM--it assumes the output based on the input and the unit's efficiency rating of 80% to arrive at the output, and both these things are subject to some variables.
It would also be worthwile to find out if your furnace is over-fired--that is, whether it is actually using gas at the rate of 154k BTU's/hr.
This, you can check yourself by clocking the meter's 1 cu. ft. dial when the furnace is firing, but no other appliance is.
If the 1 cu. ft. dial takes 60 seconds for 1 full revolution, you are burning 1 cu. ft/min. and that means 60 cu. ft./hr., or about 60k BTU's Your furnace, with its input of 154k, should use about 2.5 cu. ft/min., which would mean 24 seconds for 1 revolution on the 1cu. ft. dial. (In most places, the wintertime value of 1 cu.ft of natural gas is 1000 BTU's--you can call your gas company to get the exact conversion figure.)
The Bryant Installation Manual should have a page explaining how clock the meter, and a chart to go with it.
This process will give you a baseline to help you decide how much to reduce the gas pressure, which can usually be done at the unit's gas control valve--the installation manual will guide you thru it.
I read the post wrong ... so my comment was moot.
I misunderstood
your comment ... I was thinking of induced draft motors/fans and you were talking about induction type motors. Sorry.
I don't have any specific ideas, but >>I've taken air temp measurements in the ducts just upstream of the dampers.<<
What is the supply temp as it leaves the furnace itself?
I didn't see your reply until today.
If both dampers/zones open then temp as it leaves furnace (just before dampers, after 90degree turn into main trunk) reaches around 154F after ~ 28 minutes.
If I only have 1 zone/damper open then it will reach the 160F High limit trip switch in a shorter amount of time.
I should mention in response to other posts about it, I can easily switch to the fastest motor speed, I know how to do that. It's just that it's supposed to be for A/C and judging by the Bryant CFM/Static pressure chart it looks like it would barely make an improvement (going from med-high intended for heat to high intended for A/C)
Thanks
Supply air temps above say 110-120+ is a bit much in my experience (at the register, that is). The max setting on the furnace is just that ... a max for e.g. safety. That shouldn't imply that supply air temps should be e.g. w/in 10-20 deg of that. The sensor in the furnace will be higher than at the register, of course, but not a lot (how much is a lot? I'm not sure ... but that is a scientific measurement, I'm sure :) ). I'm thinking a 10 deg drop in temp in the ductwork might be normal?
I thought I read your house size .... but now it's elusive. What is your floor area? What is your climate? The age suggests good insulation/windows. The 154 KBtuh is a lot ... without knowing anything else, I'm guessing you are significantly oversized (as another poster suggested). 154 KBtuh tells me you got what ... maybe 5,000++ sqft?? Read another post of mine in this thread, too. What is your winter peak design outside air temp? (or what is your location?).
Supply fan motor running slower ... doubt it. I've seen 30 year old fans/motors ... still humping away. You got what ... 10+ years.
Poor man's VAV
You have the poor mans variable air volume system (common in commercial applications). It should be coupled with as some people have said a variable flow fan and maybe e.g. a two stage burner (or modulating). When dampered on the supply side, the fan energy goes up when the damper goes closed.
One approach would be to not close dampers completely, but to e.g. 90% to maintain some higher level of air flow. As with most HVAC, it should be designed as a system. The system is design for a certain air flow across the heat exchanger. Maybe a chat w/ a tech rep from the mfg would answer some of your concerns.
This is not too much different than having way oversized equipment, maybe 'cept the air flow is reduced. Oversized equipment will cycle more and ... as you are concerned about ... would wear out sooner. Generally boilers/furnaces like to run and run to maximize life and efficiency (i.e. combustion efficiency, not consumption efficiency).
When dampered on the supply side, the fan energy goes up when the damper goes closed.
Actually, that's not true in most cases. Squirrel cage fans use energy in proportion to the weight of air they move, and use less energy when throttled. This is why fan speed actually increases when throttled. (But of course the typical fract-horse AC induction motor is woefully inefficient to begin with, which is why DC fan motors are such a good idea -- they'll pay for themselves in a year or two.)
Thinking about nov141992's problem of overheating and bumping the hi limit:
Do you think he might benefit from having a tech reduce the input from 154K? This is commonly done to adjust for lesser air density at high altitudes. (Where I am, near 5000' above sea level, furnaces are almost always de-rated by 20%)
Probably no load calc was done, but 154k seems way big for most places, and he has the symptoms of oversizing. De-rating the input would allow longer cycles and improve comfort.
Maybe he could check the temp rise when both zones are open? If the rise is on the high end of the recommended range, that might indicate that de-rating could be done without reducing the temp at the registers too much when both zones are open.
This would not help problems with the cooling side, if he has a split system. But that's a problem of its own.
Yeah, he doesn't say where he lives or how big the house is, but that furnace could easily handle a 5000 sq ft house in Minnesota.
Rdesigns, thanks for the additional information.
I clocked my meter and figured 153.2 ft^3/hr of gas consumed which works out to 153.2 KBTU/hr Gas input. So it seems the gas valve is adjusted to where its supposed to be for this furnace. You were right about manual giving info on this.
I guess this means I'm screwed. I could adjust the gas rate down a little but that would be in contrast to what it is supposed to be set at, and I suspect it would burn less efficiently. The manual says to adjust the motor speed to get the right air temp rise. It is currently set for the 2nd highest speed (1st for A/C) so I'm already at the fastest for heat. Seems like the only thing I can do is something to increase CFMs, like adding ducts. Which I will be doing to a limited extent for the basement finishing.
Any one else have this issue? Any of you work in HVAC? If so is it common that the ductwork is inadequate for the furnace? I suspect it is based on my furnace and the fact that my neighbor has the same issue.
thanks again
is it common that the ductwork is inadequate for the furnace?
It's common for the furnace to be oversized.
I believe the same fan speed (high) can be used for both heating and cooling if you want. You just connect both the heating and cooling wires to the same speed tap on the blower control.
DanH could probably verify this; I have to admit to being almost ignorant of the electrical side of furnaces, and completely ignorant of the electronics.
There are several different schemes for multi-speed fan motors, and wiring one up the wrong way can make the smoke come out. So I'm a bit leery of suggesting HOW one should switch things to use the higher speed. But there would be no harm in using the higher speed, other than more fan/duct noise and a very slight increase in power use.
>>Do you think he might
>>Do you think he might benefit from having a tech reduce the input from 154K?
Only if he does a combustion analysis as part of the adjustment. As I understand it, underfired can throw efficiency off big time, especially with an 80+ induced draft furnace - remember, the draft inducer is regulating the flow of air through the heat exchanger - not 'forcing' the draft out through the flue - and it' purpose is to allow burners wto be better designed/calibrated to combustion conditions.
>>Do you think he might
>>Do you think he might benefit from having a tech reduce the input from 154K?
Only if he does a combustion analysis as part of the adjustment. As I understand it, underfired can throw efficiency off big time, especially with an 80+ induced draft furnace - remember, the draft inducer is regulating the flow of air through the heat exchanger - not 'forcing' the draft out through the flue - and it' purpose is to allow burners wto be better designed/calibrated to combustion conditions.
>>If so is it common that the ductwork is inadequate for the furnace? I suspect it is based on my furnace and the fact that my neighbor has the same issue.
Yes, probably most residential systems are poorly designed, according to the National Comfort Institute.
Lost this thread for a few days ... and it's hard to track the conversation. But I'll try to comment. 154KBtuh seems WAY big for a 2,800 sqft house. This of course depends a lot on the age, construction, insulation (particularly windows), climate, etc. I designed a house in late 70's for cool/cold climate (northern Idaho) ... 1,800 sqft and it sized out at 35 KBtuh ... That was before low-e glass, too.
Oversized furnace cannot be a good thing particularly at part loads ... which is where 75% of the furnace run time is at ... roughly/normally around 75% of your hours are at like 50% loads. That's why it's important to size right. Running 100% during extreme conditions is OK to do ... heating systems like to run full on.
So, yes, I agree ... Not sure how much you can tune a big furnace down ... that's a bit out of my expertise. I assume there is a limit, though and just guessing that his load might be say 60 KBtuh (again, I've little/no details about his condition other than the floor area estimate) .... that's a long way to turn the system down.
nov141992's location is in Mass., and the winter design temp in Boston is 12 degrees--the wdt for Coeur D'Alene is -2 as a comparison to the house you designed, so it's very likely his furnace is way big. And he thinks his A/C is 5 tons. I'm guessing that indoor humidity will hard to control with what must be a grossly oversized condenser. And what you say about wintertime part loads applies equally to summertime part loads.
His furnace is supposed to be non-condensing (80% efficiency rating), so I think there could be problems with condensation in the heat exchanger if the fire were reduced too much. OTOH, it seems there ought to be some room to reduce flame. He would no doubt be well advised to talk to the maker, but most makers won't talk to anybody that's not a dealer.
I generally try to avoid talking to my maker.
I agree ... need to know his house size, now ... but still potentially unclear w/out e.g. window area specifics, If he lives in a glass box, it might be right ... but if his house is 'normal' design, as we suspect ... then he has an oversizing problem.
Exactly right. Same goes for A/C equipment, and with A/C, the airflow is even more ctitical for efficiency and comfort.
EDIT: This was meant to be in response to DanH's comment about oversizing of furnaces being common.
We Had one for 10 years.
RD: It was a 3 zone on a 14 seer head pump with a variable speed air handler. I built the house in 99 sold in 2009. Didn't have any problems. I will say that the velosity of air comming out of the 2 smaller zones was a bit bothersome. DW probably never even noticed it, if you know what I mean. I'm 90% sure there was no bypass duct. I was very happy with the performances as far as even temperatures through out the home. There were 4 returns, and generally we keep all interior doors open. Occaisionally I would run the office/guest area zone a little hotter as I like it hot - DW likes it cold. :-)
One thng that I'd like to mention to any novices here is that only the supply side is zoned with automatic dampers; the returns are not, so there is always going to be a certain amount of air mixing between zones meaning that they are really not seperate.
I'm thinking you are a code enforement person... It may interest you to know that in NC we had a code change 1/1/10 that requires 2 story homes to have thermostatically automatic control over each level. Not sure if that is a state ammendemnt to whatever model mechanical code we use or an actual change inthe model code. Good as far as making better quality homes code required, but bad as far as raising the price of smail starter houses by about ~$900 thereby putting them a little further out of reach of the target home buyer.
The amendment in NC is not based on the IRC or the UMC (BTW,you're right--I am in code enforcement, mech and plmg).
What the codes DO require, and which is not enforced in most jurisdictions, is that HVAC systems for houses must be designed based on a load calc in accord with ACCA Manual J, equipment selected in accord ACCA Manual S, and the duct system must be designed and sized according to ACCA Manual D.
Does the new requirement mean that they must have a zoned system? Probably, since you say it adds ~$900.
Do you think it might be cheaper to install 2 systems? (2 small furnaces, less ductwork, no zone controls or zone dampers) HVAC guys in this area say so.
Longevity
on almost all variable air volume (VAV) systems equipment is shortened. This is particulary true of every zoned system based on gas heat and DX cooling., and includes all residential versions of VAV systems that I have ever seen. It is rare for a residiential system to be properly sized, and zoning a poorly sized system only exaggerates the issues of oversized components: short cycling being the greatest longevity reducer.
I've been designing and selling HVAC systems and equipment for the past 20 years, that might count as "long-term" experience. But then, some of the older, er, more experience individuals with which I work, consider me a young pup, yet.
I have seen VVT (or bypass VAV) systems where a constant volume rooftop unit (typically gas fired, DX cooled with one or two stages for each) fittted with a bypass and zone dampers, last barely beyond the standard 5 year compressor factory warranty. Many manufacturers will not honor equipment warranties if the equipment is used in a zoning arrangement.
The poster that constantly trips the high hx temp limit will be in the market for a new furnace in the next few years.The simple reality is that residential equipment is not designed for zoned operation. Modulating gas is not really modulating, but has a 5:1 turnndown ratio. The best Iv'e ever seen on a DX system is two-stages of control with a single compressor. Digital scrolls, like hot gas bypass, have the abilty to control capacity by wasting work. These features, with the exception of modulating gas, add no longevity to the equipment and typically waste energy. Exactly the opposite of what you hope to acheive by zoning.
As Dave mentioned, in "real" commercial VAV systems, they are controlled in two ways: discharge air temperature control and duct static pressure. Ideally these system are hot water chilled water systems with modulating control valves, and variable frequency drives on the supply fans. These types of zoned systems add longevity to the equipment, control comfort and humidity excetionally well, and save 40 to 60% in energy costs over standard CV stsyems.
Tim, thanks for your info on this. (It's got to be right, because you confirm what I think ;-)
Some of the guys you mentioned might consider you to be a "pup" with "only" 20 yrs experience, but methinks a good share of them have never actually designed an HVAC system in the true sense--starting with an accurate room-by-room load calc and then sizing the equipment accordingly, and finally designing and sizing the duct system right. They get good at guessing over the years, but leave a trail of problems along the way.
I think the HVAC system is by far the most complex system in any building, but in our region, the average HVAC tech is the victim of his teacher, and never has had comprehensive training in the fundamentals of the profession.
The residential
market is pretty fierce in terms of cost competitiveness (aka cheapness). In my life as an HVAC design engineer, I only designed two houses, not includung my own. There is simply no market. Every subdevelopment cookie-cutter hack out there knows well enough to throw the same 35 btu/sf furnace at a plastic coated house, round up for good measure and then add a ton of of AC for every bathroom. Flex duct, a single hallway return, 80% AFUE and a gold bug on the wall. Quick and dirty is what the market bears. And, frankly, its what uninterested and uniformed consumers deserve.
I designed more complete residential systems when I was selling integrated radiant/forced air systems for a distributor than I ever did as an design engineer.
LEED and documenting the required energy savings to get enough points to earn a Silver or better rating is driving more complete energy anaylsis and subsequently more thorough system design than ever before. I have contractor customers coming to me now want to know how to do these things (like run loads and select/size a system accordingly) and the've been in the commercial HVAC business for decades! I am a LEED AP (Accredited Professional), but agree with only select parts of the program. The energy efficiency portion is excellent and is long overdue.
As far as residential zoning goes, my preference is for forced air: don't bother excpet with separate systems. With hot water-absolutely, zone it by use and/or exposure and if applicable, supply temperature. With steam - find someone that knows steam systems and talk to them...
Before we had a zoned system we had two furnaces. I much prefer the zoned system, save for the lack of redundancy (which is not an inconsequental consideration in MN). The zoned system saves a lot of space, is basically only half as much maintenance, and gives us AC downstairs (whereas the old setup only had AC upstairs).
We did have some trouble with whistling when on one zone early on, but it turned out to be due to a poorly fitting damper.
But our system was sized properly -- both Manual J and a couple of thumb sucks all gave us the same numbers.
MY "main" system
is actually undersized (at about 20 btu/sf in N.IL). I have a "manual" zoning system. That's where I go close registers in rooms I don't often use. An undersized system is recommended for a zoned residential application.
Time will tell on the maintenance. Modern furnaces are relatively maintenance free. Depending on your zoning system, multiple small inexpensive motorized dampers and multiple zone sensors/thermostats will most likely demand more maintenance than your furnace ever will.
I put in a second and "small as available furnace" primarily because of accessibility and ducting (I didn't want to try to cut through a concrete wall) It is nice to have the redundancy. Earlier this year the inducer fan failed on one system. It was nice to be able reair the unit in days when time and parts availabilty allowed and not worry about having no heat.
Thanks for the clarification/reminder. Having a hard time following the bits and pieces as the post number increases and the conversations diverge a bit.
I tend to agree ... his furnace size is WAY BIG. I just did a heat design calc for a small (i.e. 1,200 sqft) house at a 5 degF outside design temp and got about 14,000 BTUH including the ventilation air. I'm worried I'll have some difficulty finding a small enough system to accomodate it.