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sizing choice on system upgrade

6K views 49 replies 6 participants last post by  beenthere 
#1 · (Edited)
blower sizing choice on system upgrade

3 level all above ground 2500 sqft townhouse with split system : "Builder's model" Bryant 84K BTUH input (67k output) gas furnace, 3 ton A/C. I have lived here for 16 years and I believe the system is sized properly for my heating and cooling loads. This system has a 1/3 hp blower in a size B (17" wide) cabinet. I don't see a CFM rating on the label but I bet it is 1200 or so.

So, new system I am considering is a York Affinity modulating furnace 3 tons AC, 80k input (but 95% eff, so 76K output) furnace in a "4 ton" size C (21") wide cabinet and a 1600 cfm rated blower. My plenum is actually sized so this would fit perfectly, I'd just have to have the sheet metal opened up a bit on the adapter that was made to mate the plenum to the existing 17" cabinet.

My question is the blower. The new York system documentaion shows a couple jumpers to downrate the blower speeds for high cool and low cool (it is a 2 stage cooling system) to better match the installed AC. That seems simple enough to dial in the proper airflow for cooling to match the condensing unit and the ducts. Table has about 10 combinations.

But my question is for the heat cycle... do the settings chosen by the jumpers also affect the speed ranges that the blower will use and modulate around when in heat mode? If not and it uses full speed, will the modulating feature keep the blower from running at full blast most of the time?

Not sure about the tech specs on the ducting. These were builder homes and some of my neighbors have a 4th floor and got 4 ton units. So I wonder if we all got the same ductwork that is indeed adequate for the 4 ton homes. The plenums at least are the same.

Alternatively I could get the same unit in a size B 1200 cfm unit that would more closely match what I have now, at the expense of a point of SEER for AC mode with the smaller coil.

I realize these cfm ratings are just nominal values and actual values depend on many factors including the ductwork. I understand that a tech could get velocity and pressure readings to verify if necessary. I am just trying to get a ballpark feel for now assuming the original design was decent, which I think it is.
 
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#2 ·
Slow down. To get the bang your paying for from a mod furnace. You need to know what size mod you really need. An over sized mod won't get you the comfort your paying for at milder outdoor temps. You very well may only need a 60,000 BTU mod.

With an over sized mod with a VS blower. You won't like the sound of air noise through your duct system when it comes out of recovery if you use temp set back.

On a York mod, the only heating blower speed/CFM setting you can make, is a plus or minus 10°F temp change. The mod regulates the blower to maintain a set heat rise.
 
#3 · (Edited)
No problem... I am not in a hurry. I am just trying to learn and satisfy my engineer's curiosity for now. But I'm not sure it will be possible to tell exactly how a system will perform until it is in place so best to learn as much as possible before getting any sales pitches. I know you guys always say to get an audut and calc but I believe my system is pretty spot on for sizing. Do most HVAC contractors do that or just use the eyball and table method?

The other thing I was thinking is that with a mod furnace being a tad oversize would just have it run on low burner and blower settings longer before ramping up. But that of course depends on the logic and inputs it uses. Hence all my questions.



Slow down. To get the bang your paying for from a mod furnace. You need to know what size mod you really need. An over sized mod won't get you the comfort your paying for at milder outdoor temps. You very well may only need a 60,000 BTU mod.

With an over sized mod with a VS blower. You won't like the sound of air noise through your duct system when it comes out of recovery if you use temp set back.

On a York mod, the only heating blower speed/CFM setting you can make, is a plus or minus 10°F temp change. The mod regulates the blower to maintain a set heat rise.
 
#5 ·
I would think the engineering side of you would have already developed an excel spread sheet to calculate the heat loss/heat gain of you place.

Several hours of development is all it needs and then you will know what you will need.

Mark
 
#7 ·
I think I already know what I need BUTH and tonnage wise, which is about what I have. I believe the the sizing is pretty close to being on the money. No need to reinvent that wheel. My questions are mostly trying to get at how these mod units work to vary their outputs and how to how to expect the blower speed/burner rampings to behave. Without any real control of the invisible algorithm on the Yorks I am convinced I don't want to get a unit with a blower that is a lot more powerful than what I have. I had a friend that just has his system replaced last Thursday (Trane - not sure if mod or just multi-stage) and his wife is already complaining about the roar on high speed A/C.
 
#10 ·
OK, this helps a lot. The mod blower moves the amout of air needed to maintain the specified temp difference... so it doesn't matter what size system it is. And that is adjustable +- 10 degrees.


I think you are correct about my current blower only doing 1000-1100 cfm. It is rated for 1/3 hp which is less than the York 1200 cfm nominal systems that have 1/2 hp. So the York mods (esp the 80k) might be a tad louder in full input heat mode than my current system.


So if I go York my choice is really between the 60k and 80k furnace options. It would be good to know what kind of winters to expect. I would never want to be undesized if we have a really cold one.

But like you said, the 60k really isn't but about 7k less than what I have now... plus going with a direct vent and not sucking in all the cold air would reduce that difference even further. So it might be pretty darned close to what I have.

I'll need to think about this some more.

Just for info on the 80k system what if the temp difference was set up to the max? How much would that reduce the blower cfm?
 
#11 ·
Just for info on the 80k system what if the temp difference was set up to the max? How much would that reduce the blower cfm?
1082 CFM at max set rise.

Which also puts it near its max allowable temp rise, and doesn't leave much room for the air filter to get dirty.

Do your own load calc, it can save you money upfront, and in operating cost, along with letting you sleep in piece and quiet while the A/C or heat runs. Its only 49 bucks to use and and get the results. Don't add any buffer to it for safety. Just use the outdoor design temp for your area, and realistic values for your insulation. HVAC CALC. It can also show you where else you can invest money to reduce your heating and cooling needs, to save more money.
 
#19 · (Edited)
3 level all above ground 2500 sqft townhouse with split system : "Builder's model" Bryant 84K BTUH input (67k output) gas furnace, 3 ton A/C. I have lived here for 16 years and I believe the system is sized properly for my heating and cooling loads. This system has a 1/3 hp blower in a size B (17" wide) cabinet. I don't see a CFM rating on the label but I bet it is 1200 or so.

So, new system I am considering is a York Affinity modulating furnace 3 tons AC, 80k input (but 95% eff, so 76K output) furnace in a "4 ton" size C (21") wide cabinet and a 1600 cfm rated blower. My plenum is actually sized so this would fit perfectly, I'd just have to have the sheet metal opened up a bit on the adapter that was made to mate the plenum to the existing 17" cabinet.

My question is the blower. The new York system documentaion shows a couple jumpers to downrate the blower speeds for high cool and low cool (it is a 2 stage cooling system) to better match the installed AC. That seems simple enough to dial in the proper airflow for cooling to match the condensing unit and the ducts. Table has about 10 combinations.

But my question is for the heat cycle... do the settings chosen by the jumpers also affect the speed ranges that the blower will use and modulate around when in heat mode? If not and it uses full speed, will the modulating feature keep the blower from running at full blast most of the time?

Not sure about the tech specs on the ducting. These were builder homes and some of my neighbors have a 4th floor and got 4 ton units. So I wonder if we all got the same ductwork that is indeed adequate for the 4 ton homes. The plenums at least are the same.

Alternatively I could get the same unit in a size B 1200 cfm unit that would more closely match what I have now, at the expense of a point of SEER for AC mode with the smaller coil.

I realize these cfm ratings are just nominal values and actual values depend on many factors including the ductwork. I understand that a tech could get velocity and pressure readings to verify if necessary. I am just trying to get a ballpark feel for now assuming the original design was decent, which I think it is.
1) Unit needs to be properly sized to get the best effect/performance from it

2) The Lennox SLP you can tweak/adjust the blower speeds on 2 stages / in 2 steps /scenarios/ including the mod feature , (gets a bit technical if you have never seen or set one up) + or - 15% which can get you a quieter operation. All depends on the heat rise permitting that to be allowed. If it is running too hot due to undersized ductwork then you need to put it back to the norm setting.

3) Lennox does not sell to anyone other than their approved contractors so DIY with them is out of the question.
 
#23 ·
Thanks, Yuri. That sounds similar to the adjustments with the York and I understand that if the temp rise gets too high you are operating nearer the safety cutoff. Probably don't want to push that limit to allow for variation like dirty filters and such

DIY... not sure I would take this on, but I might... unless my current system fails at the proverbial worst time, which always seems to be the case and I need to get it done fast. And one benefit of DIY is that if something isn't done perfectly I will know who to complain to and that they will listen. ;-)

I have done my own A/C maintenace forever, done my cars A/Cs forever, have my EPA cert, have some but not all of the tools etc. I'll probably get a couple of quotes but am in no hurry yet since my current system works fine. I am just thinking about how nice a new mod or VS furnace would be in the winter... not having all those cool periods in between the relatively short single speed furnace runs would be so nice....

I appreciate you guys helping me understand how these mod furnaces work. Sounds like great technology.


1) Unit needs to be properly sized to get the best effect/performance from it

2) The Lennox SLP you can tweak/adjust the blower speeds on 2 stages / in 2 steps /scenarios/ including the mod feature , (gets a bit technical if you have never seen or set one up) + or - 15% which can get you a quieter operation. All depends on the heat rise permitting that to be allowed. If it is running too hot due to undersized ductwork then you need to put it back to the norm setting.

3) Lennox does not sell to anyone other than their approved contractors so DIY with them is out of the question.
 
#20 ·
No no no,he believes he has the right size already.Believing always trumps knowing for sure.
We all know that doing the calc is almost impossible to do.Its so hard that mere mortals would have no chance at it.
Knowing the exact size his house would distroy his belief system and leave him a wounded human being for the rest of his life.
ALSO HE IS RICH AND PAYING MORE THAN HE NEEDS TO FOR ENERGY IS HIS RIGHT,no matter how high the energy costs get to in the next 20 years.Its only chump change for him anyways.
 
#21 · (Edited)
I am definitley paying too much for energy with my old system and I don't like it... but the payback with a new efficient system isn't a sure thing. First the cost of a new system is a lot of $$, then if some complicated bit fails outside of warranty the math goes south fast. So it it more a matter of comfort and not wanting to hang onto an old system too long and have it fail at the worst possible time. Isn't that how it always happens?

I have a friend who moved into their house the same time I did and got one of those fancy ground source heat pumps. He has put ~$10,000 into it for repairs and replacment parts in the 16 years, while all I have had to do with my basic split gas system is replace the dual run capacitor ($20) and put in 2 lbs of R-22 (free from a friend). Who got the better deal? My only regret with my system is that the builder didn't spec a higher efficiency furnace.

But I am still surprised their aren't codes that require that 90+ furnaces be used when the structure allows for its installation. I have another friend whose A/C failed (indoor coil blew out) last week during a bad heat wave. He ran home and his wife was on him to get 'er done. Trane contractor came out and sold him a system with an 80% furnace unit... my friend didn't even know the difference or that 95% units were available and install would have been easy in their house. But they had the existing type B vent in place so the contractor may have just decided not to try to upsell, which would also make more work for the install for them. I don't know since I wasn't there. Anyway, that's a lot of heating efficiency they are missing out on... and a lot of gas that will be wasted.
 
#25 ·
In Canada every new house has to have a 90% efficient furnace, 92% in Manitoba and 95% may be here as a standard soon. Other options are electric (very expensive) oil and propane (ridiculously expensive) and geothermal which start around $25,000. Heat pumps and elec in Southern Ontario and BC but electricity to run them is still expensive.
 
#26 ·
I think standards like that are a good thing. The equipment price difference between 80% and 90+ % isn't all that much so no reason to get 80% unless there is absolutely no way to run the direct vent for a high eff unit.... or maybe if you live in warm region that doesn't need much heat. It still bothers me to see energy just wasted, though. I am glad to live in a relatively moderate area where we actually usually have a few months where we generally need neither heating or cooling. That is the time to do system replacement if one can plan it. I can't even imagine having to heat a place in the Great White North.
 
#28 ·
Depends on how new and well built your house is up here. Buildings standards have got VERY high here with glued on vapor barrier etc etc. I have 2x6 walls R20, R40 in the attic, low E argon windows and can heat my house for under $100/month in the coldest 4 months. AC costs are cheap here as it gets cool at night and we shut em down. There are people paying $200-300/ month to heat 100 yr old houses with old hot water boilers. Some of them converted from coal to oil and then gas. Not sure how they afford that but they like that comfy feel of old cast iron radiators beaming the heat out all the time. Does not help to have sawdust for insulation in the walls and poor windows too.:no:
 
#30 ·
Just another thought on the whole sizing discussion... With the flexibility of the modulating systems I wonder why mfgs don't have adjustments to tweak the algorithms... like Beenthere and I were discussing the choice between 60k and 80 BTUH furnaces and I understand the importance of getting the correct size, esp with old "dumb" equipment. But since a mod furnace can run in a very wide BTUH range why not have a couple of selectable profiles that could fine tune the algorithm? Say you could choose an algorithm for an 80 K BTUH nominal unit to not go above 65K, 70K, or 75K BTUH, or whatever. Seems something like that could be easily accomplished in firmware and allow fewer sizes of furnaces to cover a wider range of applications. Less parts inventory, more fine tuned operation... all accomplished with a few changes in the firmware.
 
#35 ·
It would cost more in parts. The 80,000 would need to have 2 different boards, since it comes in 2 different blower sizes.

There are only 4 sizes of furnaces in the York Mod line. And i think all other brands that have a mod also only have 4 sizes.

Its best to just find out what size the house really needs, and use that size. Instead of trying to change the design of the unit.
 
#32 · (Edited)
It is not as easy as it sounds. In order to have a STABLE flame, proper ignition etc the burners have to be extremely well tuned so they cannot alter the firing rate too much w/o creating other problems. Then there is the problem of not enough temp rise and condensation in the primary heat exchanger and corrosion etc etc etc.

Bob and Doug and also the Trailer Park Boys are my heroes. Google both of them to find some hilarious You Tube vids.
 

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#33 ·
I am just talking about having it operate well within the firing rates/blower speeds it is capable of with the corresponding design blower speeds and temp rises. Just have some profiles that have different max firing rates for installs that might not need the full rated burn. Seems simple enough.

It is not as easy as it sounds. In order to have a STABLE flame, proper ignition etc the burners have to be extremely well tuned so they cannot alter the firing rate too much w/o creating other problems. Then there is the problem of not enough temp rise and condensation in the primary heat exchanger and corrosion etc etc etc.

Bob and Doug and also the Trailer Park Boys are my heroes. Google both of them to find some hilarious You Tube vids.
 
#34 ·
I agree but there would have to be a lot of $$ spent on research and testing to make sure it works well from Texas to Anchorage under a huge variety of temps etc and they don't want to do that yet. I worked with full blown Siemens bldg automation systems and chillers with VFDs etc and modulating high pressure steam boilers and know what can be done with controls.
 
#39 · (Edited)
throw in another factor. VS blowers are not perfect and CANNOT compensate for undersized ductwork. They will seriously overspeed/hunt/cavitate if they have to run wide full out due to too much resistance. the newer ones can stop or slow down the motor when that happens but it still won't get you air where you want it. therefore the manufacturers cannot design a one size fitz all unit.:no:
 
#40 ·
I agree with you guys fully that there is no way to make a "one -size fits all" unit. But you are missing my point. I am not saying have radically different profiles. Keep the "profile" the same. All I am saying is have selector jumpers that limit the max output in the usual profile to the multiples of 5k in between the nominal max output down to the next smaller unit. Then when someone gets a load calc that says 67k they don't have to choose between 60 and 80. The 80 may be "close enough" but when you can do better, why not do better? Like get the 80 nominal max unit and and set the jumper for 65 which is well within the normal range of outputs when it modulates. Then if you later find in practice that you need 70 or 75, swap the jumper and there you go.

Anyway, I'd be surprised to not see something like this in the future. Actually, I am surprised to not see it now. The equipment can handle it. Seems crazy not to take advantage of the capabilities.

throw in another factor. VS blowers are not perfect and CANNOT compensate for undersized ductwork. They will seriously overspeed/hunt/capitulate if they have to run wide full out due to too much resistance. the newer ones can stop or slow down the motor when that happens but it still won't get you air where you want it. therefore the manufacturers cannot design a one size fitz all unit.:no:
 
#44 ·
Your old PSC blower is probably not moving as much air as you think. A VS blower speeds up to move its set CFM, a PSC motor can 't do that.

The 60,000 Would be the better choice. Its easier to seal a house a little more to make it require less heat, then it is to enlarge duct work to make it quiter.

Don't size for worse case conditions, that makes the furnace over sized too much.

The size of the house doesn't determine how many CFM are needed. The BTU loss and gain does that.
 
#45 ·
Yes, the CFM numbers surprised me when I saw them in the Bryant manual. My Bryant blower motor is only 1/3 HP as compared to the York 1/2 HP in the 1.5-3 ton B cabinet models. So how exactly does it put out more CFM? Were the testing and rating standards different back in the early 1990s when these were made? Did they rate CFM based on essentially no backpressure?

The 60K York would be better in many ways... more run time which would also run the humidifier more. Might only be a little weak in zero degree conditions that don't happen that often. I will prob go with the 60K... worst case is that I have to upgrade it to the 80K if it proves to be inadequate.

I was headed that way (the 60K unit) until I saw the numbers in the Bryant manual. OK, I'll think about it some more... But I think the 60K will work. Of course, until I actually pull the trigger I may change my mind again. :-/

One thing York did that was smart was to make the furnaces compact (33" height) so that with the new taller high SEER evap coils the systems will fit into an existing installation without modifying the supply plenum. That will save a lot of work. I see alot of folks getting upgrades where the contractor sells them equipment that necessitates major plenum revisions. Totally unnecessary if you pick the right components.


Your old PSC blower is probably not moving as much air as you think. A VS blower speeds up to move its set CFM, a PSC motor can 't do that.

The 60,000 Would be the better choice. Its easier to seal a house a little more to make it require less heat, then it is to enlarge duct work to make it quiter.

Don't size for worse case conditions, that makes the furnace over sized too much.

The size of the house doesn't determine how many CFM are needed. The BTU loss and gain does that.
 
#46 ·
Look at your old furnaces CFM rating at statics/ESPs above .5".

Again, improve the homes insulation and seal it better. this will save you money in both heating and cooling season every year, year after year. A larger furnace just uses more fuel, and cost you more money year after year.
 
#47 ·
The Bryant manual only has the single topline numbers for "certified conditions" between .15 and .5 so no way for me to check that. I might try to borrow a manometer and see what my actuals are. That would be interesting.

Not much to seal here. House is pretty modern. I guess I could put a storm door on the windward exterior door. Almost impossible to do anything in the attic with cathedral ceilings. Only an 8x8 area is accessible. Not sure what R-value the flex ducts up there are but replacing them would cost way more than any savings.



Look at your old furnaces CFM rating at statics/ESPs above .5".

Again, improve the homes insulation and seal it better. this will save you money in both heating and cooling season every year, year after year. A larger furnace just uses more fuel, and cost you more money year after year.
 
#48 ·
Any switches or receps on outside walls? Also the ones on inside walls sometimes leak a lot also, if the holes in the plates at the attic and basement/crawlspace aren't sealed.If so, seal them. How about worn door seals?
 
#49 ·
Yup, door seals could use replaced. Or just add a storm door on the windward door that is the main front entrance. The south side leeward doors (2) aren't so critical. The attic is impossible. I have cathedral ceilings so no way to get anywhere but an 8'x8' "attic" in the center of the house. Otherwise need a spry midget with a light and dust mask to try to crawl in the rafters up to the wall plates..... or rip out ceiling drywall. Not gonna happen!
 
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