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Old 08-06-2011, 06:25 AM   #1
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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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Old 08-06-2011, 08:53 AM   #2
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sizing choice on system upgrade


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 10F temp change. The mod regulates the blower to maintain a set heat rise.

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Old 08-06-2011, 09:14 AM   #3
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sizing choice on system upgrade


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.



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Originally Posted by beenthere View Post
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 10F temp change. The mod regulates the blower to maintain a set heat rise.

Last edited by raylo32; 08-06-2011 at 09:24 AM.
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Old 08-06-2011, 10:02 AM   #4
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Originally Posted by raylo32 View Post
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.
Many contractors won't, or don't know how to do a load calc.

An over sized York/UPG mod won't stay in low fire longer then a correctly sized one will. The algorithm will make it increase. Plus, it will already be at a higher BTU rate to begin with. Even though a difference of 8,000 BTUs output may not sound like much, it is at mild outdoor temps, when your heat loss may only be 16,000 BTUs. The 60,000 would have a much longer run time then a 80,000 would.

On your coldest nights, how long does your current furnace have to run?

Remember, at 40%, a 80,000 BTU 95% mod is 30,400 BTUs output, which is 45% of your current furnace output. The 60,000 at 40% is 22,800 Output, and only 35% of your current furnace's output. Providing much longer on time at warmer outdoor temps.

An over sized York mod, won't go into extended run until the outdoor temp is much colder. A correctly sized one will go into extended run sooner, and provide a much tighter temp control at lower outdoor temps.

Also, the 60,000 mod, will only at most be 10,000 BTUs less then your current furnace. Less if you get one of the 97% ones.
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Old 08-06-2011, 10:06 AM   #5
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sizing choice on system upgrade


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.

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Old 08-06-2011, 10:21 AM   #6
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Here in August it is hard to remember just how much the heater runs in winter. We do have those outlier nights down around 0 degrees F but not often much below 15-20 degrees F. On the 15 degree nights it runs a good bit and on the zero nights a "lot". Unfortunately this is just subjective. Plus my old system is pulling in all that cold air for combustion.

I don't think I would find a mod running at high speed once in awhile objectionable. Heck I have a 1-speed high only system now. But that might not be the case if I got a system that had a 3O% more powerful blower. Based on your input about the algorithm I don't think I would want that C sized 1600 CFM York. The B size with the smaller blower that is close to what I have would probably be a better choice (not sure about the 80k vs 60k, though) and the noise at full speed would be what I have now... not too bad.

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Many contractors won't, or don't know how to do a load calc.

An over sized York/UPG mod won't stay in low fire longer then a correctly sized one will. The algorithm will make it increase. Plus, it will already be at a higher BTU rate to begin with. Even though a difference of 8,000 BTUs output may not sound like much, it is at mild outdoor temps, when your heat loss may only be 16,000 BTUs. The 60,000 would have a much longer run time then a 80,000 would.

On your coldest nights, how long does your current furnace have to run?

Remember, at 40%, a 80,000 BTU 95% mod is 30,400 BTUs output, which is 45% of your current furnace output. The 60,000 at 40% is 22,800 Output, and only 35% of your current furnace's output. Providing much longer on time at warmer outdoor temps.

An over sized York mod, won't go into extended run until the outdoor temp is much colder. A correctly sized one will go into extended run sooner, and provide a much tighter temp control at lower outdoor temps.

Also, the 60,000 mod, will only at most be 10,000 BTUs less then your current furnace. Less if you get one of the 97% ones.
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Old 08-06-2011, 10:32 AM   #7
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sizing choice on system upgrade


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.

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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
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Old 08-06-2011, 10:42 AM   #8
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Originally Posted by raylo32 View Post
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.
Wouldn't it be better to know and not think to know?

With such a large system investment, I would want to know exactly how much I needed.

Typical system installations of yore were as much as 25% oversized, with the higher efficiencies, continuous or close to continous running (if sized correctly) mod furnaces of today, you want to ensure the size is best suited for your load, if not you will not capture the advantages of the increased system investment.
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Old 08-06-2011, 10:42 AM   #9
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Here in August it is hard to remember just how much the heater runs in winter. We do have those outlier nights down around 0 degrees F but not often much below 15-20 degrees F. On the 15 degree nights it runs a good bit and on the zero nights a "lot". Unfortunately this is just subjective. Plus my old system is pulling in all that cold air for combustion.

I don't think I would find a mod running at high speed once in awhile objectionable. Heck I have a 1-speed high only system now. But that might not be the case if I got a system that had a 3O% more powerful blower. Based on your input about the algorithm I don't think I would want that C sized 1600 CFM York. The B size with the smaller blower that is close to what I have would probably be a better choice (not sure about the 80k vs 60k, though) and the noise at full speed would be what I have now... not too bad.
Weather a 4 ton or 3 ton blower(1600 or 1200 CFM) makes no difference in heating mode. They will both run the same CFM in heating mode.

Doubt your current furnace is moving as much air as a 60,000 BTU 3 ton blower Mod will at high fire/heat.

A VS blower will speed up to move set CFM. Your current PSC blower can't do that. So an over sized mod can make your duct system very loud when it has to use its higher firing rates.

At 38,000 BTU output, a 80,000 BTU and a 60,000 BTU 95% mod will both be moving 640 CFM, if set to maintain a 55F temp rise.

At full input, the 60,000 will want to move 960 CFm for a 55 degree temp rise, and the 80,000 at full input will want to move 1,279 CFM to maintain a 55 degree temp rise.

Very good chance your current furnace is only moving between 1000 to 1100 CFM in high blower speed in cooling mode(maybe less).

many people have found out the hard way that their duct system is under sized, after they got a VS blower.
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Old 08-06-2011, 11:22 AM   #10
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sizing choice on system upgrade


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?
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Old 08-06-2011, 12:26 PM   #11
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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.
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Old 08-06-2011, 12:32 PM   #12
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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.
Direct vent units still use outside air for combustion, either piped directly into the combustion chamber or next to the furnace.
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Old 08-06-2011, 12:37 PM   #13
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Direct vent units still use outside air for combustion, either piped directly into the combustion chamber or next to the furnace.
Direct vent doesn't dry the house out from all that dry air being drawn into the home. Plus the amount of excess air for combustion is generally less on a properly set up direct vent furnace.
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Old 08-06-2011, 12:43 PM   #14
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Direct vent doesn't dry the house out from all that dry air being drawn into the home. Plus the amount of excess air for combustion is generally less on a properly set up direct vent furnace.
Agreed, but it is still outside air, just to clarify the OP's statement.
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Old 08-06-2011, 12:45 PM   #15
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Agreed, but it is still outside air, just to clarify the OP's statement.

True, takes the same amount of BTUs to warm that air weather drawn in through a PVC pipe directly to the burner, or drawn through the house.

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