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Old 03-11-2010, 05:54 PM   #91
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Yes, do the load calc, and then you'll have the size equipment that you should have, and CFM each room should get.

For duct design. Need to know how much air to move first.
To know what frictin rate to use, you need to jknow the total equivalent length of the longest run.

Open all registers full. Then take several readings on each register to get an average reading. Then you can use the free area of the register, and multiply it by the FPM to find the CFM from the register.

That guide for zoning is ok. And its nypas formula is the correct one.

You can install the dampers as you go, as long as they are power close, spring open dampers.

As for a humidifier. Which type depends on how much moisture you need to add.
I like both Aprilaire flow through bypass and power, and EWC Autoflo steam humidifiers.

Return position, weather high or low is seldom of importance.
In a basement, I prefer to get them low. But if you use the correct supply registe, it doesn't matter.

Pretty much the same for any other floor as far as, if you have registers with good throw, high or low doesn't matter.

Outside air for combustion is semi easy. If you have a wall you can put a hole in that will except a 6" round pipe.
Make the hole insert pipe. use an intake hood on the outside. Rune the pipe toward the equipment, and run it down and into a 5 gallon bucket. The bucket helps prevent teh area from being flooded with cold outdoor air when the furnace or water heater isn't on.

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Old 03-12-2010, 10:20 AM   #92
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As always, thank you so much for all of your help.

I spent last night collecting all the measurements for HVAC Calc and tonight I will get everything entered in. I am curious to see how it comes out!

Given the other work I will be doing, adding a 6" pipe will be easy. Is PVC OK? If I do this, is that sufficient for both the furnace AND the water heater? Does it mean I can put a solid door on the furnace room, or do I need to keep the louvers? And how much cold/hot air is going to get in/out? My concern is that the furnace room is in the living room of my mother-in-law's apartment. She complains enough as it is, I don't need to add draftyness to her list!!
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Old 03-13-2010, 03:04 AM   #93
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You'll still need the louvered doors. A 6" pipe isn't enough for both the furnace and water heater.

Look at the BTU input rating of both the furnace and water hater, and add them together.

You need 50 cubic foot of volume for every 1,000 BTUs of input in order to not need a combustion air intake.

So if your furnace is 80,000 BTUs input, and your water heater is 32,000 BTUs input, thats 112,000 BTUs total.
So 112X50=6,700 cubic foot of volume that your basement needs to not require a combustion air intake.

Is the living room that big, if thats the size of your equipment?

PS: Direct vent furnaces aren't included in that since they already get their combustion air from outside.
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Old 03-15-2010, 08:21 AM   #94
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As always, thanks!

I got all my info entered into HVAC Calc last night and just got my registration code this morning, so hopefully this evening I will learn the heat gain/loss details for my house.

So, I am really glad I asked about combustion air because I think I am in big trouble! My furnace is 100,000 btu and my hot water heater is 40,000 btu, neither of them are direct vent. So, 140,000 but means I need 7,000 cubic feet of combustion air, right? The problem is, at best, the entire basement is only 4,500 cubic feet (650 sq. ft. x 7 ft. ceiling), and worse yet, the living room is only about 1,500 cubic feet. That said, it is an old house and it is not "tight" at all, but how serious of a problem is this? I would assume it is dangerous. Will it be enough to add a 6" pipe, or do I need more?
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Old 03-15-2010, 08:41 AM   #95
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If all of the space in the basement is open, or has the ability for the living room to draw air from.
Then a 6" is enough.

This includes the bedroom area if its door is under cut.
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Old 03-15-2010, 08:44 AM   #96
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Yep, the door is undercut, as is the door from the basement to the rest of the house. That said adding the 6" is necessary, right?

What happens with not enough combustion air? Is it dangerous?

Also, what does this mean for installing a return air vent in the basement? Can I? Or is it a problem given the amount of combustion air needed?

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Old 03-15-2010, 03:20 PM   #97
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Unless your leaving the door from the first floor to the basement open. You need the 6".
Or you need to make the door a louvered door.

As for a return in the basement.

You can get a way with a small one, that removes less then the supply puts in. Other wise, the return could pull from the chimney.
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Old 03-16-2010, 10:27 AM   #98
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Sounds great! I would never have thought of adding combustible air, really, but it seems like it needs it. I'll add it to the "to do" list. Fortunately, with the addition we're planning, it shouldn't be too difficult to add in.

OK, so I finally finished entering all my info into HVAC Calc and I am a bit worried. Either, I really screwed up somewhere OR my system is way, way, way oversized, worse than I had imagined. Would you mind taking a look at the attached reports?

Basically, it is showing a total heat gain of 16,151 BTUH and a total heat loss of 38,922 BTUH. The report recommends a 1.5 ton AC and I would guess a 60k BTU furnace would be appropriate based on the heat loss, right?

My current equipment includes:

Furnace: Rheem 80% upflow/horizontal gas furnace, model number 80PS10EDR01, 5 ton, 100,000 BTU(input), 80,000 BTU(output). Depending on static pressure, the low setting on the blower has CFMs of between 1,425 and 1,645. The specs/manual can be found here.

AC Coil: Rheem 4 TON 21", 13 SEER cased coil, model number RCFA-HM4821AC. The manual/specs can be found here.

AC Condenser: Rheem Corsaire 13 SEER, 3 ton condenser, model number 13AJA36A01. The manual/specs can be found here.

So, based on the HVAC Calc analysis, I am WAY oversized, right? Do you see anything funky on the report that might account for this? Did I forget anything big? I had a couple of issues when inputing my house data:
1)Do I need to enter hallways, closets, stairwells? I didn’t which is why the report sq. ft. is off from actual (approx. 1,800 sq. ft.).
2)My house is constructed entirely out of brick, the walls are two bricks thick, or about 8”. HVAC Calc had only 2 masonry options, 1) brick and block and 2) 8-12” block. I selected option 2. Was that right?
3)We have a walk-out basement, meaning that in the front of the house, we are below grade, but at the back of the house, we are at grade. I selected the option “Basement floor, 2' or more below grade” for the whole basement, but how should I have handled this?

Assuming the report isn’t totally incorrect, is there anyway that my current equipment can work in this application? Does it make sense to adjust the ductwork to work with an oversized system?

As for the duct design, assuming I don’t have to change out all of my equipment, I ran the CFM report based on my current unit, using 1,600 CFMs. That’s correct right? So, what do I do now? Is the information contained here close to correct?
1)You had indicated previously that I need to find the friction rate by determining the equivalent length of the longest duct run. Does this mean from where the trunk branches off from the plenum until it reaches the furthest register, basically? Once I have this distance, how do I figure out the rate of friction?
2)Once I have the rate of friction, I assume I use a ductolator to figure out the duct sizes, right? Is this one any good? Online Residential Ductolator
3)I have read, perhaps in the HVAC Calc manual, that the average 4x10 register only delivers 90 CFM. Is this accurate, or do you use a different rule of thumb? Are there different/other types of registers that put out more? Do you generally use a 6” duct to feed a standard register?
4) Do the CFMs on the report seem to make sense? Obviously, at 1600 CFMs, it is a lot more than I need.

I know this is a lot, on a few different topics. Thanks so much for your help!
Attached Files
File Type: pdf Stubits_CFMReport.pdf (9.6 KB, 97 views)
File Type: pdf Stubits_ByRoomEachComponentSelectionsSqFt.pdf (49.5 KB, 81 views)
File Type: pdf Stubits_ByComponent.pdf (10.7 KB, 96 views)

Last edited by stubits; 03-16-2010 at 02:15 PM.
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Old 03-16-2010, 04:03 PM   #99
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Yes, you include the rooms that are not being heated. As they add volume for the infiltration calculation.

Also, you need to add 1200 to 1500 BTUs to the misc. This is to cover cooking, showering, normal amount of electronics, etc.

A 1.5 ton will struggle to meet your sensible at design temps. So a 2 ton would be better.

Yes, your furnace is way over sized.

Brick has a higher thermal resistance then block. But using 8 to 12" will be close to 2-4" bricks.

Your basement floor selection will work fine since most of the floor will be more then 2' below grade.

A 4X10 floor grille seldom has more then 80CFM going through it, if its quiet. 100 CFM through one is tolerable noise. But over that, you'll be turning up the TV. A 6X10, or 4X14 is better for CFM's in excess of 100 CFM.

A 6" supply is fine for 80 CFM.

Supply size is determined by CFM needed, and TEL.

TEL is figured by the length of the longest return and supply added together. Along with the pressure drop of all fittings in the duct system.

EG: A register boot is equivalent to 15' of duct.
A take off is equal to 15', unless there are others beside it, then its equal to 35'(air turbulence of other take offs adds to it).
A reducing transition adds 5' to duct length besides its physical length.

At 1600 CFM for your air flows. It looks ok.

Now do that CFM report over. using 960CFM.
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Old 03-16-2010, 04:20 PM   #100
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Jim-

Thanks so much! First, just noticed you are from Lancaster. My wife and I went up there a couple of weeks ago just to get away for a long weekend. We stayed at the Inn at Loyola Village, went to the farmer's market downtown on Saturday morning and pretty much just bummed around for a couple of days. We absolutely loved it. You're lucky to live up that way.

Thank you, as always for all the info, this is really coming together! When I get home tonight I will re-run all of the reports. I need to add in the closet space and hallways, probably an additional 200 sq. ft. Also, the HVAC Calc manual suggests no including window coverings because more often than not, they are not drawn, so I will correct that too. I imagine the numbers will change a little bit, but not much.

So, regarding the equipment, both the AC and the furnace, are they just too big to work with? Or, do you think I can get decent enough performance if I set things up correctly? If I hired you, would you insist I get a new furnace and AC? Or could you make it work?

Is there a resource that has the distance value of the various fittings? I need to think through it a bit, but I know I have some 90s, some Ts, some wyes, etc. I read that it is sometimes acceptable to assume a friction rate of between .05 and .08 for residential work. Is that accurate or should I just go ahead and do the math?

I am sure I am just slow on catching on, but can you explain why I should run the CFM report at 960 CFM rather than 1600CFM? What am I not understanding here?

Once I have the rate of friction and a correct CFM report, do I use a ductolator to figure out the duct sizes? Is this one any good? Online Residential Ductolator
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Old 03-16-2010, 04:35 PM   #101
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Trying to make oversized systems work right. Is generally an exercise in futility.
So I don't bother trying it. Do it right once. Beats the heck out of try try try again.

960, is about the CFM your house should need with the proper sized equipment.

Your one room. has a CFM of 270 with using 1600 CFM. That would be 2=7" supplies min, if not 2-7's and 1-6".

Does that room have those supplies. Or does it have 2-6" for about 160CFM(which would be about right for a 960CFM through 60,000BTU input 95% furnace for heat).
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Old 03-16-2010, 04:47 PM   #102
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OK, so it seems like it would be a huge error to update the ductwork, install zones, etc. based on a seriously oversized furnace/air handler. Do you think I need to replace the AC condenser (3 ton) and/or the AC coil (3.5 ton)? It doesn't seem like either are as grossly over-sized as the furnace. Are they workable?

As for a new furnace, I've been looking at 95% efficient, 60,000 BTU furnaces and the cost isn't prohibitive. A couple of questions though:

1) Is a two stage burner worth the additional cost? How about a variable speed blower?
2) If I am able to keep my existing condenser and coil, do I need to look for anything specific to make sure the new furnaces will work well with them?
3) Am I able to replace the furnace myself? The venting for the 95% efficient furnaces seems easy enough, as does the condensation (I have a condensate pump for my AC I would tie into, and of course the gas line is already there). Is there anything that would specifically complicate me doing this myself?
4) Is there a brand of furnace or any specific features you'd recommend?

This looks interesting, as does this. Am I looking in the right direction? Are these prices reasonable?

I updated my heat gain/loss info based on your answers above. Now I have a total heat gain of 22,111 (suggested 2 ton) and a total heat loss if 42,872.

I have also run CFM report for 960 CFM. I have to say, it seems a little off to me. For example the basement is always chilly in the winter, although comfortable in the summer, so 152 CFM for the whole 675 sq. ft. basement seems insufficient, but maybe I'm just not understanding how this all works. That said, I think my current system might be skewing my understanding. Even though it is a 5 ton, 100,000 BTU, 1600 CFM furnace, all of the return air comes through a 12x10 duct, so just about 400-500 cfm, right? So, right now, in spite of the huge, high power equipment, we're only pushing a small amount of air, which explains the lack of comfort. Yes? No?

I am attaching the updated HVAC Calc reports in case they help!

Thanks for everything!
Attached Files
File Type: pdf Stubits_CFMReport(2).pdf (10.4 KB, 100 views)
File Type: pdf Stubits_ByRoomEachComponentSelectionsSqFt(2).pdf (59.1 KB, 79 views)
File Type: pdf Stubits_ByComponent(2).pdf (10.7 KB, 66 views)

Last edited by stubits; 03-16-2010 at 10:58 PM.
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Old 03-17-2010, 06:15 AM   #103
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First link, its a good furnace. Second link, says product no longer exists.

Weather a home owner/DIYer should install his own furnace. Depends on how mechanically inclined they are, and how well they can understand install instructions when they read they(there are installers that shouldn't install).

How well has your current 3 ton A/C been cooling your house? If its not too humid in the summer, you may be able to leave it in until it dies. If you have high humidity, then its probably best to change it out at the same time.

For you basement, 152CFM at a 55 difference between supply and return temp. It comes out to 9,325BTUs being delivered to the basement. So it exceeds what you need.

When you get strange CFMs like that. You have to install dampers in the supplies to help balance the system out.

A single 10X12 return duct doesn't move a lot of air. How much yours is moving, is hard to say from here. If your getting 900CFM through it. Your probably running close to a .6" static in the return alone.
Thats something that needs corrected even with a new furnace.

A VS blower is is a nice thing to have. Since it means the furnace is also 2 stage. Gives you a long run time in first stage, with a low air flow. So you tend to get a more even temp through out the house. Plus, in low heat, with the lower air flow. it is a real quiet system.

With your oversized furnace. your air is being forced out to areas that don't really need as much heat as they are getting. And the duct work prohibits the furnace from being able to push enough air to supply the rooms that need more heat. Typical bad design.
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Old 03-17-2010, 01:01 PM   #104
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Thanks so much as always, this thread is becoming a bit of a book, a manual on how to fix a totally screwed up HVAC system.

OK, so right now it seems like there are two issues, 1) replacing the furnace/air handler and possibly the AC equipment and 2) designing the ductwork for the home. I have, of course, additional questions on both topics.

Replacing Furnace/Air Handler/ AC Equipment
I am convinced, thanks to you, that I need to replace the furnace/air handler… and I managed to convince my wife as well, which is no small task.

1)There seems to be a big difference in price between Dual Stage Variable Speed furnaces and Dual Stage Multi-Speed Furnaces. What is the difference? Will the variable speed be that much of an improvement? I understand that the dual stage allows for lower heat air.


2)I think I am comfortable with the install, especially since I will just be replacing one unit with another, so the gas lines, electrical, etc. are all there. I will plan on “direct venting” the furnace, using a concentric vent setup, but I don’t see this being a problem, the distances are fine. Our furnace room doesn’t have a convenient place to tie in the condensation drain from the furnace, but we do have a condensate pump installed for our AC which pumps the condensate outside. Can I connect the furnace condensate drain to this as well?


3)I am having a hard time figuring out if I should replace the AC condenser and coil. They are brand new and so I feel bad just ripping them out, also AC is a different beast for DIY because of the refrigerant AND the need to braze the lines. Our home is actually more comfortable in the winter than in the summer and the AC has seemed to work well enough. But past performance is a poor indicator, I think because the return duct is so minimal (12x8 for a 1600 cfm system) AND because the addition with are adding will make the house much more efficient as it will replace a lot of the un-insulated exterior walls. I know it is oversized, but can I go with it for now and replace it later, if necessary, or when it dies?

Duct Design

Ok, I think I am starting to understand things here a bit more.

1)Do I need to select an air handler before I can start to move forward on this, or is 960 CFM a safe assumption for a 60-70,000 BTU, 95% efficiency, 3 ton (?), dual stage, variable speed furnace?


2)I am not quite sure what to do with the friction rate. I have added up the value of all of the fittings I can envision in the system, along with the longest supply run and the longest return run. Do I divide that by total CFM? Or the other way around?


3)I understand you’re point now about the basement. I think my understanding of HVAC has been confused by my current, poorly function system. I am anxious to get my anemometer, because I serious doubt I have 200 cfm coming from any of my registers. Based on the results of HVAC Calc CFM report, I have started to work through the duct sizes a bit, although I know I am lacking the friction rate. I am attaching it here. I am completely off or is this starting to come together? Is this and OK way to look at things, or should I be drawing a diagram? We have a small home, so none of the runs are particularly long, certainly nothing more than even 50’.


4)I have just been using a ductolator to figure out the above information. Is there a better or more precise way to go about it? I know that if velocity is too high it causes a lot of noise, but I would imagine if velocity is too low, there would not be enough throw, or is that not correct? Is there a “sweet spot” on the velocity?


5)Round duct is a better choice than square duct, right? It also seems moderately less expensive. That said, I guess it uses space a little less efficiently. I should also avoid any form of flex duct, right?


5) Are there registers and diffusers that are better than others, or are all pretty much the same, assuming they have built-in dampers?

Thanks so much! Next time I am up in Lancaster, and we hop that will be soon, I hope you'll let me buy you a beer, a coke or whatever your preferred drink it. You've gone way beyond the call of duty here.

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Old 03-17-2010, 04:17 PM   #105
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The VS blower is similar to a VFD. It monitors its torque, and adjusts its speed based on what CFM its set to. So it maintains a constant air flow. Its an expensive control set up, along with the motor costing much more. So thats what makes the big price difference.

The VS will generally deliver better air flow then a PSC/multi speed motor. Plus, furnaces with VS blowers have a setting to slow their ramp up. So that they remove more moisture when in cooling mode.

You may want to reroute the condensate pumps drain to another drain area. In the winter, the condensate pumps drain line can freeze if it goes outside.

Figure out your longest run. Add all fittings, Then subtract all other devices that cause a pressure drop. Such as the A/C coil itself, your supply and return register. Use .06 for the return and supply register combined.

A drawing makes it much easier to figure TEL.

So you would use .5" for your furnaces ASP(Available Static Pressure).
Then subtract .03 for the supply and return grille. And you would have .44" left.
Then subtract your A/C coils PD. Say .19". That leaves you with .25" ASP.

So lets say your supply and return come out to 295' TEL

Take (.25X100)/295=.084" FR. So you would use a .08"FR on your ductulator.

For supply runs, round pipe is better.

I use Honeywell mostly. And then select the type that has the throw and or spread I want.


Then

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