Best Unit for Undersized Return

[mrconsumer]

[See UPDATE on Manual J results, page 2 of thread.]

I am in a 2000 sq. foot, three story townhouse in Boston with a 19-year old Lennox hot air furnace, with 137,000 BTUs. The a/c is 5 ton. Thinking it is near the end of its life, I have started shopping for replacements. From reading this board, I am leaning toward a Rheem modulating furnace, but would consider a Trane or American Standard. Being a cheapstake, I only heat the place to 60 degrees day and night in the winter, and only use the a/c five days a year. (I use window a/c's instead a lot.)

The best I can tell, the sheet metal ductwork was improperly routed and sized when this place was built, and realistically is NOT changeable because it is all contained behind walls and ceilings.

From the top of the furnace, the two primary metal ducts going out are 18x12, and 18x9, with one flexible soft round duct, maybe 8". A few outlying registers give little or no output. The two main trunks are zoned ass-backwards with two dampers: front half of the house and back half, rather than by floor, so I basically keep them open wide.

The air returns, from what I can tell are grossly undersized. As it enters the furnace, the sheetmetal return duct is 16"x18". Working backwards, that leads up the three stories, with a few bends. With internal insulation, the actual internal dimension 12" x 17.5".

Given these issues, I have a few questions:

(1) Is the Rheem modulating furnace (RGFD) a good or bad idea? Those slower fan speeds may suck in too little air, no? Or, is it because it can self-adjust, it will be smart enough to run higher fan speed to suck in more air?

(2) Given my ducting problems, realistically NOT changeable without tearing up the walls of the whole house, do you have any other general recommendations?

(3) One installer said I should stick to the 5 ton a/c (which I think more right-sized should be 3.5 to 4 tons), because a smaller unit won't have the power for those large outgoing ducts. That doesn't make sense to me. Does it to you?

(4) From a few phone calls, and the one installer who visited so far, I get the sense that NO ONE will do a real heat load/loss calculation. They see what the ducting is, see how large the old unit is, speak to the distributor, and come up with a size. What do I realistically do, and how should my duct problems be factored into figuring out what the right size units are?

Thanks in advance.

Edgar

[Edited by mrconsumer on 08-25-2006 at 07:22 AM]

[BaldLoonie]

Somebody better do it right. 5 tons for a 2000 sq ft townhouse sounds about twice what a place in Boston would need!!!!!

Variable speed blowers don't like to be choked for air. The Mod will flash an error code if it tries to move more air than it can. You can cause motor or control damage too. Roughly calculating, your return is good for about 3 tons of air.

We don't have a contractor locator on here so I'll risk wrath and refer you to http://www.heatinghelp.com where they do. Those guys are boiler experts from the northeast but I bet you'll find someone good to handle your forced air situation.

[dash]

mrconsumer,

Have a Pro ,that can do the Manual J,S,and D calculations (hard to find ,keep calling till you do),run the calculations to size it correctly.


You may be able to add insulation,to reduce the size of the system that will be needed.Improvements to windows,doors etc ., can reduce the load as well,since your duct system is undersized,this would be the best way to go.

[mrconsumer]

>>You may be able to add insulation,to reduce the size of the system <

Dash,

The construction of the townhouse is with 2x4's in the outer walls, so they are not even 6" thick. There is insulation in the walls, and none can be added.

The windows are thermopane, and it is not realistic to say I am going to change windows.

Given the conditions I have, I really need to know the general type of furnace that will work best given an undersized air in-take.

I am sure some are better or worse than others. (With my current Lennox, I have had to keep the front metal door of the furnace open slightly to let in extra air, to keep the limit switch from tripping every 10 minutes.

Edgar

[pecmsg]

You need to size the equipment with Manual “J”
Select the equipment with Manual “S”
And size the ducts for the above with Manual “D”

If you want to size your equipment to the duct size and that’s not correct for the building, then what do you give up? Your comfort. And that’s what your paying for isn’t it?

[mrconsumer]

>>And size the ducts for the above with Manual “D”
>>

Pegmsg...

I can't say it any more clearly: my ducts are buried behind walls and in between floors and ceilings. I cannot realistically rip out all the walls of my condo to change the ducting.

I have to live with this ducting, so the question is what to replace my current equipment with given the constraint of the existing ducting.

Edgar

[dash]

First,have the load calculation done to see what size furnace and A/C you Really need ,yours sonuds oversized.A smaller system will work better on yur ducts,if it can do the job.

Or do the load calc yourself,on this site right here,

http://hvaccomputer.com/talkref.asp

for a small fee.

You should have a pro test the return and supply static ,this can be used to determine what the static of a smaller system will be,and give insight as to what mosifications can be done and what they would do.

There may be duct improvements that can be made,where the furnace connects to the supply and return duct,turning vanes would help a lot.Increasing return grille and filter size would also help.

Posting photos of what is accessable would help us help you.




Not a good idea,but if you want to replace it without looking into the improvements that you need ,stay away from VS indoor fans ,as the can self destruct on severely undersized ducts.

Find a furnace ,thru a Pro with the hisghest ESP(static)at 5 tons,one brand may be a little better then another,so check around.

[iheatncoolinnc]

Have someone size the system with manual J and find out what you need. have them evaluate the ductwork.With a manufal J calculation to determine what you need, you might need less. They can evaluate your ducting based on what you need. Maybe the ducting will be OK if you need less. All we can do is specualte on your duct size since we do not know the size of equipment that you actually need.

[kim]

You might want to consider 2 small systems.

But get an idea of how much capacity you need before doing all the work.

[BaldLoonie]

If you cycled on limit before, you WILL have trouble with a new furnace unless properly sized and the ductwork can support it. The old relics could run much hotter so if the old one needed the door cracked (very dangerous BTW) to keep off limit, you are guaranteed to limit out on a new one of similar size. AND today's limits which are $3 snap-discs at the end of a couple of posts, will not tolerate opening & closing like the old Honeywell L4064 did. A few times open and she'll stick open leaving you without heat. And today's lightweight heat exchangers don't take kindly to running hot either.

We're convinced, ducts aren't changing. So get your load done, compare that to duct capability. If you need more equipment than the ducts can handle. maybe 2 systems will be the right way. But I highly doubt you need anywhere near 5 tons of cooling. Not knowing all about the place, I'd guess that around here 2000 sq ft townhouse, especially with no outside exposure on left & right, would cool on 2.5 tons and heat with 75K 80% furnace at most. Many stand-alone homes that size get by with that equipement. Bigger is NOT better. We size to 95 summer and -10 winter.

[mrconsumer]

Thank you all for continuing comments and suggestions.

I just had a company here and insisted they do a Manual J... and he did. (Will figure results at the office, he said.)

Someone wanted pictures to help in making suggestions.

Here they are:

1. Furnace in closet in laundry room on lowest living level:

PICTURE NOTE: forum software seems not to bring you to the actual picture, but rather to my homepage. Place cut and paste the URL to view pix.

http://www.consumerworld.org/gifs/furnace1.jpg

2. Furnace with hot water heater showing (note outgoing duct splits into two (9x18", 12x18" and unseen 8" flexible):
http://www.consumerworld.org/gifs/furnace2.jpg

3. Return showing on the right:
http://www.consumerworld.org/gifs/furnace3.jpg

4. Return as it goes up into ceiling:
http://www.consumerworld.org/gifs/furnace4.jpg

Edgar

[Edited by mrconsumer on 08-24-2006 at 01:21 PM]

[icyflame]

[QUOTE]Originally posted by mrconsumer



I can't say it any more clearly: my ducts are buried behind walls and in between floors and ceilings. I cannot realistically rip out all the walls of my condo to change the ducting.

>>>>> If you had a broken water pipe in the wall would you live with that too? Chances are you would NOT have to "rip out ALL your walls or floors to fix the problem"!

I have to live with this ducting, so the question is what to replace my current equipment with given the constraint of the existing ducting.

>>>>>Do don't HAVE TO live with that ducting! I don't know of any equipment manufacturer that designs their equipment for resrticted air flow...but rather restricted air flow will void every warrenty I ever read!

[comfortdoc]

Are you an end townhouse or middle?

If you are in the middle I would think your ductwork may be closer to what you need than you think. The equipment could be oversized for your home and climate.

If you really had to insist the Man J be done I will suggest you do it yourself. It is $50 well spent and not real difficult to do especially since you are in the house.

[mrconsumer]

>>Are you an end townhouse or middle?<<

My unit is freestanding and long front to back. I am completely exposed on all four sides of the building except on 15 feet going front to back on one side (on all three floors).

Edgar

[dash]

Pics show that the return drop ,could be greatly improved,adding turning vanes will reduce the static and increase air flow.Return drop can possibly be made larger as well .Both return and supply grilles ,can possibly be replaced with ones that lower static and improve air flow.

Can't see much above the coil om the supply ,can you get a better shot?

New furnaces are shorter,but new coils are a little taller.If the new furnace and coil is shorter the supply plenum/trunk can probably be improved too.

[mrconsumer]

Hi Dash,

Thanks for your thoughts. Here are two more pix of the beginning of the two main trunks as they leave the furnace (above the plenum/coil):

Note: you must copy and paste link into your browser.

http://www.consumerworld.org/gifs/furnace5.jpg

http://www.consumerworld.org/gifs/furnace6.jpg

The fellow who was here today suggested the following:

1. Take out the hot water heater that vents into the long metal pipe shared with the current furnace (and goes up the chimney). He is afraid the small h2o pipe feeding into the larger vent pipe will lead to a backdraft, pulling in co2.

2. By putting a wall hung instant hot water heater in, that will free space on the left of the new furnace, so an additional return might be able to be added. [I see no place to run it, however, so he said he might just open a hole in the left side to get additional air in there.]

3. He will do the math on the Manual J back at the office, but while he was here, working backwards from putting in a 4 ton a/c (instead of current 5) based on a rule of thumb, he figures I will need a 100,000 btu furnace (instead of the current 137,000).

You mentioned widening the existing return, I believe, as it enters the furnace. If further away from that point on that return duct, the whole duct narrows to 12 x 17.5", doesn't that govern? (You can only get as much water through a kinked hose as the narrowest part?)

You mentioned some "vanes", I think, in the return. What are those?

Thanks,

Edgar

[mrconsumer]

Guys,

The heating contractor who visited yesterday came back with the results of his Manual J:

>>Your heat loss came to 82,543 net which is a 100,000 input furnace. Good reason to go with 2 stage. It will probably run at low fire 80% of the time. The heat gain came to 46,717 which is a 4 ton cooling unit. <<

With these figures in mind, does this suggest that my existing return is adequate or inadequate (remember, it measures 16" x 18" as it enters the furnace, but which narrows to 12" x 17.5" on the first floor of my house; and the two main trunks leaving the plenum are 12x18, 9x18, and one 8" flexible round).

Thanks,

Edgar

[cem-bsee]

what does contractor who did calc & was there say about ducts?
he has better knowledge than we

what insul in attic?
add storms will help!
sealing penetrations will help --

slab or crawl or bsmt? insulated on outside?

once again -- improve your envelope!
are any of the exposed ducts or boxes with mastic over the holes & seams?

[mrconsumer]

>>what does contractor who did calc & was there say about ducts?
he has better knowledge than we

what insul in attic?
add storms will help!
sealing penetrations will help --

slab or crawl or bsmt? insulated on outside?
<<

C/B...

Contractor said he would consider opening the left side of the new furnace for an additional return, connecting the existing right side, probably as is to the existing return.

All other suggestions don't apply to my particular place. No attic, already double pane glass, no exposed slab outside, no leaky doors, etc. (first floor is carpet on slab, no basement).


Edgar

[mrconsumer]

Hey, guys... you've left me hanging.

Please look at the questions I posed in the last three posts.

thanks,

Edgar

[m kilgore]

What do you have outside the furnace room on either side? Could the return drop be made larger and some return be piped to grills on either side? It looks like maby there is enough room in the furnace room to do this. A newer furnace will be smaller than current one.

If I read correctly the guy that came out wants to take return from withen the room? Hopefully I read wrong. Opening the front of that furnace for air will backdraft the furnace and water heater and possibly kill you.

Did the guy say he would put in a new furnace with the same duct or is he demanding duct alterations? Get a second opinion.

[mrconsumer]

<<What do you have outside the furnace room on either side? Could the return drop be made larger and some return be piped to grills on either side? It looks like maby there is enough room in the furnace room to do this. A newer furnace will be smaller than current one. <<

M/K,

Left and right of the furnace closet is outdoors!


>>If I read correctly the guy that came out wants to take return from withen the room? Hopefully I read wrong. Opening the front of that furnace for air will backdraft the furnace and water heater and possibly kill you.<<

If we go to a 90+ furnace and replace the current hot water heater with a wall hung or direct venting one, yes, he wanted to potentially open the left side of the furnace to let in additional return air. As I understand it, once you have these sealed units venting outdoors, that is not an issue. I am incorrect in that understanding?

>>Did the guy say he would put in a new furnace with the same duct or is he demanding duct alterations? Get a second opinion.<<

The same supply and return ducting will be used because nothing but what is shown in the pictures is accessible. (I am not ripping out walls/ceilings.) They may try to widen or narrow it as it meets the furnace.

Edgar

[m kilgore]

Is there any way to go all electric with a heat pump? I guess that limiting as to how to repair your problem I am running out of answers. I would NOT under any circumstances draw return air from a furnace room with a combustable appliance. Even a sealed combustion furnace and water heater could have a problem with a seal or a bad heat exchanger and cause an unsafe or life threatening situation.

One last idea. Could you build a box on the furnace room floor to raise the furnace and take return air low under the doors? You would need to cut the doors off or replace them to fit. You only have enough return air now for around a 3 ton system.

[mrconsumer]

M/K...

To answer your questions:

1. A heat pump in Boston is probably not a smart idea with high electric prices, and little winter benefit.

2. The box under the new furnace is thinking outside the box, so to speak. But, if you have concerns about pulling in air from close to the furnace, you will have the same concerns of pulling it in one foot away. The furnace is in my laundry room in a closet, behind louvered doors. See picture (cut and paste the link):

http://www.consumerworld.org/gifs/furnace7.jpg

You say that I get enough return air for a three ton system, but not for a four (which is generally what is being recommended).

What dimensions of an additional return would I need to make up the difference of what's needed?

Thanks,

Edgar

[m kilgore]

Ok would it be possible to come off the return with a 10" pipe, go through the wall into laundry, next to the door and then add return in the room behind the washer. I am just gressing now and your HVAC guy should be able to come up with somthing. I have no idea what your house layout is but would NOT take return from the furnace room. I would say you need about 400 more CFM but its just a guess without seeing the layout. Good luck.

[mrconsumer]

MK,

Nice thought to put a return in the room behind the washer, but that is the garage.

It is almost, but not entirely impossible to route a new duct anywhere out of that room because of no space above the furnace or above the laundry room itself in the ceiling, an electrical panel on the right, and only 5 inches on the left of the louvered doors. (The latter is potentially the only hope, but would have to been a very narrow, but high duct.)

Edgar

[docholiday]

Here's my take....

If the ductowrk is too small for a system and replacing the ductwork is out of the question you have two remaining options.

1. Find out what size equipment can properly run with that ductwork and install it. If it is not going to be enough then suppliment it with an alternative source when the load cannot be handled. Dash offered some valuable ways to increase the capacity of the current duct / grills.

2. Install what is required and deal with the consequenses. Just make sure to take the responsibility for it when the heat exchanger and compressors fail. Pay for those items out of your pocket since you already know you will be lacking airflow.

No equipment installed can overcome bad ductwork.

[docholiday]

I'll add to it...

14 years ago, I installed an air conditioning system and new furnace in my brother-in-law's home. The exsisting ductwork was not quite big enough for the 3 tons of cooling he needed. I told him the same thing I am telling you. He was not even able to handle 2.5 tons with his existing set up either.

I ended up installing a 2 ton system. He understood it then and still does to this day that on the hottest of days (over 92 or so) he will not have enough cooling to keep his house at 76 degrees. Well, it gets that way maybe 10 days out of the year and his temps creep up to about 82 on those days.

Now, his house is dry. That unit running all the time really keeps the humidity down. He also has only had one cooling service call for a blown capacitor in the 14 years it's been installed. Had I installed a 3 ton, he would have surely replaced the compressor at least once. His furnace is also slightly undersized and for several nights a year, they have to throw on an extra blanket, but the HX is as solid as the day it was installed. he's had to replace the hot surface ignitor once or twice, but thats typical of those things anyway.

Frankly, if he would get up off his butt and replace his windows, and do some insulating he would probably eleiminate any shortages in those few days a year. He would also see some savings on his utilities.

This summer, he complained that it was not working well enough. He had two companies come out and both offered to replace the equipment with larger equipment with no regard for the duct systems. They must not have even tried to find out why. I happened to be in town just after this and found that the outdoor coil was dirty and he started using the 3m filters. I cleaned the condenser, installed the proper filter and the system runs just as good as ever.

[mrconsumer]

Just to let you guys know, that I have had five heating contractors come by to look at my place and to provide estimates.

Today's guy blew me away! He's NATE certified and wasn't intimidated by the knowledge I had gained from reading this forum and installation manuals! He rather enjoyed the back and forth.

He will do a manual J where most others did not (but disappointingly didn't measure windows, saying with the building plans I gave him, he will call the engineer/architect who drew them).

I am sure issues will arise, and who knows how bad his price will be, but I am smitten <g> (having expected not to find ANYONE with the degree of knowledge and caring that this guy seems to have).

Thanks for your insights.

Edgar

[dash]

He really needs to know the window sizes,never heard of calling for them.

Second return ,if you ran the duct thru the garage ,can you get to a wall ,floor or ceiling ,in the living area??

definately use turning vanes in the return,and in the supply above the coil if possible..

[mrconsumer]

>Second return ,if you ran the duct thru the garage ,can you get to a wall ,floor or ceiling ,in the living area??
<

Hi Dash...

You disappeared for a few days (just when I needed you)! (I tried emailing you at work, but it bounced!)

You are getting to know my place! Can't do what you suggest without either blocking the electrical panel (not shown in the picture), or having to run the duct on the floor in between the wall and the dryer (and I don't have the room to move the washer/dryer).

From what I can tell, he is going to widen the existing return at the furnace and open up part of it to take air in directly from the laundry room. (I can tell you are yelling "no".)

I will be putting in a wall hung, sealed, direct vent tankless hot water heater, and the 90+ furnace.... so all venting in (for combustion) and out for gases, will be outside the building. I am told this will meet code in Mass.

I certainly hope this guy's bid is not outrageous (and I will give him the window measurements as soon as I do them). We can't talk price here, but people in other parts of the country would be shocked at some of the prices I am getting -- some double that of others.

Edgar

[udarrell]

Docholiday said a mouthful, and he did the absolute right thing!

The initial post information is a prime example of techs not knowing what they are doing, (unbelievable incompetent scenario).

The system will only provide the BTUH that the airflow will permit, sizing above that level is a bigtime losing game! Never size the equipment to more than the ductwork will handle at the proper static & CFM specification levels.

There are a lot of other options available to reduce the load.
I believe he said he already had window units that he uses, either using them or a ductless mini-split system with the evaporator& blower mounted high on the wall or on he ceiling will supplement on the extremely hot days.

He could install some electric baseboard heat, only if the smaller furnace will not handle the coldest weather.

You are responsible for what you & your A/C tech decide to do, NOT any of us!

http://www.udarrell.com/external_static_pressure_readings.html
- udarrell