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Old 12-29-2009, 12:08 AM   #1
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Short cycling??


I have an Armstrong 100,000 BTU furnace in my home. I am pretty sure it is way to big. my home is just shy of 1600 sq. ft. I bought big cause we were going to add a room but things changed.
Anyway I disconnected and plugged one of the burners and am running the fan on low speed on heat. The furnace is about 5 years old. My problem is that if the outside temp is 10 degrees or 40 degrees the stupid furnace runs for about 8 minutes and stays off for about 8 minutes. I changed the stat to a new Honeywell programable- Same thing. Changed to a Honeywell t86and set the anticipator to the gas valve.35-same thing. I changed the anticipater to .6 and it changed the cycle to 15 min on and 15 or so off. Moved the stat to a different room-same thing. If I shut the furnace off when it's about 20 degrees outside it takes almost 1 hour to lose one degree. It's a counterflow unit and I know I'm loosing heat in the slab cause I checked the temps at the registers and there is a BIG difference between the closest and farthest register, almost 35 degrees over about 20 feet of duct. When the furnace was installed the A coil was cleaned.

I used to have a fireplace in the family room and we used that a lot in the winter and really didn't pay attention to the furnace cause it ran very little. Now we don't have the fireplace and this furnace is driving me and my gas bill crazy. The worse part is now we are always uncomfortable.

Any suggestions. I am considering removing the counterflow and installing a conventional upflow with the ducts in the attic if I can't get this fixed.

Thanks

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Old 12-29-2009, 03:26 AM   #2
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Short cycling??


Quote:
Originally Posted by thumbkins View Post
I have an Armstrong 100,000 BTU furnace in my home. I am pretty sure it is way to big. my home is just shy of 1600 sq. ft. I bought big cause we were going to add a room but things changed.
Anyway I disconnected and plugged one of the burners and am running the fan on low speed on heat. The furnace is about 5 years old. My problem is that if the outside temp is 10 degrees or 40 degrees the stupid furnace runs for about 8 minutes and stays off for about 8 minutes. I changed the stat to a new Honeywell programable- Same thing. Changed to a Honeywell t86and set the anticipator to the gas valve.35-same thing. I changed the anticipater to .6 and it changed the cycle to 15 min on and 15 or so off. Moved the stat to a different room-same thing. If I shut the furnace off when it's about 20 degrees outside it takes almost 1 hour to lose one degree. It's a counterflow unit and I know I'm loosing heat in the slab cause I checked the temps at the registers and there is a BIG difference between the closest and farthest register, almost 35 degrees over about 20 feet of duct. When the furnace was installed the A coil was cleaned.

I used to have a fireplace in the family room and we used that a lot in the winter and really didn't pay attention to the furnace cause it ran very little. Now we don't have the fireplace and this furnace is driving me and my gas bill crazy. The worse part is now we are always uncomfortable.

Any suggestions. I am considering removing the counterflow and installing a conventional upflow with the ducts in the attic if I can't get this fixed.

Thanks

My guess from what you have described, and by making assumptions, is that your heat exchanger is plugged and/or cracked. I'm basing this on the assumption that you have a high efficient furnace and my reasoning is that you have: 1- affected the temp rise by arbitrarily setting the fan to "low"
2- plugged off one of the burners, thereby causing the furnace to operate under under conditions of which it wasn't meant to operate.

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Old 12-29-2009, 06:45 AM   #3
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Short cycling??


Just pull out that oversized furnace and install the right sized furnace in its place. You 100,000 is oversized.

Plugging a burner is the worst thing you can do. To make an oversized furnace put out less heat.
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Old 12-29-2009, 11:40 AM   #4
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No, the furnace is not a high efficiency it's a builder grade (read cheap) unit at about 80%. I wasn't to thrilled about plugging one of the burners but I was told by one of the hvac guys on the job that this would be a good way to cut down the BTU's without changing the unit right away.

Without doing a load calc. I figured it should be about 70,000-75,000 with a 3 ton drive. Presently 100,000 BTU with 5 ton drive recommended by the same HVAC guy!!!!!

There is a site that seems to get recommended a lot here about load calcs, can I get reasonably close with this? Does it also recommend the proper amount of registers and duct size per room? Since my ducts are in the slab I was wondering if they could be the wrong size or compromised in some way. I've checked them with a drop light/flash light and mirrors and they look clear. I also have a problem getting enough heat to the kitchen/family room area. Those rooms are always 3-5% cooler than the rest of the house.

However I think that I will do as said and do the load calc, install the correct size furnace, and go from there.

I'm going to stay with the 80% unit but was considering a 2 stage or variable flame unit. Any opinions????

Thanks again for your time and input
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Old 12-29-2009, 12:31 PM   #5
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HVAC Calc will do a fairly accurate load calc. And give you some idea of duct sizes.

I still recommend going with a 90% or better. Unless you really think gas is going to be 20% cheaper 3 years from now. Instead of 30% more.

Also. Take a discharge air temp at the furnace, and at some of the furtherest registers from the furnace. See how much heat you are losing in the slab. If its a lot(30 or more), then you will want a slightly larger furnace then the load calc calls for.

Good chance you only need a 60,000 BTU input 95%.

70,000 input 80% equals 56,000 output.
60,000 input 95% equals 57,000 output. And a tax credit.
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Old 12-29-2009, 01:55 PM   #6
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WOW, I just did some temp readings at the registers. Since there is four pipes coming out of the plenum in the floor there are ,what I consider four branches. I did a reading at all four corners and I'm loosing a h--- of a lot in the slab. This home is 45 years old ranch on a slab and full size attic.

Anyway would you believe 160 at the furnace and

85 Kitchen
105 Family room
110 Bedroom
110 other bedroom

and its 130 at the first register from the furnace. All this with the stat set at 90 and the furnace was running for about 5 min. before I started the readings. The first register is about 8' from the furnace.
I knew I was loosing some because when it snows I have no snow within 6" of the house.

What next???? Install all new in the attic???? or just go with the new furnace?????
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Old 12-29-2009, 06:17 PM   #7
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Short cycling??


A load calc to determine what size furnace you really need.
And then probably installed so that the duct system is in the attic.
You'll lose less heat to the attic through an insulated duct system then your losing in your slab.

For every 100 CFM moving through the supplies that are at 110 with the furnace at 160. You are losing 5,400 BTUs of heat to the slab and ground.

The kitchen supply, is losing 8,100 BTUs for every 100 CFM of air.
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Old 12-29-2009, 07:43 PM   #8
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So if i figure an 8" duct in the slab with those numbers at 1500fpm at 500cfm I'm loosing a total of about 50,000BTU??????Is that right?????

My wife has a concern that if I install the ducts in the attic,the floors will be really cold. I realize that I'm loosing my floor heater, are the floors going to be that cold? Do you think it would be worth tying in the stuff in the slab and keep the registers closed just to warm the slab? Could I use the floor registers as a return and add a small central return to make up the difference??
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Old 12-29-2009, 08:02 PM   #9
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You aren't getting 500CFM through a 8". May be 200 at most. Considering how most slab runs are.

Plus. If your furnace has 5 burners. And you plugged one of them. In therory, you downsized it to an 80,000 input. if it remained at 80% efficient. with a return air temp of 70, and a supply plenum temp of 160. That would mean you furnace is only moving about 660 CFM.

The slab will definitely be much cooler.

using those supplies as returns is not a good idea. Its been done. But can have trouble down the road.

A. Return air temp is too low for too long of a time period.
B. Moisture can collect in them, and cause mold.
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Old 12-29-2009, 09:01 PM   #10
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Thank you again for putting up with all my questions.

Since my kitchen ductwork has the most loss could I just run a new duct from the furnace thru the attic to the kitchen area and plug the registers in the slab? That would give me some heat in the slab but more heat in the kit/fam room area.

I still have to do the calc, but I'm just exploring options.

Thanks again
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Old 12-29-2009, 09:15 PM   #11
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It would have to be a fairly large duct to get enough air.
Because of the length this run would be. Coming from the bottom od the furnae, rising into the attic. then crossing the attic X distance. Can add up the length fairly quick.

If you close off the registers in the kitchen/family room area. Then little to no air will be flowing through that duct. And the slab won't be warmed up.
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Old 12-29-2009, 09:58 PM   #12
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looks like I'm between a rock and a .........

If I go in the attic=cold floors and unhappy mama.

If I do the math go with the correct furnace I'll need a small space heater in the kitchen family room area.

If I add a duct I loose closet space get mama mad and still have cold floors.

The only thing for sure is do the math and figure the correct furnace size. You really think the 90%ers are worth the extra cost?? Is there a 90%er with a variable fan speed. Those are probably really expensive.


Thank you again for your help.
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Old 12-29-2009, 10:05 PM   #13
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A 90+% with a VS blower, isn't much more expensive then an 80% with a VS blower.

An 80% with a VS blower is ok. In an attic. less install headaches.
All VS furnaces are 2 stage. So you will find that you will be more comfortable because of the longer run cycles(VS blower gets a small tax credit on an 80% furnace).

With the right register selection. your family room won't be cold.
Don't use those round ceiling registers. they're great for cooling. But are the worse for heating.

You want ceiling registers that have some downward air throw.
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Old 12-29-2009, 10:17 PM   #14
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Downward air flow and about how far from the outside wall? 1 foot? Center of the windows, front door, and patio door, if possible correct?

Just curious, what regulates the air flow on a vs unit? Are they true vs or just automatic low, med, high? Do they monitor duct pressure?
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Old 12-29-2009, 10:26 PM   #15
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You need to space them for spread and throw. So you'll need to check the register specs. Probably 4' from outside walls or windows.

VS blower is set for X CFM. And then it monitors the torque its using to determine if its moving the set air flow. If not, it speeds up to move the air its set for.

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