Single Furnace installation in attic of 2 floor home

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Old 02-12-11, 07:14 PM
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Single Furnace installation in attic of 2 floor home

Hello All,

I am doing a renovation project in my newly purchased home and had a handful of estimates and advice for hvac ductwork, Central Air Conditioning and forced hot air natural gas furnace.

Currently my home does not have any ductwork. I am planning on trashing the steam radiators and 30 year old iron boiler and replacing it with a forced hot air furnace (with humidifier) and Central-AC with ductwork.

I thought that if I am installing a 5 ton Central Air Conditioning unit I should might as well add a highly efficient 95% AFUE gas forced-air furnace along with it and throwing away my steam heating system. My house will only have 1 unit for cool and heat but it will have dampers splitting this unit up into 4 zones. Different Zones will be: 1)Basement, 2) 1st floor, 3)2nd floor, 4) Master Bedroom

The main units (A-coil, furnace, humidifier, blower) will be installed in the attic with a trunk-and-branch network of ducts and dampers.

Now here is a potential problem that one HVAC personnel pointed out regarding a forced hot air furnace installation in the attic: He said if a forced hot air furnace is installed in the attic, it will be very difficult for the hot air to travel downwards to the basement and 1st floor from a trunk-and-branch in the attic of my home due to laws of science (hot air rises, cold air stays low/falls). I could see that he is right in a way but this could be remedied couldn't it? This could be remedied by zoning dampers and a 2nd and 1st floor return? It can also be remedied by a multistage blower which is always on so the hot air is continuously flowing in the winter? Am I missing something here? Any advice is greatly appreciated.

The home resides in the northeast suburbs of NY and the temperatures do get down to 10 degrees Fahrenheit sometimes.

-Pastalover
 
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Old 02-12-11, 08:24 PM
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It's good to know that the billions of dollars dumped into public schools aren't completely wasted and that your HVAC "guru" retained something from 7th grade science (warm air rises.) However, it's called a Forced Air and not a Convection system, The only place(s) convection comes into play is after the heated air exits the ductwork into the rooms. Ceiling fans will move the warm air away from the ceilings to improve comfort.

Well, OK, I will give him that when the furnace isn't operating, the air in the ducts will cool and descend into the conditioned space, but only until the furnace fires up again. Your plan should work very well for you, and good luck with your project.
 
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Old 02-12-11, 09:25 PM
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DO NOT PUT A HUMIDIFIER WHERE IT CAN FREEZE (ATTIC)

now thats out of the way, do you have a basement? This would be a better location for the system. IMO

in your area you want low supplies and high returns.
 
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Old 02-12-11, 09:47 PM
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Zone systems have lots of issues. I'd recommend two units for greatest comfort and cheaper to run
 
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Old 02-12-11, 09:55 PM
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Thank you tldoug and hvactechfw for your helpful advice.

Tldoug,
Ceiling fans will definitely help with the circulation and air movement. All the vent setups are going to be on the ceiling of every room in the house.

hvactechfw,
I took into consideration the freeze-up of the humidifier and the cold water line. Any suggestions of how to fix the freeze-up problem other than not installing the unit in the attic? Localizing the units in the basement would be more labor intensive as opposed to installing the trunks and branches in an open and empty attic space. The joist setup in the basement are not in favor of a neat branch setup so I ultimately chose the attic for this job. Also the attic will be heavily insulated so I would not hear the trunk and blower air movement, my basement is going to be a theater room and I am pretty sure I wouldn't bear the air noise the hvac unit will create if it was installed in the basement.

Thanks again for the advice

-pastalover
 
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Old 02-12-11, 10:04 PM
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if you by good equipment and do the duct work correctly then you shouldn't hear loud noises from the mechanicals. Humidifier solution do not put it in the attic! Aprilaire Models 350 / 360 Whole-House Humidifiers

again, your area you want low supplies and high returns
 
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Old 02-12-11, 10:21 PM
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airman.1994

I figured as long as the system is installed correctly to accommodate variable air flow then I think it will work just fine. Can you give me examples of some of the issues?

From what I gathered,

According to the different hvac techs, I will have 4 zone dampers and also I will have a device called a barometric pressure reliever to work in harmony with each other. Now I could understand these can be considered extra bells and whistles but it needs to be done for efficiency and comfort levels.

You are probably correct that 2 units will be cheaper to install but I am thinking of the long run energy consumption. Given my situation and the number of family members, the 2 units would work continuously all the time. So I thought it would be better and more efficient for 1 unit to control the climate in the whole house which would draw in less energy than 2 units working almost at the same time.
 
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Old 02-12-11, 10:28 PM
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hvactechfw,

In the case of the humidifier, I guess I could look into the option of purchasing a new 83-85% cast iron boiler in the basement and running a supply and return to the attic connecting a giant hot water radiator coil which is attached to the blower. That was the other option that was presented to me. This option would eliminate the need for a humidifier since the hot water coil produces moist warm air. High efficiency cast iron boilers can get pricey, I will have to check quotes. Unfortunately these boilers can only do 83-85% max efficiency from what I've heard compared to 95-98% afue for a gas fired furnace. That margin of 10-13% loss could add up in the long run.
 
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Old 02-12-11, 10:35 PM
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no, a water coil does not produce moist air.
 
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Old 02-13-11, 07:06 PM
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Issues will be with the dampers and the controls. Two units will be cheaper to run because u might only need one smaller unit to run instead of running the 5 ton unit. I mean if u only have one zone calling the unit will run to just cool the one zone. Install will be more with two units but cheaper to operate. U will also have another unit to help cool the house if the other unit goes down
 
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Old 02-13-11, 09:20 PM
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airman.1994

I do not agree that it will be cheaper to operate 2 zones. That depends on a few variables. One main variable is that I will have family members occupying both floors during the day and night. So with that said, running 2 sets of 2.5 ton units at the same time will cost more to operate than running one 5 ton unit don't you agree? Same theory goes for 2 furnaces running at the same time compared to 1 furnace.

How often do dampers fail? That is a concern I have and I don't know many people with central air attached to dampers.
 
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Old 02-14-11, 04:56 AM
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air man is potentially correct. figure for heating your btu input to the furnace may be 120,000 for a unit that runs everything. 2 unit may only be 60,000 therefore you can potentially be ahead because if you are only heating one space at that time then you are using less gas, versus heating one zone with the 120,000 btu furnace, you are using more gas to heat the same space. A/C could be the same, A 5 ton unit uses more electricity than a 2 ton unit, same principle comes into play. The one thing that will always cost more in the long run is having 2 systems maintained versus only one system. Dampers do fail, not often, but you would want them accessible. Also you would need a very big attic for a zoning system in order to run the separate zone from that space.
 
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Old 02-14-11, 05:52 PM
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hvactechfw,

Thanks for clearing that up. Now I agree that 2 zones are better than one. Also, my home is not a total gut and renew project, only about 70% of it is so I was recommended this:
Zone 1:
1 2.5 ton CAC with 90 afue furnace with 60-70k btus
Zone 2:
1 2.5 ton CAC with 90 afue furnace with 60-70k btus

The ductwork would be simpler also since the 2nd floor unit in the attic will have vents on the ceilings and 1 main return in the common space. The 1st floor will have ductwork in the basement with vent openings on the floor of the 1st floor with a side wall return in the common area. A few vents will be added in the basement as well. As you see, the ventwork labor would be minimal compared to my previous plan with only 1 unit.

Now I have a question for you guys:
Since I have a boiler (I believe it is 80% but I will let you know precisely soon), is it better to just make this boiler have 2 zones with each connected to a hot water coil to the 2 blowers or is it better to have a gas fired furnace? I am thinking with the right tune-up that boiler could be very functional.

Which heating method will provide most comfort?:The hot water coil attached to the blower or a gas fired furnace? Assume the both have the same efficiencies.
 
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Old 02-14-11, 05:55 PM
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well, here is the problem with a fan coil or 90% in the attic. FREEZING TEMPERATURES!!!!! Coil would have water lines in the attic, not a good idea, 90% eff. forced air furnace produces condensation during heating, not a good idea for the attic.....
 
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Old 02-14-11, 06:14 PM
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I have a close friend who lives a mile away from my new home and his 2nd floor setup has forced hot air furnace installed in the attic with no problems. If 90% furnace produces condensate in the attic, then how about the rest of those with a 90% furnace in an unfinished basement? The unfinished basement can get mighty cold as well right? It can produce condensate in the basement right?
 
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Old 02-14-11, 06:39 PM
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I completely agree with the comment about putting a 90+ furnace in an attic where conditions can drop below freezing. Personally, I'd opt for an 80% (posssible 2-stage) for the second floor. Some equipment manufacturers specificaly state in the installation instructions for their 90+ equipment not to install it in such areas. I've seen more than one secondary heat exchanger freeze & split, not to mention countless internal condensate traps.

Ductwork comments: Don't install a "central return". First of all, it won't work worth a plugged nickel & secondly, it's against code in many areas. With the equipment handling both heating & cooling, you need returns in any room (except kitchen & bath) which can be closed off by means of a door. The returns should be both high (for cooling) & low (for heating). This is especially important with ceiling mounted supply registers.
 
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Old 02-14-11, 07:16 PM
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If 90% furnace produces condensate in the attic, then how about the rest of those with a 90% furnace in an unfinished basement? The unfinished basement can get mighty cold as well right? It can produce condensate in the basement right?
typically an unfinished basement will never reach freezing
 
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Old 02-14-11, 07:19 PM
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Originally Posted by hvactechfw View Post
typically an unfinished basement will never reach freezing
As far as I know, we were talking about a 90+ in an attic. Were we not? If not, I apologize for my error.
 
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Old 02-14-11, 08:25 PM
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I was under the impression that temperature is similar during the winter in an attic and an unfinished basement. I guess I was wrong.

Thank you for your concerns. Can you fine people give me some insight on whether a hot water coil attached to a blower is more comfortable or a forced hot air furnace would be more comfortable? Which one is favored and why? I have a boiler which I want to keep if it is supposed to be more comfortable than a hot air furnace. I would replace the boiler at a later time when I can afford to.
 
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Old 02-14-11, 08:30 PM
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Originally Posted by Grady View Post
As far as I know, we were talking about a 90+ in an attic. Were we not? If not, I apologize for my error.
we were, but the OP was under the impression that basements froze as well
 
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Old 02-14-11, 08:33 PM
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Can you fine people give me some insight on whether a hot water coil attached to a blower is more comfortable or a forced hot air furnace would be more comfortable? Which one is favored and why?
not much difference in hot water coil and forced air furnace, both are forced air, the more comfortable of all in heating situation is hot water baseboard and or radiators. More even temperatures.
 
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Old 02-14-11, 08:44 PM
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hvachtechfw

Steam radiators are present in the house already but my wife thinks they are ugly =( I am pretty sure their functionality is great. So I thought since I am installing central ac, I might as well just add a forced air furnace as well or utilize the boiler for 2 hot water coils for both zones, 1 in the attic, 1 in the basement. It appears my boiler can be utilized for steam radiators, baseboard, or hot water coil. I have plenty of options, just have to choose the one that's most comfortable, appealing, and efficient. I guess you can't have all 3 huh?
 
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Old 02-15-11, 03:42 PM
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frozen attic

there is another post about the guy having problems with the condensate freezing,with his furnace in the attic. if you live in a northern climate,unless the house is super insulated,including the attic,it really is not a good idea to put a plus 90 furnace in the attic.....as far as a basement unfinished being as cold as the attic,it is not true. check with most furnace manufacturers,and they will say....do not place furnace in an attic unheated space......the percentage of furnaces (90 plus) in attics in a northern climate is very low.....good luck.....let us know how it works out.....
 
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Old 02-15-11, 06:59 PM
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Being a wethead at heart, I'm naturally biased toward a boiler with multiple zones. You can use hot water coils in an attic provided you have anti-freeze in the system. The boiler gives you the versitility to have hot water coils in air handlers, baseboard, cast iron or panel radiators, or even the ultimate in heating comfort, in-floor radiant. The downside is you are putting all of your heating eggs in one basket. With multiple furnaces, if one goes down you still have some heat. As you can see there is no "correct" or "best" answer. Sorry
 
 

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