Less Risk of Oversizing with Modulating Furnace and 2 Stage Condenser?
Is there less risk of oversizing a system with a modulating furnace, variable speed blower and 2 stage condenser with a smart thermostat?
I know it's an expensive option, but with some of the rebates that favor top end systems and the tax credit, a Cadillac system is not much more than a Chevy right now.
I know that load calculations and proper sizing is very important, but so is the "wife factor" of too hot in summer or too cold. I would argue teh wife factor is more important that any Manual J output.
Can the use of this kind of equipment insure against undersizing and complaints of too hot and too cold, while still keeping the system effecient?
Oversized is still oversized, but the issues involving efficincy and comfort will be reduced, but you won't get the full benefit of the modulating furnace in more mild weather.
Oversizing the condensor is still bad if it's a 2 speed. You'll lose out on the full benefit of the system and not remove as much humidity. There are some arguments for oversizing a heat pump slightly, but not an A/C.
Proper installation and ductwork sizing also affect comfort... and are commonly one of the problems with oversized equipment. Bigger furnace = more airflow. If the ductwork is too small, it will be noisy and not distribute the air evenly.
It's pretty hard to undersize equipment. You would only notice undersized equipment on the coldest 3 or 4 days of the year, and hotest week of the year... and that's only if you use a large setback at night. A system thats sized perfectly will be running constantly on the coldest day of the year and hotest day of the year to maintain temperature... and the A/C might fall behind if you are opening and closing doors frequently or entertaining a large number of guests in the middle of hte afternoon.... but you will still remove the humidity effeciently so it will "feel" comfortable.
The reason people oversize HVAC is the same reason they "need" a V8 vs a V6. you could probably place a recording device on engine of most cars with the larger engine option and will discover that it only see's the top 20% of the enignes capability perhaps once per week at most... and that probably wasn't even nessesary. The downside of course is higher purchase price and higher operating costs.
Even if you use a mod furnace that uses the thermostat to control modulation.
If you use set back, during recovery its going to go to a higher if not high fire, and your duct system better be sized to handle the increased air flow it needs.
An over sized 2 stage A/C won't dehumidify real well, and may cause you to have to set your stat lower. Using more electric, making it an inefficient A/C.
An accurate load calc is what you need to have done.
If the load calc calls for say a 2.5 ton. Going to a 3 ton 2 stage is fine. Going to a 4 ton is not.
If the load calc calls for an 70,000 BTU furnace, going to a 90,000 BTU mod, isn't going to save you any money, and now you may not have enough duct to handle the extra air the 90,000 needs.
EG: 70,000 90% 65° rise, 897CFM, 90,000 65° rise, 1153CFM.
Don't hamstring a nice system choice like that with bad design decisions. Size it correctly, size the ducts correctly, seal your ducts, select the correct supply diffusers, provide adequate return air in every room except bathrooms and kitchens.
In short...select a contractor who insists on all of the above and more. If he's too eager to go along with an oversizing scenario, move on, you don't need that mindset going into this.
- Electricity makes refrigeration happen.
- Refrigeration makes the HVAC psychrometric process happen.
- HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.
A building is NOT beautiful unless it is also comfortable.
OK, that seems to make sense. There is general reluctance in the industry (at least in my area) to do Manual J calculations. My current set up is as follows:
In the summer, the AC runs almost constantly during the hottest and will not keep up in the afternoon if there are many people in the house. Temperature creeps up.
In the winter, on cold, windy days, the furnace runs non stop and it “feels” a little chilly. Recovery time from a set back is almost not possible. Maintenance of temperature only. I have to turn the gas fireplace on as a supplement.
One contractor commented that it was odd to have a 64,000 BTU furnace matched with a 1600 cfm blower and 4 ton coil and AC. I was also told that with older furnaces, the 64,000 was output spec and that input is 80,000 BTU at 80% AFUE.
So as a starting point, is this far off?
4 ton 2 stage AC with matching coil
100,000 BTU (input) modulating furnace
Since the modulating furnace draws combustion air directly from outside, I was also planning to close the 6” fresh air intake that goes into the cold air return.
I’m still working on finding someone to do the manual J and will try to buy the HVAC calc program to do it myself as a check.
Would those specs be that out of whack?
That York should be 64,000 output. York use to rate by output, instead of input like most everyone else.
An 80,000BTU input 90% will give you more heat then you currently have.
Using outdoor combustion air, will decrease your heat loss some.
Closing. Or controlling the fresh air intake will further reduce your heat loss.
That could reduce your heat loss by 10%.
So why would you want to grossly over size a new furnace.
80,000BTU input 90%=72,000BTU's output. A 12˝ percent output increase over your current furnace.
100,000BTU input 90%=90,000BTU's output. A 56% percent output increase over your current furnace.
If you really want high efficiency, comfort, operating savings, and a wife that is happy with the indoor temps.
Then pay the fee and do your own load calc.
While your current 64,000 BTU out put furnace has to run 24/7 to maintain set point temp during the coldest days. It is most likely sized correctly.
Good chance, that it would heat better if it had a proper tune up. (Which is hard to get in many areas)
And wasn’t pulling in as much fresh air through that 6” fresh air intake.
Your current 4 ton A/C may be sized correctly.
But, your duct work may not be sized large enough to move the air your house needs.
After a .8 static pressure, the blower of your current furnace drops off quickly, as far as its CFM ability.
Plus, with it pulling in all that uncontrolled fresh air, your adding needless load to the system.
A static check to see how much air its moving would tell you if you have duct issues.
And increasing equipment size, will NOT help if you have duct issues.
If your house feels drafty when the furnace is running continuous. Then a Mod furnace won’t help you.
You need to address the house envelope first.
I’m a proponent of Mod furnaces.
I’m a proponent of 2 stage A/C’s.
But, I hate to see anyone use, or suggest that they be used as a cure all to heating and cooling problems.
Because they are not.
You can pay once to do it right.
Or, you can listen to your wife complain you should have done it the right way.
Makes sense. Anything bigger than an 80,000 BTU input with a closed fresh air intake and high efficiency would seem like overkill.
Drafty was perhaps the wrong term when it runs continuously. With all of the work I have done to seal the windows, my neighbors comment how much better my windows are compared the theirs. It's the perception of not being cozy is better, at the same temperature as perhaps my old house.
I don't mind paying the fee for HVAC calc software at all. It a drop in the bucket for a 5 figure purchase. Want to make sure it's done properly.
Would it be out of line to ask for this:
I agree on the proposal with options for sizes with the final selection contingent on a properly measured heat loss calculation and pressure drop calculation in the duct work.
I think most contractors are reluctant to do Manual J's becuse of time investment and how many quotes they do that don't result in sales.
Don't accept any proposal that uses a 100,000 BTU furnace.
Or one that uses a A/c larger then what you already have.
Ask if they do load calcs over the phone. Saves you and them time.
If you can't find anyone. Use your load calc, and get quotes based on it.
PS: The load calc will give you a total BTU gain, in tons. This is NOT the size of equipment. Its the capacity the equipment must deliver at design conditions.
A couple of questions..
I was just thinking about buying the load calculator linked to on this page for my own house. However, if load calc's are so simple that an HVAC pro can do load calc's over the phone, why the controversy? Why bother going through all that?
The second question has already been answered, I think, in this thread. It concerns oversizing. But here goes anyhow.
If I have adequate ductwork is there any benefit to having a larger than necessary variable speed air handler? Right off the bat, as somebody that knows very little, I think that the surface area of the evaporator coil would be greater and intuitively this seems desirable. How wrong am I?
I have a couple of other general knowledge questions. Anybody care to answer?
Going to ask them anyhow..
Here's one of them. After I get back from the store I'll post some others. I really hope one of you answers them.
I have sheetmetal @8" round ductwork in the attic supplying the upstairs bedrooms. Right now, I definitely have major airflow issues so I'm going to do something about it. Would I be better off having the existing ductwork cleaned and sealed, and reinsulated as necessary ,or, buying new ductwork which will most likely be flex duct because that is what it seems many guys want to install.
Before I go to the store..
Trane split from American Standard in about 2006. Prior to that, was the engineering and quality control similar?
Originally Posted by Thebil Illpay