Do I need a 2-stage, 2-speed furnace?
Newbie here (please be as patient with me as you can and let me know if I am breaking any rules).
I am remodeling my new house. I am replacing the furnace and am moving it from the closet in the foyer to under the house. I live in Salinas, California, where freezing nights are extremely rare.
My contractor has suggested a New American Standard, Under-Floor Furnace with a new Return Air Duct with Filter Grill and a Digital Setback Thermostat.
He also suggested that I go for 2-speeds and 2-stages.
As I understand it, once the house has reached the desired temperature, the other stage would reduce the number of heating elements that came on once the temperature dropped, thus saving propane, and that the fan would come on at a slower speed, thus saving electricity.
But I don’t understand. Doesn’t the thermostat continue to ask for more heat until the house again reaches the desired temperature? If fewer heating elements are on, then, instead of propane being used quickly for a short time, aren’t I just using half as much propane but for twice as long?
Then, as for the variable speed fan, if the air is being pulled/pushed through the ducts at a slower speed, spending more time under the house, won’t there be more time for some heat loss than when the fan is at full speed? Wouldn’t the rooms at the farther ends of the house tend to be cooler then than those in the middle? And wouldn’t this effect be compounded if the air isn’t as warm to begin with because only half of the heating elements are in use? And again, wouldn’t I be trading the speed of the fan for the amount of time the fan had to be on?
I’ve been told that the 2-stage, 2-speed thing will keep the temperature more even, without large drops or over-heating the room before turning off, but the furnace in my current house (which is the same size as my new one) seems to do a perfectly good job without any of the foretold temperature extremes. Have furnaces just been changed to the point that single-stage/speed furnaces now run hotter and faster than they used to?
The cost for the 2-stage/speed is $1000 more than the 1-stage/speed version, which is a 15% increase. There are other things I can spend that on if the variable stage/speed is not necessary.
So, if my logic is all messed up, remember, I have already admitted I don’t understand this at all. Please explain it to me. I don’t like being in the dark.
Thank you for taking the time to answer.
2 stage is for comfort only. VS fan will save SOME $ on electricity. Mostly you will notice even temps and more comfort.
As above. 2 stage gives you comfort, it will not save on fuel consumption.
The longer run time in first stage will help to keep the rooms at a more even temp.
The VS blower, on a properly sized duct system will use less electric.
If the ducts are in unconditioned space, then yes, it could lose more heat to the unconditioned area in first stage then in second stage.
Here's my two cents... If you are moving a furnace from a closet, I appluad you. Moving it under the house I am not so impressed with. If you are having to redo all the ductwork (as you should), why not install a packaged unit outside so it's easier to access? You can still get multi stage in both heating and cooling with a packaged product, but 90% is unlikely.
With a packaged product you dont have to be concerned with proper venting, line-set, drainage, evacuation and proper charging. Service is easier.
As mentioned, you dont "Need" 2 stages, but you may want to take advantage of the added comfort that comes along with it.
I second Doc's answer, getting a HP and install it on the outside will give the HVAC guy easier access to the unit and at the same time solve you problem with give you more space in your home and also resolve the any issue's you may have with noise. not too mention you will gain in efficency by way of a HP
I, like Banty am on a quest for knowledge (aka not a professional).
Wouldn't the extra time the conditioned air spent in the unconditioned space while using the lower flowrate be offset by the fact that at a higher flowrate the unit will cycle more THUS extended periods of air in the ducts getting cooled below room temp.? It seems that a duct full of cold air being blown in would offset the small loss of the difference between high and low speed on the fan.
Second thought- wouldn't the extra time for the burner to reheat the HX reduce efficiency? It is alway more efficient for the burner to remain on. Correct?
That being said, the pro's usualy agree that the VS 2 stage is a comfort factor only.
Proper insulation of the duct is required when installing it in an unconditioned space.
The amount of heat the duct loses is dependant on both the amount of duct insulation, and the temp of the air in the duct, and surrounding the duct.
If the air temp in first stage is 10* cooler then in second stage, it may be losing the same amount of BTU's, or it could be losing less.
AFUE allows for HX warm up time.
I believe I'm correct about reading here that an underground basement (with house above) with ductwork is considered to be conditioned space as any heat loss will rise to the house. Well, I've been in the basement while the heat is on and haven't really felt very much warmth from the ductwork. Just took the temperature down there and it is 59, but I believe it has been cooler at times.
Originally Posted by beenthere
Unless after a long furnace cycle (morning recovery), the farthest register will be cooler than others. The run is about half basement/half outside walls, so I have been considering doing some insulating, particularly that run.
If there are no suppies for the basement. It is an unconditioned basement.
The duct loses less to it then to a vented craw;space.
This is incorrect. Heat travels to cold - it doesn't rise, warm air will rise because it is more bouyant. Ducts in a basement or crawlspace are in an unconditioned space. They must be sealed (the joints) before any insulation is applied.
Originally Posted by ampulman
Yes, this duct run is probably leaking and improperly insulated and/or it can be too restrictive. You should remove the insulation from all your ducts, when exposed make sure all the joints are tight, then seal the joints with mastic or UL 181 approved tape (not "duct tape" from hardware store), then re-apply R-6 insulation.
Originally Posted by ampulman
If the ducts have a lot of sharp bends, or loose flex duct you caould also have an air balancing problem, so some duct runs can be starved of proper airflow. The restricted ducts wont get the heat carried by the air and will be cooler than the others.
As a service technician who started as an installer it is important that your unit be servicable. Lets all be honest, is the "average" tech who is doing a PM check on a furnace going to do a more thorough job on a unit in a closet above ground or on an unit thats in a musty and maybe damp and dark crawl space? Servicability should in my opinion be up there in your priorities to ensure in the future you get good service on your equipment. Also if you have the option you should go to a heat pump. This is how I've always been taught: On an 80% furnace for every dollar you spend on heat you get .80 usuable heat and .20 goes up the flue. With a 90% its the same. However, with a heat pump (not counting electric or auxilliary heat) for every dollar you spend on heating you get $3 worth of heat. Something for you to think about. If you have concerns about ductwork, check it out for yourself. The duct MUST be insulated. If possible the joints in the ductwork should be sealed with tape or what I prefer, mastic.
Wow! You guys are fast!
A) It is a vented crawl space. I have attached a picture. The architect wanted to know about the joists and stuff, so I went under myself and took a picture because I was sure I wouldn't know how to accurately describe what I was seeing. Please do not laugh too hard if I labeled all the parts wrong.
B) The furnaces by New American Standard that we have prices for are these:
80% Single Stage
80% Two Stage
80% Two Stage, Variable Speed
90% High-Efficiency, Single Stage
92% High-Efficiency, Two Stage
95% High-Efficiency, Two Stage, Variable Speed
So it seems that there are no Single-Stage furnaces with Variable Speed. A note I wrote on the page of information might explain why. It says that when the furnace is on the lower stage, if you do not cut down the speed of the fan, the effect of lots of "wind" would overwhelm the lower amount of heat carried in the moving air. I have no idea if that makes any sense.
After reading your replies, I now have more questions.
1) I am guessing that HX is where the heat exchange takes place? But what is AFUE?
AFUE allows for HX warm up time.
2) We were originally going to move the furnace into the attic which is much larger than the crawl space, but we discovered that all the ductwork is under the house. Rather than redo all the ductwork, we thought to move the furnace to the crawl space instead and not redo all the ductwork. But you say that it should be redone anyway. Why?
3) The suggestion to go to a heat pump sounds really good. What is a heat pump, how does it work, and what might keep me from having that option?
4) What is it in the design and cost of making a furnace that makes it function more efficiently? The cost of a 90% but Single-Stage furnace is even more expensive than the 80% Two-Stage Variable-Speed furnace!
Thank you again.
Last edited by BantyMom; 03-20-2008 at 01:05 AM.
Hi, a couple of things. Is there a reason why you're going with American Standard? BTW, they are made by Trane. Secondly, why couldnt you mount the furnace in the attic and simply run a piece of duct (called a riser) from the attic to connect to the existing trunk line in the crawl? Thirdly, a heat pump does just that, in the heating mode it extracts heating btu's out of the air and transfers them inside. Now if you live in an area where it gets colder on a regular basis than 35-40 degrees, you may want to consider another option, dual fuel. This is when you have a heat pump outside and a gas furnace inside. Above 40 your primary source of heat would come from the heat pump, below 40 it would come from your gas furnace. Basically, its the best of both worlds. Otherwide if you simply had a true heat pump system with an air handler inside your auxiallary heat will come on below approx 40 to assist the heat pump and you will certainly notice that on your utility bill. Hope this helps