I have a new house that is not sold yet and I am conserned about the furnace. It is a forced air 75k btu, propane fired, 80% efficient. The air coming out of the heat registers is only 89 degrees. Its a 2500 sq ft house and it took almost 50 minutes to raise the temp from 61 to 64. The outside temp was in the upper 40s. I checked the air temp at the register at another house I have, exact same temps, and it was 119 degrees at its highest temp and only took about 25 minutes. My home has a forced air NG furnace and it puts out about 114 degrees.
I talked to my heating contractor, he came and took a look and said that everything is working as it should and that it takes a while to raise the temp when its that low….bla bla.. I aint buyin it. Anyone out there that knows what the temp should be at the register with what I have dexcribed here? And any ideas what if anything could be wrong??
Replies
first i don't know craap about this,but i thought you were suppose to get a 25 degree over air temp.i'm guessing your house at 72 = 97 degrees,so that rule might not be right.the only time i have ever checked air temp it was with the air cond. on. larry
hand me the chainsaw, i need to trim the casing just a hair.
Check the manufacturer's plate, it should give temp rise range for the unit.
With my mouth I will give great thanks to the Lord; I will praise Him in the midst of the throng. For He stands at the right hand of the needy, to save them from those who would condemn them to death.
- Psalms 109:30-31
Rusty,
My furnace just turned on when I started reading this so .... stuck an HVAC thermometer in the register and its about 95 degrees. High efficiency Miller on Natural Gas. Outside air is about 30 degrees, thermostat is set at 68.
Any chance yours has the wrong gas jets in it ? I think propane is less efficient than nat gas ? Maybe the other way around, anyway I know that they need different jets to work properly.
Bill Koustenis
Advanced Automotive Machine
Waldorf Md
Plus, you have to calculate the delta of the return air to the "made" airThat's where the plate specs come in....
With my mouth I will give great thanks to the Lord; I will praise Him in the midst of the throng. For He stands at the right hand of the needy, to save them from those who would condemn them to death.
- Psalms 109:30-31
If you've just started running the furnace, it's gonna take a while to warm the house up. Air is fairly easy to heat, but it takes time to get the walls warmed up. Give it a couple of days to stabilize.
Comparing the temperature at the registers between two houses doesn't mean much. The heat loss between the furnace and the register will depend on the length of the heat duct, how well it's insulated, having good connections, etc.
If you can determine which registers have the shortest and longest duct runs, check their temperatures when the furnace is running. You may need to partially close off the short run register to force more (warmer) air to the long run register.
Thanks for all the replies. Its a new house. Here we have to put R-22 in the walls, R-38 in the cieling and R- 30 in the floors. So the house is well insulated. I sided it and set all the windows and doors so I know there are no air leaks around any windows or doors. Also when I checked the temps the furnace had been running in the house for about a month so it wasnt trying to warm a cold house. Everything in it should be at 61 to start with. Also the air temp coming from the duct was checked about every 10 minutes and 87 was the highest it got to. The furnace is a Rheem criterion. And yes it has been converted to LP. If you dont change the jets you get lots of black soot coming out the vent on the roof. (ask me how I know that). Anyway it sounds like it may be within specks. I guess I am used to the furnace here in my house that has the 114 degree air. Thanks again for the help guys. I love this site.
Just few things...
If you do a heat loss calcualation you will be able to tell if the rating of the furnace is appropriate to your needs.
To do a heat loss you will need to check the insulation values and materials, Size and U-value of windows and doors and the general "air tightness" of the home.
Also are the ducts sealed, insulated? Are they in heated or un heated space? All these factors are important to your long term comfort.
The air temp seems low to me, especially on an 80% unit. Most of those really bake the air.
Garett
When did you measure the air temp? You need to have the furnace running for several minutes before measuring. And fan speed makes a difference. The higher the fan speed the lower the output temp will be, even though the total heat output is a hair higher with the higher fan speed.
In addition to timing temperature rise, time temperature drop. If it takes 50 minutes to raise the temp 3 degrees but only 30 minutes to drop that amount, you're on the hairy edge of having enough heat -- probably only good to 30-35. If, OTOH, it takes about 2.5 hours to drop that much then you're good to about zero.
But make sure the house is up to temp first. It takes probably 48-72 hours for all the drywall, etc, to warm up once heat is turned on in a previously cold house.
60K BTU (your net) should be sufficient for a 2500 sq ft house anywhere south of Minneapolis, if it's properly insulated.
FWIW, 2 to 4 degrees F per hour is a "rule of thumb" for a properly sized furnace - so it sounds like it's working within spec. The unit with the 119F register temperature may well be oversized and thus less efficient.
Your post made me curious, so I did some checking today. I was working at a house today that had 80% furnace installed this past Friday.
Outside temp today -10
I put oven thermometer in register closest to furnace.
TIME HOUSE TEMP REGISTER TEMP
7 am 55 87
60 90
65 100
9:30 am 70 104
2 1/2 hrs raise temp 55-70
second cycle on furnace
9:35 am 66 100
9:58 am 70 104
23 minutes to raise temp 66-70
Don't know if this means anything to you, but that is what I found out.