The model of the furnace is the Dependable 92 (I think). It is a two burner, updraft furnace with the a/c evaporator coils above. When it runs each row of burners puts out pure blue flame. The flame cones are about 1 inch high, with the total flame height at 4 inches or more. It is an open burner system where fresh air enters a screen from below. The screen is clean (this time). 5 years ago I had to clean the screen when her output temp was only 96 degrees!! There is no obstruction in the ventilation sytem(that is apparent) as the pressure switch and ventor keep working, and the ventor exhaust air outside feels to be like any other furnace that I have gone outside to feel. Also, I have inspected the a/c coils from above and below and they are as clean as new (condo history is good with regular furnace filter changing). The air output at the registers is very good.

But only 104 degree heat in the bathroom register which is just above the furnace! The lady complains about cool heat/windchills. I don't blame her. My older Miller furnace at home puts out 180 degree heat! I tell her that and she is jealous. I tell her on winter days when I come in from outside and I am chilled to the bone, I come in and sit on my register and go, "Ahhhhh". I told her I wouod NEVER own a high efficiency furnace. (Plus the fact there is always something going wrong with them as they are more complex). I will gladly pay the extra heating bills to get that nice hot heat that warms up the place so rapidly.

104 degree heat does warm up the house, but the furnace has to run and run and run...and the whole while you feel that obnoxious windchill.

I have speculated that either the factory built these furnaces real cheap with ultra thin heat exchangers and utilize the only 2 burner, low heat so that the heat exchanger does not melt...or, possibly the gas valve pressure is not quite high enough. I have also wondered if there could be blockage of the secondary heat exchanger. But if that were faulty, wouldn't that cause the ventor sensor to shut down and shut off the furnace?

There are 4 of these furnaces in 4 condos and this lady gets 104 degrees. The others only get like 110.

Shouldn't high efficiency furnaces still get around 120-130? I have worked on many that at LEAST give off 118.

Any opinions?

I am afraid to doctor with the gas pressure as I am not a licensed furnace man, for liability reasons, just incase this did affect the heat exchanger/carbon monoxide potential.

The licensed heating guy who has been in the business a long time and designs furnace systems, and has been to the condo, said that "this is normal". But I can't buy that. That temp just sounds too low. Who would want such a furnace?

 

You need to measure the air temperature about 12" downstream of the evaporator coil & at the same time in the return plenum. The difference is called temperature rise or delta T. Most 90+ furnaces are only rated for an average of 50ºF delta T. You need to check the manufacturers specs before making any adjustments.

 

I get about 120 to 130 on my two stage..

How many BTU is it? sounds like it's a small unit..

May be best to get someone to take a gas pressure reading at the furance.

 

Thanks for your input. But I would have to expect, without actually taking the readings that my reading wil be somewhat in excess of 104, only 12 inches away, and the reutrn air temp would be at somewhat below room air temp, due to cold air returns located at the floor where it would be colder than room temperature 5 feet up at thermostat level. I'm guessing, at about 66 degrees. So, I might guess at 110, compared to 66. *IF* that were the case...then what?

 

I really don't think the size of the unit has anything to do with it. My Miller is about the same input btuh at somewheres around 75,000 I think. I can't recall for sure, but I think this is about the size. These size furnaces are typical in most of the condo-apartments I work on. But some have 3 or 4 burners.

I think what I will probably most likely do, first, is call up the company and ask them if that heat about sounds normal for that furnace, or if it doesn't.
If it's way off, then I will call up our furnace guy and have him check the gas valve pressure.

You know...now that I happen to think of it... a few years ago, someone had me take a gas meter reading test as you run the furnace and see how many cubic foot (fraction of) of gas is used per given time. That way you know if the gas valve is open enough. Hmmmm. I may have to look into this again. It seems to me that the vexxing part was that this furnace was within those parameters. Hmmm. Well, I'll check with the company and hopefully they will tell me the heat output I can expect. If they will, that will save me a lot of time. I can ask them if this furnace is designed at all with an allowance where somebody can turn up the flame some more to get more heat, also. (But lots of times you ask companies stuff like this and they back off because they don't want any Joe Blow to start fiddling with the furnace. They will ask you for your license number...stuff like that.)

I get really frustrated dealing with many companies because there is a presumption that everyone is a moron.

Note: This furnace has a lot of associated literature that was left there, and there is nothing said about what amount of heat that one could expect, which I found rather odd, as THAT is what furnaces are all about...heat! And if the heat ouput deviated too much from their specs, then one coud have some idea then if something was wrong. This way, one doesn't even knwo for sure if something is wrong or not. Nothing more frustrating than calling out high dollar repair people who just take a quick look and go, "Looks fine to me".

 

If it is raising the temp 66 to 110 I think you will find it is ok.

The temp it puts out will vary according to the temp it brings in.

If you could post the airflow, and the temps and the btu rating I could tell you if it is putting out what it is supposed to, but it would take all of that info to do so without being there in person.

With just the temps (return, and supply), and the btu rating of the furnace I could compare with specs from simalar units and let you know how it stands.

 

Bonehead,

An interesting post. While not an expert, I have been told by several local dealers who sell premium lines of HVAC equipment that the 90+ AFUE condensing furnaces in fact do not deliver the same high supply air temps as the less efficient 80+ AFUE non-condensing furnaces. I suppose the question is why. Perhaps some of the expert moderators can provide an answer-surely it has something to do with the design-maybe the use of the secondary heat exchanger. A supply air temp of 104 though may be a little low.

 

Shank,
If you have a furnace with a delta T of 110º something is wrong, big time. I know of no furnace, regardless of efficiency or fuel source which is designed to operate at over an 80º delta T and those are few and far between.

Tiger Dunes,
Some 90+ furnaces will produce the same temperatue rise as some 80% efficient equipment but that is because the temperature rise specs on the 80's have dropped over time. I don't think you will find any new gas fired furnaces which produce the same air temperatures we saw on furnaces 20+ years ago. Old equipment produced temps of 120-140º at the register after going thru uninsulated ducts. You'll not find that today, I don't think.

 

Where did I say anything about a delta T of 110.

I did mention the difference between 66 and 110 which is equal to 44 degrees.

My point was supposed to be the supply air temp, or temp differance alone doesn't mean a thing.

If it is too low to suit you slow the fan down.

To find out if it is putting the right amount of heat in the air you need to know airflow and btu input rating

[(delta T) x 1.08] x CFM = BTU output

compare that with the input rating, and the effeincy rating and see if it is close.

And anyone who trys to tell you it is doing what it is supposed to be doing with out doing all of this doesn't know if it is doing what it should be doing.

 

What is the "airflow" test?

I should write a note to myself to do what you ask, at that rental, and get back to you. In fact, I am doing that now, before I submit this, so I don't forget.

I also am intrigued by the later post that says that more heat could be had by slowing the blower down. That makes sense. But if one did that, a person would want to be certain that this maneuver is okay for the design of the heat exchangers.

But then, it would be nice to know if the gas burner is putting out what it should...because, if one were to slow down the blower motor on say a furnace that is not putting out the gas that it should, would be akin to lowering all the specs, would be akin to you having a smaller btuh furnace in the house.

I just got through writing the note.

 

the blower is more than likely already on low speed for heat.

 

I just have an annual service on my Carrier Infinity 96. The temp rise was 66 degrees.

 

From the above statement, I took it to mean raising the temperature by 66-110º.
If you meant raising the temperature from 66º inlet to 110º discharge that is a whole different ballgame.

 

The furnace theoreticly puts out a fixed number of btus.Slowing the fan speed down does get you more heat, it only raises the temp of the lower amount of air higher.

In the formula I posted you will notice as airflow increases temp differance decreases, and visa versa. That is allowing that the btu output remains the same.

The btu output of the furnace will actually be greater at lower return temp due to heat transfer increases as temp difference in creases (the greater the temp difference between the air and the heat exchanger the faster the heat, and the more heat can be absorbed by the air.)

 

Just got back from 'fixing' a high efficiency gas furnace. It took me about 2 minutes to diagnose, but I could have saved myself a trip and had the problem solved in 3 seconds. So, I said to myeslf, "Bonehead? (lol) Never forget the gas meter or gas line shut off valve or gas valve!" That's what was wrong. Somebody shut off the meter outside. Oddly, the pin was not in the meter. So I'm not sure who would have just shut it off.

When I arrived there, about 40 minutes ago, I first walked up to the side of the house, because I could hear the ventor gas discharge coming out of the pipe on the side of the house near where i parked. I felt the discharge and the intake (sealed combustion furnace, obbviously). Then I went in the basement, and took off the cover. Reset the furnace. Turned it back on. The ignitor glowed . Then I heard the click of the gas valve, and felt it too, as I have told people here. Then, I said to myself that it's like there is no gas coming out. So I went back upstairs, and outside, and found the meter off.

If a hot surface ignitor comes on, and you hear the click of the gas valve, that means gas is trying to get to the burner. There is no pilot light or any other sensor at that point. (Something for everyone to remember). Only, to my knowledge, it's the gas furnaces that have the spark ignition that have the pilot light and the flame sensor is part of the whole pilot/hod unit. So, if you hear the gas valve open and there is no fire, even though there is glow, about all it could be is a 'weak' ignitor Glow...but insufficient glow), or there is a gas flow problem.

I had to run and shut off the furnace 5 times before the gas valve bled out the air and the gas took hold and it fired. I also found the furnace filter sticking out 1/2 inch so I pulled it out and found a piece of school paper lodged inside that I got out.

Just thought I'd share my little story here this morning.

I love working on furnace and love trying to figure out the problem. This is probably my favorite job in the maintenance field. it's like puzzle solving.

 

Grady and Shank...let me ask you guys something. I have really never got into the furnace flow rates and other calculations that furnace men do. I am mostly a bad parts diagnoser/ charger to existing set up furnaces where all the bolt on parts with a furnace fail. I am pretty good at figuring out all those problems, like the ventor motor, condensate back up problems, pressure switch, ignitor, ignition control board, motor out, etc. But from the engineering aspects of the furnace, I am probably as green as anybody else.

You mean that with these furnaces, that because of the differential between the heat output at 12 inches outside the furnace, and the cold air return temperature at the furnace, that, in theory, that if the furnace where shut off and the house was cold...let's say only 33 degrees in the house... that the furnace would only be capable of putting out say 70 degree heat at a register , until the house warmed up? (on the basis that cold air returen is cooling down the heated air in the furnace, and mixing with it and diluting the temperature capability?)..and as the house warms up, the register temperature would keep climbing? Is that it?

Like, in the case of the 104 degree heat, the lady has it at 66 in the house. Maybe that is why it is 104? Like if she had it at 75, the register temp would then be more like 114? Is this like what you are trying to say?

 

Exactly.

btu = amount of heat to raise the temp of one pound of water one degree F.

Crude example follows:

so if you have 10 pounds of 66 degree water it takes 440 btus to raise it to 104 degrees.

that same 440 btus will raise the temp of 70 degree water to 114. and if you raise the initail temp to 75 you would end up with 119.

at the fixed btu rating of the furnace the output temp will vary acording to input temp

Just remember that you don't care what the supply temp is, you just care about the temp differance from supply to return. and that only tells part of the story.

the last Rheem 90% furnace I was looking at the specs for called for a temp rise between 40 and 70 degrees.

 

So if the rise difference can vary from 40-70 degrees does that mean that the output heat temperature could vary by 30 degrees?..like instead of being 104...be 134 degres instead? If so...or if this is even close to being true..who wouldn't want the 134 heat over the 104 heat?

I just came from a rental I am doing a go-thru on. It is 43 degrees out right now and the furnace was shut off and it was 51 in the house. It is a Tempstar 9000 model (it has a fan switch also, I might add )...75,000 input btuh. And THAT furnace puts out 142 degree heat in a few minutes, at a register!!!!, and raised the large 2 bedroom condo inside temp from 51-61 in 20 minutes. (The 104 degree furnace would have taken over an hour to.or something like that). (The Tempstar is a sealed combustion chamber model and the Dependable 92 is an open one.)

This got me thinking. That other Dependable 92 that only put out 104 degree heat is in about the same size condo, built about the same time (late 80's_ and they are only 1 block away (same neighborhood, obviouisly.). I *believe that Dependable 92 is also about a 75,000 input btuh furnace. Since the Tempstar furnace puts out way greater heat per hour, that obviosly means the output btu's are way more than the Dependable. And shouldn't that automatically raise red flags as to something being amiss with the dependable, being they both say they are 75,000 input btu's? (I think most furnaces I have seen will say that the input btu's are about 10,000 higher than the output btu's)

 

Depends on the airflow. If you speed up the fan on the Tempstar the Temp differance would go down even though you would be putting the same amount of heat in the building (More CFMs=less heat per CFM). and, if you slow down the fan on the other unit the temp differance would increase (less CFMs= More heat per CFM).

Now I am not saying that there is, or is not a problem with the unit, only that the supply temp and Temp differances aren't the only things you need to know before determining that there is something wrong with it.

 

That's very true, what you say. But I noted when I felt the air coming out of the register, that the amount of air was at least as great or *greater* with the Tempstar that the other one. Which, if what you way is true, that the *Tempstar* should be the one putting out the cooler heat because it is blowing more air past the heat exchanger per given unit of time. But, in all actuality, I believe the air coming out of the registers is close to being the same.

I wish I could have seen the burner area better. I saw 2 burners for *sure* but one or more others may have been blocked out of view...I'm not sure. I guess whan I go back there (tomorrow?) I am going to see if the manual (duh...I should have thought of that) is there and look in it and see. Lot's of times they show an exploded view) I also noted that the cone of the blue flames was at least 1 inch high and maybe 1 1/2 inches high. I could not see how high the total blue flame was though. But I already realize even THIS does not prove anything because unless the burner sizes are the same, then we aren't comparing apples to apples.

But none the less...I believe both are 75 000 btu furnaces and one is putting out great heat while the other puts out windchilling measely heat.