Poor New Hot Air Furnace Performance

Terry_14

Looking at your gas meter while only the furnace is running will help determin if you are burning the correct amount of gas.

I time the 1/2 foot meter with furnace on. each 1/2 foot of gas burned equals 500 BTU count the revolutions for 60 seconds multiply that total by 500 then multiply result times 60

example 4 revolutions X 500 = 2000 X 60 = 120,000 BTU Hour

a check of the lable should indicate the max gas pressure in " many times it is 3.5 "

Hope this helps
Happy new Year
Terry

Bob Mc

Poor Perfromance - New Hot Air Furnace

About 2 weeks ago we replaced an old Heat Controller Model GHJ-125-DH due to a blower motor failure. Due to its age approx. 25yrs, seemed like a good time to just replace the whole furnace.

The replacment furnace is a Philco Model G6RA 096C 16B. This was almost a drop in replacement, the only change required extending the outlet duct by 11 in. due to the lower profile of the new unit.

Right from start-up I have been very unhappy with the air temp's I'm feeling at my outlets. Since start-up I have reduced the blower speed from the factory setting of Med Hi, to 1st Med-Lo (with a little temp rise) to then Low (with a little more temp rise). On Low speed I still am getting unacceptable heating and we feel the unit is runing at least twice a much to maintain a 70F thermostat setting.

My "professional" advisors all tell me that this is more than enough furnace to heat 1700 sq. ft. here in central MA.

Can anyone offer some advice on why I not getting any "real heat" out of this new furnace that seems to be burning up lots of natural gas?

Thanks in advance for your replies.

Bob

Mark Hunt

Well


newer gas furnaces operate on a time delay now rather than the old type that activated the fan on temperature. The fan comes on sooner in the newer units and I typically see outlet temps around 110 to 120 degrees.

That being said, unless the installer performed a combustion test AND a temperature rise test on the unit, they will not know if the unit is underfired or if the duct work is adequate.

There is more to this than just screwing tin together.

Now this may sound crazy but.......if you have a couple digital meat thermometers....stick one in the supply duct and one in the return duct near the furnace. IF you can do this, post your findings here. This will give us a pretty accurate temperature rise.

After that though, the only way to determine if the unit is under-fired is for a digital combustion analysis to be performed.

Where are you located?

Mark H

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rick_35

are you not happy with the outlet temperature at register. Was the old unit gas .check temp rise to see if it is within spec,

Steve_35

What temp rise does the manufacturer recommend?

This is a common issue with new gas furnaces. With old furnaces a temperature rise (difference of the temperature of the air coming off the furnace vs the temp of the air going in)of 80F wasn't uncommon. 35-55F is much more common today. That means the air temp coming off the furnace may be as low as 95F.

The recommended temp rise should be in the manual or on a data plate on the furnace or both. The supply temp should be taken out of line of sight of the heat exchanger.

rob

Manual J.....

Could be a sizing issue and blower capacity. Where is the manual J load Calc and what is the delta T. It is possible that a standard blower with 125K furnace will not obtain an acceptable delta t because the blower sizing for anticipated cooling needs with such a high capacity furnace.
This is why a firm foundation is needed for all heating designs and it begins with a Manual J right on thru to Manual D and T. You may have been happier with variable speed unit has most brands now run as low possible delta T in order to obtain the highest heat extraction and therefore relative steadt state efficiency....

Tony_23

G6RA 096

That's a 96000 BTUH input and 80% efficient. Your old one was 125000 input and about the same effiency.

Was a heatloss calc done ? Maybe the old one was right-sized and this one's too small. Without the calc's you're just guessing.

Changing fan speed too much will cause HX issues. I'd stay on Med-Lo at least.

Sounds like a "quick" job to me

S Ebels

Pretty standard unfortunately

AFA the low tem rise goes anyhow. Most new furnaces are rated at <55* rise across the HX. At max that gives you about 120* plenum temp, minus duct loss along the way. Not real warm when you're standing a few feet away from the register. The exception is ThermoPride, they allow up to a 70* rise on their units.

The cycling problem, which I think you have from reading between the lines of your post, is possibly due to your thermostat's anticipator setting. If you have a T-87 or other mechanical stat, you need the anticipator setting to be around .8 to produce longer cycle times.


There is a heating company in these parts named B&R Heating. The inside joke around here is that the name stands for Burn & Run. Maybe they opened up a branch in your neighborhood.

G Lyons

Bob Mc; If you want to know how your new furnace is doing. Set the thermostat at 70 and see if the room temperature comes up to 70 and stays there. Stay away from the hot air registers, they won't blow hot air like they used to.. Your new furnace is just fine and will save you a lot on your gas bill.. We need to warn people up front that their new furnace is not going to blow as hot.. But it will get the job done and save money. As you think about it, if the furnace is going to get nearly all the heat out of the fuel it's got to blow at a lower temp. The furnace can't give off any hotter air than that of the flue gas. and the flue gas in a 90 plus furnace is just warm. But your getting the heat instead of it going up the chimney. So set the blower speed back to where it was and relax.

G Lyons

Bob Mc; If you want to know how your new furnace is doing. Set the thermostat at 70 and see if the room temperature comes up to 70 and stays there. Stay away from the hot air registers, they won't blow hot air like they used to.. Your new furnace is just fine and will save you a lot on your gas bill.. We need to warn people up front that their new furnace is not going to blow as hot.. But it will get the job done and save money. As you think about it, if the furnace is going to get nearly all the heat out of the fuel it's got to blow at a lower temp. The furnace can't give off any hotter air than that of the flue gas. and the flue gas in a 90 plus furnace is just warm. But your getting the heat instead of it going up the chimney. So set the blower speed back to where it was and relax. G Lyons, Dispatch P&H

Steve_35

Furnace

Here's a link to data on your furnace.
Philco furnace

This is an 80% furnace and allows for a temp rise of 50-80º so the air temp shouldn't be that much different unless your old one was too high to begin with.

Another poster mentioned setting the t-stat at 70, or where ever you want it, and leaving it alone. IMO, that's good advice. Your old furnace had more than 25% more input so the new one will cycle longer. Something most folks don't understand is that if we've done our job exactly right on a design day your furnace should run non-stop.

That said, ask the installing contractor if they could set the furnace to the maximum temp rise. Ask them to check the gas usage by clocking the meter. And as someone else mentioned if you have a t-stat with an aticipator, make sure it's set properly or, better yet, go to a digital t-stat.