8 replies to this topic

[Carleton]

Carrier 58GS150-3 Sereis 301, s/n 2284A10270 upflow, gas furnace
with a working vent damper, and electronic ignition.
Installed January 1985

A-coil and outside unit installed July 1989.

Recent service: New limit switch Feb 2003 & Dec. 2005
Outside motor for fan Aug 2005
Sept. 2006 - blower motor replaced.

Asode from normal cleaning and filter replacements, this unit hasn't given us much trouble - I've asked for a PM to check the heat exchanger condition as well as the general condition of the furnace.

Last winter was mild, but on really cold winters, we usually take out the limit swich at least once. And we can't use thicker filters as the limit switch will trip. [this is a new condition over the last couple of years]

What should we expect in terms of a PM type call? Should the tech evaluate the condition of the heat exchanger and maybe find out why the limit switches are going out?

Thanks for your insight,

- carl underwood

[hvacdrd]

150,000 BTUH - this better be in one big ol' house, as in 3000 sq ft or larger.

A standard PM should include at the very least a heat exchanger inspection, cleaning/adjustng of burners and temperature rise measurement. The temp rise will tell you if you are moving enough air accross the heat exchanger to prevent tripping on the limit.

Given the fact that the unit is 20 years old and probably in the 70% efficient range, tripping on the limit frequently could mean heat exchanger fatigue. So prepare yourself for the probability that whoever does the PM is going to push you in the direction of replacement. With an increase in natural gas prices of 300% (yes 300% - 0.28/ccf in 2000 and currently 0.87/ccf this month) in the last 5 years it would be wise to get as much out of it as you can.

[Carleton]

I don't know what you define as a big house but it's a 2 1/2 story american four square, brick circa 1913 - heated basement, first and second story - about 2700 sq ft not counting the attic.

We're in Ohio - Zanesville - middle of the state off I-70.

Not as cold as Cleveland but still it can get cold!

Thanks for your insight and I'll be armed with that on Wednesday when the tech is to show - hope it's cooler.....


BTW, any brands of high efficiency furnaces to steer clear of should a replacement be in the future?

Thanks very much,

- carl
 

[Carleton]

Upshot of PM on 9-27-06

Heat exchanger inspected, no cracks, holes, soot.

Ignighter(?sp) was a bit dirty although it checked out electrically - he cleaned it.

Burners cleaned, no rust fell from the burners.

Cycled the furnace a couple of times to observe startup and shutdown. Vent damper worked as expected, good draft.

Tech remarked that the startup was smooth. Shydown was clean.

Temp checkout was fine -
ran the furnace to bring the house to 80 degrees - stat and separete thermometer agreed - temp at register was 165

Guess we won't replace for a bit - unless the heat exchange fails at some point.

As to whether we're staying in this house - I'm betting until our last child goes off to college - at least - and she's nine now!

Thanks for the help,

- carl underwood

[hvacdrd]

Just a thought...wasn't he a little concerned about the temp rise? 165F at register minus 80F return air is 85F. What is the recommended temp rise listed on the name plate? I would expect 35 to 65. Typical limits on these are 180F, 165F is awful close.

[Carleton]

Nameplate says 45-75 with a CFM of 1851 (heating)

He didn't seem concerned but I am.

Cooling CFM is 2110.

Would there be any harm in switching the fan taps between cooling and heating once cooling season is over?

Cooling is currently set on the Hi tap (black) and heating is set on Med-Hi (yellow)

Is my logic correct in faster fan speed pushes more air over heat exchanger resulting in a lower temp rise?

I did indicate to him that the filter that is in place now is what I use during the cooling season (one of those denser 3M filters - takes a 16X25X1 ) and that usually for heating season I use a fiberglass (blue, not as dense 16X25X1 ) filter.

I guess I can do my own temp rise measurements with the less dense filter in place before I go switching fan speeds.

Thanks,

Carl

[hvacdrd]

Increasing the fan speed will certainly help, air flow is directly linked to temperature rise. BTUH = 1.08 X CFM X temp rise or CFM = BTUH/1.08 X temp rise. Given your numbers it looks like this: CFM = 150,000(75%efficient) / 1.08 X 85 = 1225 CFM - even a high estimate of 80% efficient is only 1307 CFM

Like I said when we started here the unit seems a bit on the large side. The CFM ratings are for a given static pressure(resistance to air flow - ie. ductwork, registers, filter, a-coil) the more resistance the less air flow.

If increasing the air flow lowers your temp rise within an acceptable range and the noise(air velocity) levels are tolerable,  I would simply wire the control that way permanently so you do not have to switch it from summer to winter. If the motor taps are on the control board (usually cool - heat - park) then remove the heat speed tap (yellow) & place on park, jumper the heat & cool terminals with the high speed motor tap still attatched (black wire).

If yours is not set up this way let me know what you have & I'll walk you through it.

 

[Dan Webster]

I prefer manual type heaters to electronic control anythangs