Trane XV95 fan speed?

[tumbleweed]

We have acquired a used Trane XV95. No info as to why it was pulled out but it was free so no complaints there. We have built a new farm shop and intend to install in floor heating so this is a temporary installation.
It appears to work normally with this exception. When it switches from stage 1 to stage 2 the fan speed picks up but it will only run for about a minute before the limit switch opens (4 red flashes) and won't restart until both the fan and inducer have shut down.
My first thought was the limit switch must be faulty. The best shot I have to check the temp of the hot air is with a meat thermometer with a long probe. With it sitting just above the heat exchanger it shows right at about 180F. When stage 2 cuts in the fan speed picks up but it only runs for about a minute until the limit switch opens. The usual reason for the limit switch to open is restricted air flow, but that can't be the reason in this case. We have no ducting on either the cold or hot side and the incoming air is only about 10C or less.
A Trane fact sheet covering this model shows a max fan speed of 5000 rpm. Using a digital tach I read just over 700 on stage 2.
I have the thermometer wired as a 2 wire system using R and W. I see both the thermostat and the furnace have G terminals, which provide some sort of fan control. Should these be wired up?
Any ideas? 

[Admin]

Are dip switches 7 and 8 ON or OFF on the furnace control board?  Maybe the fan speeds are set too low.  Turning them both OFF would engage the high speed fan for both 1st and 2nd stage heating.

180F seems like a valid limit trip although a furnace without ductwork should not overheat.  I would check the gas manifold pressure or clock the gas meter to see if the furnace is over fired.

A simple fix for a temporary situation would be to remove the jumper between WI and W2 on the furnace control board.  The thermostat should be wired to R and W1.  This will force the furnace to stay is low fire and not overheat in high fire.  Maybe it won't trip the limit in low fire only.  Wiring G to a thermostat with fan control would engage the continuous fan speed and give you less CFM but would only run when the furnace is not heating.

[tumbleweed]

Thanks Admin. The dip switches were the first thing we checked. When we turned 7 and 8 off there was a noticeable increase in fan speed but it didn't solve the problem. CFM of course is the metric that removes heat from the heat exchanger but it does seem odd to me there seems to be no available specs on what the fan speed should be.

We had already removed the jumper between W1 and W2 and I think it did make a small improvement in over all heating because when the limit switch opened we were loosing more heating than we were gaining during the short time it was firing on stage 2.

I too was wondering about the gas manifold pressure. I see 2 ports which I expect are for making adjustments but I don't see any provision for attaching a manometer.

Could gas pressure coming into the building be a problem or is the gas manifold pressure compensated?
It is an old regulator but it was working fine on an application we just took it off of.

[Admin]

Download the Trane XV95 Service Manual here and the CFM is listed on pages 9 to 19.  Match your model on the top of the charts.

https://www.hvactechgroup.com/hvacforum/index.php?topic=1788.0

Download the Trane XV95 Installation Manual and look at pages 32 and 33 for manifold adjustments.  Slide your 1/4" manometer tubing inside a piece of 5/16" tubing and that will slide over the gas valve ports.  It's not like the older valves where you removed a plug and installed a threaded barb fitting.

Is your gas supply propane or natural gas?  Is it possible you're using propane on a natural gas furnace?  You would need a propane kit if so, Part # BAYLPKT210A.

https://www.amazon.ca/Trane-BAYLPKT210B-by/dp/B01N1IUX5E