Reznor Rdh300 Problem

[mrhvacmechanic]

REZNOR M# RDH300 NATURAL GAS 100% OA HAS HEAT EXCHANGER

Spent the day on this unit!! Outdoor air was 25F in NYC today

Unit starts up and runs with no problem in low fire all the time!!!After low fire it gradually comes up to high fire and then it always fails??????????

Gas trian: Honeywell valve then Maxitrol M520R down stream.Down stream of Maxitrol a pressure switch NO and makes at 1.7" changes ventor motor speed at high fire.

Gas pressure before HW is 7" During low fire between HW and Max valve a constant 4" as per factory manual. As it goes into high fire the pressure falls from 4" to 1.5" and climbes back to 4". (I guessing the regulator starts to bring up pressure again?)

As soon a it reaches 4" it fails and then goes thru another cycle???Then it refires???

No limits are opening,supply set for 95F with room sensor for 6F ofset up or down of set point. Supply fan, dampers, ducts ok. Only happen after it goes to high fire.NO INDICATION ON BOARD LIGHTS OF A FLAME OR ANY OTHER FAILURES??

0 volts dc valve is closed 20 volts dc valve open. At low fire valve is 3.14volts dc. when it goes to high it jumps up to 19.8 vdc. There is no graduall clime in dc volts. (Is this correct?)

Any ideas would help.

[teckster]

Is this a newer unit?? had similar troubles on a new unit reznor. does the unit have a low and hi pressure safty switch? is this the unit that changes the venter motor pressure switch while in operation when the gas pressure increases? let me know.

[slimwoodie]

ya don't gotta enuuf .... Gas :rolleyes:

[chillersandcontrols]

Sounds like you need to swap places of the Gas Valve and Regulator...Gas train should start with the regulator. Check if gas pipe is correct size and main is correct pressure for high fire flow needs....Good Luck! wear your long underwear in that warm 25° !!!!!

[markwolf]

ya don't gotta enuuf .... Gas :rolleyes:

second that.I would check the modulating valve.maxtrol has decent info on thier web site.

[mrhvacmechanic]

Teckster,
that is correct, unit is brand new!!! heat was only just started up! Ther is a gas pressure switch that is located just before the orifice. It is factory set at 1.7" wc. juat as soon as it goes to high flame it hits switch and motor changes speed then flame drops out!!!!!!!!!!!!!!!!There are two air pressure switches on ventor motor also?

When in low fire if you look in furnace window you can see the ignition rod and sensing rod glowing orange. Just after high speed on the ventor motor, you can't see rods glowing orange anymore. Sensing rod is in back and can't be reached unlis I disassemble burner.We thought ventor motor may be pulling flame off sensing rod causing flame shut down??? However the green light on board doesn't show a flame out. No flashing??? The yellow light just goes out telling you flame is off?

Chiller, regulator is upstream of Maxi valve.

[rich pickering]

Set your high gas pressure switch higher than the max manifold pressure.

[mrhvacmechanic]

Rich,
Good point, but switch is set at 1.7" wc as per factory documentation and its also labeled that way. Adjustment screw has factory sealer on it so it's not tampered with. I was told when unit goes into high fire, the pressure switch makes to speed up ventor motor???

[mrhvacmechanic]

Teckster, please see my reply on REZNOR problem??

[teckster]

Sorry I have been busy in the field. Have you checked the gas pressure on the inlet side of the gas valve when it goes to hi fire?? I had the same issue and the gas meter set was under sized causing the unit to trip on low pressure only if all the units were running.It would recycle every couple minutes. The one I worked on had a low gas pressure switch before the gas valve and that was the problem. let me know and I will try to help you if I can. thx teckster

[mrhvacmechanic]

No gas pressure switches on the gas train at all. Only the 1.7" wc to change speed of fan at high fire.

[teckster]

If I rember right then there are 2 pressure swithes one for hi fire and one for low fire and they switch on the pressure switch on the gas manifold before the inshot burner. I am also recalling that there are 2 leds one is red and the other one is for flame which is amber or yellowish. Also is the damper for the inlet fully open? There is a pressure switch on the fan proff in the mag starter cabnet. Let me know if this is true. thx Randy

[mrhvacmechanic]

Going to job this morning.
Two pressure switches on venter motor to prove high speed and low speed. Green and yellow light.
Spoke to engineer at reznor wendsday evening and he told me to check three items. I will get back to you this evening when I come home from work.

[teckster]

Going to job this morning.
Two pressure switches on venter motor to prove high speed and low speed. Green and yellow light.
Spoke to engineer at reznor wendsday evening and he told me to check three items. I will get back to you this evening when I come home from work.

I will be in and out of the site tonight. I will look for your response. teckster

[mrhvacmechanic]

Spoke with reznor engineer and was told to check the following:

Check to see if heat exchanger sags in the middle, they had problems during shipping causing the bottom tor sag. Exchanger whould have to be replaced!
Found no problem!!!

Check to see if ventor motor high speed switch makes when changing from low to high fire!
Pressure switch good!!!

Check timer that bypasses high press switch during its change from low to High!!
Timer OK!!!

So I pilled burner out to check flamr rod location!
Rod OK and in flame, otherwise it wouldn't stay on in low fire!!!!

Slim suggested not enough gas, but regulator maintaines 4" to Maxi Valve????

Still unit trips out on high fire??????????????

[Moose]

Could it be that your flame is lifting off of the burner on high fire? = lockout?

[mrhvacmechanic]

Moose,
This was my first opinion originally. There is no way to control the air quantity of the ventor motor. When I removed the burner to check the sensor rod it looks ok. I think the high speed is pulling the flame from rod!!! Can't see rod due toits location at rear of burner.

[Moose]

Perhaps some kind of orifice belongs @ ventor intake, or wrong ventor?

[Moose]

OK... go to www.Reznoronline.com, manuals/media, find your Mo# ... complete IOM there. Good luck! :)

[mrhvacmechanic]

No help there, we have book and no help from reznor except for the tree tings they recommended.

[superfittertech]

your low gas pressure switch is opening & it is an automatic reset control. put manometer on the gas valve. inlet pressure has to between 7" and 14" during both low & high fire. I'll bet that your gas pressure is dropping off as you approach high fire. since it is an auto reset control you won't get any alarms at all. you said that you have 4" between the gas valve & the modulation valve, but a momentary drop @ the low pressure switch will drop off the burner.

[Moose]

As per Slim? :)