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Re: Rheem 90 Plus Problems


Your ignition system is like many others out there; there is no separate flame sensor....because it doesn't need one. Many gas clothes dryers use this same system, as do gas ovens.

It works like this; voltage drop/current draw. Yup, it's that simple. The gas valve control to the main burner will be either a solenoid or a bi-metal valve which will only activate/open when voltage/current is applied to it.......in a very narrow range. Too high... no go. Too low.... no go.

This reduced voltage/current comes from one of those two wires you see on the HSI. One of them is input and the other is output, so to speak. The HSI element itself is a resistance device, of course. If the resistance is correct, the element will not only glow sufficiently hot, but will also have the correct amount of "thru-put" to the gas valve.

No separate "flame sensor" is required because the correct "thru-put" not only "tells" the gas valve control that the HSI is present, but also tells it that the HSI is hot enough to ignite the gas. Since the igniter continues to glow the entire time the furnace, clothes dryer or oven is firing, we have a reliable "re-ignition" source if there should be a momentary lapse in the gas supply for some reason. It's a very safe system.

As these HSI igniter elements age, the resistance changes/increases. That's why seeing one glow doesn't mean that all is okay in that regard. Doing a resistance check on a cold element also doesn't tell you everything is okay with the HSI. That's because the resistance will begin to change as soon as voltage is applied and the element begins to heat up. A current draw test while the element is heating will tell you a bit more in that regard. A Norton flat element should show a draw of 3.2 - 3.6 amps while a carborundum round igniter should show a draw of 2.5 - 3 amps.

Several times I've run into HSI elements that are ever so slightly fractured. Often this fracture is so minute that you need a good magnifying glass to see it and continuity is still present when the element is cold. This fracture can/will cause problems because when the element begins to heat, the element swells and the crack opens up a bit.....or maybe a bunch. If it opens just a tad, but stillo maintains continuity...it alters the draw a bit and consequently the valve won't open. If it opens a bunch, continuity is totally lost and the game is over.

Fact is that your furnace guys should have a suitable/matching *known-good!* or brand new igniter on their truck with them. If they don't...I gotta wonder why. Also gotta wonder why they haven't tried this yet. It only takes a couple minutes to swap yours with the replacement and then you have an answer......and hopefully a working furnace. If it works, you buy the new igniter. If it doesn't, they put yours back in and look elsewhere for the troublesome culprit.

Maybe they're trying to convince you that you need that new furnace. ??? As in......sorry, but yours can't be fixed.

Re: Rheem 90 Plus Problems


One more thing........

If replacing the current igniter with a new one doesn't resolve the problem...then it's time to suspect/inspect both the wiring from the igniter to the controls and/or any associated wiring connections to the valve.

If the right voltage is for sure arriving at that valve from the igniter and the valve still isn't opening ...... then the valve itself becomes very suspect.

Re: Rheem 90 Plus Problems

Wayne ... if it's not too late ... I bounced this off a couple of HVAC guys and here's their comments:


Temporarly jump out your pressure switch and see if it helps. You may have a weak switch or a blockage. Check it out. Then look at the ingiter real close, look for a discolored spot where it may be starting to crack. Go ahead and replace it, it's cheap

What he meant by blockage is to specifically check the hose from the IDM to the pressure switch. Remove the hose from the barbed fitting on the pressure switch and clean there.



What resistance reading did you get? Most of the standard Nortons are between 50-70 ohms. If it is higher then it is going bad and needs replacing and may be why it takes too long to warm up when cold.

Re: Rheem 90 Plus Problems

Information forwarded to Wayne is from licensed , certified and experienced HVAC technicians.

Re: Rheem 90 Plus Problems

LOL ... wouldn't you know it ... yesterday I had to troubleshoot my own furnace not igniting.

The induced draft fan would come on but no ignition ( intermittent).:rolleyes:

So there I go taking the covers off the furnace to start checking things then all of a sudden ... the power goes out.:(

After about 1/2 hour or so without any power and with flash light in teeth I decided to remove the IDM to clean it out.

I discovered the hose that connects the IDM to the pressure switch had a small split. It seems the end of the hose that connects to the fitting at the blower end had become brittle from the heat and developed a split.
It became apparent that as the split had began it disrupted the negative pressure for the switch intermittently but as the split grew it became more constant.

So .. cut the hose to good flexible rubber and re-attach and waited 2 hours for the power to come back on. When it did and performed the testing everything was back to normal. Yippie :D

Side note .... regarding the discussion of the original topic .... the HVAC techs I've had discussions regarding that problem had mentioned that the majority of furnaces have a separate flame sensor and HSI. The rare ones that use the HSI as the flame sensor are problematic in that there are conversions that install a separate flame sensor. This eliminates the problems of ignition and firing issues.

Re: Rheem 90 Plus Problems

Goldhiller and all that have inputed to this; I am grateful thank you all: I have been stonewalled by the RHEEM company!
No one has responded as of today's date!

The furnace is still failing and I have a new HSI on order to replace! but also on the IC pnl the HSI has to go thru 2 relay points relay K7 and K6! but have no idea how these relays get picked and they are soldered on the board!

So thank you again and sorry for the delay getting back as I could not get signed on;;;had my wrong sign on;;;but finally got that fixed! lol ok later my good friends! byeee

wayne:o :(

Re: Rheem 90 Plus Problems

I have a Rheem 90 plus Gas Furnace. It wouldn't light. I contacted the authorized dealer in Rolla, MO and put in a service call. The unit was 4 years old in June of this year. The problem seems to have been fixed by installing a new single stage DSI control board (part number 62-241140-04). The service person said the warranty on this part expired in June of this year. Total cost was $326.32

Any feedback from anyone?



Re: Rheem 90 Plus Problems

Just thought I'd toss my 2 cents into this discussion. I also have a Ruud 90 plus of the same vintage and was having ignition problems starting LAST season. I never has a "no start" problem but I occasionally got some blow-back on the 3rd try/ignition. I had resistance on the HSI but at the time I didn't know much about the heater. I had a tech check it out. He said he didn't know why it was doing that and "maybe we should consider a new heater". Wouldn't you know 2 weeks later the HSI quit but not before I learned all I could about it. My HSI had cracked so I got a replacement. The new one had a nice healthy glow and the burner lit on the first try. After 2 minutes it shut down and tried to relight 4 more times, then it locked out. It took 3 more lockouts to finally start working correctly, each time running a bit longer.Unfortunately I couldn't find any info on what resistance I should see hot or cold. I figured the HSI resistance was just a bit off since it was not a Ruud part. It probably needed some "burn-in" time before the resistance was correct for the control board to sense the correct value for flame sensing. I might be a bit patient with a new part.

Re: Rheem 90 Plus Problems

Maybe someone can shed light on my problem? I am at a loss...

My Rheem 90 Plus is not igniting. The HSI glows bright red but the furnace is not igniting. I replaced the flame sensor with a new one and also replaced a rotted rubber hose (one of two), which went from the draft inducer into a black unit positioned to the right of the control board (sorry for the awkward explanation). But, anyways, the furnace ignited. I then proceeded to toggle the furnace on and off by raising and lowering the thermostat setting, so it seemed I solved the problem. However, the furnace worked for about 10 minutes, then not reaching the thermostat setting the flame went out and since then I have been unable to ignite it again. I turned off the power to it, and when I turned it back on, the HSI glows red but no ignition happens. This occurs four times, then i'm assuming, the system locks out.

Also, before I replaced the rubber hose and the fuel sensor, the gas used to shoot straight out of the PVC pipe into the backyard. I didn't realize that until I went outside and smelled gas coming straight out of it. Now it doesn't do that at least.

Any ideas at all?

Francois Servais
Re: Rheem 90 Plus Problems

I have a Rheem Imperial 90 Plus Natural Gas Furnace. When it was installed I went through all these problems and was ready to have the contractor replace the furnace. As a last ditch effort on his part he talked with the engineering dept at Rheem and eliminated every possibility. They said the only other possibility would be the air supply. So we found that the 2" supply line (reccomended in specs) was not large enough to supply enough air to the furnace and they recommended a 3" line to supply air. The 2" PVC pipe was already installed so we removed the hookup to the furnace and let the air be supplied from the air in the furnace room. That solved the problem and the furnace works fine has been trouble free since 1993.


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