Hydroflame Furnace

Just to update, got the new limit switch installed, and the furnace back in the motorhome. It seems to work properly, but have some concerns on the surface temperatures of the plenum, or case surrounding the heat exchanger. As I mentioned earlier, the furnace is installed with a bottom discharge. The new limit switch is rated at 170*, and the surface temperature of the top, and back of the case are in the 140*-160* range, but the sides of the case are reaching 275*, hot enough to burn your fingers.

There are partitions protecting anyone from touching this area, but thought I would feel the furnace before I reinstalled them. This is when I found it to be hotter than expected. Read the temps with a infrared thermometer.

Could this be normal??

Steve

Thanks Chris, and others who helped get me through this. The new limit switch is ordered, I'll put it all back together and post the results when I'm done.

It's great to have a site like this available, especially for those of us that prefer to do work ourselves.

Steve

The replacement will be 37021- and yes. there is only one limit switch. The recall changed it to the gas valve circuit, so that if it ever cycles on the limit, you will know something is wrong- the cycling is what cracks the heat chamber.

I took apart the furnace, the unknown device is a limit switch part # 36176, and the only limit switch on the furnace.

3Lakes Steve wrote:
...snip...Some where in the past, the lead at the board (red) to the Gas valve has a splice in it that goes to the unknown switch or device, and returns to the gas valve...

What you said and Chris, I wonder if the unknown wire leads to a melt-able fuse like on a hot water heater. :h

This link shows it as a 37021
Parts Breakdown

Hey Chris, I've been looking at furnace documents on the internet, and just realized their from your website. Great Info! You have a parts cross reference for my furnace, and shows both a limit switch, and a thermal cutoff 8900-II, The limit switch I have now has 36176 inked on the back. My parts list that came with the unit shows 34781.
your doc shows 37021. Are these different #'s with the same specs??

Steve

Thanks for the advice. I pulled the furnace, and removed the combustion chamber. It looks to be in very good condition, very solid, and hardly heat tarnished at all. Also, the limit switch that runs through the gas valve line, is the only limit switch on the furnace. It shows no continuity between the posts.

The way this is wired, I would think it screws up the sequence of operation, as there will be power sent to the igniter board only thru the sail switch. I think the limit switch cycles regularly, anytime its calling for heat.
If power is sent to the igniter board thru the sail switch only, its going to try and light even if the limit switch is open. What happens to the 3 try delay??

I'll pick up a new limit switch and put the unit back together.

I was relieved to see the chamber in such good condition, considering this is a 21 year old coach. It would be nice to see the replaceable parts a little easier to access.

Steve

It would be a good bet that the limit switch is defective. I don't think that switch is a manual reset- what makes it require manual intervention is its place in the circuit- the regular limit switch is in the main power line, so when it cools down, the furnace fires again. With the switch in the gas valve circuit, the board sees the lack of flame and locks out, so when the switch cools down, the furnace still does not fire.
At the time, Hydroflame sent out a tool kit with large rubber stoppers and an lp detector- one of the stoppers was rigged so you could inject straight propane in to the chamber- if you look somewhere on the bulkhead, there is a small metal plug. You filled the chamber with LP, stuck the detector probe through the small hole, and you could tell if the chamber was cracked without pulling the whole thing.

To inspect it, it would probably be easiest to pull the whole thing, and not try to pull just the chamber- it's been a while though, so I could be wrong.

That recall really has me scared now. I would think a tripped limit switch would be open until manually reset. I'm getting erratic voltage thru it, so I'm wondering if it has malfunctioned, or if that is normal. I'm not sure how to even get at it?

It looks like the installation meets the supply, (bottom discharge) and return air requirements.

I'd like to remove the combustion chamber to visually inspect it myself, as well as replacing the added limit switch. I'm pretty handy, and would be comfortable doing the work. Looking for some advice on the disassembly/assembly.

Thanks again
Steve