Generic LP to NG gas stove conversion ?????

[KeithO]

I bought a use DESA gas stove through FB marketplace.

The stove cost $200 and came with some venting, since it is a direct vent stove. I picked it up at night, after work a 2 hour drive one way. It had been sitting under a lean-to roof next to a barn for an undisclosed time. The lower half of the castings was pretty rusty, but it was intact. I brought it home, took it apart, wire brushed off all the rust, drilled and re-tapped some broken off mild steel bolts. Repainted it with the high temp stove paint.

Then I removed the glass from the firebox and vacuumed and brushed out all the cobwebs. There was no tile or firebrick inside the firebox. There was something that looked like sand and gravel which I didnt know what to do with, so I chucked it..... I painted the inside of the firebox with the same high temp paint and laid the ceramic logs back in.

I hooked the stove up to a 20lb propane tank (previous owner had run it on propane) with a 11"WC LP regulator on it. I got the pilot lit in a few minutes after venting some of the air out of the hook up line. The controls has a ON-OFF-AUTO 3 position switch and the main burner lit up almost immediately when I put it in the "on" position.. So far so good, right ???

I installed the stove at a house I am renovating since I plan to remove the furnace and all associated ducting since my wife suffers from Asthma and any forced air system is problematic, let alone one that dates back to the 1950's. Well, I got the pilot lit on the stove, but regardless of what I did I could not get the main burner to light. I figured that the pilot jet and the main jet would likely need to be increased in size with the change to natural gas, but I guess I wasn't prepared for the gas valve simply not working at all due to the reduced pressure of the natural gas...

So I started investigating what is typically needed to convert from NG to LP and vice versa an found a kit listed on tesstools.net which is the exact kit needed for converting this stove from LP to Nat gas. As can be seen, the majority of the kit parts are conversion parts for the gas valve. I ordered the kit but as it turns out they had no stock and it is obsolete so they refunded my payment. I'm not convinced that the burner tube is essential, but clearly the pilot and main jets are included.

So now I'm in a bit of a jam... I have the stove fully installed and hooked up and it is not wanting to work with my gas supply. I have the impression that the gas valve that is used on the DESA stoves is obsolete. I haven't yet removed the gas valve, but I don't find any identification markings on the outside of my valve.

I have the impression that there are relatively inexpensive gas valves like the Robertshaw 710-502 which come set up for use with natural gas. Right now there is one listed on ebay for $61.33 with free shipping. I think I can drill out the main jet myself once I establish the correct hole diameter. The pilot is too small for DIY so I am going to have to try to buy one that was intended for NG use. I'm pretty sure the thermopile is working since it worked fine on propane and the output is probably just low due to the difference in flow between LP vs Nat gas. I measure the output as 260mV with the pilot lit and sustaining. If I go to turn the valve "ON" then the voltage drops down to about 110mV with a load, which is probably not enough. Perhaps just a bigger pilot jet will provide a more powerful pilot and give more voltage to work with to operate the gas valve....

Anyone with more experience might want to comment on whether the idea of using a current millivolt gas valve (that is likely to be supported for some time looking ahead) along with the jetting changes would be the way to go to get this stove operational again ?

 

[wooduser]

Unfortunately, you've made so many changes in the equipment that it's hard to say what the problem might be, and I would be hesitent to comment lest some issue create a safety hazard.

DIY repairs can be justified when the scope of the repair is limited, but wholesale changes in equipment is probably to be recommended against, even if someone knows what they are doing, in my opinion.

But someone else may choose to comment....

 

[KeithO]

Not sure what changes you are referring to ? Just a change from LP to NG ? I removed the gas valve today with associated pilot assembly and main burner from the stove. The valve is an SIT 0820651. Copies of this valve still appear to be in use today. I removed the 2 jets, pilot and main and will measure the orifice diameters under the microscope at work tomorrow. It appears the pilot jet can be had on ebay, sized for NG for under $10. The main jet will need to be drilled out, I just need to find a table for drill size vs effective BTU for NG. The stove is designed for 30kbtu so that would be the design point.

On a separate note, as per this listing https://www.efireplacestore.com/cui-18817.html it seems to be possible to get the conversion parts for the valve to change the supply pressure to suit the NG application and I assume I will still need that.

If I had been able to obtain the proper kit as a complete package as described in the initial posting, this job would just be about exchanging parts. But I am sure there are many people in a similar situation who have to piece together their own conversion kit due to obsolete hardware or manufacturers who have gone out of business in the last 15 years.

 

[KeithO]

I forgot the jets at home this am....doh! But in looking at the main jet last night, it was marked "55". Based on this link (broken link removed to http://web.go-spi.com/sinclairdev-new/wp-content/themes/sinclair-supply/pdf/Gas-Orifice-Capacity-Chart.pdf) the btu output of a #55 orifice for LP at 11"WC is 18.8k btu. That seems a bit odd since the stove is rated for 28k btu on propane and 30k btu on NG.

Its possible that someone already drilled out the #55 jet to the size for the required btu, but it seems unlikely that would be done at the factory. But the referenced table is clear, for 28kbtu at 11"WC of propane, one needs an orifice diameter of 0.0625" or a #52 drill. For Nat gas at 4" WC and 30k btu one needs a #36 drill or 0.1065. Thats a size of drill that I feel fine using in my drill press. Now I just have to get the pilot jet on order and figure out what to do with the gas valve.

 

[KeithO]

Measured the LP orifices today. Pilot 0.0137" Main 0.0574" so I suspect the main was indeed a 55 which would be way underpowered. NG pilots appear to run 0.021" on Amazon and Ebay

 

[KeithO]

Looks like I lucked out on the valve conversion.
https://www.woodmanspartsplus.com/Search.aspx?key=0907201
Thin conversion kit is made by the manufacturer of my gas valve and the output pressure is controllable from 1.6 - 3.5" WC. Their shipping is pretty steep, but overall cost is the same as any other online seller I found. Now just have to order the pilot.

 

[KeithO]

I made some unsuccessful calls to local hearth shops regading a new pilot orifice. If it does not come to them in a kit from a stove manufacturer, it does not seem to interest them.

Finally I had a look at the 2019 Robertshaw catalog
https://www.robertshaw.com/WorkArea/DownloadAsset.aspx?id=2147496739

On page 109 are shown the pilot orifices of the type I have in my stove. And part # 10-209 was a orifice for NG with a hole size of 0.026". Given that the LP pilot was already just about 0.014, I think I will be OK with the 0.026 given 1/3 of the operating pressure.

I did a google search for that, which lead to www.mccombssupply.com who wanted $5.25+ shipping. So now I need to wait for parts to come in which should be next week....

 

[KeithO]

First snag today, the pilot orifices arrived and it looks like I should have ordered 3/16" diameter and not 1/4" because the one I got is too large to fit into the pilot... So I need to make another plan for this item... Looks like part number 10-066 in the Robertshaw catalog would have been the right item with 0.018" orifice.

 

[KeithO]

To fix my booboo, I had to drill out the orifice on my existing pilot, which is not an every day task. I loaned a few 0.020" drills from a work college and tried to pick the sharpest one under the microscope. The point grind on drills of this sort tend not to be the best. I was using 100x magnification for this as well as taking the picture of the finished hole.

I chucked the drill in a small pin vice that I had, this is usually most of the problem with drilling small holes, is finding something to grab a hold of the drill with. Forget about drilling machines or regular drill chucks. I then drilled the orifice by hand, checking progress with the microscope. The drill I picked seemed not to be cutting much, so I picked another promising one and with that had the hole finished in just a few seconds.

The hole ended up being 6/10th over the nominal drill size. I used a 60 degree countersinking cutter from harvey tool which has a far superior grind to deburr both edges of the hole. One can obviously not see what is going on so it is is by feel. The carbide Harvey cutters are sharp as all getout so one needs a light touch not to destroy the hole, just take the edge and any burrs off.

I think it turned out pretty good, so now I wait for the valve conversion kit to get the pressure down to the level needed for Natural gas. I figure that if the pilot flows too much I still have a manual adjustment on the gas valve for it, rather a bit too strong than too weak which is where it was with the original 0.014" pilot hole. Would love to get this finished in the next few days so that I can get that nasty furnace out by the weekend....

 

[KeithO]

Well, my gas valve and re-jetting conversion from LP to NG has been a success. I did not drill out the main orifice to 7/64 in the first round, I did just 3/32 for starters. Now that the stove has been running for a bit, I think it will be fine to open it up some more. My phone wont let me upload to flickr without installing their app, so pictures will have to follow.

I do have all the parts needed to convert a Robertshaw 710 series gas valve from NG to propane and it should work for similar NOVA valves too. For $7 to cover shipping, it can be yours. Send a pm if interested.

 

[KeithO]

Sorry, did not take a pic of the pilot alone. I was able to dial the pilot back some and it was still robust. I think I need some more gas flow to reach 30k btu though, I'm definitely not getting that kind of power yet with the main jet 1/64 undersize compared to the table I found. Photographed through the glass, I have to put my front casting back on.

 

[KeithO]

Final pic, cranked all the way up with 7/64 orifice on the main jet. Surely throwing off some heat,,,

 

[k0wtz]

congratulations you have done a lot of study and work to make it work. Yes you can!! I just fixed an old vermont castings gas stove with a new regular valve instead of a discontinued honeywell sit valve. I had never heard of b-type venting and it took quite awhile for me to figure that out but bob helped out a lot. at 77 i am sort of hard to get information through my brain!

i too had to figure that final part for the pilot but i found it after more study. Our Vermont Castings stove is a beautiful green color unfortunately I think its a gas hog so I am starting a new search for something more efficient if there is such a critter made in the gas heater line.

there is a ton of these stoves out there that the service guy has told owners that it costs too much to fix but the real truth many of them dont know how to fix them! I spent 150.00 on mine took me about a month to really figure out what made it tick but it was a fun project and we do have a beautiful appliance in our family room!

bob

 

[k0wtz]

mr keith what is the output of your stove meaning are you getting good heat to the front. that burner looks like the one that was in my vent free stove ti would run you out!

 

[KeithO]

I drilled the main jet in stages. Initially 6/64 or 3/32. That produced some heat but the flames were not very high (second to last picture). Once I saw how that was running I went up to 7/64 and now the stove will really crank up some heat. With the gas valve turned all the way down, it is holding 53F in a house with no insulation in it and interior drywall removed. Outside temperature in the high teens.

When I show up to work on the house, I dial the gas flow up when Im there, and back to low when I leave. It seems to be doing everything Im asking of it and it will only get better as I get insulation in the walls.

 

[k0wtz]

Got it!! Thanks Bob