The situation
A customer was replacing a Suburban SF-35Q and wanted more heat out of the new unit than the old one gave him. He asked what we'd suggest.
The short answer
The Suburban SF-42VHQ is the exact replacement with more heat. Dimensions, cutout, ducting, and wiring are all the same as your SF-35Q. It runs on slightly more amperage and uses a little more propane — that's what the higher BTU rating costs you.
If you'd rather stay at the same output, the SF-35VHQ is the straight-across replacement for the SF-35Q.
Why
Suburban builds the entire SF-VHQ line on one cabinet. Every model in the series shares identical cabinet dimensions, which is exactly why you can step up a BTU class without touching the opening, the ducting, or the wiring. Both furnaces measure 7.5" H × 17" W × 20" D.
What actually changes going from 35K to 40K:
- Heat output — 35,000 BTU/h → 40,000 BTU/h
- Amp draw — 8 A → 9.0 A, still 12V DC
- Weight — 29 lbs → 32 lbs
- Propane use — up proportionally with the burn rate
One naming quirk worth knowing: the "42" in SF-42VHQ is not the BTU rating. It's a 40,000 BTU/h furnace. Suburban's model numbering and its actual output don't line up in this part of the line.
Everything else carries over. The SF-42VHQ is a Q variant like your SF-35Q, so the gas connection is in the same place — not at the front. It's the same LP fuel, same automatic direct spark ignition, same sealed forced-draft combustion, and it installs horizontally or vertically. Duct connections are available from the top, front, sides, and bottom, with 4" round outlets on the sides and bottom plus a 4" × 14" bottom outlet.
What to do
- Confirm you have a Q, not an FQ. Your SF-35Q is a Q variant, so the SF-42VHQ matches. If the furnace you're pulling connects gas at the front, you have an F-series unit and want the SF-42VHFQ instead. This is the one place people order the wrong unit.
- Account for the extra amp. Nine amps instead of eight is a small change, but check that your 12V feed and fuse are sized for it rather than assuming.
- Have your installer confirm the return air opening is adequate for the higher output while the furnace is out. Easier to address now than after it's buttoned up.
- Before you replace the whole furnace: if the symptom is failure to ignite rather than a dead blower, the culprit is often the igniter electrode. The 525051 electrode kit is the SF/SFV Q and FQ specific part, and the 525009 assembly covers a broader SF/SFV range. Both cost a fraction of a furnace and are worth ruling out first.
- Order the SF-42VHQ. It ships with vents, the installation manual, and the original manufacturer box, backed by Suburban's 2-year warranty. An exterior access door is not included — if yours isn't reusable, add one in white or black.
- Have a qualified RV technician handle the install and leak-test the gas connections afterward.
Still not sure?
Send us the full model number off your furnace data plate and tell us where the gas line connects, and we'll confirm the right unit before you order. Ask us here.