720-400 – GAS VALVE

Robertshaw 720 Series 24 volt dual (six-function) combination gas valve for standing-pilot gas heating equipment. It combines a manual valve, safety shut-off magnet, dual automatic valves, main gas regulator and pilot adjustment in one body, and Robertshaw states it is available only in a 24 volt model used with a constant (standing) pilot. This is the straight-through version with a 1/2″ inlet and 3/4″ outlet and no side outlets; the regulator is factory set at 3.5″ W.C. for natural gas and converts to L.P. using the included conversion kit.

Specifications published by the manufacturer. Source: manufacturer documentation.

Specs

• Series720 Series 24V Dual
• Ignition SystemStanding (constant) pilot only - not for intermittent/direct ignition
• Inlet Size1/2"
• Outlet Size3/4"
• Side OutletsNone
• Capacity150,000 BTU
• Electrical Rating24V AC @ 0.5 A, 50/60 Hz
• Pressure Regulation3.5" W.C. natural gas; convertible to 11.0" W.C. L.P.
• Pilot Outlet1/4" tubing
• Maximum Inlet Pressure14" W.C. (1/2 PSI)
• Ambient Temperature-40 F to 175 F
• MountingAny position except upside down

Can I put any 24 volt gas valve on my furnace?

No. The ignition system has to match. A standing pilot valve, an intermittent pilot valve and an HSI or DSI valve are built differently inside, and the manufacturers state plainly that they are not interchangeable. A standing pilot valve has a pilot outlet and a thermocouple circuit. A hot surface ignition valve has no pilot at all and expects the board to prove flame through a sensor. Substituting one for the other either will not run, or runs without proving flame, which is the part that matters.

What is the difference between 24 volt and millivolt?

A 24 volt valve needs a transformer and a control circuit. A millivolt valve runs on roughly 750 millivolts generated by a thermopile sitting in the pilot flame, with no outside power at all. They look similar on the shelf and they are not swappable. If the appliance has no transformer anywhere in it, it is millivolt.

Single stage or two stage?

A two stage valve has two regulator adjustments and two 24 volt hot terminals so the board can fire low then high. Hang a single stage valve on a two stage furnace and you lose low fire and usually pick up board faults. Bring the old valve or the furnace model number.

Can I convert a natural gas valve to LP?

Only with the manufacturer's kit for that furnace, and the kit is more than a spring - you replace every burner orifice and reset the regulator. Ballpark manifold pressure is about 3.5 inches water column on natural gas and about 10 inches on LP high fire, but the rating plate is the number that counts. Most of DFW is on Atmos natural gas, and LP mostly turns up out past the county lines.