Monessen Natural Blaze See-Through 27" Vent Free Gas Burner — 39,000 BTU millivolt burner for a two-sided fireplace
No chimney and no household power, and a fire finished on both faces. The Monessen Natural Blaze See-Through is a 27" vent-free gas log burner built for a two-sided fireplace, listed as an unvented room heater and as a vented decorative log set: 39,000 BTU/hr on natural gas (26,000 minimum) or propane (30,000 minimum), a self-powered millivolt standing pilot lit with a piezo igniter, and a minimum fireplace opening of 32" wide, 19" deep and 21-1/2" high — and because a see-through opens into two rooms, both faces are front faces, so each one has to measure the full 32" wide. It ships as the burner with its grate assembly, on/off switch log, lava rock, rock wool, flex line and anchor screws; the NBST27-F see-through log set it requires is sold separately. from $1,219.
Choose it if you have — or are building — a see-through masonry fireplace, UL127 factory-built fireplace or a listed vent-free firebox such as Monessen's Lo-Rider See-Through, and you want one burner whose flame reads correctly from both rooms without a chimney run, a gas conversion or an electrical circuit. The opening needs 32" of width, 19" of depth and 21-1/2" of height — and since a see-through has a front on each side, Monessen's firebox table asks for the 32" of width at both faces, not just the one you measure from. The millivolt valve keeps working through a power outage and takes an on/off or thermostatic remote. Dual listing means that if a mantel or wall clearance cannot be met, the same burner runs vented with the damper open.
It is not right for a bedroom or bathroom — at 39,000 BTU/hr it is far above the 10,000 BTU/hr that unvented appliances are allowed there. It is not a primary heat source, it takes its combustion air from the room and puts about an ounce of water vapor into it per 1,000 BTU each hour, so a very tightly sealed house without added ventilation, or a room smaller than the manual's 50 cubic feet per 1,000 BTU/hr (1,950 cubic feet at full input), is the wrong place for it. It is not a single-sided burner: the log stack is built for a fireplace open on two faces, and a conventional fireplace shallower than 19" cannot take it. Nor is it a stand-alone appliance — without a masonry fireplace, a UL127 factory-built fireplace or a listed vent-free firebox that meets its minimum opening there is nothing to install it in, and a glass-door frame or lip rising more than 3" in front of the burner is not recommended by the manufacturer.
Specifications
The fireplace or firebox opening must be at least 32" wide, 19" deep and 21-1/2" high, and because a see-through is open on two sides Monessen's minimum-firebox table prints the same 32" for the rear width as for the front — both faces are fronts, so both must clear 32". A glass-door frame in front of the burner changes the depth: no restriction needs 19", a 0"-3" lip needs 24", and anything taller is not recommended. Sidewalls stay 9" from the side of the opening and the ceiling 42" above it; the material above the opening must be non-combustible — 12" or more of it needs no hood, 8"-12" requires the optional CABL hood, less than 8" is not allowed. A combustible mantel sits 14" above the opening if it is 3" deep, 17" at 6", 20" at 9" and 23" at 12" (8", 11", 14" and 17" with the hood); if those cannot be met the burner runs vented with the damper open. Combustible flooring other than carpet or vinyl may come right to the opening, but no carpet or vinyl within 14" of the face, and vinyl running to the wall calls for 14" of floor protection or an opening raised 12". Gas pressure is 5.0"-10.0" w.c. inlet and 3.5" manifold on natural gas, 11.0"-13.0" and 10.0" on propane, through a 3/8" line to the right-rear inlet; the burner weighs 36 lb and anchors to the floor with two screws.
Features
- See-through burner with Natural Blaze technology — instant yellow flames and a glowing ember bed on both faces of a two-sided fireplace
- 39,000 BTU/hr on either fuel, with an ODS safety pilot that shuts the burner down if room oxygen drops
- Dual listed: an unvented room heater (CSA/ANSI Z21.11.2) or a vented decorative log set with the damper open (ANSI Z21.60)
- No household power: a self-powered millivolt valve lit with the piezo igniter, the on/off switch log in the box and optional remotes
- Ships with the grate assembly, on/off switch log, three bags of lava rock, rock wool, anchor screws and the flex line with shutoff — add the NBST27-F see-through log set
- Anchors to the floor of a masonry fireplace, UL127 factory-built fireplace or listed vent-free firebox at least 32" wide, 19" deep and 21-1/2" high — 32" of width at each of the two faces
Answers to common questions
What’s included, what’s required, and what’s optional?
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Included in the box: Natural Blaze See-Through gas log burner assembly with ODS safety pilot; Grate assembly (two halves) that positions and supports the logs on both sides; On/off switch log and wire harness; Three bags of lava rock and one bag of rock wool embers; Flex gas line and pipe fittings with 1/2" T-handle shutoff valve; Fastener pack with two anchor screws; UL listing to CSA/ANSI Z21.11.2:19 and ANSI Z21.60-2017; HHT limited warranty — 5 years on the burner.
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Required to complete the installation, purchased separately: Log Set - the NBST27-F Natural Blaze See-Through set, ten ceramic fiber logs finished on both sides; it is the only log set listed for this burner, and the burner must never be operated without it; Fireplace or Firebox - a solid-fuel masonry fireplace, a UL127 factory-built fireplace or a listed vent-free firebox whose floor is at least 32" wide, 19" deep and 21-1/2" high, measuring the 32" at both faces since a see-through is fronted on each side; Gas Line - a 3/8" supply run and purged by a qualified installer or licensed plumber, with a regulator upstream if line pressure exceeds 1/2 PSIG (an external regulator is required for propane).
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Optional: C-CLAMP damper clamps (carton of 6) - only if the fireplace forces you to run it vented. Vent-free, the burner works without them and the damper stays under your control; if the wall or mantel clearances cannot be met the manual sends you to a vented installation with the chimney and damper open, and there the clamps are required; The RCB on/off remote, TSST thermostat remote with the TS thermostat sensor, HILOKTN / HILOKTP variable-flame kits and RRCM receiver cover; the CABL adjustable hood (needed only when the non-combustible facing above the opening is 8"-12") and extra VR1000A volcanic rock — a vent-free installation runs without any of them.
What size see-through fireplace does it need, and which Monessen firebox takes it?
The opening must be at least 32" wide, 19" deep and 21-1/2" high. A see-through faces two rooms, so treat both sides as the front — Monessen's minimum-firebox table lists a 32" rear width next to the 32" front width, and the opening has to make that 32" on each face. If a glass-door frame or a recessed hearth stands up to 3" in front of the burner, plan on 24" of depth instead; anything taller than 3" is not recommended. Monessen pairs the set with its Lo-Rider See-Through vent-free firebox (LSTF36-B), whose price-book page lists this burner and the NBST27-F logs as the pieces that complete it; in any vent-free firebox the firebox maker's own clearances govern.
Does it need a chimney, and what does dual listed mean?
No chimney is needed for vent-free use: the burner is listed as an unvented room heater, an ODS safety pilot shuts it off if room oxygen drops, and a damper stop on the fireplace damper is required so it cannot close fully (one is provided with the appliance, but its carton list does not name it, so check the carton and ask us if it is missing). The same burner is also listed as a vented gas log set — run with the chimney and damper open when a mantel or wall clearance cannot be met, or when you prefer to vent the moisture outside. Vented, the damper must stay open to the free area in the manual's tables (22.1 sq in at a 10 ft factory-built chimney, 33.8 sq in at a 6 ft masonry chimney), a carton of C-CLAMP damper clamps is required, a thermostat is not allowed, and Massachusetts requires the damper permanently removed or welded open.
Does it need electricity or batteries?
No household power is needed, and line voltage must never be wired to the valve. The millivolt valve is self-powered by its thermopile and the standing pilot lights with the built-in piezo igniter — no matches, no batteries — so the fire keeps working through a power outage. The decorative on/off switch log in the box runs it; an optional wall switch, thermostat (vent-free installs only) or battery remote wires to the same terminals, and the receiver's piggy-back terminal lets a remote and a wall switch share the valve.
How high must a mantel or shelf above the opening be?
That depends on how deep the mantel is: a combustible mantel 3" deep sits at least 14" above the fireplace opening, 6" deep at 17", 9" deep at 20" and 12" deep at 23" — or 8", 11", 14" and 17" with the optional CABL adjustable hood installed. The wall directly above the opening must be non-combustible (slate or marble at least 1/2" thick): 12" or more of it needs no hood, 8"-12" requires the hood, and less than 8" is not allowed. Sidewalls stay 9" from the side of the opening and the ceiling 42" above it. If a clearance cannot be met, the burner must be installed and run as a vented appliance with the chimney and damper open. Send us your mantel depth and we will check the table with you.
Is there any difference between the natural gas and propane versions?
Both are rated 39,000 BTU/hr at full input, and the only differences are the minimum input (26,000 BTU/hr on natural gas, 30,000 on propane), the orifice, and the gas pressure the valve expects — 5.0"-10.0" w.c. inlet on natural gas, 11.0"-13.0" on propane, with an external regulator required for propane. Each burner is built for one gas and cannot be converted in the field, so choose the fuel that is piped to the fireplace. Some sooting on the logs is normal in use and tends to be more visible with propane.
How does the warranty work if something goes wrong?
Hearth & Home Technologies backs the Natural Blaze See-Through with its limited warranty: 5 years on the burner and logs, 2 years on valves, pilots and remotes, 1 year on other parts, transferable to a later owner of the home and printed in full on pages 4-5 of the manual. The Torch Guys is an authorized Monessen dealer, so buying from us does not affect it — keep your invoice and the model and serial number from the rating plate in the burner's control area, and we handle the claim with the manufacturer for you.
