Short answer: Yes, an electric fireplace insert can go into an old wood-burning fireplace, and it is one of the simplest ways to bring a dead firebox back to life. Three things decide whether it works: the unit has to be smaller than your opening at every point you measure (front, middle and back, top and bottom), you need power at or inside the firebox, and the chimney has to be blocked off so it stops pulling warm air and letting in drafts and moisture. Expect supplemental heat, not a replacement for your furnace: a typical 120-volt insert makes about 5,000 BTU.
What "electric insert" can mean
Three kinds of product get sold under this name, and they fit an old fireplace in different ways:
- Electric log sets sit on the hearth floor like a grate of logs. Dimplex describes them as designed to fit most existing fireplace openings, which makes them the most forgiving choice for an odd-sized or tapered firebox. The masonry stays visible.
- Insert-style fireboxes are a complete box with a glass front that slides into the opening. A trim or surround covers the gap between the box and the masonry. Dimplex notes that a firebox has to fit the opening closely to avoid gaps, or you pair it with a trim kit for a finished look.
- Built-in fireboxes are designed for a newly framed wall opening. Most of the larger electric fireboxes we carry fall here. They can make sense if you are rebuilding the fireplace wall, but they are not a slide-in product.
Step 1: Measure the opening properly
Old masonry fireplaces are rarely a simple box. The sides usually angle inward and the back often slopes forward, so the opening at the front is bigger than the space a few inches in. Dimplex recommends measuring the height and width at the opening and again about 9 inches and 12 inches into it, at the top and the bottom.
Write down:
- Front width at the bottom and at the top.
- Width 9 inches and 12 inches back, at the bottom.
- Rear width of the firebox.
- Front height, from the hearth floor to the underside of the lintel or face.
- Height 9 and 12 inches in, if the back wall slopes.
- Depth, from the front face to the back wall, at the floor and near the top.
- Where the damper sits and whether its handle or a lintel bar hangs down into the opening.
The insert has to clear the smallest of those numbers, not the front opening. If the firebox tapers a lot, a log set may be the easier fit.
Step 2: Compare against the unit's required opening
Here are the two electric fireplaces we carry whose manufacturers describe them for an existing opening, with the minimum opening each needs. Specs are from the manufacturers' current documents.
| Model | Minimum opening (W x H x D) | Power | Heat at 120 V | Warranty |
|---|---|---|---|---|
| Dimplex Revillusion 24 | 24 5/8" x 22 3/8" x 12 1/2" | 120 V plug-in, can be hardwired | 1,500 W, 5,118 BTU | 2-year limited |
| Napoleon Element 42 | 41 1/2" x 31 1/4" x 13 3/4" | 120 V plug-in, or 240 V hardwired for more heat | 1,450 W, about 5,000 BTU | 2-year limited |
Dimplex has sold the 24-inch Revillusion as a firebox that can "update and finish an existing fireplace," and it is the only Revillusion size that plugs in as standard. It suits the smaller fireboxes common in older homes and bedrooms. Napoleon describes the Element 42 for an existing mantel opening or a wall, so it fits only larger openings. Because both are boxes with straight sides, a deeply tapered firebox may be too narrow at the back for either one, which is why the measurements above matter.
The larger Revillusion fireboxes (30, 36, 36 Portrait and 42 inches) and the Dimplex Ignite Aspire are documented for framed openings. Their manuals do not describe installing them inside a masonry firebox, so check with us before planning that.
Step 3: Plan the power
Most plug-in inserts need an ordinary 120-volt outlet, and the cord has to reach it directly. A few points to settle before you buy:
- Outlet location. Dimplex's insert manual gives two options: an outlet installed inside the existing fireplace, or the cord run along the front of the hearth to an outlet nearby. An outlet inside the firebox is the cleaner look, and it is a job for an electrician.
- No extension cords. Dimplex says the Revillusion is not intended for use with an extension cord. Plug it directly into a wall receptacle. Do not run the cord under a rug or where it can be pinched.
- Circuit. A 1,500-watt heater draws 12.5 amps at 120 volts. For its larger Revillusion fireboxes, Dimplex recommends a dedicated 15-amp circuit. If the outlet you plan to use shares a circuit with other loads, ask your electrician.
- 240 volts. The Element 42 can be hardwired at 240 volts, which Napoleon rates at 2,800 watts and up to 800 square feet, versus 400 square feet on the 120-volt plug. That needs a new circuit from the panel.
Our electric fireplace electrical requirements guide covers circuits, outlets and 120 V vs 240 V in more detail.
Step 4: Deal with the chimney
An open flue keeps doing what it was built to do: pulling warm room air up and out. Dimplex notes that a chimney no longer in use is usually capped or blocked off during installation to stop drafts. Its insert manual adds specifics worth following:
- Seal drafts and vents with a non-fibrous insulation material.
- Cap the top of the flue so rain can't get in.
- Do not install an insert in a fireplace that is prone to dampness.
Before sealing anything, have the chimney and firebox cleaned and looked over by a chimney professional, especially if the fireplace burned wood for years. Soot and loose debris falling onto an electric unit is a maintenance problem, and a damp firebox is a reason to stop and fix the cause first.
Step 5: Doors, screens and the finished look
- Glass doors: Dimplex says not to use existing fireplace doors that close the insert off from the room. If doors stay, they must stay open while the heater runs, because the heat has to get out.
- Screens: a screen can stay if it lets air flow freely in front of the unit.
- Trim: a slide-in firebox will leave a gap to the masonry on most fireplaces. Plan a trim kit or a surround to close it. The Element 42 is sold with a black trim option; for the Revillusion 24, ask us which finishing options suit your opening size.
- No more wood: once an electric unit goes in, the fireplace is no longer a wood-burner. Dimplex's manuals say plainly not to burn wood or other materials in the unit.
How much heat and what it costs
An electric heater turns 1 watt into about 3.412 BTU per hour, so a 1,500-watt insert puts out about 5,118 BTU and a 1,450-watt unit about 4,950 BTU. That warms a room. It will not replace central heat. Dimplex and Napoleon both position these as supplemental heat.
Running cost on high: 1.5 kW x 18.21¢ per kWh = about 27¢ an hour, using the U.S. Energy Information Administration's 2026 average residential electricity rate. Four hours a night for 30 nights (120 hours) comes to about $33 a month on high. Run the flame without heat and the cost falls to a small fraction of that. Swap in your own rate from your bill. See how much it costs to run an electric fireplace for the full method.
Is an electric insert the right choice?
| Electric insert suits you if... | Look at gas instead if... |
|---|---|
| You want the fireplace to look alive without smoke, ash or a chimney fire risk | You want the fireplace to be a real heat source in cold weather |
| You want a flame in summer with the heater off | You want heat during a power outage |
| You'd rather avoid a gas line, liner and venting work | Gas is already at the house and the chimney can take a liner |
| Budget and simplicity matter most | You're comfortable with a licensed gas installation |
If gas is the better fit, our guide to converting a wood-burning fireplace to gas walks through direct vent inserts such as the Kingsman Capella. Replacing an old gas unit with electric instead? See replacing a gas fireplace with electric.
Checklist before you order
- All seven measurements written down, including width 9 and 12 inches in.
- The smallest width, height and depth are larger than the unit's minimum opening.
- Outlet plan agreed with an electrician (inside the firebox, or along the hearth).
- Chimney cleaned, inspected, and plan in place to block drafts and cap the flue.
- No dampness or water staining in the firebox.
- Glass doors removed or kept open; screen allows airflow.
- Trim or surround chosen to close the gap.
Next step
Send us your measurements through our free electric fireplace insert fit check and we'll tell you which unit fits and what trim you'll need. You can also browse electric fireplace inserts, the Dimplex Revillusion range and Napoleon electric fireplaces, or contact us with photos of your fireplace.
Sources
- Dimplex, Electric Fireplace Inserts (dimplex.com): inserts fit existing masonry or wood-burning fireboxes; measure width, height and depth; unused chimneys are usually capped or blocked off.
- Dimplex, Converting Your Old Wood Burning Fireplace into an Electric Fireplace (dimplex.com): measuring at the opening and 9 and 12 inches in, tapered fireboxes, log sets vs fireboxes vs trim kits.
- Dimplex, Electric Fireplace Insert Owner's Manual, models DFI2309/DFI2310: sealing drafts, capping the flue, dampness, outlet location, cord routing, doors and screens.
- Dimplex, Revillusion brochure (2026): REV24 installation dimensions, 120 V, 1,500 W, 5,118 BTU, plug-in with option to hardwire, 2-year limited warranty.
- Dimplex, Revillusion 24" sell sheet (RBF24DLX): "update and finish an existing fireplace."
- Dimplex, Revillusion Built-in Electric Firebox Installation and User's Guide (7214620200R03): extension cord warning, dedicated 15-amp circuit recommendation.
- Napoleon, Element 42 (NEFB42H-BS) product page and specifications: minimum framing, 120 V plug-in and 240 V hardwired ratings, heated area, 2-year limited warranty.
- U.S. Energy Information Administration, Short-Term Energy Outlook, October 6, 2026, Table 2: 2026 U.S. residential electricity price, 18.21¢/kWh.
This guide is for planning and product selection. Follow the installation manual for your exact model and local electrical code, have outlets and circuits installed by a licensed electrician, and have your chimney inspected by a qualified professional before converting the fireplace.
