Roundup · masonry openings
Electric fireplace inserts for an existing fireplace
You have a real firebox that no longer has a fire in it. Here is what goes into it, what to measure first, and why the back of the opening matters more than the front.
By Scooter M.Published September 2, 2026Last reviewed September 2, 2026
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Can you put an electric insert in an existing fireplace?
Yes — it is the most common use for one. A decommissioned masonry firebox needs no venting, no gas line and no chimney work, because there is no combustion. The insert sits inside the opening and plugs into a standard receptacle. The only two things that stop the job are an opening that is too small at the back, and no outlet within reach.
Measure four numbers, in this order
Masonry openings are not rectangles. They taper toward the back, sometimes dramatically, and the rear dimension is the one that decides whether a unit physically fits. Write all four of these down before you look at a single product.
| Measure | Where | Why it decides the purchase |
|---|---|---|
| Front width | At the outer face of the opening | Decides how the finished result looks, and whether you need trim |
| Rear width | At the back wall of the firebox | The real constraint. Almost always smaller than the front |
| Height | Front lip to the top of the opening | Usually generous in masonry; check for a lintel that intrudes |
| Depth | Front face to back wall | A log set needs a few inches; a full insert typically wants 8 or more |
The receptacle problem
Fireplaces built for wood do not have outlets in them, because wood does not need one. This is the second most common obstacle after dimensions, and it has three answers, in descending order of tidiness: have an electrician add a receptacle inside the firebox; run the cord out along the hearth to a nearby outlet and accept seeing it; or use a floor outlet if one happens to be close.
What you should not do is run it through an extension cord or a power strip. At 1,500 W the unit pulls 12.5 amps continuously, which is most of a 15-amp circuit, and continuous high-draw loads on cords are exactly what manufacturers warn against. The arithmetic and the reasoning are on watts, amps and your circuit.
What it costs to run in a masonry recess
The same as anywhere else — the recess does not change the wattage — but the heat delivery does differ. Warm air rising inside a brick recess partly warms the brick, and brick has a great deal of thermal mass. Fan-forced units in deep openings can feel less effective than the same unit on a wall for this reason. Infrared units are less affected because they radiate outward rather than relying on convection.
| Usage | Hours | Heat on (1,500 W) | Flame only (~100 W) |
|---|---|---|---|
| One evening | 4 | $1.10 | $0.07 |
| A week of evenings | 28 | $7.70 | $0.51 |
| A month, 4 hrs a night | 120 | $33.01 | $2.20 |
| A 90-day winter, 4 hrs a night | 360 | $99.04 | $6.60 |
| Per hour | 1 | 27.5¢ | 1.8¢ |
#1 · Keeping the original brick visible
Duraflame DFI030ARU 20" Infrared Electric Fireplace Log Set
A log set with an infrared heater. Nothing to fit, nothing to trim — it sits in the opening you already have.
The great advantage of a log set in an existing fireplace is that there is nothing to size. It is 20 inches wide, it goes inside the firebox, and the masonry frames it exactly as it framed the fire that used to be there. No trim kit, no gap, no filler panel.
The heater is infrared quartz, which Duraflame rates for supplemental heating up to 1,000 square feet. Infrared radiates onto surfaces and people rather than blowing hot air around, and in a masonry opening that is useful: a fan-forced unit set deep inside a brick recess is partly heating the brick.
This is the option to start with if your fireplace is attractive. If it is not, skip to a full insert that covers it up.
What it does not do: The 1,000 sq ft figure is the manufacturer's supplemental-heat rating, not a claim that it will hold temperature in a 1,000 sq ft room. See our BTU page for what the wattage actually buys.
Published specs
- Heater:
- Infrared quartz, rated by the manufacturer for up to 1,000 sq ft
- Width:
- 20 in
- Form:
- Drop-in log set for an existing firebox
- Flame:
- LED with resin cast logs and glowing embers
- Control:
- Remote with adjustable thermostat
What it costs to run
- 27.5¢/hr on full heat
- $99.04 across a 90-day winter at 4 hrs a night
- 5,118 BTU/hr · 12.5 A on a 120 V circuit
Computed at 18.34¢/kWh (EIA, June 2026). Your rate will differ — work out your own.
#ad We earn a commission on this link at no cost to you. How this is funded. Price shown on the retailer's page.
Specs from: Duraflame DFI030ARU listing — infrared, 1,000 sq ft rating; Bed Bath & Beyond product page — 20 in infrared log set.
#2 · Covering an opening that is not attractive
PuraFlame Klaus 33" Electric Fireplace Insert
A full insert with its own glass door, mesh screen and brick-look interior. It replaces the view rather than sitting inside it.
When the existing firebox is stained, crumbling, or was never nice to look at, the answer is an insert that brings its own interior. The Klaus is 33 1/16 inches wide and 25 9/16 inches high with a three-sided brick-effect lining, so what you see afterward is the insert, not the hole.
Two heat settings, 750 W and 1,500 W, which matters more here than elsewhere: a masonry recess is a good place to run something on low for a long evening rather than blasting it and switching it off.
Check the rear width of your opening before you order. Masonry fireboxes taper inward, sometimes by several inches, and an insert that clears the front lip and fouls the back does not go in — this is the single most common return in the category.
What it does not do: No trim kit in the base version — an existing masonry opening wider than 33 in leaves a visible gap unless you buy the trim separately.
Published specs
- Heater:
- 750 W / 1,500 W, two settings
- Width:
- 33 1/16 in
- Height:
- 25 9/16 in
- Flame:
- LED, three intensity settings, runs with heat off
- Front:
- Glass door, mesh screen, three-sided brick interior
- Log:
- Resin cast log, 11 13/16 in high, on a grate over the ember bed
What it costs to run
- 27.5¢/hr on full heat
- $99.04 across a 90-day winter at 4 hrs a night
- 5,118 BTU/hr · 12.5 A on a 120 V circuit
Computed at 18.34¢/kWh (EIA, June 2026). Your rate will differ — work out your own.
#ad We earn a commission on this link at no cost to you. How this is funded. Price shown on the retailer's page.
Specs from: PuraFlame Klaus insert listing (specifications and dimensions); Walmart product listing — 750/1500 W rating confirmed.
#3 · A small or awkward firebox
Duraflame DFI021ARU 20" Electric Fireplace Log Set
The cheapest thing that will make a dead fireplace look alive again, at 20 inches wide.
Small period fireplaces — bedroom grates, cottage openings, anything under about 26 inches — rule out most full inserts entirely. A 20-inch log set fits where nothing else will.
It is rated for 400 square feet of supplemental heat, has an LED flame with resin logs and a glowing ember bed, and the remote separates flame from heat so you can run the look on its own.
It is the least impressive product on this site and it solves a problem nothing else solves. Sometimes that is the whole review.
What it does not do: No enclosure, no glass, no trim. It sits inside the masonry opening you already have, so the old firebox is still visible around it.
Published specs
- Heater:
- Rated by the manufacturer for spaces up to 400 sq ft
- Width:
- 20 in
- Form:
- Drop-in log set — sits inside an existing firebox, no framing
- Flame:
- LED with resin cast logs and a glowing ember bed
- Control:
- Remote with independent flame and heat settings
What it costs to run
- 27.5¢/hr on full heat
- $99.04 across a 90-day winter at 4 hrs a night
- 5,118 BTU/hr · 12.5 A on a 120 V circuit
Computed at 18.34¢/kWh (EIA, June 2026). Your rate will differ — work out your own.
#ad We earn a commission on this link at no cost to you. How this is funded. Price shown on the retailer's page.
Specs from: Duraflame DFI021ARU listing — 400 sq ft rating; Mantels Direct product page — 20 in log set.
Skip this one
Duraflame DFI-5010-01 3D Infrared Fireplace Stove
Freestanding stove, not an insert. Put it in a masonry opening and you have a stove in a hole.
It is a competent stove — 5,200 BTU per hour, 120 V at 12.5 A, a layered 3D flame — and it is repeatedly listed alongside inserts by sites that have not looked at the form factor.
It has feet. It has a 13.1-inch depth and needs clearance in front of its door. It is designed to stand in a room, not to disappear into a wall.
If your goal is to fill an existing fireplace, this is the wrong shape and no amount of specification comparison changes that.
What it does not do: It is a freestanding stove, not an insert. It will not fill a masonry opening and it needs floor space plus clearance in front of the glass door.
Published specs
- Heat output:
- 5,200 BTU/hr
- Electrical:
- 120 V, 12.5 A
- Dimensions:
- 24 in W x 23.4 in H x 13.1 in D
- Weight:
- 28.6 lb
- Flame:
- Layered 3D flame effect, five brightness settings
- Safety:
- Overheat shutoff, adjustable digital thermostat
What it costs to run
- 27.5¢/hr on full heat
- $99.04 across a 90-day winter at 4 hrs a night
- 5,118 BTU/hr · 12.5 A on a 120 V circuit
Computed at 18.34¢/kWh (EIA, June 2026). Your rate will differ — work out your own.
#ad We earn a commission on this link at no cost to you. How this is funded. Price shown on the retailer's page.
Specs from: Duraflame DFI-5010-02-3A user manual — 120 V, 12.5 A; Walmart product page — 5,200 BTU, dimensions, weight.
One thing to check on the chimney
You do not need the flue for an electric insert, and capping or blocking a disused chimney is a reasonable thing to do for draft reasons. But an open flue above an electric fireplace pulls conditioned air out of the room continuously, which quietly costs you more than the fireplace does. If the chimney is genuinely finished with, a chimney balloon or a proper cap is cheap. If you might ever want the fireplace working again, talk to a chimney professional before you seal anything permanently — that is outside what we can advise on.
Questions people actually ask
Do I need to block off the chimney?
Not for the fireplace to work — an electric insert vents nothing. But an open flue draws heated air out of the room all winter, which costs more than the fireplace itself. A removable chimney balloon is the reversible option; permanent capping is a job for a chimney professional.
What if my opening is bigger than every insert I can find?
Two options. A trim kit or filler panel closes the gap around a smaller insert — the usual answer. Or fit a log set instead and let the original masonry frame it, which suits a large period opening better than a small insert floating in the middle of it.
Will an electric insert damage the brick or the hearth?
No. There is no combustion, no soot and no creosote, and surface temperatures are far below anything masonry cares about. Electric inserts are frequently used specifically to stop further deterioration of a firebox that is no longer being burned in.
Can I still burn wood if I change my mind?
If you have not permanently sealed the flue, yes — the insert lifts out. This is one of the genuine advantages of an electric insert over a gas conversion, which is much harder to reverse.
Sources
Every figure on this page is either taken from one of these documents or computed from a formula shown on the page. Nothing here is recalled from memory.
- EIA Electric Power Monthly, Table 5.3 — US residential average, June 2026
- PuraFlame Klaus 33 in insert — 750/1500 W, dimensions
- Duraflame DFI030ARU — infrared quartz log set, 1,000 sq ft rating
- Duraflame DFI-5010 user manual — 120 V, 12.5 A
- NFPA — National Electrical Code (NFPA 70), the standard behind US branch-circuit limits
Read next
- The best electric fireplace inserts
Six inserts ranked by opening fit, flame mechanism and computed running cost — plus the one we would not buy.
- How to install an electric fireplace insert
Eight steps for fitting an insert into an existing firebox, and the point at which you stop and call an electrician.
- What installing an electric fireplace insert really costs
Trim kits, receptacles, framing and electrical work — the costs that arrive after the unit does.
- Watts, amps and your circuit
12.5 amps on a 15-amp circuit. Why the ceiling exists, and where we stop and an electrician starts.