Cast Iron Fireplace Assembly
You can either:
- Hire a registered installer who is legally allowed to self-certify that the work the performs complies with local Building Regulations.
- Install your new cast iron fireplace yourself, and save a lot of money (if you have the proper skills needed).
In most cases, it is better to use a registered, accredited installer. They can supply a Cerficitace of Compliance that complies with your local Building Regulations.
We have several experienced writers on staff that spend hundreds of hours researching and compiling information for our fireplace guides. I hope you find this blog informational and helpful and wish you many years of comfort with your fireplace.
Before You Install a Cast Iron Fireplace
Keep in mind that the installation process might vary depending on the fireplace type, size, and other factors, and considering this is very important if you decide on DIY installation.
In general, there are three main things to consider and prepare before the installation process:
- Check your local regulations whether there are certain things from adding a hearth, removing your house’s original chimney breast, installing new appliances, and others need to comply with certain Building Regulations or other legal requirements.
- Downdraught—-exhaust smoke being blown back into the room—- is a very common issue with improper installations. This is mainly caused by inadequate height, the low temperature with the fireplace flue, or existence of high trees and/or buildings around it
- In most cases, you will need to line the chimney before you can run an open fire and/or insert a new appliance
After considering these three factors and making adequate preparation, below we will discuss the proper steps to install your fireplace according to the standard safety procedures and local regulations.
Preparation for Supporting Floors
We have to ensure the existing construction can withstand the new fireplace’s load, and so the fireplace must only be installed on floors with adequate load-bearing capacity. Make sure the hearth can accommodate the full weight of the fireplace. Also, if the chimney is not supported independently, make sure the hearth can also accommodate its weight.
Hearths should be made of non-combustible and solid material such as brick or concrete with at least 125mm of thickness. This thickness should include the non-combustible floor or when there’s any decorative surface.
The fireplace must be wholly supported by the hearths that are made of non-combustible tiles or sheet materials with at least 12mm of thickness. The fireplace must have been tested to an applicable standard so it cannot cause the temperature of the supporting hearth (the upper surface) to exceed 212F or 100C.
Maintain Safe Distance From Flammable Materials
If the fireplace is positioned near a combustible/flammable wall, maintain a distance clearance of 0.8 meters (32 inches). Alternatively, we can use a heat shield that is at least 100mm thick and can be made of brick or concrete/stone.
Any household furnishings should be at least 1 meter away since they might be affected by the fireplace’s heat. If the fireplace is recessed, a permanent free air gap of at least 200mm must be made available around the sides and the top of the fireplace, and at least 50mm at the back. This configuration can allow a maximum heat output of the fireplace while also allowing us to access the rear.
Installing The Chimney and Flues
Since the chimney is going to be the endpoint where the smoke is going to be exhausted, the installation of the chimney and the nearby systems deserve our extra attention.
In general, the fireplace should not be connected to a shared flue/chimney system. Never use flue pipes that feature a smaller cross-sectional area compared to the fireplace’s outlet. Make sure the flue pipes do not project directly into the chimney’s connector, which can restrict the gas flow.
- If the fireplace is under 9kW in power, use a minimum flue size of 125mm diameter or comparable cross-sectional size for rectangular flues. Maintain a minimum dimension not less than 125mm for flues with offsets (bends) and 100mm for straight flues. Check the fireplace’s product specification for the correct flue pipe size for each model.
- Fireplaces up to 20kW in power output should use a 150mm diameter of lues or similar cross-sectional area for rectangular flues. The minimum dimension shouldn’t be less than 125mm. Check the fireplace’s product specification for the correct flue pipe size for each model.
Install chimney liners according to the manufacturer’s instructions. When forming the flue, the main principle is to keep as few joints as possible. Avoid cutting the flue unless it’s absolutely required. Offsets/bends should be formed only with factory-made components made for the specific flue. Chimney liners should be placed with the sockets to contain moistures and condensates in the flue. Flue joints should be sealed properly with refractory mortar or fire cement and/or installed according to the manufacturer’s instructions.
Make sure the joints are completely sealed to avoid any air leakage that might compromise performance.
Bends in Flues
In general, bends or offsets in flues are only acceptable when:
- The chimney system features no more than 4 bends in total: 2 bends in the flue pipe connection and another 2 in the chimney
- A bend should only provide no more than 45 degrees of change in direction from the vertical
- The maximum length of the horizontal flue must not be above 150mm or 6”
- The run of the chimney between bends should not be above 20% of the total length of the chimney
Flue Height for Cast Iron Fireplace
The chimney height and the position of the chimney should follow the flue manufacturer’s recommendations. In general, flues should be high enough to clear the smoke exhaust (products of combustion), and different appliances might require different height. Also, obviously the height of your building will play a big factor, along with other factors like the number of bends/offset (as discussed above), the type of flue, and local wind patterns.
A flue height of around 4.5 m (14.7’) should be sufficient in most cases. In general, the outlet from the flue should be just above the roof of the building where the smoke exhaust can discharge freely without meeting any obstacles (especially flammable obstacles that can present a fire hazard).
Inspection and Cleaning of Your Chimney
Inspect the chimney thoroughly and make sure it is in good condition: totally dry, free from any obstructions and cracks. The diameter of the chimney shouldn’t be less than 150mm (around 6.2”) and no more than 230mm (9”). If not, then we must line the chimney with a suitable method.
Sweep the chimney thoroughly before connecting it to the fireplace. Also, it’s recommended to sweep the chimney again within a month of regular use after installation. Higher flue gas temperature from the fireplace might cause flue blockage, and this is why a second sweeping might be necessary.
After we are sure the chimney is totally clean, the chimney flue should be inspected from time to time to establish the sweeping schedule required. Consult your local dealer if you aren’t sure about the suitability of your chimney.
Air Vent Size for Cast Iron Fireplace
Any space (or room) containing the fireplace should have a permanent air vent opening of at least:
- If air permeability is below 5.0m3/(h.m2) then 550mm2/kW of appliance rated output above 5kW
- If air permeability is above 5.0m3/(h.m2) then 550mm2/kW for the balance of appliance rated output
We need to maintain a permanent means to provide air for combustion into the room where the fireplace is installed. Maintain at least 550mm2 for every kW that is rated above 5kw for the permanent vent, which should be connected directly to the outside air. Don’t fit an extractor fan to this room.
The connection to the chimney must be approved by your local authority before you can use the fireplace, or alternatively this work should be carried out by an accredited person according to your local Building Regulation.
How to Backfill a Cast Iron Fireplace
Backfill is the process of filling the (empty) chamber behind the fireplace with mainly cement and vermiculite. The purpose of the backfill is to prevent the heat generated from the fireplace to heat the chamber behind (which won’t be valuable for the owner) and instead, we can push most of the heat forward.
Backfilling is an essential step of cast iron fireplace fitting, and not doing it correctly can damage the cast iron back.
It’s very important to be careful in attempting to backfill a cast iron fireplace: the in-filling should be solid enough not to crack, but yet not too strong that it might break the cast iron back.
In general, mix an insulating mix that consists of 6 parts vermiculite and 1 part cement. Or alternatively, you can mix 4 parts broken brick, 2 parts sand and 1 part lime.
Put a corrugated paper, cardboard or strawboard around the back of the lower fireback half before you attempt the backfilling. The idea is that so this cardboard or paper will quickly burn away when you light the fireplace, leaving a void. This is done to create an air gap between the vermiculite and the cast iron.
Once it’s done, finish off the top at a slight angle and also its sides with a weak mortar mix.
How To Attach a Cast Iron Fireplace to a Wall
Below is a step-by-step guide on how we can attach a cast iron fireplace to a wall:
- Remove the skirting board. Cut it to size if necessary to fit around the fireplace.
- Remove the blocks covering the fireplace opening (if any)
- Uncover the hearth and rebuild it. You can make some timber shuttering and fill it with a wet concrete mix: 1 part cement, 5 part ballast. Level with afloat.
- Install the metal insert carefully. Most cast iron fireplaces and especially smaller ones have holes just below the mantelpiece that will enable you to easily attach the fireplace to a wall.
- Use four fixing plates screwed to the back of the surround to attach the surround the wall. In most cases, you’d want to chisel out a channel in the wall for this purpose. You can fill the holes to smooth it out.
- Tile the hearth after the process is finished
How To Fit Cast Iron Fire Surround
In fitting a cast iron fire surround, the first thing that needs to be considered is to ensure a big enough opening which can accommodate not only the cast iron back of the fireplace but also to ensure the tile cheeks can fit properly inside the builder opening of the chimney breast.
With that being said, there are the essential steps in fitting a cast iron fire surround:
- Preparing the Opening
Measure the opening and check whether you”d need to cut any of the existing brickwork for a bigger opening. Older houses in the U.S. commonly feature an original opening of 36 by 36 inches. It might have been plastered up over the years to reduce openings to accommodate the previous fireplace.
A typical arched insert requires an opening of 22″ wide 30.5″ high and 6.5″depth. For a tiled fireplace, the average opening is usually 18″ wide 26″ high and 9″ depth.
- Laying the hearth
Lay the hearth on a thin layer of sloppy mortar. Make sure it’s perfectly level since the hearth must properly support the whole fireplace. Once you are sure this is level, fill in the chimney breast with a cement mix up to the level of the hearth. Allow this filling to set properly before we move on to the next step.
If, however, you are going to place the cast iron fireplace in the bedroom, you might want to consider not using any hearth since they tend to look better when they are level with the floorboards. This is also important if you want more compactness for one reason or another.
- Cast the insert filling
Position the fire surround against the wall, properly. Make sure it’s flat at the outer legs. Measure the distance from the wall for the rebate. This will determine how far the cast sits off the wall.
Once you’ve checked this properly, it’s time to fix the insert (arched or tiled) to the wall. If your rebated mantels are 1”, then you can use bonding plaster to fit the 1” gap behind the cast iron fireplace. For 3” rebated mantels, use brick slips on either side to build two “pillars” on the hearth. Form a small lintel across the top then seal it against the back of the cast. You should be able to form an airtight seal this way.
You can check the previous section on how to backfill a cast iron fireplace. Make sure the insert filling is 100% dried before you attempt any backfilling.
- Mantel Fixing
Fix the mantel in place, over the fireplace insert. We can use concealed fixing brackets to accommodate the timber surrounds. For marble/limestone mantels, glue the L brackets to the inside legs, and use screws and raw plugs to fix the mantel to the wall (two at the bottom, two at the top of the legs). Use a strong adhesive to fit each piece together and also to the wall.
Sand the mantel carefully to remove any small marks and chips, then wipe it clean with a lint-free cloth. Apply the sealer coat evenly to remove any excess, so that it won’t soak into the fireplace. Let it dry completely for a day or two before you light the fireplace.
Make sure to use qualified filters for both solid and gas fuels.
Using The Cast Iron Fireplace
The amount of emitted heat is controlled by:
- The primary air supply is controlled using the air regulator that is located on the front ash door and by the regulator located in the feed door
- The rest of the air supply is controlled by turning up or down the butterfly damper that is mounted at the top of the fireplace. This is done to control the draught to the chimney.
WARNING: IT IS RECOMMENDED THAT PROTECTIVE GLOVES ARE ALWAYS WORN WHEN HANDLING THESE OPERATIONS.
THE TEMPERATURE REACHED BY THESE ITEMS DURING OPERATION MAY CAUSE AN INJURY TO AN END-USER. AS THESE ARE INTENDED TO BE ADJUSTED DURING OPERATION.
Types of Fuel
Fuel should be dry at all times. Maintain moisture content below 20% for the timber. Damp wood will leave more soot and tar deposits in the chimney, flue, and the fireplace itself. As a result, this will decrease efficiency as the thermal energy is used to drive out the excess moisture instead.
We can use other types of fuels to combine with woods, but woods or compressed blocks must remain as the base of the fire. Don’t use the fireplace to incinerate other materials (including wastes).
Avoid using unseasoned wood and/or treated woods (plywood, chipboard with glues and resins, etc.). They might pollute the environment and also compromise your fireplace’s efficiency. These materials can produce excessive creosote and tar which can cause a fire inside the chimney and can be damaging.
Check for radial cracks and even splits which can be good signs of dry and seasoned logs. The maximum length of wood fuel suitable is around 25cm so they can lay fully flat over the embers. Split logs that are greater than 6cm in diameter.
After installation and before any regular use, season the fireplace properly to prevent cracking of the cast metal. Repeat this procedure if the fireplace has not been used for prolonged periods (a few months). Here is how we can season the fireplace:
- Light a small fire right in the middle of the fireplace, make sure it’s far enough from the sides.
- Allow the fire to warm up slowly and evenly, keep the air controls almost closed to avoid the intense heat.
- Keep the fire burning for around three hours, gradually increase the heat by using larger logs (so they burn slowly and naturally). Keep the fireplace on a low setting.
- Keep this practice for a couple of days, then increase the fire intensity and brightness over the next 10 years.
Don’t use the full strength of the fireplace until around 20 days of mild use. Only after this time will the fireplace be totally ready. Do this procedure at the beginning of each season of use, so you can properly remove the retained moisture during non-use. Also, during this seasoning time, it’s normal if the fireplace might smell a little since it’s releasing gas. The gas is non-toxic, but make sure the room is properly ventilated.
How To Light With Wood
- Open the air controls
- Lay rolled-up newspapers or firelighters on the grate. Make sure there are enough dry woods, and place 2 to 3 smaller logs on top
- Light the firelighters or newspaper (use a long taper), and close the fireplace door
- Add further logs when the fire is burning steadily, up to 10mm in diameter
- Once the fireplace is burning really hot, close the front air control door. Ensure there are enough lasting flames until all the woods become charcoal.
Refueling The Fireplace
1. Always open the doors slowly to avoid a sudden rush of air intake. This might cause smoke to escape into the room.
2. Open the air controls and leave open for a few minutes to allow the gasses in the wood to burn completely.
The rate of refueling can vary depending on individual use and preferences.
When re-loading, it is normally necessary to burn the fireplace fast initially to drive off any gasses and moisture, before closing the air vents to control the burn rate.
Keep the fireplace free from the accumulation of ash from time to time. How often you’d need to do this will depend on the quality and type of fuels being used.
In general, we should remove ash before it builds up to the grate’s underside, which will cause the grate to burn really hot on both sides. This can significantly shorten the grate’s life expectancy.
Make sure the ashes have cooled down properly when you are going to dispose of them. Remove ash only when the fire is at its lowest point (usually, first thing in the morning). Use a metal container to collect the ashes.
How to Backfill a Cast Iron Fireplace?
Backfill is the process of filling the (empty) chamber behind the fireplace with mainly cement and vermiculite. The purpose of the backfill is to prevent the heat generated from the fireplace to heat the chamber behind (which won’t be valuable for the owner) and instead, we can push most of the heat forward. Backfilling is an essential step of cast iron fireplace fitting, and not doing it correctly can damage the cast iron back.
Is there a guide for Cast Iron Fireplace Installation?
Keep in mind that the installation process might vary depending on the fireplace type, size, and other factors, and considering this is very important if you decide on DIY installation. In general, there are three main things to consider and prepare before the installation process. Check your local regulations whether there are certain things from adding a hearth, removing your house’s original chimney breast, installing new appliances, and others need to comply with certain Building Regulations or other legal requirements.
More information on Fireplace safety