We have a big old gas-fired hot water heating system. The boiler is around 100 years old, converted from coal to oil to gas. It has a round firewall inside the main chamber that keeps the flames from touching the back of the boiler. The firewall has crumbled and we are looking at options to replace it. The boiler works quite well and our heating costs are pretty reasonable so we would prefer not to replace it with a new boiler as that would be very expensive. We are considering putting some concrete fire pit blocks in the boiler to make a new firewall.
We bought some curved concrete BBQ / fire pit blocks and they are about 6 inches thick. The old firewall was only about 2 inches thick so I am considering cutting the new blocks down to make them more manageable.
Do you know of any reason why using concrete fire pit blocks would not be a good idea?
There are many old cast iron conversion boilers still in use today that are fully functional with various modifications done over the years. They may not be as energy efficient as newer boilers, but hot water heat is a very cost effective heating method once the radiators get up to operating temperature. I agree that the high cost of upgrading your boiler will certainly outweigh the gas saving cost. However, your proposed modifications inside the burner compartment are not a good idea and I will elaborate on this further.
I often overly simplify my description of old cast iron boilers, to my clients during home inspections, as a very large kettle. The water is contained in and flows through the upper sections of the heat exchanger, which is heated by the flame in the combustion chamber below. Once the water reaches a certain temperature a circulation pump, which may have been retrofitted many years after original installation, pumps the hot water through the heating pipes and radiators or convectors. The older units, like yours, may have had an initial coal fired combustion chamber, which was converted to heating oil and then to natural gas. The “firewall”, as you describe it, may have been integral to the original unit or may have been added when the oil burner was installed.
The purpose of this refractory is to help deflect the wide oil flame to more efficiently heat the boiler inside the combustion chamber and at the same time protecting the sides of the chamber from the very hot flame. Fuel oil burns at a much higher temperature than natural gas so the secondary protection function of this refractory is not as critical now as in the past. For this reason, I often see a considerable amount of deterioration in these areas that is not typically repaired during regular servicing.
Regardless of this last point, any modifications to gas fired heating equipment such as your old boiler or a furnace should only be attempted by a licensed gasfitter. A hot water heating system is a pressurized system and amateurish modifications can be very dangerous. You may inadvertently cause damage the burner or other components when attempting to remove or replace the old refractory. These components are typically composed of ceramics or special firebrick held in place with special heat resistant mortar. Regular concrete blocks may not last very long without cracking from the heat of the flame. Also, modifications of pre-cast concrete will not be easy and may cause voids or small air pockets in the concrete, which could be a major safety hazard. If the air is heated inside a small air pocket in the concrete it could expand and cause a small explosion, damaging the boiler combustion chamber.
Concrete blocks designed for outdoor barbecue pits are in no way acceptable for the purpose you propose. Repairs with proper firebrick or special ceramic components designed for hydronic heating systems are the only option. There are several heating or HVAC contractors that specialize in hot water systems that should be contacted for inspection, servicing and repairs to your old boiler. I can’t stress enough the possible serious health and safety problems that could be created by improper modifications like the one you suggest. Don’t put yourself and your family at risk to save a few bucks, call a professional.