CONVERTING A STEAM HEATING SYSTEM TO HOT WATER – THE WHYS AND HOWS

Steam Heating Systems were the Cadillac of heating options for residential applications for about a century. Pricey, tending to be a bit fuel-thirsty (regardless of the fuel used), they were extremely simple, durable and provided a superbly comfortable heated environment. Economics have gradually forced steam heating into the commercial and industrial process realms alone. So where do you go with that residential steam system? It depends upon your goals.

When do you stay with steam rather than change over to hot water or some other heating form?

1. If you have a nice, period home that suits your needs excepting to lighten up on your wallet a bit, just upgrade the boiler to a modern, high efficiency unit. Older boilers typically are large, with open heating passages to suit both wood or coal fires that when upgraded to gas or oil result in very poor fuel efficiencies. Presuming the system piping and radiators are serviceable there is little incentive to change over the entire system. (Steam heating distribution is arguably more efficient than hot water. It requires no distribution energy, being naturally convective, as does gravity hot water!)
2. Similarly, if you like those decorative radiators that warm your hands, food, dry clothes on, etc. and take up less footprint and wall space than hot water baseboard, think again.
3. If you plan an addition or heated area extension and envision running steam piping everywhere to heat it, there is the little known and under-utilized steam boiler “bottom water” forced hot water heating option. By circulation looping the lower water below the steaming chamber (top of the boiler) provides extended heating system flexibility. Furthermore, forced hot water extends capability to attics, garages and additions with baseboard, Unit Heaters (fan forced radiators) and Air Handlers (a ducted FHA Furnace with an internal radiator that heats your hot air vs. using a gas or oil fuel source). You must however convert zero-pressure steam water into approx. 15PSI heating water for circulation to new radiation. A correct plate-to-plate heat exchanger is required and circulation both from the boiler and to radiation added. A separate water supply source and an expansion capability must be provided for the pressurized heating water circuit as well. Note: Remember to size your now “two-state energy” Steam/Hot Water Boiler accordingly.

There is an interesting “middle ground” where you can convert your existing, newer steam boiler to hot water operation while keeping those aesthetic steam radiators. You must however replace all the old steam system piping in doing so. Steam radiators work well with hot water, but at a moderately reduced heating (temperature) capacity. More importantly is the higher water volume content of steam radiators and how to supply them properly for even distribution.

Referring to our separate blog on FHW Heating Loops, you cannot pipe cast iron steam radiators in series and get even heating! Even a split loop will not work but for a couple of radiators at best.  The only effective options are the “mono-flow loop system” or the “two-pipe, reverse return piping system” with either branched for both supplies and returns to each radiator. Both will require increased piping and circulator capacity.

Despite the challenges, converting steam radiation to hot water operation provides some attractive opportunities, heating-wise.

1. You can maintain your prior heated area aesthetics and functionality with few perceptible changes.
2. You can now re-pipe and “zone” the prior area with multiple thermostats, even down to individual room level if you desire.
3. Obviously you can add additional heated areas (zones) as well.

Fully converting a steam boiler to hot water operation and then replacing or adding all heating distribution components is the last and most complete option. Scenarios:

1. You have an excellent steam boiler with an economic incentive in mind. If you just wish to swap this unit out for your existing, inefficient or failed FHW Boiler as a one-for-one, be careful. Make certain that the conversion components and labor (as applicable) justify the changeover.
2. Changing your existing, older steam boiler to FHW in our view is questionable. You are trading off operational efficiency against upgrade costs.
3. Steam Boilers typically and Weil-McLain Steamers (our expertise) in particular have several advantages over their sister Hot Water Boilers. The front and rear boiler sections are notably heavier and bulkier, containing more cast iron and water and contribute to both durability and theoretically capacity. Can’t speak for other manufacturers, but the Weils are heavier and tougher. Check their Specifications. Also if you are using a DHW Coil (immersion coil in the boiler to generate your domestic hot water), steam boiler coil(s) have nominally higher capacities. Larger (Weil-McLain 6-Section+) boilers sometimes have two coils, or provisions for them for greater DHW capacity delivery. Check.

A recent phenomenon is the Outdoor Wood Boiler. You know, that thing that sits beside a house that looks like a Metal Garden Shed with a Smoke Pipe sticking up out of it and a woodpile alongside. They are typically owned by rural folks that have a great wood supply and don’t mind tripping through the snow to keep themselves warm. These boilers are also “zero pressure” systems. They must be adapted to a pressurized FHW System through a Plate-To-Plate Heat Exchanger, utilizing circulators and controls. (Note: You must maintain constant electric service to these systems or it can get exciting and cold, or both.)

Coupling an Outside Wood Boiler to a Steam System is dubious at best. The only deliverables in this scenario are preheated boiler water that must be then fired and converted into steam by the central boiler, but which can also provide DHW through its internal coil (if equipped) or by an Indirect Water Heater (Insulated DHW Storage Tank) as an option. It just doesn’t make sense except to generate a lot of Domestic Hot Water. Therefore, in order to utilize the Outside Wood Boiler effectively you must do a complete steam boiler conversion (or a hot water boiler substitution) with the appropriate scenarios as previously detailed. There is no “easy road to glory” on this one.

So procedural, to convert a steam boiler to forced hot water operation you must:

1. De-plumb all iron and other piping right to the boiler. It must be “bare” as we say.
2. Remove all of the electric components and associated wiring.
3. Remove the Boiler Jacket (usually sheet metal) and place aside for reassembly.
4. First, locate and substitute a 30 PSI (FHW) Pressure Relief Valve for the 15PSI (Steam) Valve. VERY IMPORTANT! Forget, and you’ll get wet — and surprised!
5. Remove the Water Sight Glass, LWCO (Low Water Cut Off), Pressure Switch, etc. (Clean off the front of the boiler, in other words.) Dope and plug all affected boiler taps.
6. Check Immersion Coil (DHW) Gasket(s) and Blanker Plates for leaks. Fix them.
7. The smart guy plugs, fills the boiler and pressurizes it to 30 PSI (until the Relief Valve opens) and then checks for ANY LEAKS! Remember, steam boilers operate at about 0.5 to 5 PSI in use. You may have sectional leaking issues and not see them at that pressure. Sectional leaks between boiler castings are usually catastrophic. Stop and rethink your options. But, assuming it passes …..
8. Find the manufacturer’s boiler piping diagram and locate the preferred aquastat front tapping and insert an appropriate “Spud Well” to receive the aquastat.
9. Reassemble the boiler jacket and provide the opening for the Aquastat “Spud Well”.
10. From the Manufacturer’s Hot Water Boiler Manual, identify the control components and hardware necessary to refit. Present this info to your Qualified Heating Engineer or Technician.

Pay particular attention that your Master Aquastat selection compliments your application. There are several operational options available and should be qualified prior to final selection. Our preferred is the Hydrolevel “Fuel Smart” 3250-Plus with appropriate “Electro-Well” for all conversions.

You now have a tight boiler ready to reconfigure for your application. Your further risk is minimal, save a hot operation leak(s) that may or may not be seal-able. Now consult and utilize a knowledgeable source.

Be mindful that in converting any steam system to forced hot water you reduce the capacity of that system by 10% or more, if that is a consideration. Steam operates at a significantly higher system temperature in its vapor state than can be safely achieved with heating water, kept safely below its boiling point.

It may be implied from the above that we discourage steam to hot water boiler conversions. We have done it very successfully, once with an almost new Weil-McLain Gold Steamer and we’ve never been back, but do your homework!

The option of acquiring a near-new FHW boiler instead of converting your steamer, particularly with the preponderance of on-going fuel conversions can also make very good sense. Check your local classifieds and internet resources such as Craigslist & Facebook Marketplace. (We have found some great deals there.) Please read our blogs on buying a used boiler, etc. in our Boilers On Demand Heating Blog Library beforehand.

Hope this has helped you assess your particular situation.

Updated: 04/27/2020 pdm