It has been estimated that by converting from an old steam system to a high efficiency hot water system could save you between 15% and 60%. They usually average this around 25%. I personally think this should be closer to the high side with the products on the market today. That is of course if the new boiler is properly sized and properly piped. This is a good system for the newer mod/con boilers operating in the mid to high 90% range. This is a more expensive boiler which requires more maintenance but, is an excellent choice for this type of application. The mod/con boilers are more efficient while cooler water returns to the boiler. The steam to water conversions usually operates at a greatly reduced water temperature. To address the second question it could cost as high as $15,000 to $20,000 or more to convert to a hot water system. It all depends on where you are located and what needs done for the conversion. Is there a pay back or break even point at these costs? You need to decide that for yourself. How long are you going to live there? How high are your fuel bills? What can be done to update you current system and at what cost to savings ratio. Look at the cost and the fuel usage. Calculate a savings and look at pay back.

    We will look at this from a few different angles. The first will be what to look for in a steam system before thinking too hard about converting it, how to size the new hot water boiler, what we need to do before we convert it, and how to convert it.

    1. What to look for before you get too involved in this project.
        Let's look at the radiators first. If you look at the bottom of the radiators you will see all the vertical sections are all connected (touching) at the bottom. Are they the same at the top? If not, you may want to avoid this conversion. It will add anywhere from $2000 to $10,000 more. Do the radiators have two pipes connected to them or just one? In only one you will pay more for all the return piping to be installed and run back to the new hot water boiler. Next do a heat loss for the water boiler. A heat loss will tell you what size boiler to install and also you can determine if you will have enough radiation. You more than likely will but do the heat loss and measure the radiation to verify. The radiation heated with steam puts out 240 btu's per square foot of radiation. When heating with hot water the radiation will only put out 150 btu's per square foot of radiation. Do the heat loss and measure the radiation. Divide the heat loss by the total square feet of radiation and verify the answer is equal to 150 or less. If so you have enough radiation for the conversion. Size the new boiler off the heat loss not the radiation. If you size the boiler off the radiation you will put in a bigger boiler than required and it will short cycle costing you more in fuel and maintenance.

      Do not get rid of the cast iron radiation! Cast iron radiation has a big percentage of heat output is radiant heat. Radiant heat does not create air currents. Air currents increase heat loss. This is not a huge difference but every little bit of savings helps. Cast iron radiation produces less air currents than any other heat including copper tube aluminum baseboard.

2. What do we need to do before we start?
         1. First run the steam pressure up to 10 psi and check for leaks. I usually do this with a flashlight as it will reflect off the steam. Some people use a mirror to see if it steams up.
         2. If there are return lines on the radiators are they big enough to carry the water back to the boiler?
         3. Determine if you have wet or dry returns. If you have wet returns plan on replacing them. They have collected a lot of dirt through the years.
         4. If using the same boiler due to not being very old verify with the manufacturer if it can be converted to water and do they offer a conversion control kit.
         5. Determine the water pressure you will need to be operating at.
         6. How many zones you want. Is it doable. Be careful here. Many times the mistake made here is over zoning and causing the boiler to short cycle. 
   

3. How to Convert

          1. Pay special attention to piping. I suggest the use of primary/secondary piping. There will be way too much water in the system for all the flow through the boiler. It will work better to mix the water from the boiler to that water volume rather than send all the water through the boiler.

          2. Size your circulator to the heat loss. If doing multiple zones size the zone circulators (if used) for the heat loss of that zone. If using zone valves size the circulator for the total heat loss and use a differential bypass valve. 

          3. Size the piping properly. See chart.  

          4. Size the expansion tank according to water volume and water temperature. Do not use the manufacturers charts for these installations. The charts do not work well with steam to water conversions and gravity hot water installations. You can determine the system piping water volume here , add in the radiation than add the boiler water volume to the gallons from the system.

                5. Rest the heating curve to match the system load as compared to the heat loss.

4. What to expect when conversion is completed.

             1. Good even heat throughout the entire zone or system if not zoned. Make sure the thermostat cycle rate per hour is set to gravity hot water or steam. One may be better than the other depending on the amount of the radiation in your system.

                2. Noise free operation. There should be no expansion noises. The steam to water system should always have an outdoor reset control installed so we do not overheat the systems.

                3. Very little, if any pressure fluctuation on the pressure gauge.

               4. Increased comfort levels 

               5.  Substantial fuel savings.

5. Improving your existing steam system efficiency.

                 1. Make sure all the steam piping in the basement is insulated. No need to insulate the return pipes. If you have insulation removed from the steam pipes for basement heat replace it. This is an expensive way to heat even a small area.   See this link, than click the back button.

                2. Verify the near boiler piping is correct. Incorrect near boiler piping produces either short cycling or wet steam. Both of these conditions increase fuel bills. See the link above.

                3. Verify all system vents are working properly. Every one pipe system requires working radiator vents. NO AIR VENT SHOULD EVER VENT STEAM!!!!

                4. The main line vents also need to work. They must also vent fast enough. This may require the use of two or more main line vents in the same location. See vents on the above link.

                5. Reduce the steam pressure. The Cut-out or main steam pressure should never be set above 2 psi. Most systems today are operating lower than 2 psi even in the ounces range. If the setting is too low it may cause short cycling which will increase fuel consumption.

   `       6. Maintain the water level where the manufacturer determines it should be, even if you have an automatic water feeder.  Remember an automatic water feeder does not take the place of you maintaining water levels. The automatic feeder is for use when you are not able to check and maintain the proper level.

           7. Replace your thermostat with an electronic thermostat. The old thermostat calibration may become out of adjustment or a dirt/dust buildup on the mercury bulb affecting the accuracy of the thermostat. It may cause the heat to run longer than necessary or causing an uncomfortable temperature swing. Make sure you set the thermostat for a steam system in the menu or dip switches.

           8. Make sure the return lines are working properly. The condensate coming back could be slowed down causing the need for more makeup water. The more makeup water you add the more fuel is used to bring the makeup water to 212º. Test this yourself. Let the pipes cool down and start the boiler. When the steam steam gets into the system, away from the boiler piping, make a mental note of the time. Now, feel the system return piping near the boiler but not the boiler piping. You should start feeling a temperature difference with in about 15 - 20 minutes.

                9. Clean and inspect the existing boiler. This should include a gas pressure check or fuel pump pressure check and combustion analysis.

           10. Replace the boiler with a newer steam boiler if the existing boiler is over 10 years old. The older the boiler is the less efficient it is. If the boiler is a coal converted boiler to natural gas or oil this is a must. The large flue passes allows too much heat to leave the boiler and never touch the iron. Therefore less heat transfer.

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