Adding Baseboard Heat to a Steam Boiler
Important - All the following applications require an aquastat on the boiler to prevent the boiler from producing steam and overheating the home when the demand is for DHW or baseboard heat only.
During my years as a boiler technician, I have piped a steam boiler located in the basement and supplied hot water to upstairs radiators using boiler water and no heat exchanger. It always seemed wrong to put hot water radiation above a steam boiler without a heat exchanger, but I found out it worked well if you do not exceed 30' above the water line of the boiler.
This practice does work well with no problems if done properly. If you are adding hot water radiation to a steam boiler, it is a good practice to size the radiation for a 160°F to 170°F water temperature to help prevent the water in the piping from flashing to steam after the circulator turns off. Following the drawing below will allow for a good operation.
When sizing the steam boiler, you have the option of adding hot water baseboard and/or an indirect water heater. If not done properly your DHW or baseboard loop can affect the steaming of the boiler. Let's look at options.
All prioritizing is done with add on controls like these options
Baseboard Only Options
- Size boiler for the steam and baseboard load combined so both can run at the same time
- Size the boiler for just the steam system and prioritize the steam over the baseboard
- Size the steam boiler for steam system and prioritize the baseboard over the steam boiler
- Size the steam boiler for the steam system and do not prioritize the DHW demand
- Size the steam boiler for the steam system and prioritize the DHW
- Both of the above will work the same. The IWH will collapse the steam cheat and stop heating the home. I would not add the IWH tank size to the boiler
- Size the boiler for the steam system and prioritize the baseboard loop, but not the IWH. The IWH will self-prioritize
- I would not prioritize both baseboard and IWH as it will affect home heating too much
- Size the boiler for the steam system and baseboard load, no need to prioritize either or IWH
Standard piping for hot water radiation above the boiler
The water flow through the boiler will need to be reduced to allow return water to go through the bypass and temper down the supply water temperature to the baseboard. I suggest 160°F to 170°F. This is achieved by partially closing the return water inlet to the water valve shown below.
Piping with purge hoses attached, notice valves that are now closed
After the installation is done, you will need to purge the air out of the heating loop. You can use a double-ended hose to connect a regular hose to a faucet and feed water into one of the boiler drains. Close the ball valves to prevent water from bypassing through the bypass and returning to the boiler, as shown in the drawing below. Ensure you remember to open the correct valves and bleed air from the bypass loop. You will not need a water feeder or expansion tank on the loop, as it is open to the boiler. The boiler acts like an expansion tank. The water will remain in the system with a vacuum, so it cannot drain back to the boiler, which means there are no automatic air vents on the radiation upstairs. There is no reason for any air vents at all.
Using a heat exchanger, tank type
While the above drawing will work, some prefer the use of a heat exchanger. This would include either a flat plate heat exchanger, tank-type heat exchanger, or DHW coil in the boiler, all of which would require a pressure relief valve installed on the heating loop, usually near the boiler.. In my opinion, if I were going to choose a heat exchanger, it would be the tank-type heat exchanger. The heat would be more evenly distributed due to the stored water in the tank, which would supply the hot water to the heating loop. This would be more expensive than the other choices of heat exchangers but has fewer problems down the road.
Using a flat plate heat exchanger
The flat plate style heat exchanger has very narrow passages between the plates that could foul or block. I would choose a good strainer installed before the flat plate exchanger. This application will probably require a larger circulator.
Using the DHW coil in the boiler
Last but not least is the domestic hot water (DHW) coil in the boiler. While this is the cheapest of all heat exchanger styles, it will work well as long as the pipe diameter of the coil is 3/4". There are two main downsides:
1. The DHW coil for boilers is produced of extruded copper. This means many ridges on the boiler water side of the (DHW) coil. These can collect a lot of dirt over the years and reduce the radiation output upstairs.
2. A (DHW coil in a steam boiler is seldom 100% submerged in the boiler water, so if the boiler is not steaming, you will not get full water temperature to the radiation.
IWH and Baseboard on the same level as the boiler
If the heat or an indirect water heater is on the same level as the steam boiler, you can pipe it as usual. Check the boiler manufacturer's directions for information on which boiler taps to use for the supply and return pipes from the radiator.
This application would not require the use of a pressure relief valve on the heating loop. This would work for the water heater or the basement heat or both at the same time. It does not matter whether the heat is going upstairs or on the same level; it is essential to ensure that when you leave the boiler, the piping runs down near the floor and the circulator(s) are positioned below the outlet of the boiler. Disclaimer: The information found on this website is for informational purposes only. All preventive maintenance, service, and installations should be reviewed on a per-job basis. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc or its personnel accepts no responsibility for improper information, application, damage to property or bodily injury from applied information found on this website as it should be reviewed by a professional.
