Steam radiator above steam Boiler

Adding Baseboard Heat to a Steam Boiler

Important - All the following applications require an aquastat on the boiler to keep the boiler from making steam and overheating the home when the demand is DHW or baseboard heat only.

During my years as a boiler tech, I have piped a steam boiler located in the basement and supply hot water to upstairs radiation 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 found out it worked well as long as 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 the hot water radiation circulated above a steam boiler, it is a good practice to size the radiation on 160F to 170F water temperature to assist in the water in the piping not flashing to steam after the circulator turns off. Following the below drawing 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 basebaord 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
DHW - Indirect water heater only
  • 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
IWH and HW baseboard loop
  • Size the boiler for the steam system and prioritize 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 to 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 160f to 170 f. This is done by partially closing the return water entering 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 a regular hose to a faucet and feed water into one of the boiler drains. Close the ball valves to keep the water from going through the bypass and back into the boiler, see drawing below. Make sure you remember to open proper valves and bleed air from bypass loop. You will not have to have a water feeder or expansion tank on the loop due to being open to the boiler. The boiler acts like an expansion tank. The water will stay up in the system with a vacuum so it cannot drain back to the boiler which means no automatic air vents on the radiation upstairs. There really 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 plat heat exchanger, tank type heat exchanger or DHW coiler 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 was going to choose a heat exchanger it would be the tank type heat exchanger. The heat would be more even heat due to stored water in the tank that 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 exchager has very narrow passages between the plates that could foul or block. I would choose a very good strainer installed before the flat plat 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 down sides;

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 through the years and reduce the output of the radiation upstairs.
2. A (DHW coil in a steam boiler is seldon 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 just pipe it as usual. Check the boiler manufacturers directions for information as to which boiler tappings to use for supply and return pipes from the radiation.

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 mater of the heat is going upstairs or on the same level it is important to make sure when you leave the boiler the piping goes down near the floor and keep the circulator(s) below the outlet of the boiler.

Disclaimer: The information found on this website is for informational purposes only. All preventive maintenance, service, installations should be reviewed on a per job situation. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc or it's 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.