Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Heater choice for old cast iron radiators

Criso Member Posts: 1
Hi there,
I need to install a new heating system for my house, 2 floor of 700sqft each and a basement. I chose gas heating and I would like to keep my old cast iron radiators. I asked to several contractors about the system I should install and got away very confused. Some recommend tankless combi heater, others say the iron and sludge will damage the unit. Others prefer the indirect water heater and other the conventional water tank/boiler for the radiators and so forth. My question is: which water heater should I use for cast iron radiators? Thank you very much!!


  • Jamie Hall
    Jamie Hall Member Posts: 23,436
    I assume this was/is a hot water system? This isn't a steam system you are trying to convert (if it is, don't -- get the steam system repaired and possibly a new boiler -- if you tell us where you are, we can probably recommend two or three contractors who are qualified -- which very few are).

    That said. Now. Do NOT use a water heater. Of any kind. If this really is a hot water system, what you need is a hot water boiler -- and if possible a modulating/condensing boiler. They are designed to heat buildings. Water heaters are not; they are designed to heat water for showers etc. -- a very different use.

    The proper procedure -- still assuming a hot water system -- is to first determine the heat loss of the house. Any competent heating contractor can and should do that, and you can do it yourself; there are several very easy to use heat loss calculators available on the web. I happen to like the one from Slant/Fin, but there are others.

    Once you have done that, you select a boiler which can meet that heat loss. If you are working with a contractor, it is wise to see what he or she recommends and likes to work with -- you'll get better results. There are many good makes, and, like cars, each has its advantages and disadvantages.

    It is essential that it be installed correctly and the controls set properly!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • mikeg2015
    mikeg2015 Member Posts: 1,194
    Combi isn’t a bad option since it’s a smaller load. A good mud leg using oversized pipe, with a lot point drain, along with a dirt separator and strainer should prevent gunk from reaching the boiler. Good economical solution and 90%+ in mild weather, 88% is colder weather.

    However bet option and more reliabile is a high effecincy atmospheric cast iron boiler with outdoor reset, variable speed pump and a small indirect tank for DHW. The radiators (espeailly upstairs) can get thermostatic radiator valves so you have zoning in each room.

    2nd option will cost more, but last 2x longer and have less things to fail.

    Combi gets sized to hot water demand. Cast iron boiler you size to heat loss, which is likely pretty small at 1400sft 2 story. Probably a 50-70k input boiler I’d guess depending on climate and insulation.
  • Steamhead
    Steamhead Member Posts: 16,893
    @Criso , post some pics of what you have and we'll have a look. Also, where are you located?
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service