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Radiant Floor addition to Steam system

NO2nice Member Posts: 26

I am performing a renovation on my house to add to the 1st floor and 2nd floor. I currently have a 1 pipe steam system fed by a Weil-Mclain EG-55 gas boiler and hot water is provided by a standard gas water heater. There is an existing extension 12x16’ that is heated by baseboard heating fed off of the right side of the boiler. I measured every pipe and radiator in the house when I first moved in to get the heating under control with correct venting and this website. Right now its working pretty good through this unseasonably cold winter in Long Island, NY.

The new construction will be to expand the extension to 16x24 and create a 550sqft kitchen/family room area of which 16x24’ will be on concrete slab and the rest over wood subfloor over the basement. We want to use 1/4sawn oak for most of the area with tile in the kitchen area. The kitchen area will be cooled by a split unit w/heat pump (18000Btu rough guess because I did not do a load calc yet and the AC contractor said that is what he would go with).

If that’s not enough we are also expanding over the unheated garage creating a 10x26’ area that will be split into a bathroom and bedroom. I was planning on installing a steam radiator in the bedroom but then I wonder if its better to heat that entire floor as 1 radiant zone since it will be over the garage and probably need its own heating?

If your still reading then I thank you!


- Is radiant flooring a robust solution to heat this new area on the 1st floor? Given the cold temps I am worried that at 0deg this large open area may not heat up enough and the split unit will not be much help to supplement at that temp.

- I looked into warmboard and quicktrac, are there any other good alternatives? Suggestions?

- If the radiant heating is a good solution to my heating problem can I use my boiler to provide the hot water with the right mixing valve setup? One local guy said its too hot and I should get a stand alone unit.

- If I go with a stand alone unit what should I research and could I use that same unit to provide potable hot water since my HWH is on its last legs?

- Can I keep the wood and tile on the same zone or does the different rvalues of the flooring require different zones?

- While I’m at it I have my master bath that needs the waste and steam line relocated do to the renovation. Should I just heat it with radiant at this point?

- Any programs or apps out there for calculating heat loss? Sizing AC the right way not rule of thumb?

Any help is REALLY appreciated.


  • Snowmelt
    Snowmelt Member Posts: 1,416
    Steam boiler

    You don't want to mix the domestic water with the radiant floor. There are a few options to choose from for heating the floor and doing your domestic water.

    You can use a tank, with a a taco product cAlled x-block,

    How much domestic hot water do you use in gallon per minute at a single moment . (2 gpm or 8 gpm)

    You may want to install a combi unit.

    It's not a good idea to use the same water temp, let a pro do the calculation for you. Tile vs wood has a different r value. The proper way to do radiant floor is aggressive zoning. More zones the better and eventually you will get a smaller heating bill with confort.

    Here is the best sinareo giving to me.

    You go to buy a family top of the line mini van. How many comfort zones do you have? Answer - 2, 3 sometimes 4. Two in front 1 in back, and the size of a minivan is like a 4 x 10 sheet of plywood. That's 40 square feet.

    How many zones do you have in your 1970's home? 1 sometimes 2. Also to make it worse the thermostat is located in the hallway where you don't live you just pass it to get from bedroom to kitchen to family room.
  • NO2nice
    NO2nice Member Posts: 26

    I have a 1930's house with 1 big zone that has worked great for 80 years but the addition will not be heated the way I want unless it has a separate zone. I fully agree that calcs have to be done but the people I've been talking to (recommended plumbers) all work on rule of thumb and that doesn't fly with me. I'm an engineer so I want to back up any decision with some numbers. I was thinking about the heat exchanger to keep the radiant loop isolated but i was curious about efficiencies using it for a dual purpose of potable water as well.

    I looked up my boiler manual and its made to tap a low temp hot water loop (under 140deg) so with a mixing valve I don't know why it wouldn't work to feed a radiant manifold.
  • Zman
    Zman Member Posts: 7,579

    If your boiler has enough capacity to do the job it was designed for plus the additional load you are proposing there is nothing wrong with your idea. You will have to add up the EDR of all you radiators an apply the correct "pick up factor" for the piping losses and check that number against the boiler output rating. If you steal too much heat, the boiler will not be able to produce enough steam to satisfy the radiators.

    You are correct to use a  heat exchanger. The radiant loop should be a pressurize,closed loop.

    You could heat your domestic water the same way. It needs to be another separate loop with it's own heat exchanger.

    Both loops would be set up so that the boiler aquastat  would fire the boiler to a lower temperature setting when there is no demand for steam.

    Don't be fooled into the common thinking that you are somehow getting "free heat" because the boiler is firing anyway. A boiler that large has a great deal of jacket, flue and piping heat loss. You will be running pretty inefficiently when just the smaller loads are calling.

    Dan's books on steam heating are very informative. I would recommend reading all of them before undertaking this.

    It would also be a good idea to post this in the "strictly steam" section. You want to get the attention of the true steam gurus (I am not one of them).

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Snowmelt
    Snowmelt Member Posts: 1,416
    edited March 2014
    Local supply house

    If you stop by one of your local supply houses, but a good one you have to feel them out. Personally I would you my return line of my steam boiler which is always wet and but a b&g 100 on the boiler side and feed a 50 gallon indirect water heater and you that tank for radiant. In order for you to get the better of a heat load calc you have to give them as much information as possible. R or u value of window, insulation, material of house, doors and floor, if you by there material they will do it for you. They might give you pointers like using plates for in between joist, how many loops per zone, if you can or should use a bigger mannifold and use powerheads to control the zones on the manifolds.

    There is a webinar on taco-hvac site that is very very informational on the layout of the radiant floor and what temp for what zones.

    Where are you located?

    Also the post above me is right, it won't really run efficient heating up a steam boiler is just a lot of metal to heat .

    Do you have gas going to the house?
  • NO2nice
    NO2nice Member Posts: 26

    thanks for the info and the reality check about using the steam boiler for more than it was intended. I sometimes focus on certain points without taking a step back and looking at the amount of heat lost in warming up the entire boiler. after reading this post and a few others i looked up an Argo electric heater which is extremely small footprint and outputs 30000BTU with no venting issues and and super efficiency rating. It might be worth setting up my radiant and a hot water loop off of this boiler since I dont need super hot water for either application. I will post this in the steam section and I've been meaning to buy one of Dan's books ever since i moved into my house. Since time is hard to come buy which book would you recommend me starting with?
  • NO2nice
    NO2nice Member Posts: 26
    local supply

    thanks snowmelt- so i was looking at an Agro electric unit to supply my radiant and domestic HW. I have Gas in the house and I live in Long Island NY. I looked at the taco website but couldnt find the webinar, I'll have to try again. I'm learning more on this every day and will be doing a heat loss calc this weekend after i dig up my books from school. I was hoping there was an easier way with all the free online stuff.

    Having the ability to control the zones and dial in temperatures is exactly what i have been dreaming of for the past 3 winters.

    I'll look up some of the local supply houses and see who sounds friendly.
  • Snowmelt
    Snowmelt Member Posts: 1,416
    edited March 2014

    The real question is do you have gas in your house? Since this is for your floor, they may run the gas for free to your house. Then if I was you get a navien water heater with out the pump. Set I it up like a boiler except the pressure reducer put primery /secoundary piping and you will be able to run the radiant floor no problem.

    You will have a great system above 92%eff.

    I did that for my driveway and so far so good it worked out well.