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best temperature for Cast Iron Radiators

I live in a large 5,800 sq ft victorian with three floors and basement. I swapped out my 4 separate 4 inch mains and replace them with individual supplied 1/2" and 3/4" pex to each radiator. System worked great on oil but I cut my heating cost from 13K to 3 k when oiled prices soared by installing a coal boiler.

I utilized my dads way of reasoning by increasing my subject matter expert (not) recommendation of a 6kb boiler to the 8kb boiler. Thinking Id have adequate coverage. Not! Needed the 10K. When weather is sub zero water temp drops to 140 and the radiators cant maintain. I'm installing radiant heat this winter on the first floor so it can heat more effectively at 140 deg. Radiators call for water temps around 180 degrees while radiant flooring around 90 degrees.

The first few years in the house we followed the goverments recomendations by installing electronic thermostats and turned heat down at night. I had to shut it down at 7 pm because the heat idled until 11 pm.At 12 pm the heat came back on to heat the house back up for 6 am. You could hear the oil being sucked in. My oil man/plumber at the time said the radiators were constantly calling for heat because the water temp was to low. He increased the water temp and the result was great heat and less oil consumption. At its best I never burned less than 550 gallons every 21 days. OUCH! Wasn't bad when I was paying .50 cents a gallon. But when it soared to over 4.00 a gallon I was forced to go to coal to survive.

However with coal you dont have the luxury of instant fire to heat up water. What would be the best overall setting to maintain the first floor at? 70 degrees, 72 deg or what? At some point the radiator itself has to be at its most efficient levelof output versus maintaining its idle state. Does anyone know what temp level that would be for Cast iron radiators?

I know their is a complex formula for determining heat loss etc. Which will not change anything for me.


  • Catskill123
    Catskill123 Member Posts: 2
    best idle temperture

    I guess What is the best idle temperature for a radiator? is what I'm looking for. I only have a single thermastat on each floor. I can get a temperature reading from each radiator if needed
  • JohnNY
    JohnNY Member Posts: 3,231
    There's no "best".

    Most hot water radiators' published output ratings are based on 180° degree water, but that doesn't really help you. That's only an industry standard. If you knew the heat loss of the areas where the radiators reside, you could look up their ratings and see for yourself at what water temperature and flow rate they meet the heat demand of the rooms they serve.
    Contact John "JohnNY" Cataneo, NYC Master Plumber, Lic 1784
    Consulting & Troubleshooting
    Heating in NYC or NJ.
  • Jason_13
    Jason_13 Member Posts: 304
    figure it out

    Do your heat loss per room, measure the radiation. and use this chart.

    Enter the second row to the btu per sq. ft. of radiation, look up and that is your water temperature. Set to the highest required. If zoned you would calculate by zone.


    For example.

    your room is 12,000 btu's and the sq ft of radiation is 80 sq ft,

    12,000/150 sq. ft. of radiation = 80 btu's per sq ft.

    From the chart you see 80 is between 70 and 90. The water temperature is between 130f and 140f. The water temp you need is 135f.

    This gives you a very good starting point.
  • Bob Gagnon plumbing and heating
    How About Insulation

    Could you add to your insulation, or do "Air Sealing". How about shutting down some of the rooms, do you use them all?

    Thanks, Bob Gagnon
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • Robert_H
    Robert_H Member Posts: 141
    edited November 2013
    The formula isnt too complicated, even I figured it out :D

    I'm currently installing so old cast iron rads in my house with a modcon boiler. I purposely oversize the rads meet design day heat loss at 140 supply temp. I built a spreadsheet that given the EDR of the rads provides a table of BTUH vs supply temp. I plan to some day offer it to the community for putting up with my questions for the last 8 months. 

    Its not ready for the public yet but If you can tell me the manufacturer, number of sections and column/tube count, of your radiators,  Ill would be happy to run the numbers and send you the table.

    You will still need to do a heat loss estimate on your spaces to help you dial the right heat in. 

  • Robert_25
    Robert_25 Member Posts: 530
    boiler capacity

    Sounds like your boiler is undersized for the load.

    Do you have a Keystoker boiler? A KB-8 is rated at 160k Net btu's...their recommended load is not more than 900 sq. ft of hot water radiation. I'd be amazed to see a 5800 sq. ft Victorian in the Catskills come in with a head load under 160k btu's with subzero outside temperatures.
  • Weezbo
    Weezbo Member Posts: 6,232
    edited November 2013
    so , let me see if i 'Get it"

    when a chunk of iron heating a space seems like its too cold as does the room it is in , it may differ from another room that seems comfortable because the size of the chunk and configuration of the iron , heat loss of the room , wow . i think we are on to something here ...

    ; )

    i can Do this ..

    the other guys said he is pretty certain it has something to do with the boiler or heat source being undersized for the 6000 sq ft house .. who would have thought that would have anything to do with it ?

    what i do not seem to get is if all the available heat is going into the house now and it isn't warm , how come when the boiler shuts off it stays the same temp in the building for quite a while ..

    : )

    this is really how folks think , that is when Dan's great story about the train delivery system comes in handy.

    the loss of moose every year is in the tonnage here, because trains don't stop on a dime ... and that is also why the speed of the train is not on the 90 to nothing scale in the first 1/4 mile.

    it is lucky to see 12 miles an hour in the quarter .

    really we get too wrapped up in the particulars as human beings at times ,

    because the problem we are having is predominate in our mind and lives . someone giving good information is dismissed as over taxing our brain with too many details and is unaware of it therefore whatever they are babbling about is not getting that answer we want right now!.


    that is what it is ,

    sometimes, i think if there was a small interactive paint program we could draw coloured pics red , blue , purple , black and white ...folks would see the "Picture" go wow ! i Get it ! ,

    and maybe there would be some more answers that were more quickly arrived at in the overall scheme of things.

    heat goes to cold , = a house drawn inside an ice berg ...

    moving water carries away BTU's, = a river drawn alongside a house with arrows lol...

    i think it would help anyway ... because then folks would see that answer and may care to listen to the ever changing picture .

    Then they can see the house in the ice berg melting the water,

    that turns into a river and melts a way out to a river , with a house along side the river with arrows , showing the heat being carried away from that house too .

    while maybe a poor example, at least , it would give folks an immediate chance to depict a problem. Dan said he was considering making a few tweaks to the site

    i offer this idea up ...as maybe like part of a new tool set ... it is no where near the level of the interactive tv chat buh i think it as simple as it may sound to be a functional tweak..
  • Jason_13
    Jason_13 Member Posts: 304
    Heat loss

    Can't agree with the 5800 sq. ft.. home in the Catskills would need more than 169k boiler.

    I have a friend in northern MA and his home is a bit over 5300 sq. ft. and he is heating it with a 150k Alpine boiler de-rated to about 90K. He is also doing it with @ 32f outside with 127f water temperature.  His heat loss at 0f is 117k. Over the weekend he hit 17f and still did not hit 150f water temp and has copper tube baseboard.
  • gennady
    gennady Member Posts: 839

    just install TRVs on each radiator and set your temperature room by room.
  • nicholas bonham-carter
    Lucky with insulation

    Your friend is lucky to have a large place with a very low heat loss- let his situation be a lesson for all!--NBC
  • Robert_25
    Robert_25 Member Posts: 530

    Jason - That is great that you friend's home is so energy efficient, but a

    drafty 100 yr old house is a different story.  Those big Victorians are

    energy hogs...especially if the house is loose and has a lot of wind

    exposure.  The original poster has already proven that a 160k btu boiler falls on its face when the mercury drops below OF. 

    Step one is to try and stop the heat from leaving the house.  Infiltration is the killer for those old homes, so I would spend some time sealing drafts in the foundation, sill, doors, windows, etc.  It there are fireplaces, make sure the dampers are closed properly.

    Step 2 is to make sure the coal stoker is setup properly.  The feed/air/draft settings are important. 

    Radiant heat on the first floor will probably improve the comfort, but I would leave the radiators in place on the first floor.  adjust the radiant to keep the floor comfortable, and let the radiators pick up the slack on the nasty days. 

    What is the feed rate on the stoker?
This discussion has been closed.