Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.

If you've found help here, check back in to let us know how everything worked out.
It's a great way to thank those who helped you.
Need to contact us? Visit

How to cold start my furnace to avoid cracks? Oil, forced hot water.

LeonardLeonard Posts: 718Member
edited March 2018 in Oil Heating
To avoid boiler cracks Is there a special way I need to start my house oil furnace from cold start, about 60 deg F. It's operating fine now , but I'm thinking if loose power from ice storm and it cools to 60 deg.
I'm thinking thermal shock/stress to it's cast iron during heat up.
In past I just turned it on, but now I'm wondering.

Forced hot water
60 year old National Sunray IV , boiler # 5-19A/O/OP , rated 128k BTU/hr 1.6 gal/hr
Has original burner, National -US model NB2-K2, firing rate .85-2.0 gal/hr, 1.1 gal/hr nozzle

Chimney is most 4 stories from cellar floor to top of it
Tankless coil in water jacket
Cast iron baseboards, 1ft tall .


  • newagedawnnewagedawn Posts: 549Member
    that boiler is a tank, it can handle it
    "The bitter taste of a poor install lasts far longer than the JOY of the lowest price"
  • lchmblchmb Posts: 2,891Member
    im sure after 60 years it's been cold a time or two.. I'd worry less on the old and start thinking down the road to new...
  • DennisDennis Posts: 92Member
    Boilers crack because either they are not installed correctly, they freeze up, or are fired with out water. On a few occasions burner techs who need a little extra money, might over fire the boiler to hasten its demise. Oil boilers are easy to crack by stepping the nozzle size above the rated firing rate. I will admit I did so on a boiler the customer refused to pay for. That was in my impetus youth now days I just foreclose on your house.
    Just do it, right.
  • HVACNUTHVACNUT Posts: 1,899Member
  • newagedawnnewagedawn Posts: 549Member
    beware of wolves in sheep clothing, dennis sounds like a real great guy,...not!!!! those of us that care about our reputations dont do anal things to our customers, thats why our businesses grow and people are happy to use us
    "The bitter taste of a poor install lasts far longer than the JOY of the lowest price"
  • LeonardLeonard Posts: 718Member
    edited March 2018
    Was just that I remember dad saying to not add water to system unless circulator was running, to avoid thermal shock to boiler cast iron.

    Have run out of oil ~ 5 times in last 4-5 years and boiler cooled to ~ 70 deg F till I discovered it. Never froze, nozzle is 1.1GPH --- boiler rated at 0.8-2.0. Fire does not touch cast iron. Firebox hard ceramic insulation box is intact.

    LEAK.....Well that comes to next part of the story, think I've got a very small crack in boiler water jacket. ~3-5 years ago after I ran out of oil and boiler cooled to room temp ~ 75 degs. I heard a small water leak from fire box. Did not see any water, but had to be boiler water since there's nothing else pressurized in there. Got oil, fired it up, and no more squirt sound when it was hot.

    Had to add a little water now and then, Past few months seems to need water more often, maybe once a week. If I filled it to ~ 10-15 psi then it would bleed down and stabilize to ~ 3-5 psi.
    Top of tallest baseboard is only ~ 5.5 ft above gauge, so that pressure is enough to be sure it's always full of water. (Circulator is on outlet of boiler. )

    30 psi pressure only relief valve is OLD and leaks sometimes, plan on replacement in summer. I put a 5 gal bucket under it and water doesn't seem to be going there. Although some times it will fill with 4 gal water. But I'm loosing pressure even when it doesn't fill

    Ideas , comments. Can I just keep topping off the water? Or am I running into problems. Maybe oxygen in water rusting issues of baseboards? Copper piping to 1ft steel pipe into cast iron baseboards.
  • SuperTechSuperTech Posts: 934Member
    edited March 2018
    Time to budget for a new boiler. You got your money's worth out of the old beast. A properly sized and installed boiler will provide you with significant energy savings and superior reliability.

    Oxygen from fresh fill water is the enemy of the hydronic system.
  • Big Ed_4Big Ed_4 Posts: 1,100Member
    If you have a brick chamber , you should candle it to dry out the moisture ... If not the moisture turning to steam will break the brick into crumbs .. The older National original came with a metal chamber , we changed them out years back ...The Sunray , I am not too sure ..It may be brick .... To candle a chamber , slowly fire the unit up on temperture by turning it on or off
    I have enough experience to know , that I dont know it all
  • DZoroDZoro Posts: 716Member
    FYI didn't know the site could have two of the same user names.
    I do not agree with the above Dennis. @Erin Holohan Haskell
  • Erin Holohan HaskellErin Holohan Haskell Posts: 915Member, Moderator, Administrator
    Dennis said:

    FYI didn't know the site could have two of the same user names.
    I do not agree with the above Dennis. @Erin Holohan Haskell

    I'm sorting out the multiple usernames situation. Dennis, please send me a private message via your Inbox here. Thanks!
  • LeonardLeonard Posts: 718Member
    edited March 2018
    Sounds like internal rusting will occur from adding water periodically. What will go bad: boiler, 1 ft high cast iron baseboards, piping is copper , except for ~ 1ft steel pipe into baseboards, and steel pipe from boiler to circulator.


    I may be adding water less frequently than I thought.

    17 days ago I got distracted and added too much water, was ~ 15 psi.

    2 days later it was 10 psi.
    5 days later it was 7 psi
    2 days later it was 5 psi
    7 days later stayed at 5 psi
    NOW ...2 days later ~ 4.5 psi

    I'm GUESSING this is not a serious leak as far as rusting/damaging the system.

  • JohnnyGlocalstpJohnnyGlocalstp Posts: 10Member
    Hey OP! Not sure if you got a satisfactory answer yet. In Minnesota we install a bypass on all boilers that runs between the supply and return as the first before pump piping. We always install a valve to either modulate the mixture or completely isolate. This is done to reduce the “thermal shock” of excessively cold water returning to the boiler on a cold start up as it mixes the hot and cold return and supply before it can enter the boiler, tempering the water so to speak. Hope that helps! Also 60 years! Love to hear the old ones still rocking. Make sure to have a CO test done around the seals every year.
  • LeonardLeonard Posts: 718Member
    edited March 2018
    Boiler seems oversized, but nozzle GPM was reduced (boiler was sized to also heat 2nd floor, that remains unfinished and unheated). On coldest day here (-10 deg F this January) boiler runs 50% duty cycle, 1 hour on , 1 hour off.

    On 40 deg days the burner on time is only ~ 2 hours/day. So off time is likely 4-5 hours between firings. When circulator starts, return copper pipe to boiler feels cellar room temp cold, (65 degs F)
  • LeonardLeonard Posts: 718Member
    edited February 16
    Boiler doesn't need much makeup water, guessing 3 gal a month. Hopefully problem is the weeping T&P valve and circulator, they will be replaced, but for now:

    Thinking may have excess air in expansion tank ( no bladder), from oxygen in makeup water. Thinking would be good to remove excess air from piping to tank to help keep oxygen out of rest of system.

    How much air should be in expansion tank.? Guessing empty when 0 psi, but water right at bottom of tank, then pressurize.

    Way it's plumbed air in boiler can accumulate in expansion tank, so that might help keep released oxygen out of rest of system. Rare but sometimes have to bleed a little air out of my cast iron baseboards. So I'm thinking my expansion tank piping may be full of air.

    Below expansion tank is a "airtol tank fitting ATF12" ( valve), no sight tube on tank

    No air separator in the system, circulator is pumping OUT of boiler
  • EBEBRATT-EdEBEBRATT-Ed Posts: 5,264Member
    Normally the expansion tank is 1/2 water 1/2 air but it will vary. As long as you have an air cushion your ok. Do not use any automatic air vents with a compression tank. Seems like it's time to budget for something new if your adding water
  • LeonardLeonard Posts: 718Member
    edited February 16
    I can see weeping from OLD T&P valve and circulator. So some hope it's not the boiler, ~3 gal/month leak , maybe weep and evaporate. Cheap parts so I'll replace and recheck for leak.

    I only heard water leak in combustion chamber when it ran out of oil and cooled to ~ 80 degs ~ 5years ago. Never heard it again even at 15 psi. Joint thermal contraction? Kept pressure at ~4 psi for few years hoping if was a crack it'll rust over.
  • DZoroDZoro Posts: 716Member
    Turn the blue gate valve off to the tank and drain the tank via the drain valve at the bottom of the tank. This can take some time if there is no way to let air into the top of the tank. When done open blue gate valve.
  • LeonardLeonard Posts: 718Member
    edited February 17
    Was more worried about excess oxygen accumulating in tank and pipes leading to it . I opened bleeder on that blue valve, was happy to see water come out, so maybe not as much oxygen coming out of solution as I worried about. I know there is some air in expansion tank, can hear turbulence in it as I add water to system.

    Have to replace 60 year old blue valve first, it's a rubber bullet shaped plunger design, not rubber washer. In past rubber plunger in it's "brother"' on cold water feed fell apart when I used it. Guess gotta replace a few parts.....out of sight out of mind.
  • SuperTechSuperTech Posts: 934Member
    Normally you should keep the boiler pressure around 12-15 PSI, I don't consider 15 PSI over filling it, unless you are concerned about the leakage.
    Yes the air will damage the cast iron baseboards and the boiler. The boiler is on its last legs and should be replaced, but you don't want to damage those nice cast iron baseboards, that could lead to a costly repair.
  • LeonardLeonard Posts: 718Member
    edited February 19
    Hopefully circulator and relief valve are the leakers, not boiler.

    New sound, circulator shaft started chattering. Oiled it and noise stopped, seems bearing is worn. I'll get circulator midsection tomorrow, before seal wears out and leaks. Old midseaction is B&G # 189134 (labled booster). Is there a generic replacement with similar pump curve that fits the flanged casting, have monoflow tees . ( Sometimes exact # replacements are $$$$ overpriced)

    While have it drained thinking of replacing old valves with ball valves and adding boiler isolation valves on circulator lines. I'm running ~ 200 deg on boiler water for HOT DHW needs. Do I need special hi temp ball valves for boiler use. I assume I'll have to disassemble ball valves for pipe soldering so I don't damage internal seal during soldering. right?

    Will replace weeping relief valve. Boiler only has 30 psi pressure relief valve , not Temp&pressure. Is this OK? Primary burner safty is exhaust over temp switch ( ~ 1 ft spiral), located ~4 inches above heat exchanger. NO fire light sensor ( cad cell)

Sign In or Register to comment.


It looks like you're new here. If you want to get involved, click one of these buttons!