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Post-fire purge

Good day,

Find this site really helpful and interesting for even naive homeowners like myself. And hook on it ever since I discover it.

We have a 14-yr old Weil-Mclain gas-fired 82% efficiency 100,000 btu output cast iron boiler to run our alum.plate suspended under-joist radiant system and DHW to heat our 2700 sq-ft 2-story 14 yr-old house with 7 zones.

No fancy control (basically just with zone valves and 2x4-zone controller as per attached).

Since at the end of the cycle, a lot of heat remains in the boiler (eg supply temp at 180 & in temp at 110, it's not uncommon for us to see high delt-t for our system especially if the zone is small), my question is how or what simplest way for us to get some of the residual heat out in the boiler to the zone(s)?

It is an one-pump system.


  • Steamhead
    Steamhead Member Posts: 15,991
    No reason to heat the boiler to 180

    on any but the coldest day of the year. What you need is an outdoor-reset control that will limit the boiler temp based on the outdoor temp.

    With such low return-water temps, you may be getting condensation in the flueways or chimney. An injection- or mixing-type reset control setup will let the boiler stay out of its condensing range while limiting the outgoing water temp. It will also let the boiler ramp up to full temp on a DHW call.

    The idea here is to not generate more heat then you need, so you can save some fuel and a purge-out shouldn't be needed.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • icesailor
    icesailor Member Posts: 7,265
    Post-Fire Purge,

    If you are willing to spend more money than you will save, you can make this get better.

    That said, take the front cover off the boiler and look up at the front left and find the high limit thermostat. Turn it down as low as you can. It may not go below 150 degrees. Fine. Set it there. If everything still works fine and the house stays warm the way you like it, leave it. Plan on saving your money for the expensive stuff. If it gets really cold again and the house won't come up to temperature, turn up the high limit until it is warm.

    My ancestry traces back to an area of Northern England. Unless someone can show me a cost/payback that is more than a year or two, I'd spend my money on insulating all the domestic and heating pipes. That will pay back quickly.
  • 1sttimeposter
    1sttimeposter Member Posts: 39
    Great points

    taken from all of you. I totally agree with you that, at this point, I shouldn't spend more than I can save particularly this on/off type of cast iron boiler is 14 yr-old already.

    While the temp range is rarely at this extreme, it is very common to see a delta-T of 30-50F on the zone heating side at the end of a cycle, (this cast iron boiler's temp keeps on shooting up another 10-15F after heat call ends), but never for DHW heating (15F max). Can never understand that. Just thought there's an easy way to purge some of the residual heat to the zone.

    The only reason I keep it at 180F high-limit, with 15F differential, is for the indirect although our under-joist suspends do require fairly high temp as well.

    I will play with the high-limit thermostat as suggested by Icesailor to see how far I can set it down without bothering anything or anybody. Just worry about DHW recovery...

    Attached is a couple of our set-ups in our crawl space if anyone's interested.
  • RobbieDo
    RobbieDo Member Posts: 131
    Reset Control

    A reset control is great. As far as recovery that you state for DHW, the controllers have a priority for that, if you wire it properly that won't be an issue. So boilers though, will not like it if your return temp is too low, some manufactures will not warranty them if low return temps unless it is capable of low temps. I know Buderus allows low return temps on cast iron 80% boilers and doesn't void warranty.
  • 1sttimeposter
    1sttimeposter Member Posts: 39
    post-fire purge

    indeed, our existing valve controllers have DHW priority. but I'd think it still needs 180F min. for hot water recovery

    No, the boiler is Weil-Mclain and there's nothing we can do about the low temp return situation without investing some money in that now.

    It'd be nice to know an easy way to do purge.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,840
    No way Jose...

    Unless you are willing to COMPLETELY change the near boiler piping, a purge cycle is not in the cards for you. You have zone valves, and those zone valves have end switches on them, and they would have to be open before you could circulate water through the boiler in order to post purge, and if the zone valve is open, it means it needs HEAT and PUMP...

    See what I'm sayin'?

    There are controls available that will do an outdoor reset whenever a space heat call is calling, and raise the temperature for DHW production. You will realize a 15 to 20% reduction in energy consumption in doing so.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • RobbieDo
    RobbieDo Member Posts: 131
    What Controller?

    What type of unit do you have that controls the zones and your DHW? If it is setup right it will control the DHW so your water heater will produce got water efficiently.
  • 1sttimeposter
    1sttimeposter Member Posts: 39
    My controller

    is merely a zone valve control with DHW priority built-in (jpeg attached in original post#1). It doesn't really regulate temperature. So max. DHW temp will be whatever high-limit boiler aquastat set at.

    ME is right. Either I bite the bullet to buy a boiler control or move on and wait until this old monster dies......

  • RobbieDo
    RobbieDo Member Posts: 131

    I guess I have never seen a boiler control like that. I either control them with PLCs that I program myself or I use a controller for boilers. Never seen one like that? If your not good with wiring and controls the easiest way is to buy one that is made for your application.
This discussion has been closed.