Cold Return Question

Discussion in 'Boiler Forum' started by Richtdow, Nov 3, 2019.

  1. Richtdow

    Richtdow New Member

    Nov 3, 2019
    New England
    I was wondering about the effect of cold returns on a boiler that stays above 140 F. What I mean is that I have the boiler set between 150 and 180 F. When it is heating my home the lines are all fine with a low delta and a return that is near or slightly above 140 F.

    However, once the home reaches the set temperature of 65 F, the water in the radiators and associated lines decreases as the heat dissipates from them because of a lack of circulation from the boiler. Once the boiler restarts to answer the call for heat, the return lines are now at a temperature significantly lower than 140 F resulting in cold water being introduced into the boiler itself. The boiler is not setup as a cold start and it will maintain temperature at the boiler of between 145 - 180 F while waiting for a call for heat.

    How does this affect the boiler? Should I be concerned about thermal shock and corrosion from condensation? If so, should I convert this boiler to a cold start to remediate this issue or something else?

    Home Size: 1200 sqft with average insulation
    Boiler: WGO-2 (#2 Fuel Oil)
    Burner: Beckett AFG with GeniSys Model 7505
    Aquastat: HydroStat Model 3250-Plus
  2. Dana

    Dana In the trades

    Jan 14, 2009
    If the entering water temperature is below 140F for a large fraction of every burn there will be excessive exhaust condensation in the flue. If the flue has a stainless liner that's not necessarily a disaster, but if it's a terra-cotta lined chimney the acids in the condensate will slowly destroy the mortar and ruin the chimney from the inside out.

    If the entering water temp is chronically below 130F (for most oil boilers) there will be excessive acidic condensation on the heat exchanger plates themselves, which could destroy the boiler from the inside out in one or two heating seasons.

    The WGO boilers are cold-startable, and at least somewhat tolerant of condensation during the warm-up, but not chronic low-temp operation. The 3250-Plus default settings hold off the circulation pumps until the boiler has risen to at least 125F to raise the boiler temp quickly and limit the amount of time the boiler is operating below 130F. If the radiation is all low-mass fin-tube baseboard or something that should work just fine, but if you're working with the thermal mass of the higher water volume + cast iron in cast iron radiators other measures need to be taken, such as a boiler-bypass loop in the near-boiler plumbing, to mix the return water with direct boiler output, raising the entering water temperature at the boiler:


    ...or a primary/secondary approach (which adds a pump):

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