Early heat exchanger failure

So these were high mass/ water volume systems? So I gotta ask... what were you using for boiler protection from thermal shock?

now that i,ve re read you post and there HW boilers .I would look to see if any loops where ran under the slab .I once had a service call where they had any issues with air re appearing in the system which can indicate a leak .I had a HO that unknowingly would use the bottom boiler drain for washing there cars what a nightmare .Went there like every 2 weeks for air in system and it was a older system drove us crazy finally one day i pull up and long behold they got a garden hose hooked up to the boiler washing cars .Would have never fiqured that one out .But a big clue was bright orange water that would come out when i went to purge the boiler out .I have also witenessed this with under ground loops .Personally i would do a system pressure test to make sure she s water tight also don t leave feed valve open and make sure they have a low water cut off and or a pressure switch to ebnsure there water in boiler .Peace and good luck clammy

yeap those boiler don t hold much water volume may have been better to ps pipe them and put a danfgoss esbe thermic valve on the return to keep those water temp up and prevent flue gas condensation .Seems some small low water content boilers don t like those low temps durning a cold start .It sucks but on a boiler replacem,ent take a look at thoose esbe thermic valves or pipe in a system by pass off of your main system pump ,just do a little math to fiqure out what size to use it usually either 1/2 or 3/4 on most small to mid sized residental homes .I learned this trick from a smart german ,a small bypass help bring your return temp up as the boiler warms ,i bypass on the pump outlet and use a globe valve to adjust flow of course pump on supply and usually a small spring chk in the by pass w union for ease of replacement or repair.Works great on oil boiler with triple aquastat and low limit on circ ,helps those retrun temps and cuts down on the circ cycling on low limit good little trick to put in the bag .Remenber to leave a note if you do installa bypass alot of guys have no idea and close them ,i usually mark the valve stem and remove the handle and tag the valve so one one completly closes it .Again hope this helps peace and good luck clammy

Yes I agree.

What's wrong with two zone valves? As long as the circs has the fizz to handle the job ZV work great, less power consumption, 100% shutoff, no check valves required.

Better yet a couple PICCVs

to answer your question installing a by pass with more then 1 circ .I m talking about a system by pass which is piped off the supply on the pump discharge and ties into the return for that loop or zone .A simple spring chk and a globe is installed in this line usually with a ball valve and union for ease of replacement of spring chk,.I know this is not the greatsest but when your boss is cheap and you get tried of cleaning the same soaked oil fired boiler because of prolonged low temp you pipe in a by pass .It simple and it does work .In cases with multi zones there s usually the original hi volume zone and then a bunch of mirco base board zones,It s a tool that i use here and there when you just can t talk them into a thermic valve some people and contractors just won t listen so you this and it s better then nothing and it s worked for me in a pinch . good and luck clammy

Also have seen it and done it with a simple zone valve and aquastat on some 3 boiler systems tied into a extreme gravity system it worked great .Used 3 small 007 and they had a hi vee series 60 on the return on the existing system .No tekmar stage control either just 2 aquastat to bring them on and when she got to 160 1 boiler would drop our .Work for some cheapos who would not pay for a stage controller there lose.There s alkways more then one way to skin a cat peace and good luck clammy