Aluminum Con Boiler Thermal Shock Protection from Buffer Tank return - Cold boiler OR Hot Boiler.

stetsoncow

W/M Ultra 155, (aluminum con modulating boiler) retrofitted 5 yrs ago into our 60 yr old 3750 sf home, 180 degree F/T baseboard system, (one big loop/zone), three branches, proportioned with valves for proper flow and heat distribution. Boiler loop is 1-1/4", main system piping is 1-1/4" to 1" branches. Works pretty well for what it is.

Have since installed a 750 sf radiant Warmboard system in part of the home over exposed carport to help with chilly floor. (no baseboards in there, just a free-standing gas stove that gets things too hot after awhile!) After realizing the boiler is too big to run such a small area without constant short-cycling, installed 115 gal buffer tank. Running the radiant system 100 degrees, (with mixing valve), the buffer tank 140 degrees (with a 120 degree aquastat closing temp). Tied the 1" radiant system main piping into the boiler loop with injection type closely spaced tee's. Running full 180 degree water thru the tank to heat it up. Using a variable (3-speed) Grundfos Alpha 15-55 to supply the buffer tank, running at full speed.

A short time later I started experiencing air introduction problems. While researching hydronic subjects here and elsewhere, I became aware of and deathly afraid of thermal shock, thinking I may have cracked $1300 heat exchanger (somebody's making money!) due to 65 degree main house water entering my 180 degree boiler that just finished heating up the tank. (turns out it was low expansion tank pressure allowing an auto vent to suck air on cold system start-up, being it's only 3 ft ahead of the circ in the return piping before the boiler loop) But now I'm pretty sure I need to install boiler thermal shock protection mixing valve on the main house system. Will use a Caleffi 280 with 140 degree (+18 degree final closing) to keep the return temp to the boiler loop up. That part is easy!

My question is: how would I protect the boiler from the 'high' (120 degree) buffer tank 'call-for-heat' return water temp into a cold 65 degree boiler loop, while also protecting it from a 'cold' (120 degree) return temp when the boiler has just finished heating the 180 degree house system? Thinking I may need another loop between the boiler loop and the tank loop? Double mixing valves? Or maybe I just need to slow down the buffer tank pump? Scratching the bald head, since all the hair's been pulled out!

Also, was wondering, does making an aluminum con mod boiler heat up so fast with a protection valve hurt the heat exchanger?; presently it takes the system quite awhile to get to target heat (180 degrees) when it all starts cold, I've watched the temps on the unit display and notice it does a good job keeping the supply and return temps within 20 degrees all the way up to target.

There certainly isn't much info on aluminum heat exchanger thermal shock protection - In the boiler manual or anywhere!
Am I worried about nothing?

Long question! Thanks for you time!

Ironman

A couple of questions:
1. Why don't you have the buffer tank piped into the radiant and not the boiler loop?
2. Are you using outdoor reset on the boiler? That would keep the system water temp well below 180* most of the time.

stetsoncow

Sorry, I meant to say the tank is tied into the boiler loop for reheating the tank. I have the tank set to hold 140 degree water. Aquastat set to reheat the tank down at 120 degrees. That all works great! My only concern is how I re-heat the tank without shocking the boiler. Tank return temp will always be around 120 to start. 120 into a 180 degree exchanger seems a bit abusive. As does 120 degree tank return temp into a 65 degree exchanger. These are the extreme ends of possibilities, but they're there none-the-less!

My fin tube baseboards need hot 180 degree water to work! Most of the house is this type system. My boiler can't tell the difference between the two systems, I have it running thru a Taco relay panel. This boiler isn't real smart, only meant to run one system (temp) at a time, giving pref to one over the other.

Thinking I should run both systems thru a separate loop using the same return piping thru the protection valve? But not sure if this would protect against hot tank return water to a cold boiler.

Thanks!

Brewbeer

Are you sure your baseboards need 180F water to work? At 20F outside, the baseboards heating my house are running 110F water. You might find that your house will heat just fine with supply temps lower than 180.

stetsoncow

Well now!......never gave that idea any thought! I could try reducing it to 140, that would give me the 20 degree DeltaT the boiler seems to want. But I still can't get my head around how to simultaneously protect from hot water hitting a cold boiler.

Ironman

Mod/con boilers are made to take cold water. The colder the water, the more efficient the boiler becomes.

Again I ask: why aren't you using ODR?

njtommy

Your not utilizing your boilers potential running the supply water temp at 180f. You may only need that high of a supply water temp on a 0 degree day or even max 160 supply water temp depending on how much radtion you need and have. I would highly recommend doing a heat load calculation on your house and baseboard to figure out your max supply temp. Also an out door reset control that ties directly it to your boiler. It will pay for itself in no time at all.