Thermal shock question

John_34

How is the issue of thermal shock approached in a multi-zone system. i.e. I have zoned my hw cast iron radiator system into two zones, my project is pretty much complete-but I just had this thought......If one zone is calling for heat, then shuts off when the thermosat is satisfied, then the other zone kicks on shortly after--much cooler water is going to be introduced into the boiler. Will this create thermal shock? If so is there a "standard" way to avoid this scenario? My boiler is newer, about one year old, and in the project I included flo-control valves to avoid the possibility of thermal "ghost" flows from one zone to another when just one calls for heat.

Ted_9

Its not an issue if its short turn, like at cold boiler start up. Its only a problem if its constant low water temp.
Patriot Heating & Cooling, Inc.

Boilerpro_3

Yes you probably have a problem

If you have cast iron radiation you already need some form of low temp protection, unlessyour running a condensor. My favorite, more economal method is to pipe the boiler P/S to the main loops and use a Danfoss thermix valve on the boiler return. It will respond to changes is boiler return temp to protect it from low temperature condensation and also help greatly to protect the boiler from sudden drops in return temp. I probably install about 65% of my boilers this way, and another 25% with a variable speed pump.

Once you have this protection in place you can also add full outdoor reset, which is helpful on any zoned system, even though not so much so on a high mass system like yours.

If you are running a condensor, some still should have thermal shock protection. This seem to be a problem that few manufacturers want to talk about. I suspect that condensors with more flexible heat exchangers like tube types, deal with sudden temp changes better than molithic block types.


Boilerpro

Ted_9

Please clearify. Is this a gravity hot water system? Yes in that case you should look to protect the boiler.
Patriot Heating & Cooling, Inc.

John_34

My system is a two-pipe forced-circulation system. I've just finished doing more reading on the net, and realized I should have piped in a boiler bypass line. I'm going to be adding that in tomorrow. I added 7 new radiators to the system, for a total of 18 radiators. I split the system into 2 zones, 1 for main floor, 1 for second floor. I want to avoid any problems with condensation and/or thermal shock, hence my reason for installing the bypass. Any advice would be greatly appreciated.

Ted_9

Was it a gravity system. Are all the pipes large, say 2"?
Patriot Heating & Cooling, Inc.

John_34

The two original main supply/return lines running thru the basement are 2".

Boilerpro_3

Bypass helps

but does not ensure full protection from cold temps. Typically you will still get a fairly long period of time that the boiler is below 140F. Also A bypass is not so good if you have a high mass system that needs to run at high temps....ie 180F... on design days. You may not be able to get those high temps because the boiler will be off on limit. I only use bypass on older boilers that have no protection and I want an cheap way to help the boiler get a few more good years.

Boilerpro

dave connors

If the boiler was shockproof

Shame you didnt know about thermostream design in a boiler. Thermal shock would not have been a question and return water temps would not be so critical.

Weezbo

John...Taco makes a set point controled zone valve i=Series

The i-Series set point zone valve can be used in the bypass loop to regulate the temps back to the boiler...by wiring it through the recirc pump and a sr501 relay then any call for re circ pump the 501 is energised modulates out to the set point ,when another zone opens it opens even more to satisfy the set point if both zones close it is always reacting off the recirc pump so it acts a little bit like a cuircit setter as well...

Joe_30

A SYSTEM bypass, say a 3/4 inch line which takes hot water off the supply manifold [like a zone does] but just runs right back into the return side of the boiler would temper the cold water from that cold zone as it comes on line, would'nt it? Put a globe valve in it to regulate flow. Granted, it cannot sense the temp or vary with it.

Joe_30

Weezbo, would that Taco setpoint zone valve work in a system bypass line as I spoke of? Would there be a heat sensor on the boiler return line.? Thanks. joe

Weezbo

Yes ...thats it exactly.

the boiler SUPPLY TEMP SENSOR goes on the return after the T into the return...then any time water comes past it the sensor tells the valve ho wide it should open to maintain the set point. they are made in 1/2" 3/4" and one inch . it is a two way esp zone valve body. with a Green head(Zone valve controller) on it. i am very happy with this controllers preformance as a means to temper the water and protect the boiler.

Mike R.

Revisit Thermal Shock

On September 14, John asked about Thermal shock due to his multi-zoned system with hot water cast iron radiator. Does a single zone also experience thermal shock?

I have 137.7 feet of Baseray baseboard hot water cast-iron radiators which hold 41 gallons of water (0.3 gal/liner foot). I am getting a new oil fired hot water boiler, probably with indirect tank (decesion still pending). If priority zone is given to the tank and the other zone to the radiators, do I still need a SYSTEM bypass? What about boiler cold start?

Chris_32

??

I believe that the scenario that you are addressing is condensation due to prolonged low return temperaturtes. Thermal shock occurs when a hot boiler recieves a blast of cold water causing rapid contraction of the cast iron. Cracks will manifest on the return side of the boiler in intermediate sections and are most often parallel with the top of the crown sheet.

Chris