Thermal shock in Radiators?

DaveC

I am just putting the finishing touches on my wood boiler installation. I have a converted gravity feed system and am currently using a gas boiler. I re-piped the system using primary secondary piping. In the loop the gas boiler comes first and then the wood boiler, each has a Taco 007 pump pushing water toward the boiler. The primary loop has the same pump at it is pumping away.

So far I have been running the gas boiler and everything is working fine. I am running the primary pump continuously right now and using the original thermostat to control the gas boiler. When there is a call for heat the gas boiler kicks in and gradually warms up the system. When the wood boiler is fully operational I will set the gas thermostat at about 65 or so so it can run as a backup.

What I am worried about is what happens with the wood boiler? It will be operating at about 180 degrees and the circulator will be running all the time for freeze protection since there is no glycol in the system and the boiler is located out in the garage.

My intention is to use a separate thermostat to control the primary pump and turn it on and off to control the temperature in the house. Since there will be hot water available all the time in the common piping the system will not heat up gradually as it does now. Am I likely to break a radiator by applying heat to quickly? The pipes in the house are really big so they do hold a lot of water, but I don't know it the temperature change will be too much too soon.

Any help would be appreciated.

Mark

Mike T., Swampeast MO

Very doubtful. Remember that radiators at room temperature have no problem with live steam.

Paul Pollets

Buffer zone

The wood boiler should have a make on rise aquastat that always turns on the circ pump when the temp in the wood boiler exceeds 140 degrees. A "dump" zone is often used to make sure the heat goes somewhere. If you don't dump the heat, wood boilers can easily put out many more BTU's than needed, and start blowing the relief valves from overfiring. There are schematics on correct installation by John Siegenthaler that have been published in PM Mag.

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Mark Mathys

Paul, the circulator pump for the wood boiler runs all the time. The aquastat on the boiler controls a draft blower and air intake valve so it maintains the boiler around 180. When there is no demand the fire almost goes out. Is this heat dump still required?

Mike, I know steam is a lot hotter than water but does it transfer heat to the iron as fast? I sure hope not because I don't want to lose a radiator.

Thanks

Mike T., Swampeast MO

If anything, I would presume that steam transfers heat significantly faster--remember there's a LOT of latent BTUs that are given up as it condenses.