Dialing in New Boiler

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John Molyneux

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One of the things I'm noticing about my new Bosch Greenstar 57 is that it will shut down sometimes even though the thermostat is still calling for heat. For example: it'll be 50F outside and the boiler will be sending out 90F water based on where I've set the curve. (Base Point 80F, Design Day 130F, cast iron radiators, WWSD disabled.) When the t-stat first calls, the boiler runs for awhile (at least 10 minutes or more) but then the boiler and secondary circ pump will shut down even though the zone valve is open and the t-stat is still calling for heat. Then after awhile it fires back up until the t-stat is satisfied.

I'm confused why the boiler wouldn't just run until the t-stat shuts it off. I have the WWSD disabled and the max boiler heating temp set at 175F. I've never seen the boiler get hotter than about 110F unless it's charging my indirect hot water tank.

Any thoughts?
 

Jadnashua

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On some, there's a max fire duration timer, and it shuts down after that max time of running continuously. But, I think on mine, that time is about an hour...10-minutes seems like way too short of a time. But, at such low temperatures, the delta T is quite small, and therefore, the heat transfer is slower, and if the return water isn't sufficiently cold, continuing to run the boiler would cause it to get much hotter than your setpoints...IOW, it may not need to run while still providing heat to the house for awhile. The theory of operation section of the user's manual should give you some insight on your specific operational functionality.
 

John Molyneux

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Thanks Jim. The delta T explanation makes sense.

I've read all the manuals backwards and forwards. Bosch doesn't provide a whole lot of explanation on 'theory of operation' :)
 
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