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Re: Temp. overshoot


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Posted by Rick on March 29, 2005 at 00:23:50:

In Reply to: Re: Temp. overshoot posted by HeatPro on March 27, 2005 at 16:44:36:

: At the end of a call for heat, most simple control systems turn the circulator off with the burner; thus the heat that is on the fireside has time to transfer into the still water inside the burner after the shutoff. Check again to see if your control system shuts the circulator off with the burner; if so, then your observation doesn't occur.

: If your burner goes off BEFORE the heat is satisfied, that shows that there is more input into the water than there is radiation to get rid of the heat, so the water is hitting the high limit, thus shutting off the burner; but not the circulator. If that happens in the coldest weather, your burner is larger than necessary, otherwise said that your boiler is OVERSIZED. As you can't make the water flow through the radiant section faster without causing other circulator problems, you might have to provide a loop-subloop piping so the true water temperature within the boiler distributes the heat evenly inside the boiler tripping the high limit earlier so the overshoot doesn't occur.
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Thank you Heatpro very much for your help and this accurate analysis as if you are at the site.

Indeed, youíre right in that when the burner shuts off and heat is satisfied everywhere, the circulator is off. And even with the overshoot, thereís no concern of overheating the Ipex cross-linked orange tubing while all zone valves are closed off as well.

My dilemma lays in if the burner goes off when thereís still need for heat, the circulator will deliver the super-hot water up the Ipex tube due to overshoot. You bring up an interesting point if that occurs only because weíre getting milder in weather.

I donít know whatís involved in your subloop remedy or how expensive itíll be. But I have set the high limit down to 160F on the aquastat which allows the overshoot to reach at around 190F (still higher than what I prefer) for radiant floor heat or 175F (not hot enough) for DHW which is on priority switch. I donít think I want to lower the high cut-off any lower because we have run out of domestic hot water in last two evenings. However, even at 190F, Ipex indicates this kind of tubing can only handle 180F or 82C max. Iím stuck in between!

I donít think the aquastat or tmp/press gauge is going because the burner stops exactly at +/- a couple of degrees at 160F of the high limit that I set. One thing Iíve forgotten to mention is that my differential is set at 5F which should make the low cut-in at 155F. But the boiler wonít start until it dips further down to 148F and this wonít help my DHW recovery neither. Same heat delayed transfer phenomenon?

My wife thinks I have been hiding my mistress in the crawl space since I spend a lot of time lately observing my boiler down there that I seldom do before. (very new and intriguing to me!)





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