Dormant Secondary Boiler in a Pri/Sec Loop System

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BunkerdCaddis
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Post by BunkerdCaddis » Sun. Mar. 15, 2015 9:25 pm

First, I want to say thanks to all who share their wisdom and knowledge on this forum, the education I've gleaned over the last few months has been infinitely worth more than the price of admission. :D

My goal is to add a coal boiler to an existing oil-fired hydronic system while doing a necessary rezone of the house we moved into three years ago. I have waded thru most of the threads on piping and read much of the suggested literature.
The one question in my mind regards the cold dormant secondary or backup boiler in a pri/sec loop where both boilers are tied into the pri loop w/ close tee's. The reason most often stated for this is to eliminate parasitic heat loss up the flue. This is my plan at present. But with the concern of keeping a shut down boiler from rusting by putting a light in it over the summer, does this become an issue in a setup such as this in the winter? The alternative then would be to run the oil-fired boiler or heat it occasionally from the coal boiler or make the dormant boiler part of the primary loop, leading to the parasitic loss issue. John S's book doesn't seem to address this, as most of the multi boiler setups are alternate running or cumulative. My secondary boiler will be for emergency or going away so will need to be in a state of ready to go, although it may sit dormant for a month or hopefully more.

Or maybe I am just over thinking this...

 
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Post by oliver power » Sun. Mar. 15, 2015 9:38 pm

I've considered doing the same exact thing you're talking about. Haven't given it any deep thought, as it's not priority. Interested to hear what others have to say. I don't know this for fact but, I'm guessing some kind of by-pass loop might keep the dormant boiler water warmed enough through natural circulation. Or maybe something like a devirter "T" put in line.

 
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Post by Scottscoaled » Sun. Mar. 15, 2015 10:04 pm

In a coal boiler, moisture is the culprit IMO. During summer the light is put in to make heat and prevent moisture from condensing on the cooler boiler surfaces. The moisture and the sulfur left in the ash residue makes for a powerful acid. Boiler eating acid. With a oil ,propain or natural gas boiler, I don't think the acid making parts are there in enough of a quantity to cause problems. They might be a problem when it is happening every day from low boiler temperature condensate and continual firing. But I think when they are shutdown for long periods of time, the corrosion from acids isn't much. The water inside the boilers shouldn't be anymore of an issue to the stand by boiler then the operating boiler because they are pretty much on the same water. Just my opinion.

 
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Post by BunkerdCaddis » Sun. Mar. 15, 2015 10:40 pm

Scottscoaled wrote: The moisture and the sulfur left in the ash residue makes for a powerful acid. Boiler eating acid.
Well, that sheds light on the issue. I am new to coal just this year and the ash/acid issue was something I was not aware of, I suppose I was thinking more along the lines of humid air flowing thru the boiler and up the chimney as the problem, So I see how it would be more of a problem w/ an inactive coal appliance. That makes sense. The water inside the castings or jacket would be ok because it's a closed system still.


 
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Post by waldo lemieux » Mon. Mar. 16, 2015 7:34 am

There is very little if any heat loss up the chimney on an inactive oil boiler. In my opinion no reason to worry about that or the condensation. If your still concerned about it you could put a switch on the oil burner and put your pump on a timer or an aquastat to maintain a min temp :gee:

 
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Post by Sleeper735 » Mon. Mar. 16, 2015 10:00 pm

My question would be are you going to have the stand by boiler capable of turning on its own. Or would you have to manually turn it on. You could set up the circulator on a aqua stat to keep the boiler warm. It would be better to keep the boiler warm if not hot. It will help with expansion and contraction. Depending if your boiler has push nipples or gaskets that may be the bigger issue. Plus you have to remember to exercise your pump on the boiler and make sure the fuel oil in the filter and pump don't go bad in no time. And I mean in a year or 2. Any heat loss would be pennies vs keeping the boiler up to temp and using the pump and etc.

 
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Post by Scottscoaled » Mon. Mar. 16, 2015 11:06 pm

Sleeper735 wrote:My question would be are you going to have the stand by boiler capable of turning on its own. Or would you have to manually turn it on. You could set up the circulator on a aqua stat to keep the boiler warm. It would be better to keep the boiler warm if not hot. It will help with expansion and contraction. Depending if your boiler has push nipples or gaskets that may be the bigger issue. Plus you have to remember to exercise your pump on the boiler and make sure the fuel oil in the filter and pump don't go bad in no time. And I mean in a year or 2. Any heat loss would be pennies vs keeping the boiler up to temp and using the pump and etc.
Not true. I came into basement in middle of winter day ad saw my barometric damper for the oil boiler swinging back and forth. Stuck my hand in there and could feel the warm air pushing by. Caused me enough concern to take it out of the system. Once it gets a gravity flow thing going it will suck a lot of heat out.

 
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Post by coalkirk » Tue. Mar. 17, 2015 7:28 am

I run the hot water from the coal boiler through the oil boiler. I also remove the vent pipe from the oil boiler to the chimney cause there was a large amount of heat escaping. Cap the vent on the oil boiler and disconnect a wire from the oil burner so it can't be fired accidentally.


 
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Post by coaledsweat » Tue. Mar. 17, 2015 8:57 am

Lift checks on the plumbing and a stack damper on the oil burner flue. It will be ready to rock and roll when needed.

 
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Post by Sleeper735 » Tue. Mar. 17, 2015 9:11 am

Be carful with that stack damper. I believe they are illegal on oil. And if you have one and you have a co problem or puff back or something the insurance will not cover it and cancel your policy.

 
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Post by Rob R. » Tue. Mar. 17, 2015 10:46 am

As long as you get the oil boiler heat exchanger is clean and dry before you shut it down, I don't think it will be an issue. If you do decide to use the oil boiler, make sure it runs long enough to get up to operating temperature.

Lots of people shut oil boilers down for the summer months...one benefit of the reduced sulfur levels in today's heating oil is that the boilers seem to handle the off season better. Having a coal boiler keeping the basement warm and dry will also help prevent corrosion.

As for stack losses from the oil boiler, it depends on the burner. I have a Riello burner that closes the inlet damper when not in use, so no air can be drawn through the boiler.

If you have the space and budget to hook the oil boiler up as a secondary loop, it seems like a good way to have a backup boiler that can be brought online automatically - with no stack losses while sitting idle.

 
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Post by BunkerdCaddis » Tue. Mar. 17, 2015 10:48 pm

Thank you all for the insightful discussion. It is good to have the wisdom and expertise of others to fill in the blanks and solidify my own thoughts.
Sleeper735 wrote:My question would be are you going to have the stand by boiler capable of turning on its own. Or would you have to manually turn it on.
The oil boiler would be automatic, when the coal boiler would not be keeping up or not available. Yes, leaking might also be a concern to watch for.
coalkirk wrote: I run the hot water from the coal boiler through the oil boiler.
If corrosion were an issue this is what I would have done. I work for a township, during emergency weather I can be gone for a couple of days so the oil unit has to be RTR. The coal boiler will be remote from the oil boiler, I have no extra room in the basement for a coal unit nor coal storage as it is a finished basement.
Rob R. wrote: As for stack losses from the oil boiler, it depends on the burner. I have a Riello burner that closes the inlet damper when not in use, so no air can be drawn through the boiler.
If you have the space and budget to hook the oil boiler up as a secondary loop, it seems like a good way to have a backup boiler that can be brought online automatically - with no stack losses while sitting idle.
Mine is a beckett burner, don't know if an inlet damper is available?? I'll check the lawyerese re: a stack damper.

Boilers on secondary loops is the way I'm trending, I can always set aqua stats for low limit to maintain a base temp on either boiler. Now to finish planning this project and to decide on which solid fuel boiler.

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