An Easier Parallel Boiler Connection?

Stoker Coal Boilers automatically feed the coal and have controls and pumps just like any conventions boiler. They are intended to be used as a primary heat and often have domestic hot water coils as an added bonus. They can be set up independently or in dual sytem with your existing oil/gas boiler. They can accommodate both hot water base board or steam plumbing.
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jfgovern
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Post Fri. Jul. 10, 2009 1:14 am

In reviewing the AHS parallel boiler configuration and looking at my current oil-boiler configuration (looks just like the right-hand side of the AHS diagram), I'm wondering if I can add a Ka-6 in parallel with my existing oil boiler by simply adding the AHS-indicated "new check valve" between the Ka-6 and oil burner supplies (allow flow from coal boiler toward oil boiler, up to radiant heat but not back toward Ka-6) and installing a zone-valve (with limit contact) on the return to the oil-burner. The zone-valve on the oil-burner return would remain closed when the Ka-6 is operating but open when the Ka-6 is below operating temperature ("failed-over to oil" condition) to allow return to flow through the oil-burner. As a precaution the oil-burner control power wouldn't be allowed to "make" unless the zone-valve limit contact was closed. My existing return pump is located prior to the oil burner, and the Ka-6 return would be plumbed in parallel with the oil-burner return. This setup would look just like the AHS parallel-boiler diagram, except the "new circulator" pump would be eliminated and a zone-valve (with limit contact) would be added on the oil-burner return. Plumbed this way the Ka-6 wouldn't constantly heat the oil-burner (and send hot air up the chimney), and the oil-burner wouldn't heat the Ka-6 when it's shut-down. The downsides would seem to be cold-starting the oil-burner at the start of failed-over operation, and pumping water through the coal boiler when shut-down (wastes heat) which I wouldn't expect to happen more than a couple times per year. At the end of the winter season I could close a manual valve on the return to the coal boiler to eliminate pumping oil-burner heated water through it during summer months.

What drawbacks exist with this scenario? What am I missing? Any advice would be appreciated. Thanks in advance.

http://www.alternateheatingsystems.com/boilers_in_tandem.htm

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steamup
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Post Fri. Jul. 10, 2009 11:55 am

I prefer to use pumps as zone control vs. zone valves as I have found pumps are more reliable than zone valves.

Attached is a Glitch/Fix article that I saved that shows the ideal dual boiler piping (in the solution) to the problem.

It will cost more money but is a more solid solution to dual boilers.

Good luck.
0108-rh-GlitchFix.pdf
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Steamup
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Yanche
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Stoker Coal Boiler: Alternate Heating Systems S-130
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Post Fri. Jul. 10, 2009 2:30 pm

The page 3 solution shows the circulators
Dual_Boiler_Priping.jpg
on each boiler pumping water flow into the supply side of the boilers. Can anyone suggest why? I've seen this illustrated this way before. Usually in some article authored by John Siegenthaler a noted expert on hydronic heating. I think it's wrong. While the pump should be on the supply side the flow should be out of the boiler. Why? For example my Weil McLain oil fired boiler has an internal casting that is an air scope near the supply tapping. This would not work if flow were reversed pumping flow into the boiler via the supply tapping. Also pumping into the boiler, from either the supply or return tapping increase the pressure on the boiler, reducing the margin on the safety pressure relief valve. Pumping away from the boiler reduces the pressure on the boiler increasing the pop off margin on the relief valve. A good thing. Anyone else agree with me?

I also believe the boiler isolation valve (supply side) is in the wrong place. As I read the plumbing codes the isolation valve needs to be directly on the boiler supply and return lines.
Dual_Boiler_Priping.jpg
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Alternate Heating Systems S-130
Stoker Boiler burning Anthracite Pea Coal

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coaledsweat
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Post Fri. Jul. 10, 2009 2:41 pm

Yanche wrote:Anyone else agree with me?
I do.
Nothing is impossible for people who don't have to do it themselves.

jim d
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Post Fri. Jul. 10, 2009 11:25 pm

yanche; if those diagrams are the page 3 you mention the flow is going to the return with the circ on the return I know they for the last 15 years or so they keep saying to pump away from the boiler but with the expantion tank where it is the diagram looks acceptable to me on the flip side pumping of the bottom return side and returning thru the top used to work well with the larger h2o capacity boilers from the 50s 60s &70s ,with a single air vent at the boiler you could vent the whole system by just turning on the water and the circ on a ranch or raised ranch no purging no air noises ever flooded suction!

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steamup
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Coal Size/Type: pea, buck, rice
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Post Mon. Jul. 13, 2009 12:55 pm

The diagram for these mutiple boilers is the most correct that I have seen.

The theory of pumping can be somewhat complex. Without going into great detail, I will try to explain some points here.

1. FYI, the glitch/fix article was written by John Siegenthaler, P.E. He has written alot about small hydronic systems and has several articals of this type.

2. Pumping towards or away from the boiler is not an issue in the particular diagram, due to the short primary loop. The added head to the boiler is minor. If your pressure is running that close to the relief valve setting, your expansion tank is probably undersized.

3. Heated water give up air more readily. Therefore the air separator is best placed after the boiler. On boilers with built in air separators, an automatic air vent on the air outlet of each boiler would eliminate the need of an inline air separator. However, since not all boilers have built in air separators, it is best to add one inline.

4. The expansion tank in relation to the pump suction is the most important details. Attached is an article from Bell and Gossett that explains why.

5. A minor benefit of pumping return water is the cooler water is less damaging to the pump and seals. Also, there is less chance of the heated water flashing into steam at the pump impeller (this is usually not a concern for smaller residential low head pumps).

5. For a point of reference, most packaged boilers have the pump mounted on the boiler pumping INTO the boiler. All packaged boilers that I have purchased are set up that way.

6. The mechanical codes I deal with require shutoff valves for each boiler. If the pump is dedicated to the boiler, it should not matter if the valve is before or after the pump. Most inspectors would not have a problem with this.
MagazineArticle1.pdf
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Sting
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Post Mon. Jul. 13, 2009 7:38 pm

or

Simply you pump away to heat

and you pump at the boiler to host temperature reducing or primary manifold loops
When you turn your boiler on -Does it return the favor?
I have finally lost my mind. Don't bother to return it. It wasn't working properly anyway!

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Yanche
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Stoker Coal Boiler: Alternate Heating Systems S-130
Coal Size/Type: Anthracite Pea
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Post Mon. Jul. 13, 2009 11:08 pm

steamup, thanks for pointing out that the diagram shows the flow into the return of the boiler. I missed that detail. I have Siegenthaler's book and he has several dual boiler piping diagrams in it. I've looked at them many times and puzzled why he had the pump on the return when in other areas of the book it's always on the supply. I just missed the important detail of the pump's flow direction. Funny how you mind can convince you it's wrong when it isn't. :-)

I agree your posted dual boiler diagram by Siegenthaler is correct and is what I would also recommend.

I would add an immersion well in the boiler loop with an aquastat. The aquastat contacts could then be used with external relay logic to shut off power to the coal boiler when the fire goes out and bring the non-coal boiler online and set an alarm.
Yanche
Alternate Heating Systems S-130
Stoker Boiler burning Anthracite Pea Coal

jfgovern
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Post Tue. Jul. 14, 2009 12:50 am

Thanks for the feedback everyone! In the end of the day I chose to plumb them in series - if the heat loss up the chimney of the oil boiler bothers me enough I'll get an automated flue-pipe damper for it. :-) The coal stove feeds into the return of the oil burner, and the supply of the oil burner feeds the house. This way the coal burner keeps the oil burner warm so if the system fails-over to oil the oil burner is already warm (no condensation).. Curious to see how it runs later this week (installer is shooting for a Friday completion)..

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