Dual Boiler Hookup: Is Parallel Better for Year Around Use?

Yes, but the converse is also true as more zones demand heat the flow slows making the response slower. With individual zone pumps that is not the case. Properly plumbed the zone flows are independent. My choice is to use individual zone pumps because they are about the same cost as zone valves.

There are complex ways to use a single pump for multiple zone valves. Some zone valves have switches on them that indicate their position. That switch contact is used to change the speed of the circulator pump, keeping the the flow more uniform. Other ways include a differential pressure sensor on the supply or primary to set pump speed. Guess what, they all usually cost more than the low cost cartridge pumps?

hehehe, ok thanks. I guess my thought was, size the pump big enough for when all zones are calling for heat. When less zones are calling for heat, the heat to those zones just gets their faster. I probbably oversimplified it, but it looks really good in my head!

I've attached Rev D. I got this idea from the Tekmar d423 controller manual.

Basically, the primary loop circulator only kicks on when a call for heat happens. If it's just a call for DHW, the pump for the DHW loop and active boiler kicks on.

This would eliminate the use of the 3rd pump for summer use!

Thoughts?

I don't think you have the fundamental principle of the primary loop quite right just yet. It's just that, a loop, one with it's own circulator. The capability of its circulator only needs to over come the piping resistance of the loop, let's call it loop P. There must be a circulator in the loop, it can not work without one. Now suppose there are four pairs of closely spaced tee's, Pairs A, B, C and D. We can feed heat to the loop or remove it from the loop via any one of the pairs. Suppose secondary loop A is from a boiler feeding heat into the loop P. When it's suppling heat it's circulator A must run. How else can the heat get to the P loop? If the P loop circulator is not running, the boiler water will just return via the second pair A tee to the boiler return. Nothing accomplished. If the P loop circulator is also running, you are moving hot boiler water around the P loop. Now let's also remove some heat, let's say to an indirect hot water heater piped to tee's B. Turn on circulator B and presto we are feeding heat to it's heat exchanger. Understand this before you go any further.

You can add or remove heat from the loop by turning on or off secondary loop circulators. The primary loop circulator MUST run whenever any other circulator runs. Each of the circulators must only be sized to overcome the friction resistance of it's own loop pipping and the little bit of primary loop piping between the closely spaced tees.

Why do all of this? Because flow around the P loop is independent of any secondary loop circulator being on or off. It just doesn't matter, the only addition flow a secondary circulator contributes to the primary loop is the flow between the closely space tees. This flow independence is an important concept.

Let's suppose we have a second boiler our coal fired boiler that's located in a remote outbuilding. It's connected to tee's C. When boiler C is up to temperature you just turn on circulator C and you are pumping heat in the P loop available for distribution to all the secondary loops. In summer you just shut down the coal boiler and because it's circulator C doesn't run there is absolutely no heat loss through it or the piping to it. Your boiler A, in the house continues to heat your indirect hot water heater. It does not bleed some heat out to the outside boiler.

Secondary loop D could be radiators, low temperature radiant slab, baseboard, etc. Where you inject and remove heat in the loop matters because when a secondary loop is drawing heat it cools it's return water to the loop, leaving less heat for the next set of tees.

As you can imagine controlling and sequencing all these pumps is an exercise in careful thought. The expensive Tekmar like controls put it all in one package but it can be done with simple interconnected relays and time delays. I'm not at all familiar with the high end controllers but I would be surprised it they have examples of coal stokers, where the control logic has to deal with short and long term power outages, some method of coal being fed to an out fire, etc.

Depending on what you are trying to do and how automatic and failsafe you want it a primary-secondary piping solution may not be for you.

Yanche, thanks, I understand what you wrote.

I like the concept of the primary loop with secondary loops hanging off of it. I just don't like the thought of 3 circulators running to provide heat to a single zone.

The modularity of the primary/secondary loop really excite me! It's an awesome concept.

Perhaps when I start to read some of these books I ordered, I will be at peace with the many pumps or perhaps I will decide on a different route!

....................... on an other note.

What size pipes do I use for the primary loop?

I plan on using 1-1/4" black pipe for the primary loop and the secondary loop to the boilers. The TEEs for the heating zone will turn into copper for the baseboard and probably PEX for the radiant (assuming we put the addition on this year).

The only issue I see is that the primary/boiler loop pipes should be well insulated so they don't give off too much heat in the summer time. My cousin ran black pipe from his outside wood boiler acroos the floor joists of his entire house to his oil boiler. He said the heat from the black pipes heated his entire house. (not good when it's warmer out)

To answer the question about what size pipes you need you first need to know the heat in Btu/Hr. want to convey, supply temperature and temperature drop caused by your heat emitters. A lot of heat can be conveyed by a small diameter pipe if the difference between the supply and return temperatures are great. It's likely most residential heating systems with remotely located coal boilers will work well with 1" PEX-AL-PEX feeding a primary secondary loop system. You need to do an analysis.

When I installed my AHS S130 3-4 years ago all this analysis detail was unknown to me. I designed by rule of thumb and conservatism. My garage to house boiler run uses 1-1/2 copper. It's way oversize, likely has laminar rather than turbulent flow of the water, which is not necessary. It was a needless expense. One of the advantages of multiple pumps is the size of the pump only has to be sized to it's specific pipe flow segment. Since even small circulators are more than enough, so you can reduce the size of the pipe to save money.

There are large warehouses heated with radiant heat that use 3/4" copper as supply piping. The very hot supply water (200 deg +) is mixed with return water to get the desired slab water temperature. The resulting boiler return water temperature is also low. So a lot of BTU's are transfered. Pump fast, large water flow and small pipe sizes. Google "John Siegenthaler" and "mini-tube systems"

Use all copper/PEX-AL-PEX and avoid iron pipe. Primary-secondary loops depend on smooth interior walls, iron pipe and fittings are not smooth.

ahhhhh.... I quickly read "Pumping Away" last night and Yanche's statement are becoming MUCH more clear now....

I read some of "Primary \Secondary Pumping Made Easy" also. I will reread both of these again and take some notes!

THANKS!!!!

OK, So I've read, "Pumping Away" and "Primary-Secondary Pumping Made Easy" twice. I finished reading "Hydronic Radiant Heating" last night. One of the examples Dan talks about is putting the indirect hot water heater directly off of the boiler's secondary loop. this way you do not have to heat the entire primary loop for just hot water (summer use).

I've redrawn my piping schematic again to show this. I've also shown what size and what kind of pipe I propose to use on each loop. Now that the indirect hot water is not coming off of the primary loop that eliminates 5 GPM from the primary loop. I may be able to reduce the primary loop to 1", but if my existing baseboard needs 6GPM that only leaves me with 2 GPM for everything else. This may be OK for Radiant and the 6 GPM for the baseboard is for worst case, but I think I would rather stay with the 1-1/4" just to be on the safe side. And I don't think it ever hurts to go 1 pipe size bigger, does it? It's just the cost that increases right? But for the 12' or so of primary loop I need, the cost will not be too much greater.

I plan on using the "pumping away module" in the primary loop which consists of, boiler drain, ball valve, exp tank, air scoop, and circ. pump. It's not completely shown on the drawing.

All comments welcomed!!

EDIT: I plan on putting the boiler pumps on their return side so I will be "pumping away" from the primary loop. This way they will not add any prssure to the primary loop.

Pumps should be as shown on the supply side of the boiler. Putting pumps on the return side increases the boiler pressure, decreasing the margin on the 30 psi safety blow off valve.

Having the indirect DWH as indicated provides no path to an air scoop when only the DHW is used.

Look again at your drawing. Now with the knowledge you have gained

How can you reduce the complexity of your design???

Hint: you have two primary loops.

Here is a re-draw with both boilers integrated into the primary loop. I'm still pumping away from the exp. tank!

I can pump into the boilers as long as I have a low enough system pressure (which I do since I have a 1 story house.) The pump into each boiler would have to produce ~18psi to blow the valve.

Yanche,

what are your thought son this? It eliminates 1 pump and a lot of piping.

Yanche wrote:Pumps should be as shown on the supply side of the boiler. Putting pumps on the return side increases the boiler pressure, decreasing the margin on the 30 psi safety blow off valve.

Having the indirect DWH as indicated provides no path to an air scoop when only the DHW is used.

Yanche,

I thought as long as you has ~12psi system it was OK to pump into the boiler. The higher 18psi+ systems were the ones that you couldn't pump into.

Also, see attached on pg 145-146 of "Hydronic Radiant Heating". Dan shows the indirect HW plumbed in like I showed it above. He doesn't make mention of an air scoop.

I supposed I could put one on the HW loop just in case.

It's desirable to have as high a system static pressure as practical. The minimum pressure that must be maintained on a liquid to prevent it from boiling is called its vapor pressure. For a liquid at a given temperature, cavaitation will occur anywhere the liquid's pressure drops below it's vapor pressure. In a hydronic system when the liquid flows into the pump impeller it experiences low pressure. If the pressure is too low cavaitation occurs which will over time destroy the impeller.

You have an air scoop in your primary loop. Good. But as I understood you proposed summertime operation for DHW production it's pump would not operate. When that's the case there is no path to remove trapped gases in the water. It may be ok, especially if you have a tight system. During winter operation the gases will be removed and the only additional gases will be those coming from the make-up water.

got it. I could modify the rev E drawing to put the cic. pump between the expansion tank and the main circ. as John did here.

EDIT: See the SECOND figure in the link below.
http://www.pmmag.com/articles/84478-an-indirect-q ... ?v=preview

Yanche,

What did you think of that last drawing I did where I put both the boilers in the primary loop?

Thanks again!

Jeremy

What you suggest for the two boilers is fine. I don't feel the detail piping of a backup boiler to the primary loop is all that critical. After all in most cases only one is operating at any one time. For designs where the entire heat load cannot be met with the coal boiler alone and the second boiler will also operate the piping is more important.

The PM Magazine indirect DHW article by John Siegenthaler is a good one. Be sure to follow his suggestions on check valves.

Good job on taking your time and understanding the design principles. You will have a good system.

Yanche wrote:What you suggest for the two boilers is fine. I don't feel the detail piping of a backup boiler to the primary loop is all that critical. After all in most cases only one is operating at any one time. For designs where the entire heat load cannot be met with the coal boiler and the second boiler will also operate the piping is more important.

The PM Magazine indirect DHW article by John Siegenthaler is a good one. Be sure to follow his suggestions on check valves.

Good job on taking your time and understanding the design principles. You will have a good system.

Yanche,

Thanks for reviewing my design! I humbly admit I wanted someone to say "hook it up like x..." but I'm glad I bought these books and educated myself. Now, I have more confidence in this design and I will be more equipped to make modifications!!

Thanks to all for their advise! I listen to all of it!