Munchkin MC80 & Gravity Tee System

Ted G

Make sure the boiler gets 8gpm.

M. Hase

Munchkin MC80 & Gravity Tee System

I am replacing an old American Standard HW Boiler with a new Munchkin MC80. My question is with the existing gravity tees in the system and how to get the most out of the new boiler. As it presently stands the supply off the boiler comes out the top of the boiler and is routed to (9) hot water radiators throughout the house back to a inline pump that pumps through the boiler. There are no flow control valves on teh system. The main is sized at 1-1/4" throughout with 3/4" branch lines with the gravity tee on the "return" side of the radiators.
What is the best way to pipe this system without repiping the system to add a return pipe to a convert it to a true primary/secondary? There is approximately 9" of elevation difference between the supply (low) and the return (high) at the location of the boiler. I'm envisioning utilizing the small boiler pump for the boiler and spacing two tees close to one another then adding a pump for the radiator loop. I'm concerned however given the present elevation difference in the piping configuration that I'm opening myself up to problems at the closely spaced tee locations. I do not wish to add any control valves to the system at this point if I can avoid it. I have no experience with a mono-flo tee system & a condensing boiler so any help in this matter would be greatly appreciated. Thanks

Mike T., Swampeast MO

To my knowledge, there's no such thing as a "gravity tee". The term you're looking for is "diverter tee" or "monoflow tee". These devices are used in one-pipe systems and their purpose is to divert a carefully engineered amount of water through each radiator while allowing the majority of the flow to continue through the main. While I've heard tell of one-pipe gravity systems, I can guarantee you that the main size would be larger than 1 1/4".

You seem a bit confused regarding the nature and purpose of primary/secondary piping. The purpose of this is to separate primary (boiler) flow from secondary (emitter) flow. This is done by creating two separate loops that only meet between closely spaced tees. Piped this way, flow through the secondary loop(s) cannot interfere with the primary flow and vice-versa. It has NOTHING to do with "adding a return pipe to create 'true' primary/secondary." In one-pipe systems supply becomes return as it progresses through the main(s)...

As you suggested in your system you would use one circulator for the boiler loop and one for the radiation. If your current boiler is cast iron, it was essentially considered to have zero head loss and your existing circulator--provided it's appropriately sized--would be moved to the secondary loop with no affect on flow through the radiators. Copper tube boilers do have head loss that must be considered.

Primary/secondary piping in the form described above (closely-spaced tees) is really about keeping return temperature UP. This is the OPPOSITE of what you want with a condensing/modulating boiler where the objective is to keep return temperature DOWN.

For this reason, whenever hydraulic separation is required* with a condensing/modulating boiler it is best to use a device called a low-loss header. It performs the same hydraulic separation function of primary/secondary, but it utterly guarantees that return temperature will be as low as possible.

Your message almost leads me to believe that you're a homeowner who wants to install a mod-con into an existing system. I can't fault you--I'm one myself--but I will tell you that such requires some SERIOUS study and you'll need to make GREAT modifications to the near-boiler piping. Your concerns about "elevation differences" make me think that you just want to throw in a mod-con. Do so at your own risk and when things screw up expect to pay a premium to have it done properly.

*Condensing/modulating boilers can definitely be installed without primary/secondary or a low-loss header--even in a one-pipe system, but you will have to COMPLETELY reverse engineer the system to ensure proper flow rates and even if possible you may not be able to find an appropriate circulator.

Some here call gravity systems "the most highly engineered." I find them first nature and supremely simple. One-pipe diverter tee systems, to me, are the most difficult to engineer.

BTW, two-pipe systems (separate supply and return connections either at each rad or to central manifold(s)) are greatly preferred with new systems designed for condensing/modulating boilers. Such boilers work fine--when properly installed--with one-pipe systems, but you cannot fully realize their energy saving potential.

M. Hase

Mike,
Thank you for your time to respond to my question. I probably should have made myself a little clearer on the "adding a return pipe" to the system statement. If I were to have to do so I would utilize the exisiting piping for the new supply, eliminating the mono-flo tee on the current main and then pipe the return branch from each radiator into a new return main. I would have to some sort of flow control device at each radiator in this situation. Given my present situation I am looking to eliminate this additional cost if I can avoid it. I don't plan to be in the house any longer than 3-5yrs and the present boiler is toast.
You did lose me a little on the primary/secondary and the no loss header. The munchkin manual shows a primary/secondary system as the preferred piping method in their IOM manual. Closely spaced tees with P1 serving the boiler and P2 serving the heating loop. Any idea why they don't suggest a no loss header like you described above or am I miss interpreting it? I did a little digging around and came across the Caleffi 548 Hydro Separator which seems to fit the bill from what you have described. I'm trying to come to a compromise between keeping cost down yet making sure the boiler operates as designed even if that means not reaching the boiler's maximum efficiency. I've attached a couple sketches to help clear up what I'm dealing with. The first is the existing setup, page 2 is the piping arrangement I intially was looking convert too, the 3rd page would be the use of the Caleffi 548 in lieu of the closely spaced tees shown on page 2. Once again thanks for you help & input it is greatly appreciated.

Ted G

Munchkin perfered drawing

Warning: In the Munchkin Preferred piping diagrams, P2 is not to be electrically connected to the P2 wiring terminal in the boiler. Notice; there are no wires drawn from the P2 to the Munchkin. In these drawings, P2 refers to that circ as the 2nd pump. The P2 connections on the Munchkin are for using Vision 1 to control an indirect water heater using the priority feature of the Munchkin. Also note, P1 and P2 will never run at the same time. If there is a call for heat on the TT connections, P1 will energize 110volt to power the P1 circ. P2 will not energize.

If there is a call for DHW on the DHW sensor terminals, P1 is de-energized and P2 is energized with 110volts. This is how the Vision 1 works for DHW. Please refer to drawings that say Vision 1 in the title.

For your drawing to work, you would just use the P1 terminals and use a separate pump relay to control your monoflo loop circ.

Refer to Munchkin Contender drawing 1C

Mike T., Swampeast MO

Condensing/modulating boilers are fairly new and are a SEVERE departure from the "normal" way of thinking--both on this side of the pond and the other. BTW, North American companies assemble mod cons--and nobody on this side makes the heat exchangers.

Primary/Secondary is the simplest and least expensive way to achieve hydraulic separation as long as you follow the VERY simple rules. I believe this is why many mod/con manufacturers illustrate such in their suggested, simple application drawings. Low-loss headers or "hydro-separators" do the exact same thing, but they will add to material cost.

In a very basic system like yours with one secondary (emitter loop) operating at a FIXED flow level and one primary loop (the boiler) also operating at FIXED flow level, either primary/secondary or a low-loss header will ensure the lowest return temperature to the boiler. Low-loss headers do however offer two very nice features that can be quite useful. You'll get an air bleeder (often just where you want one) at the top and the ability to trap debris at the bottom. The body is a comparative HUGE spot in the road where velocity is very low. Particles of any size will fall to the bottom where they can likely accumulate for years without any problem. They'll offer a way to "drain" the sludge, but something tells me that most will just forget about it... You should CERTAINLY install a wye strainer into the supply to the mod-con and I would also suggest that you drain and flush your system--under constant circulation a few times before installing the boiler. That's the kind of thing that homeowners can do...

In that vein, please be aware that mod-cons REQUIRE combustion analysis upon installation. Not only does this require the proper equipment, but it requires the understanding of how to read and how to produce the proper conditions for an accurate reading. If you're "just a homeowner" I highly suggest that you enlist the service of a pro BEFORE you attempt to DIY.

p.s. Your illustrations are nice. Now add actual length of the main, and the exact diverter tees used (they're tagged) and EDR of each radiator.

M. Hase

Gentleman,
Thank you for all of your help & suggestions they have cleared up a lot for me. I have enlisted the help of an union service tech friend for the startup itself and I do have the help of a local pipefitter & electrician for the main piping & electrical work. Gas, combustion air & exhaust, and condensate piping are in. I will let you know how it goes.

Kool Rod

I'm not sure I understand why your looking to make a primary/secondary system out of the one pipe system. Is there something else your looking to add in before the new diverter t's?

In general with diverter T system would it not be best to just pipe it as it was to the new boiler? Is there a special need with a mod/con to make it primary/secondary?