so maybe I can't use Munchkin on my conv gravity house?

Plumbob

An 80MBTU Munchkin is big enough to heat our house. However, if I understand correctly, there is a flow problem. There is an excess of radiation, so according to an article elsewhere on heatinghelp: <a href="http://www.heatinghelp.com/newsletter.cfm?Id=125">http://www.heatinghelp.com/newsletter.cfm?Id=125</a> we need about 20 gpm in our converted gravity zone. We have a second zone, new construction, that uses less than 5 gpm. So the primary flow needs to be ~20gpm.

Problem is, the 80M Munchkin has a nasty head-vs-flow curve (figure below). HTP didn't even bother to measure it above 10 gpm, but if I guesstrapolate it to 20 gpm, there is about 35 feet of head at 20gpm just due to the boiler!

To overcome that, and get >20gpm @ 35 ft head, I would need a very big primary pump (say the B&G PL-55, 2/5 hp). In other words, in order to have a small Munchkin we'd need to overcome a very high head, and we'd be stuck running a huge, power-hungry and maybe noisy pump all day.

Question 1: Am I doing this right? The conclusion would be, one can't efficiently use a Munchkin in a house needing high flow because of gravity conversion, even if the BTU output is matched to the house, because the electric bill would go up. Or is there a way around this?

Question 2: Do all low-mass boilers (Trinity, MZ, Baxi, Vitodens) have such high head? Is the Munchkin boiler made of capillary tubes?

Mark J Strawcutter

use primary/secondary piping and pumping

Size one pump to the boiler loop, the other(s) to the system loop(s)

Mark

ScottMP

Marks Right

HTP requires this anyway.

A Munchkin is a perfect fit for an old gravity system !!

Scott

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Ted_9

Exaclty. And in Primary Pumping, yo size acording to that loop. So the result should be smaller pumps.

Plumbob

of course, but problem is unchanged

Of course; if you look at my original post, I did say "primary pump".

The primary pump may not be the pump that is directly sending water into the gravity zone, but it still has to move as much water as is needed by all the zones (since water arriving at the tees = water leaving the tees).

So it has to be a big, power-hungry pump because of the resistance of the boiler and the large quantity of water needed by the gravity zone.

There is no problem with the zone pump...the gravity zone has very small head. I am talking about the head due to the boiler, and therefore I am talking about the primary pump.

Summary:

Gravity zone: negligible head, but need flow 20gpm (see article whose link is given in original post)

Primary: 35 ft head (see 80M Munchkin graph and extrapolate) because if gravity zone needs 20gpm, primary must have ~20gpm.

Again, I am an amateur, so if I am missing something, please clarify.

Plumbob

> HTP requires this anyway.

>

> A Munchkin is a

> perfect fit for an old gravity system !!

> Scott

>

> _A

> HREF="http://www.heatinghelp.com/getListed.cfm?id=

> 237&Step=30"_To Learn More About This

> Professional, Click Here to Visit Their Ad in

> "Find A Professional"_/A_

paul lessard_3

I see no problem

"but it still has to move as much water as is needed by the zones" . Change the word "water" to btu's. Think delta T

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Plumbob

I see no solution

Yes, the primary pump needs to move as many gpm, and also as many BTUs as the zones.

So if the gravity zone needs 20gpm, are you saying the primary pump can move less than 20gpm? Where will the rest of the water come from?

If you are saying the gravity zone doesn't need 20 gpm, well, this is the heart of the matter. See the link to a heatinghelp article in the original post. I asked about this in another thread some weeks ago, because the flow rates in that article are way more than required for ΔT = 20F.

In that thread, people agreed that gravity zones need much higher flow than ΔT=20 would require, because otherwise there would be no flow to the second floor or even to first-floor radiators that were far away; all the flow would go through the shortest path since the pipes are so big. Dan Holohan also said this, obliquely, in a recent thread . Quote: "Gravity systems have different needs."

Dan Peel

Solution

Move the boiler in your piping. You seem to see the boiler as part of the primary loop. It won't be. Install the munchkin with it's required circulator to feed in and out of the primary loop on a couple of closely spaced tees. You can use a 2" primary loop for your 20 plus gallons of flow and use 1" supply and retu.rn to the sidestreamed munchkin. Enjoy...Dan

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DanHolohan

Which i exactly what I would do.

Darin Cook_3

Just curious

Where did you come up with those numbers for the heating system? When it was gravity it did not move at 20 gpm. Take the size of the gravity main decrease it by half, then drop one pipe size. Then go with the circulator required by the boiler which in this case is a Taco 007 or a Grundfos 15-42. Unless this is some really wacky job this little formula will work. This piece of information is from Mr. Holahan also. It has never failed me.






Darin

Mike T., Swampeast MO

Flow in a Proportional Gravity System

You're forgetting that you MUST decouple a wide-open gravity system from the boiler.

The key concept there is WIDE OPEN. When wide open there is no pressure loss at any reasonable delta-t.

Say your gravity portion looses 50,000 btu/hr at design. At 20 gpm flow delta-t is only 5°!!! But you NEED this high flow on the radiation side to ensure complete flow through a system where the highest, farthest radiator likely has at least 2½ times the flow restriction of the lowest and closest. Remember--EXACT opposite of a system designed with a circulator in mind.

You DO NOT WANT your modulating, condensing boiler to only have a 5° temperature gain!!!!! If fact, there's no way you'd ever do this with any reasonable pump given the flow restriction of the boiler.

You MUST decouple the gravity system from the boiler. You do this with primary-secondary piping. Here your primary is the boiler--your secondary is the gravity loop. You make your primary (boiler) pump move MUCH less water than the secondary...

How much less???

Look for post titled "Flow in a Proportional System".

R. Kalia_3

that's it! now another question...

That's it. Makes sense. Thank you. That gets the BTUs from the boiler, but doesn't send the entire flow through the boiler.

My problem was that I thought all water had to be heated by passing the boiler. In fact it does not. I also thought "primary loop" and "boiler loop" were synonymous.

So now I have another question. Suppose I want completely continuous circulation through the radiation. Of course I will have outdoor reset, but still there will be times when the thermostat stops calling for heat.

What if we have the thermostat stop the boiler and boiler circulator, while letting the zone circulators run? That way, no heat is being added to the water going into the radiation, and the water will slowly cool, but it will never stop. This would be like the plans mnetioned by Siegenthaler
in his article Zoning with constant circulation
where the idea is to "Stop the heat, not the flow".

Would this work or did I miss something?

R. Kalia_3

> Where did you come up with those numbers for the

> heating system? When it was gravity it did not

> move at 20 gpm. ,BR.

I provided the link in the original post. It is an article here, on heatinghelp.com.

I understand the solution now (see above). I am slow but I get there.

R. Kalia_3

thanks!

> You MUST decouple

> the gravity system from the boiler. You do this

> with primary-secondary piping. Here your primary

> is the boiler--your secondary is the gravity

> loop. You make your primary (boiler) pump move

> MUCH less water than the secondary...


Makes sense now, many thanks!

Dan Peel

Yes BUT

In practice, for your converted system, why chase 20 GPM for no purpose? Once the demand is satisfied I'd stop the circulation and save the electricity. With radiant floors I much prefer constant circulation, not so much with rads. Enjoy.... Dan

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R. Kalia_3

two reasons

Two reasons:

(a) the usual arguments...lower noise, smaller temp fluctuations. Remember that with outdoor reset, the pump will be running most of the time anyway, the change from most to all is not a big increase in the electric bill.

(b) when you have a lot of warm water in those big gravity pipes, continuing to circulate the water even after the boiler is off saves some energy, by delivering more of that residual heat to where it is needed rather than to in-wall spaces and basements.

Mark J Strawcutter

ditch the thermostat

run the system loop on continuous circulation with outdoor reset and add TRVs to each of the radiators. Don't forget a differential pressure bypass for when all the TRV shut.

Mark

R. Kalia_3

too expensive

Too expensive...our contractor will charge $350/TRV and we have a good number of them. I agree that this would be a better solution in principle.

Mike T., Swampeast MO

Do the \"grunt\" work

of disconnecting the radiators and removing the old supply-side spud and/or bushing with spud and you'll probably bring that cost down considerably without offending anyone...

Guy_5

Thank You

You folks are good. I can keep my job, right??

Guy

Mike T., Swampeast MO

Really don't need to keep it running.

(a) Gravity systems are known for their absolute silence--even when converted to forced flow. About the only noise you could hear is harmonic resonance from the circulator. If you ever hear such, first look for loose main pipe hangers, then check impeller for damage, then if obnoxious, do something to change the piping and/or circulator (and thus the resonant frequency).

(b) Just let the mass of the system and the ability of the radiation do its job. There will still be a small amount of gravity flow and the heat already favors leaving the system via the radiators. As the burner learns to modulate to keep thermostat calls as long as possible (given the supply temperature and flow) off time will be as slight as you can make the "headroom" in the reset curve. On a full-flow gravity conversion you might have to keep reset temperature somewhat higher than necessary because the closer you get to "perfection" of heat delivery, the more any imbalances in the radiation will magnify.

RS_2

Condensing Boilers & Constant Circulation

I heard that a three way zone valve is key to Zoning With Constant Circulation. I have a few questions. Please feel free to put your answer with any comments right next to my question since this would be faster for you.

1) Is it a fact that this is a more efficient way to heat i.e. considering gas burned by the boiler and electricity consumed by the circulating pump will it cost less to operate?

2) How reliable is a three way zone valve? Who's a proven manufacturer of them and can anyone tell me if they had any problems with them?

3) How easy/difficult is it to troubleshoot?

4) Has anyone utilized this with a high efficiency condensing boiler employing baseboard heat? If yes, which makes and models?

5) I've heard from a tech at Buderus that using a condensing boiler with baseboard lowers it's efficiency because baseboards require a higher temperature water which exceeds the "condensing point" of a condensing boiler. Is this true or false?

6) Does anyone know if it is cost effective to use an outdoor reset with constant circulation on a condensing boiler? Which is a bigger money saver, the outdoor reset or the constant circulation?

THANKS!

Bill Nye_2

RK

Yeah but, that chart is for 180° water and a 10°^T. With the Munchker you want way more ^T. like 20 or 25° the lower the return the better, more condensing.

Somebody way smarter than me needs to explain this for you. You DO NOT need the 20 gpm. You need a system pump and a pump that can pump the boiler

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Wayco Wayne_2

I came late to this thread

but using a Munchkin with an old gravity system is one of my favorite things to do. I use constant circulation with a Grundfos 4 speed and use the lowest speed. The gravity system had way low velocity and GPM generated only by the pressur of hot water rising. I use outdoor reset since the old radiators when sized for gravity end up being oversized for a circ driven system. Sometimes by as much as 300%. This means you have great big heat emmitters and can get by with lower water temps which means more chances for condensing and higher savings with the Munchkin. I have 2 systems in DC that saved one Homeowner 50% on previous heating bills. The other was an oil conversion to gas and they spent only 30% of what they had spent before. (oil being historically more expensive than gas in this area) I was able to get by with 150 degree water on the reset curve when it was 10 degree outside temps. On one house I had to tweak it up to 155. I see no problem with constant circ. The circ on low speed uses very little electricity, and provides beter uniform comfort. Learn to put in your own Thermostatic radiator valves. They dont cost much then. Good luck. WW

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Ron Schroeder_3

Well, since no one else has responded to this question

Let me take a stab at it from our perspective.

We too use three way valves with condensing bolers but not as a zone control device. We utilize the three way valve primarily as a comfort/control measure as the mixing valve can achieve much tighter supply temperature regulation than the boiler is designed to.

We want the boiler temperature to fluctuate as widely as possible so as to minimize the start stop cycles and to be able to draw down the boiler temperature as far as possible with maximum turn down in view. We want the supply water temperature to be as tighly regulated as possible to avoid discomfort.

Our three way valves are as reliable as our four way valves and both are operated by the same motors. Both carry the same warranty and repair parts are stocked for both. These are not throw away components.

I cannot asnswer your troubleshooting question directly because I do not know what you mean. Please clarify.

Yes, we have attached our condensing boilers to existing fin tube base board heating systems and that with gratifying results. Fuel savings in excess of 38% are being reported.

Perhaps the tech you spoke to has this experience with his product. All condensing boilers achieve their best efficiencies while they condense but not all condensing boilers condens equally. Baseboard emitters do not require high supply temperatures all year long. Many baseboard emitters never require the high supply temperatures that they are commonly fed with that is why they need to be controlled with zone valves. So the answer to question 5 is, "Well, that depends."

It is always cost effective to use outdoor reset. With high mass heating systems it is always advantageous to use constant circulation. With many low mass emitters it is advantageous to use constant circulation. Outdoor reset saves the most fuel, constant circulation improves occupant comfort.

Shalom.

Steamhead (in transit)

Darin, that article

which I wrote, essentially re-stated the principles of sizing circulators for gravity conversions as originally researched and published by Bell & Gossett and Taco in the 1940s and 1950s. Back then they didn't have the wide variety of circs available that we do now, nor did they have cast-iron packaged boilers where they just throw on whatever circ is cheapest. So I wrote it in such a way that you can use it with any manufacturer's circs. And before I shared this with anyone I verified that it worked- on as many different sizes of converted gravity systems as I could get my hands on, including the one in my own house.

The only thing a circ does on a gravity conversion is move the water thru the boiler or in and out of the primary loop on a P/S job. The system will circulate itself quite well, as designed, so it doesn't need much help.

The method of reducing pipe sizes that you describe was once about the only way to approximate the gentle gravity flow in an old system. But I've never liked the idea of throttling down a large circ when a smaller one which is less expensive to buy will do the job on less electricity. So now that we have smaller circs, it makes sense to use them. I believe the method in the article is more exact than playing with pipe sizes.

If you read the article, you will see that the Dead Men who converted these systems knew they had to take care of the extra water in the large pipes of a gravity system. This is why the GPM listed for a gravity conversion is higher than that for a more modern system.

With some boilers of which the Munchkin is a good example, you must maintain a certain flow thru the boiler. This is the reason they specify certain circs for these boilers. The difference between the boiler's flow characteristics and those of the system is rectified by the use of primary-secondary pumping, as stated earlier in this thread.

But the usual cast-iron boiler comes, as I've said, with whatever circ is cheapest. This may or may not be a good match for the system you're installing it in.

Try this: Next time you're in a small rowhouse with a gravity conversion whose boiler has a B&G 100, Taco 110 or similar- or even the ubiquitous Taco 007- check the delta-T across supply and return mains with an infrared thermometer. If the system is properly bled and all rad valves are open, you'll find almost no delta-T, which begs the question: Why is the boiler not adding much heat to this water, and why is the heat not being shed in the radiators? The answer, of course, is that the water is moving too fast.

Then change the circ to one that will give you the flow at the head specified in my article. It's easier to do this than to repipe the near-circ piping. Watch how well it works.