Gravity Conversion System

Mike T., Swampeast MO

If converted to forced flow, the attic expansion tank is usually replaced with a compression tank near the boiler.

Some indicators:

1) Usually two pairs of mains, one each for supply and return leading in different directions and closely paralleling each other. Smaller homes may have only a single pair.

2) No branch lines smaller than ¾". (At least I've never seen original branches smaller.)

3) Branches serving upper floor radiators smaller than those serving ground floor radiators (with similar size radiators).

4) Supply branches serving upper floor rads taken off the side of the main; branches serving ground floor rads taken off the top of the main.

5) VERY large mains (2½" minimum unless the house is very small) that gradually decrease in size as they wind through the basement. Mains often look large enough to heat a modern elementary school.

JIMBO_2

What is the easiest way to id a gavity conversion system?

Dealing with many older homes with all sorts of re-worked heating systems, what is the easiest way to identify a gravity system conversion? Some folks "claim" that their system was once a gravity system, but it certainly looks like an old, slightly rusted 2 pipe hot water system. What is the easiest clue?

Dan_15

Expansion tank in the attic?

JIMBO_2

Very interesting

This house has 2" supply and return running toward opposite ends of the basement. They tee of right after leaving the boiler. And yes, there is an old drainable expansion tank nestled between the beams above the boiler. And the radiators on the second floor are fed by 3/4" lines rather than the 1" that feeds the first floor.

But, here is my question, now that it has been converted, will it function "normally" when installing a new boiler? It has been working for 50 years the way it's presently piped. The headers in the basement just seem too big for a small house, but they do throw-off nice heat into the basement and to the first floor flooring, as inefficient as that is. Still, I would think the new boiler could be installed right where the old one was piped to the tees and let it flow as before. Right?

kevin coppinger_4

if the person...

installing the new boiler knows what they are doing and uses the right controls it should work even better.kpc

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brucewo1b

Jimbo

A little more than half way through this article it goes into pumping a Gravity conversion.

http://www.heatinghelp.com/heating_howcome1.cfm

JIMBO_2

Controls that are specific to gravity converted systems?

The right controls? Other than outside air reset, I am not sure what controls would assist the system to make it even better. But I am always eager to learn from someone else's experience and know how. What controls would help?

Mike T., Swampeast MO

I won't say these measures are specific only to gravity conversion, but their nature requires some special attention.

The near boiler piping should be reduced by a simple rule-of-thumb. Take 1/2 of the largest pipe size, then reduce one more size. For 2" mains, the near boiler piping should be ¾". It looks funky, but it works. The most common circulators used for residential gravity conversions are a B&G 100 and Taco 007. Both move lots of water with very little head loss. As far as a circulator is concerned, head loss in gravity systems is essentially zero. The only restriction the pump sees is from the reduced near boiler piping and the boiler itself.

While there will be MUCH more water moving than under gravity, heating balance is [usually] OK. If upper floor rads underheat, look for flow restrictors (usually at the hand valve--sometimes at the angle union). If you find them, remove. If downstairs rads overheat, you can make your own flow restrictors (nothing but a non-corroding metal plate with a hole in it about the size of a kindergardner's pencil).

Because so much water is moving, temperature drop across the radiation is very low. That's why it's best to use system bypass piping--much of the flow will "short circuit" right back into the boiler allowing it to come up to temp. Thermostatic bypass valves are available that won't allow flow to the radiation until some temperature (usually 140°F) is reached in the boiler.

Too much system bypass can cause balance problems as there won't be enough flow in the radiation. This happens because of the ****-backwards way that gravity systems were piped. The highest (and longest) branches have the smallest branch pipes, while the lowest and closest to the boiler have the largest. This can cause the water to favor the low, close radss with little or no flow to the high, far rads.

Thermostatic radiator valves (TRVs) are a WONDERFUL addition to converted gravity systems. Not only do they allow room-by-room zoning (without having to change the piping, add circulators, add wall thermostats, add zone valves, etc.) but they allow the flow to be MUCH more similar to what happened under gravity.

The gravity systems were originally used solid fuel. This allowed the size of the fire to vary by adjusting the draft. More draft meant a bigger fire. A bigger fire meant higher temps which in turn meant faster circulation. A smaller fire had the opposite effect. While FAR from automatic, these old systems had reset and modulation!

With TRVs, reset and preferrably a condensing/modulating boiler the system flow rates will be VERY similar to the original. The colder the weather, the hotter the supply, the faster the flow and the higher the delta-t. All this allows for EXCEPTIONAL efficiency when using a condensing/modulating boiler.

JIMBO_2

Thank you for the insight

I am ready to install an Ultra-80 at this point, heat loss figures to be about 50K BTUs. The old boiler is a Weil-Mclain P-OB18, MBH= 100.5, shipped in 1956. It's almost funny to hear you mention the TRVs, because a few years ago I brought home a dozen Danfoss 3/4 & 1" we had left over after doing a public school. Regretably, they're down in the basement collecting dust. Never got around to it. Originally, a few weeks ago, I had resigned myself to instally a WM CGs-3 to avoid all the fancy-dancy stuff with the con/mod units. It seems that the deeper I dig into this, the more complex it's becomming. I am now fairly certain this is a converted gravity system, but some of my friendly plumbers have said YES, while others have stated NO. After closely looking at the heel-tees that feed the 2nd floor off the run, in 3/4", and the 1st floor out the bull, in 1", I am pretty much convencied.

Mike T., Swampeast MO

The 2" mains [seem] a bit small for a gravity system, but with two pairs in a not-so-large house I can definitely see it working. I doubt that even the dead men liked working with larger pipes and may well have preferred using two pairs of smaller mains over one larger pair--even if it technically meant more pipe.

Larger branches to the ground floor rads compared to the upper [seems] a dead giveaway. I can't imagine anyone doing that if the system originally used forced circulation.

If you use the TRVs and the Ultra-80 (or other mod-con)--HIGHLY RECOMMENDED--here are some suggestions:

1) Install the TRVs on ALL radiators.

2) No need for a wall thermostat unless you want a simple way for WHOLE HOUSE setback via "starvation". Done this way, set the wall thermostat HIGHER than any temperature you want to maintain in the house. This produces a continuous call for heat.

3) If you don't use a wall thermostat, just jump the T-T connections. I know it sounds strange, but I've talked with W/M and other boiler mfgrs about this sort of system and there's no problem. This works for "American" mod/cons--not the Vitodens (has no thermostat connections) or the Buderus GB (use the RC-10 with a TRVd system).

4) DISABLE any "boost" setting on the boiler. Default behavior of the Ultra and other "American" mod-cons is to boost the supply temp by some amount after some length of time of a thermostat call. Remember--with either a thermostat set higher than needed or the T-T connections jumped you are intentionally producing a CONTINUOUS call for heat! The last thing you want to happen is for the boiler to continually boost the temperature! You want the system to work COMPLETELY via the temperature available from the reset curve.

5) For the secondary (radiators) circulator, you will need VERY little flow volume. With a design loss of 50 mbh you'll NEVER need more than 5 gpm! Actual flow will likely be significantly lower. The TRVs will handle the flow for you, but you can get noises and excessive wear if the circulator is trying to produce higher flow. A Grundfoss "Super Brute" on minimum speed will be more than adequate and preferrable to the more common B&G 100 or Taco 007 typically used on a gravity conversion. If there's such a thing as a low-volume, low head circulator, US IT!

6) The secondary (radiators) circuit MUST have a differential pressure bypass valve. It simply connects between the supply and return of the secondary circuit AFTER the secondary circulator. It's function is to give the water some place to go (in a circle) in the event that all of the TRVs are closed. Without, the circulator can "dead head" and overheat.

kevin coppinger_4

controls...

Mike given some good thought further down...one contol I have used w/ good sucess in the Buderus 2107 w/ the room senosor...very happy customers...TRv's definately help. I have found that the Grundfos 15-58 is great...don't be surprised in spd! is all you need you are trying to mimic gravity flow...more/excess flow is bad. In tech topics Steamhead has a chart to help in picking out the right circulator...kpc

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JIMBO_2

Thanks, again

That sounds very interesting, Mike. I have 7 fairly good size rads that have been here forever. About 25 years ago, my father had a "plumber" remove a radiator from the kitchen and bathroom which were being remodeled at the time. They added 10 feet of CI baseboard in the kitchen and 2 feet in the bathroom. On the whole, the house is definitely over-raiadted, especially considering the added insolation, storm windows, and general tightening. I never really thought of constant circulation, though it was mentioned in the Ultra paperwork sent to me. It iss always nice to have options. Ok, here it is in the primary/secondary piping guide. They show the supply leaving the boiler, imediately teeing off (looking like a bypass of sorts) with a flow control valve and balancing valve on both sides . . . and the circulator must run continusly. Where's the noose????? I'm about ready.

Mike T., Swampeast MO

With All Deference to Steamhead

If you're using TRVs, I'd cut those flow requirements for gravity conversions in half if not by a third or a quarter.

If not using TRVs and a conventional boiler, then the recommendations are very reasonable--just be CERTAIN that you use system bypass! Such is the ONLY way to ensure that the boiler operates anywhere near it's best efficiency.

A thermostatic bypass valve similar to the Danfoss ESBE type TV [seems] close to ideal. It FORCES the boiler to come up to temp (140F) before allowing flow to the rads. Of course a significantly oversized boiler (comparing net output to a Manual-J heat load calculation) will mess with anything you try to do regarding efficiency unless you add mega buffering.

JIMBO_2

Okay, I like it so far

But, Mike, I do have one concern that "she," the Boss, has raised. Normally when no one is at home, during the day, the thermastat was lowered to save energy consumption. Even at night, for sleeping, the temp was lowered to the low 60s. How would that be accomplished with this new system of a con/mod boiler, TRVs and constantly running circulator? As she says, Why heat the house when there's no one home? Is there a happy medium?

Mike T., Swampeast MO

Kitchens & Baths with Gravity

When the original systems was installed there's a very good chance that NO radiator was installed in the kitchen.

Why? BIG IRON SOLID FUELED COOKSTOVE! The FIRST job for the wife or maid was to build the fire in the cookstove. After the biscuits were baked and the husband off to work and the kids off to school, chores or play, she'd tend the boiler fire. Then "air" the house, do her housekeeping chores--including daytime entertaining (JUST ANOTHER CHORE)--then build the fire nicely so that hubby arrives home to a COZY space after his walk or trolly ride.

I give this aside only because you need to understand the the CONTEXT of gravity hot water heating systems.

BTW, in warmer climates nearly everyone had a "summer kitchen" on the back porch. Usually fueled by "coal oil". The cookstove [might] have been fired once a week for baking...

As the generations progressed things changed.

The cookstove went bye-bye. Can you blame anyone?

To be continued...

kevin coppinger_4

that is why...

you have trv's...you can adjust as needed...if you go w/ good outdoor reset controls you will save a lot of fuel...kpc

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Mike T., Swampeast MO

What You're Really Talking About Is...

...Setback

A topic DEAR to my heart...

Mike T., Swampeast MO

While difficult if not impossible to prove, I sincerely feel that the savings from setback are an illusion in many cases. The enemy is time. Spaces don't magically and instantly jump between temperatures just because you change the thermostat. The better your insulation/weatherization and the more moderate the weather, the longer it takes for space temp to fall. If you're using a significant amount of setback regardless of the weather conditions, there's a very good chance that you never maintain the lower setting for any length of time.

The savings from setback only begin once you're maintaining a lower setting. Otherwise it takes just as much (if not more) energy to raise the temp back to the "normal" setting. It CERTAINLY takes more with a condensing/modulating boiler because of their very nature. The efficiency of a condensing/modulating boiler increases as BOTH output and supply temperature decrease. (The efficiency difference between output levels is usually not much, but it is there.) The efficiency difference with supply temperature however, is quite significant.

The supply temperature required to maintain a space is automatically the lowest and thus the most efficient with a mod-con. To raise space temperature the supply temp MUST be higher. Again, time is the enemy. The faster you want to raise space temperature the higher the supply temp must be above that required to maintain space temp. It's really not out-of-line for a mod-con to have to operate at both maximum output and the maximum possible supply temperature to satisfy such a demand--particularly if it's for the whole house and you want quick heat-up.

Time plays yet another roll and it has to do with mean radiant temperature (MRT). When you raise the temp of a space a great deal of the initial energy goes into convection--yes, even with "radiant" systems... The increase of MRT lags significantly behind the increase of air temperature. Say the house dropped down to 62° overnight and you crank the heat in the morning to get it to 70°. Even if you've used a "smart" setback thermostat to have the house heated up to the thermostat setting right before you wake, the house will not "feel" like it's 70° because it takes significantly more time for the MRT to rise. It's fairly well accepted that it takes multiple solar cycles to achieve a balanced MRT!

In your case where you're likely using a quick morning heat-up followed by daytime setback, followed by another quick heatup in the evening then nighttime setback, you're likely never achieving a stable MRT. While the air temp may be at your high setpoint, I can nearly guarantee that the house "feels" cooler.

With the TRVs, you can set each space at the desired temp. If you have rarely used rooms, keep the doors shut and turn the TRVs down as low as you want--that REAL setback that TRULY saves because it's nearly constant. As long as you have at least decent insulation/weatherization and you're also using a mod-con, I sincerely believe you'll be just as (if not more) comfortable if you maintain a constant room temp somewhere between the "low" and "high" you're using now. As long as you adjust the reset curve such that you're barely maintaining the setpoints on all of the TRVs, I also believe that your fuel efficiency will be maximized.

If you go away for days, set everything down--again--that's the type of setback that REALLY saves.

If you insist upon using daily setback, there really are better ways in such a system. It all has to do with supply temperature--done correctly you don't even need to adjust the TRVs! If you want the house to "feel" warmer for some period, increase the supply curve by 10° or so. (This is easy with some mod-cons, not so easy with others.) While TRVs are quite precise, they're not perfect. If your "normal" heating curve (and I consider "normal" to be the REDUCED curve) is just barely adequate, raising the curve will result in slightly higher room temperature (if you wait long enough)--without adjusting the TRVs. More importantly, with the big cast iron rads of a converted gravity system, you'll "feel" warmer before you can measure the difference in air temperature! I cannot fully explain this, but I can CERTAINLY feel it!!! It really seems to center around radiation and MRT. While the average temperature of the radiator may stay unchanged, I believe its level of radiation increases--in other words it's slowly increasing space temperature mainly by raising MRT--not by mainly raising the air temperature! You'll "feel" this long before you can measure it!

George_25

two pipe system

My system has a large pipe and a smaller return pipe.
Is this normal for a steam boiler?
Why do you need a return pipe...couldn't the condensate just return to the boiler from the same pipe that it rises in?
Thanks

brucewo1b

George

My system has a large pipe and a smaller return pipe. Is this normal for a steam boiler? yes

Why do you need a return pipe...couldn't the condensate just return to the boiler from the same pipe that it rises in? yes, but also needs twice the pitch back and you need even bigger pipes so the steam doesn't pick up water on it's way north as the water is heading south.

Your Welcome

George_25

return pipe

Thanks for the info....do you know of a company that begins with W that made coal fired steam boilers about 1910?
George

JIMBO_2

Thank You, indeed

Mike, that was truly interesting. I really like the TRVs, we use them a great deal. Granted, a few rooms, with the CI baseboard, I cannot install because there is no room. On 2 rooms, i can install the valve body below the floor, in the basement, and run the temp sensor up into the desired room. As it presently stands, we have learned to live with more "setback" time than higher temps. Since no one is home during the day, we tend to leave the stat at about 63 degrees. When we all arrive home, we kick it up to 66 or 67. When bed time arrives, it gos back down 4 or 5 clicks. It's not ideal, but we've gotten used to it.

Now, considering the function of the Ultra, mod-con, I will no doubt have to alter the way we heat the house. Aside from cost, this is one of the reasons I was considering the Weil-Mclain CGs-3. It's a cast iron thing, I guess, simple in function, yet still 85% and EnergyStar. To be perfectly honest, I've been beating the heck out of myself trying to make the final decision and just DO IT. My head is spinning like a cartoon. HeLp . . .

rav3 heating

gravity conversion

Heres one of my gravity conversions with the wonderful 93%EFF. Lochinvar:-)