Gravity system questions

Steve_190

I am looking at replacing the boiler in my 1910 home. The current system is a converted gravity system with a 1960's Crane boiler. The house is two stories with a fully finished attic. I currently maintain about 20 psi pressure in the boiler. There are only two zones in the system. One being only two radiators in the basement and the other being the rest of the house. All radiators are cast iron.

My quetion is, would I be able to pipe the Prestige directly using the included Grundfos 3-speed UP15-58
system circulator? What other modifications would be required to use the Prestige (or any other mod-con) in this application?

Unknown

C 1920 system conversion

I have a call to look at a conversion from the original boiler to I would guess a mod con, more than likely a TT Prestige if I can get one. How are you guys doing these? I'm a little leary of introducing problems during the process of adding active circulation to such an old system. One thought I have is to connect the supply and return together and tee in the boiler so that active circulation can take place in the new piping and allow gravity to then work as it always has throughout the rest of the system. I'm afraid if I start forcing water through these old pipes I'm going to start seeing leaks developing over time.I assume I'll have to remove the expansion tank in the attic and replace it with a bladder type in the basement. Also he wants to remodel and eliminate some radiators and replace them with something more modern. I know we don't like to mix copper baseboard and CI radiators but I don't recall why. Any and all input and experiences are welcome. Thanks!

Unknown

FYI..

Left The Wall and went to resources and did a quick read on the gravity systems. It brushed a lot of the dust off the course I did with Dan some 10+ years ago. Including the mercury story and thw whole thing about the restictor plates. I also have the answer to why we don't mix old with new. Tricky stuff!

My thought is to connect supply to return, twice if there are two of both, and then tee into the vertical piping to heat the water in the risers and allow it to flow by gravity. OR might the advanced age of the system mean it's too rough in there now for gravity to work effectively anymore?

marco_7

best way?

its a lot more work, but personally i feel the best way is to cut out all the old pipe where you can and go to a home run system;virtually guarantees good flow. with pex/pex-al or what have you, its not as labor intensive or expensive as you might first think.

Unknown

So,,,

my leak concerns are valid? Or are we talking strictly about flow issues? Seems to me a 90 yo iron pipe system that has been open to the atmosphere all these years would be prone to start leaking sooner or later. When you say home run you mean take each radiator to a manifold in the basement separately?

Unknown

The reading,,,

also got me to thinking about another one I just looked at that had been coverted years ago. I wonder if the people that did the conversion addressed all the concerns I found. Or did they just slap another boiler in there without looking any farther into it and what kind of mess I might be inheriting if I get into it.

Mike T., Swampeast MO

Early 20th century gravity conversions are the only systems I've worked on and I've yet to see any leaks caused by the conversion to forced flow either recently or many decades ago.

Sounds like you want to use forced flow for the primary and gravity flow for the secondary. Many have questioned and pondered but nobody to my knowledge has accomplished such a hybrid beast.

The problem you would face is that the forced flow would fight with the gravity flow and forced being forced, it will win. Result would be a boiler (be it mod-con or conventional) hitting high (or reset) limit with extremely little energy making its way to the radiators.

Copper fin baseboard and standing iron radiators are the worst possible mix in a system and a nearly guaranteed disaster in the same zone. Iron radiators have lots of mass and lots of water content and will give off heat LONG after the boiler stops firing and/or the circulator stops running. Fin baseboard on the other hand has comparatively no mass and no water content and will stop giving off heat very shortly after the boiler stops firing and/or the circulators stops running.

Dan Holohan's book "How Come" gives GREAT information for converting gravity systems to forced flow. You are correct that you will bring the supply and return pairs together using a very simple and very safe rule of thumb. Take the largest pipe size, cut in half then drop one size. For example, if the largest supply mains are 2 1/2" you would use 1" for the new piping. IMHO it is best to strip the mains back to horizontal and use only bushings (not couplings) to reduce the pipe size. This is one place where it doesn't seem to matter in the least how you split the flow between the two branches--bullheads don't seem to cause any problem.

Do though be VERY careful about making changes to the piping. If you follow "monkey see, monkey do" you will have NO problem--if you start reducing pipe sizes to "modern" standards, watch out!

Some say they prefer to rip out the piping and re-do with home runs or other modern piping. Utterly unnecessary unless the homeowner insists that the big pipes MUST be removed.

Your finest conversion will find TRVs on ALL radiators and a mod-con well suited for a single circulator system (and the TT Prestige is one). FAR cheaper than replacing the piping. Homeowners will be astounded by the comfort, energy savings and versatility. Just never, ever, ever, oversize a mod-con and keep pestering for smaller...

Timco

sweatin' to the oldies...

I use a separate circ for each old loop leaving the gravity boiler, and have never had a leak or a flow issue, but all have been 2-pipe set ups. Takes very little effort to move the water. Hot Rod did an article about not over-pumping these gravity conversions, may be in the library. My customers have questioned tieing a 1" copper into a 2.5" Iron pipe, but it always works great...

Tim

Unknown

I wonder,,,

if you put like a 40 gal tank in place of the boiler and piped correctly and then heated the water in that with a modcon if it might work. Just curious though, I wouldn't try it. For one thing I guess the connections on the tank would have to be the same sixe as the piping, meaning big. Thanks for the info! I have to look through my Dan library and see if I have that one, I might cause it sounds familiar.

Unknown

Thanks,

It's reassuring to hear you both say leaks have not been an issue. I feel more comfortable tackling these now than I did when I first started thinking about it.

nicholas bonham-carter

open to the atmosphere?

we have a gravity system circa 1890-1900 and i am sure the same water is still in it [with all oxygen boiled out].
there is probably little rust in those old systems!
so hopefully no weak spots for leaks.--nbc

gasfolk

Our Gravity Flow System...

works fine with a small "front-end" boiler injecting into the bottom of the old boiler and drawing return from the top. The old boiler is a W-M EGH-85, which now functions as a mixing tank. No problem with heat getting to the whole house. The system was and remains well-balanced. Absolutely silent. In a pinch we can fire the EGH-85 for rapid recovery or supplemental heat.

A couple years ago, we converted back from an injection mixing setup with three pumps (boiler loop, injection, and system circ), which required several relays, used more electricity, and never gave the anticipated fuel savings.

FWIW

gf

gasfolk

After years of consideration...

this is essentially what we finally did. Our 1928 gravity system had a relatively new (installed 20 years ago) but grossly over-sized W-M EGH-85 boiler (with four large tappings). So, we kept it and piped the output of a small "front-end" boiler into it. Mixing tank, "home for the heat," driving force for gravity circulation, and backup boiler all in one.

(A few more minor details in an earlier post below.)

gf

Charlie from wmass

This is how Robert Starbuck and Sons 1953 book Modern Heating Illustrated says to pipe it. It is the right hand illustration.

Unknown

Interesting,,,

apparently I'm not as crazy as it would first appear.

Unknown

Thanks!

Nice illustration, I appreciate you posting it for me.

gasfolk

Front-end Boiler Piping

We thought about this a lot and ended up piping the reverse of the Starbuck & Sons scheme (our "front-end" boiler sends output to the top of the old boiler and takes return from the bottom).

Our reasons were 1) to avoid thermal stratification in the mixing tank (old boiler), which might have sent most of the injected heat up only one of our two (huge) gravity zones; and 2) to raise the return temp to the "front-end" boiler, which for us is a low-volume, atmospheric, CI boiler.

If you only have one zone, our first reason seems unimportant. If your supplemental boiler can handle cool return temps, then our second reason disappears.

When we re-converted to gravity flow, we restored the 2 1/2 inch pipes connecting the old boiler to the two zones, reasoning that they would allow heat to leave the mixing tank faster. Cannot say whether the reduced pipes recommended for gravity conversions would substantially slow the response of the system, but that was our fear. At steady state, it probably does not matter.

Our lack of fuel savings on the first conversion (from gravity to injection mixing) likely reflects gross over-sizing of the old boiler, which was otherwise in good shape. We are glad we kept it, because it now provides a "home for the heat" from the low volume boiler and also backup.

Finally, this fall, heat calls have generated temps in the old boiler ranging from from about 90 F at cycle start to 108 at end. The "front-end" boiler has a system bypass adjusted to run at 140 to 160. These system temps have so far worked well with our radiant floor zone (kitchen and master bath) and another zone of CI baseboard emitters (Burnham Baseray). We have an aquastat on the supply to the RFH zone, which cuts power to the RFH pump if the supply temp drops below 75 F. So far so good.

No long-term fuel use data yet, but the house is comfortable. Of course, ours is only one data point. YMMV.

Cheers,

gf

gasfolk

Front-end Boiler Piping

We thought about this a lot and ended up piping the reverse of the Starbuck & Sons scheme (our "front-end" boiler sends output to the bottom of the old boiler and takes return from the top).

Our reasons were 1) to avoid thermal stratification in the mixing tank (old boiler), which might have sent most of the injected heat up only one of our two (huge) gravity zones; and 2) to raise the return temp to the "front-end" boiler, which for us is a low-volume, atmospheric, CI boiler.

If you only have one zone, our first reason seems unimportant. If your supplemental boiler can handle cool return temps, then our second reason disappears.

When we re-converted to gravity flow, we restored the 2 1/2 inch pipes connecting the old boiler to the two zones, reasoning that they would allow heat to leave the mixing tank faster. Cannot say whether the reduced pipes recommended for gravity conversions would substantially slow the response of the system, but that was our fear. At steady state, it probably does not matter.

Our lack of fuel savings on the first conversion (from gravity to injection mixing) likely reflects gross over-sizing of the old boiler, which was otherwise in good shape. We are glad we kept it, because it now provides a "home for the heat" from the low volume boiler and also backup.

Finally, this fall, heat calls have generated temps in the old boiler ranging from from about 90 F at cycle start to 108 at end. The "front-end" boiler has a system bypass adjusted to keep the boiler at 140 to 160. These system temps have so far worked well with our radiant floor zone (kitchen and master bath) and another zone of CI baseboard emitters (Burnham Baseray). We have an aquastat on the supply to the RFH zone, which cuts power to the RFH pump if the supply temp drops below 75 F. So far so good.

No long-term fuel use data yet, but the house is comfortable. Of course, ours is only one data point. YMMV.

Cheers,

gf

[Edited to correct the description of our piping scheme.]

gasfolk

Correction

I edited the above post to correct the description of our piping scheme. gf

tim smith

Done lots of gravity to pumped conversions and never a leak

on old piping. Don't over pump. Been using the TT prestige for these w/ small of a pump allowed for boiler. Works great. No primary secondary. Tim

Unknown

Ahhh,,,

simplicity, now we're talking my lingo.

Andy Stern

Gravity Conversion

Before / After Photos My house - circa 1928. Its been working OK about 3 years with no problems and I'm very pleased with the savings and the freed-up space in the basement... I did it myself with a lot of "Dan Research". It was one of those jobs I waited far too long to do. I installed lots of balancing valves, union disconnects and a lot of plumbing fittings that thankfully I had on-hand. This is an excellent site and I lurk it regularly.

Andy

Andy Stern

My gravity conversion

Before / After Photos My house - circa 1928. Its been working OK about 3 years with no problems and I'm very pleased with the savings and the freed-up space in the basement... I did it myself with a lot of "Dan Research". It was one of those jobs I waited far too long to do. I installed lots of balancing valves, union disconnects and a lot of plumbing fittings that thankfully I had on-hand. This is an excellent site and I lurk it regularly.

Andy

Eileen Olive

Interisting

I just installed a TT, replacing a half-century old WM which was a converted gravity system. Had serious concerns about "feeding" the 2 inch supply and return headers runing around the basement. Plus I have always been warned not to bull head my piping. Yet the previous conversion (1956) from a coal-burning unit bull headed both supply & return right off the boiler. How it works, I cannot say, but it does??? The weather is not yet cold, but the delta T right now is 20, and it heats up right quickly. The 1" boiler takeoffs are piped directly into the 2" existig headers, no primary/secondary. I'm thinking about adding a few CI rads in the basement. What do you think?

Unknown

Nice,,,

clean job, great work!

Timco

Depending on how big your house is, and depending on how many zones leave the near boiler area, I put a separate circ on each big zone. The circ in the boiler is the same I use, but is intended to be a primary circ for P/S piping.

Tim