Replacing gravity heat boiler.

Mike T., Swampeast MO

Nothing at all wrong with using the existing piping. It's just the piping <I>right around the boiler</I> that will have to be replaced when you convert to forced flow.

Mark Berry

Gravity heat boiler

I went to bid a boiler replacement today and found a gravity hot water boiler in place now. Two 2" mains heating 8 radiators. I have measured all radiators to figure my BTU's but am wondering how I should pipe the boiler. Should I install a ciculator or leave it gravity. The boiler is used for heat only so hot water is not a factor.The boiler is a closed system with an expansion tank. This would be my first gravity boiler swap out and am looking for any helpful hints. Thanks Mark

Timco

You need to do a heatloss calc on the entire house to size the boiler first...see heatloss calcs under the resources tab at the top...it's a free download. Then go from there. Those large pipes will have almost 0 head (great flow)...Now is the time to sell zones and ODR and such...do yourself a favor and read some of the HW books found in the online store...

Tim

Al Letellier_9

gravity replacement

Mark, I have had great luck doing primary secondary set-up. The new boiler will undoubtedly be a lot smaller and with all that water content in the system, you can avoid thermal shock very easily. The great thing about gravity is that the rads stay warm throughout the heating cycle and the customer will notice a big difference if you go to circulated water. This is a perfect opportunity to set the system up with constant circulation and outdoor reset. P/S the boiler into the heating loop and you'll have an ideal setup and a very happy customer.

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Mike T., Swampeast MO

No modern boiler of even remotely suitable output will have sufficient tappings (either in number or size) for a gravity system so there's almost no choice except converting to forced flow.

A VERY safe rule of thumb for near-boiler pipe sizing when you convert a gravity system to forced flow is to cut the pipe size in half, then drop down one more size. In this case with 2" mains you will use 3/4" pipe. It will look strange but there won't be any problem regardless of the type or fuel of the boiler.

You cannot size the replacement boiler by measuring the radiators. That only works for steam systems!!!! You must conduct a heat loss calculation. If you don't already have, a quite reasonable heat loss program is available at this site for the perfect price of "free".

You don't mention fuel.

If gas (natural or propane) limit your replacement boiler choices to cast iron or condensing/modulating. Copper tube boilers are not well suited to converted gravity systems. If oil, your choices are conventional cast iron, condensing cast iron and "near condensing" cast iron. "Near condensing" oil boilers can operate safely at any nearly any temperature, but do not recover energy from the condensation. VERY few true condensing oil boilers are currently available. Slant Fin now offers a near condensing oil boiler with a really sweet control system. Viessmann makes true top-of-the-line near condensing oil boilers but they are quite expensive.

Gravity systems are almost always open. You're correct saying that there is an expansion tank--it's usually in the attic. The tank itself will be open to the atmosphere (this was an important safety feature). An overflow pipe usually goes from the tank out through the roof. When you convert to forced flow you will remove the open expansion tank and replace with a close compression tank. Water volume of the system will affect the sizing of the tank more than anything. If using modern bladder-style tanks, be aware that multiple tanks may be required. The larger tanks tend to be ASME certified and quite expensive and there's nothing at all wrong with using smaller tanks in multiples.

If gas is the fuel I would HIGHLY recommend using a condensing/modulating boiler. Labor will be much of the cost for the conversion and the added material cost when using a condensing/modulation boiler will be RAPIDLY recovered in fuel savings. Size and install the boiler properly and you can nearly guarantee the homeowner that fuel costs will be cut by more than 50% as long as they don't keep the place warmer than before...

Thermostatic radiator valves (TRVs) are WONDERFUL when used in a gravity conversion--particularly with a condensing/modulating boiler. TRVs will essentially provide room-by-room zoning with NO additional costs and NO piping changes save a differential pressure bypass valve near the boiler. While you must verify, I can nearly guarantee that a single circulator can be used with a condensing/modulating boiler if you have TRVs on ALL radiators. If you don't use TRVs, you MUST use either primary/secondary piping or a low-loss header. If the homeowners are willing to go all-out in the conversion, the Viessmann Vitodens 200 has my HIGHEST recommendation. Combine the Vitodens 200 with a fully TRVd gravity conversion system using only the built-in variable speed circulator and the homeowners will be truly astounded with the fuel savings. If your heat loss calculation comes in much less than 50,000 btu or so however, I would not use the Vitodens 200 as even the smallest model will wind up grossly oversized.

If you must use a conventional cast iron boiler you really should include some form of boiler protection against low return temperatures. Assuming the system operates via a SINGLE thermostat (e.g. no zones), use an ESBE thermostatic bypass valve. This is the least expensive and most effective protection and primary/secondary piping will not be required and the system will work perfectly with a single circulator (probably a B&G 100 or Taco 007).

Eileen Olive

Already converted gravity system

Your system sounds very much like ours, 2-inch black pipe running both supply & returns in both directins teeing out from the boiler. We have a 51-year old oil-fired hot-water boiler that was installed in the early '50s to replace a coal-fired unit, which I don't recall seeing (I was too young to remember much more than being yelled at for playing in the coal bin). We do not have a tank in the attic. We do have a rather large expansion tank hanging above the boiler between the beams, though. We also have large cast iron rads, 8 of them. I have been told that a new mod/con would be great for my situation. I have been trying to decide whichh one to go with (Triangle, Ultra-80, or Knight). Since my system was converted a half-century ago, I am hoping all I have to do is remove the old boiler and install the new one, with whatever associated piping is required. Yes?? Or no??

Mike T., Swampeast MO

Your current 1950s boiler likely has decades of remaining life and provided you have a good oil company that provides true service its original efficiency for the system will remain quite stable.

Natural gas suppliers give ZERO service after their "free boiler/furnace" inticements... Yes, natural gas is "clean" but annual or at most semi-annual service of a gas-fired mod-con is REQUIRED!

Boilerpro_5

Still some gravity boilers available, I believe....

The Weil Mclain EG series probably can run gravity (3 inch tappings) as can the Burnham Independence. I would check the installation manuals. Peerless probably also has something available.

Boilerpro

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zeke

What is wrong about using the existing piping ( and save a bundle on not repiping() and the gravity feed system, even with a new boiler, the smaller fittings, notwithstanding?
They can be transitioned to the larger piping with minimal loss of friction and therefore gravity flow, especially, given that the older systems with huge radiators were probably way oversized.

A system natural convection flow analysis with the existing radiation should be made to confirm this. I suspect that the natural convection flow analysis is not easily done and, accordingly the contractors are reluctant to do such a ssystem. Even so, why would they repl;ace larger pipes with smaler ones? Does that make sense?

All you would need are a few zone valves and/or TRV's for control.

Please comment or correct me if I am wrong.

Mike T., Swampeast MO

By dogs, it does look like the W-M EG series would work for a smaller gravity system with 2" mains. 3" supply tapping and 2 1/2" return.

zeke

Why must it be forced flow? Why not use the existing gravity system?

Eileen Olive

Not wortht the switch?

You don't think it is worth replacing the 50-year old boiler with anew mod/con? At what point in its life might the aging oil burner finally quit? The only noticable problem is that the fire chamber's wall directly opposite the gun has worn out. The service fellow (oil delivery company owner) claims it costs almost as much to replace the chamber as it does to install a new oil-fired boiler. I cannot see how. I understand, though, that he would like to keep us as customers; hence,he wants to install a new oil burner. I, on the other hand, would rather install a new gas boiler, if need be, and be rid of the 275 gal tank in the basement. What do you think?