Gravity Conversion Newbie Question

Lordniacin

Hi, this is my first post here, though I've been reading it for some time, along with many of the excellent technical papers and articles recommended here.

OK, here is my system: 1924 house in Wisconsin. Two floors of 1800 square feet each. Originally a single coal-fired boiler feeding a gravity hydronic system. About 20 big cast iron freestanding radiators. Huge 4-inch mains running the length of the basement with those clever fittings coming out at either 45 degrees or sideways, depending on what floor the water was headed towards.

Well, in the '70s the house was divided into three apartments: one on the first floor and two smaller ones upstairs. The lines from the upstairs radiators were sawed off in the basement and reduced to 1" copper. The big boiler was replaced with three AO smith copper boilers at 60K, 60K, and 100K. Looked like three R2D2's having a meeting in a maze of pipes! All the original giant mains were left serving the first floor system. So at this point in time the first floor system is like: 35K of actual heat loss, 70K of radiation, and 150 gallons of water in the system! You can guess that this is very harsh on boilers. The boiler fires, the hot water trickles out to the radiators and floats to the top. By the time the radiators are half full of hot water, the heat demand is satisfied. At NO TIME does warmer water actually make it around to the boiler return! Talk about a condensation problem.

OK. How to fix this. Recently I was practically gifted a Burnham 204, which is 80K output. That is probably about right for the entire house, as I want to eventually un-divide the house. But first floor first. Seems I need p/s piping AND a form of return water tempering, like an ESBE thermic valve. I drew up some diagrams in the Moose Antlers configuration. (The other apartments can later be added as more antlers with zone valves). One circulator, running all the time at 14 GPM would spin that 150 gallons of water through the radiators about once every ten minutes. This would help even out the temperature between rooms. When heat was called, another circulator at 8 GPM would extract the 80K BTU from the boiler at a 20 degree rise. When starting from setback, or in the shoulder season, the return water in the system might be at 60 degrees. So the thermic valve will bypass MOST of the output water around again to get the input up to 140, which the boiler raises to 160. Really, only a couple GPM of that 160 degree water is coming out to the system under those conditions. The trickle of 160 water goes out to the tees, where it mixes in with the 15 GPH flow spinning through the radiators. Slowly they warm up, about a degree a minute I figure (!) After half an hour-ish, the 60 degree system water is more like 90, and maybe the heating call is satisfied. I have observed that in the shoulder season the radiators just barely get warm, and even in midwinter maybe make it to 150.
Does this make sense so far?

Here is my question. The Burnham 204 has 1-1/4 water connections. At my desired 8 GPM that is a flow velocity of only 2 feet per second. I though it was supposed to be between 2 and 4. If I reduced the pipe size to 1 inch, the flow velocity would become 3.2 feet per second. Also I could do the near-boiler piping (circulator, ESBE valve, etc) in cheaper 1" copper. Is there any reason I can't come off the boiler and immediately transition down one size to 1" pipe?

Related question: best size for my moose antler manifold. Maybe go with 1.5", considering I could have three 1" returns coming in one end, then a set of closely-spaced tees with 1" bulls coming from the boiler, then up to three 1" supplies leaving via zone valves at the other end? And yes, at that point I'd probably consider a delta-P circulator or pressure bypass valve to keep from deadheading the circulator too much when only one zone is calling.

Thanks for any input you can give!

SWEI

We love these conversions, because they are quite technically demanding but when done properly, they truly do shine.

Let's start with some really basic questions:

• Have you done a heat loss calculation on the building?
  
• Any envelope upgrades planned?
  
• What are your gas prices like?
  
• What kind of zoning arrangements do you need?
  
• How much of this do you intend do yourself?
  

Lordniacin

Thanks for the comments. To answer these questions:
--I have done several heat loss calculations for the first floor and even datalogged the boiler on/off times at different outside temperatures. The figures converge at around around 35K for the first floor. Haven't formally done the 2nd floor yet, it has a colder ceiling and warmer floors but is otherwise the same size.
--In the future I'm considering making the attic into conditioned space. I have tracked down and sealed zillions of little leaks all over the structure. An IR thermometer and stick of incense are helpful in finding cold spots and air leaks.
--gas prices here are about .37 per therm.
--I need at most three zones, one for each apartment.
--I want to do as much of the pre-planning as possible then have a contractor solder the bits together. I'm originally an electrical engineer, and at first just wanted to understand what was really going on here, why only one of three near-identical boilers was being eaten alive. Like a boiler murder mystery. As I read more and more, wow, this is quite the rabbit hole! Compared to a freshly designed PEX/ baseboard system designed straight from the book, a converted gravity hydronic system in an up-insulated house is a fascinating engineering challenge. No two factors really match any more.
So far, it seems that just by right-sizing the boiler, adding an ESBE thermic valve to temper the return water temperature and piping it P/S with the right flow rates I would be miles ahead of where I was before. Radiators more even from room to room and boiler not eating itself alive.
Really, I guess I just wanted some feedback as to whether my reasoning makes any sense at all. What am I missing? What other concern may I be missing?

SWEI

Where are you located? $0.37 per therm is close to our nominal rate here, but seems on the low side for a net rate -- even in the current market. Did you divide the total at the bottom of the bill by the total usage to get that number?

The secret to making a high mass (of both water and steel) system is to keep the water circulating whenever the outdoor temp is below the WWSD regardless of demand. This requires proportional zone flow (TRV's being one option there) paired with a properly set ODR curve (generally on a mod/con boiler or a motorized mixing valve.) Conventional on/off control of zones simply can't deal with the long lag times that mass creates.