Gas and Oil Home Heating Furnaces - Triad boiler liquid volume

I need to know the volume of my 1950's Triad boiler (still going strong!) to size a diaphragm-type expansion tank for my system. I'm adding baseboard heat loops to my current in-floor hot water heat. Why? because I love hot water heat, especially in the floor, but the 50-year old iron pipes in the concrete floor are starting to go (I've fixed one and taken another off the manifold). I can use the remaining zones as long as they work, and still have hot water heat when I finally have to give up on the in-floor system.

The boiler is a Traid 0-75-DK, about 4-3/4 feet tall and 2-1/2 feet diameter. The operating manual also calls it a G-75-DK I believe. It has a non-diaphragm expansion tank that looks to be about 5 gallons. I am getting rid of that and replacing with a diaphragm-type tank because I'm adding a Spirovent air resorber. I am adding 2 loops of fin/tube of about 4.7 and 5.8 gallons (176 and 217 ft of 3/4" copper) but have no idea what the volume of the boiler is, and since the temperature is -2 F today, I really hate to drain it to try to find out, and dont know what the pitfalls would be with that anyway.

The plate on the side of the boiler says 1 gallon, but that can't be the volume of the boiler? It has a secondary coil inside the boiler for domestic hot water. Maybe that's one gallon, but it still sounds small.

Any help?

Thanks,
Garp

1. area : central Illinois typically -15 to 95 F
2. House : frame w steel siding over wood sheathing, some FG batting, plaster inside.
3. equipment: 50 yr old Triad boiler (converted from oil to gas burner, range 70,000 to 150,000btu/hr) and steel heating pipes (also 50 yrs) in concrete floor.
4. Fuel type: gas
5. Water temperature: 170 to 220 F, runs at 12 psi - dont know pressure max rating.
6. Thermostat type: honeywell - the common round one.
7. Anything else: see above

I figure that the 75 means 75,000 btus. Regardless of whether is is gross or net, you should be fine with a #30 extrol or equivalent tank. The tank you now have is probably not a 5 gallon tank. If you measure and calculate I think you will find it to be a 15 gallon. The 30 on the extrol means that it will accept expansion equivalent to a 30 gallon regular tank. You will love that spuirovent too. They are great. Screw the #30 right into the bottom of the spirovent.

Good luck.

Thanks for the advice, KField -

My email went down and I'm just now getting back to the forum.
I called and emailed Triad and they were no help. There's some old manuals around there in storage somewhere, but too deep to dig out I guess. I would have thought there was some kind of archive or something on the web. I took guesses on the high side and the low side for the boiler volume and did the sizing calculation from Siegenthaler's book. What I found was that the extrol #30 you recommended would handle that entire range. Also you were right about the 75 being kbtu/hr, although it now has a 70,000-150,000 btu/hr gas burner in it. In addition, for what its worth in boiler trivia, the model #O-75-DK is for the oil-fired boiler (it's an O not a zero 0) and naturally, then, the G-75-DK is the gas version.

The new installation is almost ready to switch over, fill, and bleed. It has 3 independently-pumped zones (including the in-floor as one), globe valves for balancing between zones, bimetal thermometers on heated water out from the boiler to the pumps and on each of the returns, so I can tell what's going on, recycle valve to maintain high enough return water temperature to the boiler, and enough other valves that I can isolate all portions for servicing or modifications. Floor and one baseboard loop are on one thermostat and the loop for the back part of the house on another. All fin and tube emitters have monoflo tees (1) and vane valves for proportion between flow-through and bypass. All pretty simple right now, but set up so I can modify later as needed if I want and can afford more automatic control.

I did want to ask one other question. Several people have asked me why I replaced the expansion tank with the rubber membrane type. I assume that the spirovent would eventually deplete the air in the plain (no membrane) tank, and it would fill up with water to where it couldnt act like an expansion tank any more. Correct? Also, it seems like the old style expansion tank makes an awfully good air reservoir to keep on putting dissolved air back into your water to promote corrosion.

If I hit any serious snags, I'll report them, since there seems to be a lot of interest in hot water heat right now.

Thanks again, KField

GarP