Best Of

The sound could be from the air separator. You could have some gunk in there that is occasionally restricting flow when it makes it to the air sep.

DIY boiler replacements have been done by others, with good success, on this site. You know your abilities.

Looking at the price of a GV90 and the price of a mod-con like the Weil-McLain Ultra, there is a significant price difference. The GV90 is a complicated cast iron boiler, and the Ultra is a complicated mod-con boiler. Both have sophisticated control systems, and both can accept cooler return water. I would give serious consideration to the Ultra, since it is a lower-cost boiler.

When I look at a boiler job that comes into the five-digit price range—and that first number is not a one (1)—I also add over $2,500 in hydronic accessories, piping, and fittings before I even begin to consider the total job cost to a customer. Those quotes were not unreasonable, considering the total parts, equipment costs, and labor needed to complete such a job. (I didn’t see them before you removed them, but I can imagine the prices were a real sticker shock.)

You can download the installation manual and review the installation procedures before selecting the boiler you want to install. You will still need to hire a professional with a combustion analyzer for commissioning the boiler. There are adjustments that require measurements you can only obtain with the proper instruments, and unless you plan on spending another $1,000 for those tools, you will need a pro for startup.

In your position I might opt for the Ultra and use the piping arrangement in figure 4 on page 11 of the manual

You will need to consider if you plan on using the existing B&G 100 circulator as your system circulator to save on costs. I always include a new thermostat with the job. You can use the existing thermostat and save.

You need to account some $$$ for venting the system. That will include PVC pipe and fittings. Some Electrical parts like a new switch, and some wire and wire connectors. All this adds up a little at a time.

If you’re considering going back to steam, you need to think hard about what’s happened to the piping over the years. Those hangers have been carrying water weight, not steam conditions. Any sagging or loss of pitch is going to show up fast as water hammer once you put it back into steam service. The question isn’t just “will it run,” it’s “will it run quietly?” Pooled condensate in the mains or runouts will make it noisy in a hurry. @Steamhead and @gerry gill would have good insight there, but based on what you’ve described, I’d lean toward staying with water since that’s what the system has been operating as.

On the hydronic side, the load calculation is the key starting point. That will define the proper boiler size, and in many of these conversions you’ll find there is more radiation than you actually need at 180°F. That opens the door to lower water temperatures and better seasonal efficiency. If you introduce outdoor reset on a cast iron boiler, you must protect the boiler from sustained low return temperatures. You want to see 135°F+ return within the first 6–10 minutes to stay out of the condensing range. Given that you may only need 110°F supply on mild days and ~140°F at design, some form of boiler protection/mixing (bypass, 3-way, or variable-speed injection) becomes necessary.

Whether you go with something like a Weil-McLain GV90+ boiler or keep the existing unit, the load calc is really about right-sizing the replacement. The existing boiler is already past its prime, so it’s a candidate for downsizing. One option—if you want to experiment before replacing—is to stage the input. A two-stage gas valve can effectively “resize” the boiler by operating primarily on low fire. That said, this is not DIY territory. It requires someone who knows what they’re doing, with a combustion analyzer, to verify safe operation at low fire and high fire if needed.

If you undershoot slightly, high fire is still there as a safety net—but in most cases like this, the original boiler is so oversized that even low fire will cover the load.

Bottom line: You are going to pay for a properly sized boiler one way or another—either through higher fuel costs over the next 10 years or upfront with a replacement that actually matches the load. The difference is whether you end up owning an efficient boiler at the end of that period.

A load calculation is not required if you return to a Mouat steam system. Steam systems are sized based on the connected radiation, not the building heat loss. This is because you must produce enough steam to heat all of the radiation in the system in order to achieve proper balance.

With a hot water system, however, you can reduce the output of the radiators by lowering the water temperature. This allows you to select a smaller boiler that more closely matches the actual building load.

Hope this helps to make this as clear as MUD!

Ed Y.