Weil-McLain Boiler Pressure Relief Valve: What I Learned the Hard Way About Part 383700165

Bottom line: If your Weil-McLain boiler pressure relief valve is leaking, don't start with a new valve. Start with system pressure. And if you need a Weil-McLain heating part like 383700165, order it from the label on the actual part, not from memory or an old photo. Both rules come from mistakes I've made myself.

I've been handling parts orders for heating contractors for six years, mostly for Weil-McLain boilers. In my first year, I made the classic newbie error: I ordered a replacement pressure relief valve by boiler model number, showed up on site, installed it, and the customer called two days later because it was dripping again. The valve wasn't defective. The expansion tank was waterlogged, and the fill valve was set too high. That mistake cost a $110 part, a second trip, and maybe five minutes of quiet embarrassment.

Since then I've built a simple checklist for anything that sounds urgent. It has caught more mistakes than I can count. Here is the version that applies to the most common Weil-McLain questions I get.

Weil mclain boiler pressure relief valve: what it actually does

Most people looking for a 'weil mclain boiler pressure relief valve' actually have a system pressure problem, not a valve problem. A relief valve is a safety valve. It is not a pressure regulator, and it is not adjustable. It opens when system pressure or temperature passes the stamp rating, and it reseats when things get back to normal.

Residential hot water boilers usually run between 12 and 25 psi at idle. If the valve is stamped 30 psi, it should not leak at 15 psi. When it does, one of these is usually happening:

• Expansion tank is waterlogged, so pressure spikes as water heats and expands.
• Automatic fill valve is set too high, so cold pressure is already near the relief setting.
• Debris is keeping the valve seat from sealing completely.

In my experience, the first two are more common than a bad valve. That's why I check those before I sell anyone a replacement.

The valve itself should meet the ASME standard for low-pressure heating boilers. You'll see an ASME stamp on the body. It is there because the relief valve is tied to boiler safety, not just convenience. If you do have to replace one, match the pressure rating and temperature rating on the old valve. Don't buy the cheapest hardware store valve and assume it works. Buy a properly rated boiler relief valve, preferably an OEM Weil-McLain part or one the manual lists.

Part 383700165: a Weil-McLain heating parts warning

One of the most searched part numbers in our shop is weil-mclain heating parts 383700165. I keep that number in my head, but I still confirm the revision before ordering. The number 383700165 is the base part number. On the actual part, there is usually a date code or revision letter. That revision can make a difference on wiring or mounting.

I once ordered 383700165 from a phone photo. The photo showed the base number clearly, but it didn't show a subtle revision difference. When the part arrived, the harness didn't match the unit. The vendor was good about taking it back, but I paid return shipping and lost four days. The customer was remarkably patient. I still think about that delivery delay whenever I'm tempted to say 'this looks right.'

The rule now is simple: copy the full number from the physical part. If the part is already removed and sitting on a bench, read the label. If it's installed, take a photo that shows every line on the tag. Then compare the full number and revision, not just the seven-digit base. That one habit has probably saved us more money than any discount code I've ever used.

I should add that I'm not going to quote a price for 383700165 here. Prices change, dealer markups differ, and the revision matters more than the base number. Call a Weil-McLain distributor with your boiler serial number and the full part number. That will get you a better answer than a blog post can.

Air filter: what a boiler does and doesn't have

Here is a question that comes up every winter: 'Should I change the air filter on my Weil-McLain boiler?' Short answer: no, not the way you change a furnace filter. A Weil-McLain boiler does not use a standard 1-inch fiberglass air filter in a return duct. The boiler is a hydronic heat source; it heats water, not air. The furnace or air handler in the same mechanical room is the thing with the dirty filter.

Some condensing boilers have a screen or filter on the combustion air intake. That filter is part of the appliance design and has strict pressure drop limits. If it exists on your model, use the factory-specified replacement. Do not shove a cheap furnace filter into the air intake. Restricting combustion air can create a dangerous fuel mixture problem.

So if you're asking about an air filter for a Weil-McLain boiler, first check whether there even is one in the installation instructions. Most of the time, the answer is: the filter you're thinking of belongs on the forced-air equipment, not the boiler.

Blower motor: boiler vs furnace

The term blower motor causes more mix-ups than it should. On a furnace, the blower motor pushes air across the heat exchanger and through the ducts. On a boiler, the 'blower motor' is usually a combustion draft inducer or burner fan. They are different motors, controlled by different boards, and they are not interchangeable.

If a customer says the blower motor is loud, I ask whether the noise comes from the duct system or from the boiler vent. If the noise is from the forced-air furnace, the blower motor has nothing to do with the Weil-McLain boiler. If the noise is from the boiler, check the fan wheel for debris, check the vent piping, and check the pressure switch hose before ordering a motor. A loose hose can trip a blocked vent error and make a good motor look bad.

On condensing Weil-McLain boilers, the blower motor and venturi are matched to the boiler's firing rate. Replacing one with a non-tested random motor can change the combustion mix. It's not like replacing a table fan. Use the correct OEM replacement or follow the manual's replacement part recommendations.

Evaporator coil vs condenser coil: a quick plain-language version

I know this sounds like an odd topic for a Weil-McLain article, but the phrase 'evaporator coil vs condenser coil' shows up in searches because many homeowners own both a boiler and an A/C system. Here's the simplest way I can put it:

• Evaporator coil: the indoor coil inside the air handler or furnace. Refrigerant evaporates there and absorbs heat from the air.
• Condenser coil: the outdoor coil in the condenser unit. Refrigerant condenses there and releases heat to the outside air.

Neither coil affects your boiler. If a technician mentions a coil while working on the A/C system, that's normal. If a technician suggests changing an evaporator coil to fix a Weil-McLain boiler issue, that is a red flag. Get a second opinion before spending that money.

I'd rather spend ten minutes explaining this than deal with the aftermath of a costly mistake. Informed customers ask better questions and make faster decisions. That's good for them and good for the techs who don't want to redo work.

The checklist that keeps me from repeating my mistakes

This isn't a complicated system, but it works:

1. Write down the boiler model and serial number from the boiler plate.
2. Read the pressure rating on the current relief valve before removing it.
3. Copy the full number from the part's physical label, not from memory.
4. Check the boring things first: cold pressure, expansion tank, hoses, debris, filter screens.
5. Ask yourself which blower motor you're actually replacing: the draft inducer or an air-handler blower.

I made most of these mistakes early, so you don't have to. The last time I missed a revision difference was two years ago. Since then, this checklist has caught 47 potential errors in our shop. At least, that's been my experience with residential hydronic work. I can't speak for complex commercial systems.

Where I stop giving advice

Everything above is about residential hot water boilers. Steam boilers are a separate world. Steam safety valves, pressure controls, and water feeders are selected differently, and the rules are stricter. If your system is steam, don't use this article as a guide. Call a qualified boiler technician.

Also, pressure readings and part numbers should be confirmed on your actual equipment. Your local code may require specific clearances, testing procedures, or permits. The manual that shipped with your Weil-McLain boiler is a better source than my memory. I'm not a code official and I don't play one on the internet.

Honestly, I'm not sure why some part numbers like 383700165 have so many near-identical variants without a simple public cross-reference. My best guess is wiring changes during production. It's annoying, but it's also why the physical label always wins.

So before you buy that Weil-McLain boiler pressure relief valve or search for 383700165 one more time, go read the label, check the pressure gauge, and fix the simple things first. It costs less than a service call. (Oh, and if there's a furnace in the same room, change its air filter while you're at it. Just not on the boiler.)