A friend of mine passed away and I am fixing up the house for the family as it is readied for sale. The house has a newer oil boiler but old cast iron radiators. All of the radiators are constantly releasing steam from the pressure relief valves. They all look to be very old and I suspect need to be replaced. What do I need to know to be able to choose proper replacements for these? As it is now the boiler keeps running low on water and shutting itself off, I assume because so much steam is being released throughout the house through the relief valves. I have no experience with these old systems. I am afraid the boiler is going to shut down some really cold night and we end up with frozen pipes if I cannot get up there for a few days. Thanks Trent

You should have the whole system checked by a professional. Constant low water levels are a bigger symptom in my view.

The furnace is only a few years old, there are no leaks, it just loses the water via faulty pressure release valves. Every one of them look to be several decades old at the very least and should be replaced. I am looking for information on how to determine what the correct replacement relief valve would be.
The last time I hired a "professional" the bastard charged me his full hourly labor rate x2 for both him and his teenage apprentice who just stood and watched what he was doing.
I know there are honest and skilled professionals out there but it is a gamble whether you get one or not and this is not a complex task, I just need to know what criteria to look at to make the correct selection. Posting here hopefully would attract answers from knowledgeable people and provide me with what I need to complete the job myself.

Are you talking about air vents? Normally you won't have relief valves on the radiators.

Sounds to me like your auto fill valve is not working properly. A pressure relief valve should never deplete the water supply of your boiler tank. NEVER. Your logic is flawed.

I can send you the boiler specs if you give me your make and model number.

The devices to which you refer are NOT pressure relief valves. They are air vents for steam systems. Here's a place where you can learn more about them. Air Valves , Hoffman Air Valves , Gorton Air Valves - SupplyHouse.com

Yes, you are correct they are air valves. They are all old and apparently stuck open as they continue to spit and hiss even after the system is hot. They all need to be replaced.
I do not believe the furnace has an autofill, they have a ball valve on the water line to fill it so it has to be done manually.

As I said, there do not appear to be any leaks anywhere in the system. All the pipes are easily followed in the basement. It is possible that if there were a leak in a pipe that it is being absorbed by the insulation and dried back out by the heat. All the pipes are wrapped in asbestos and then sealed off on the outside so I do not want to disturb them if not absolutely necessary. If I end up disturbing them then I might as well have all of the asbestos removed and then replace the one pipe steam system with forced hot water.

The furnace is a Weil McLain P-SG0-5 Series 3.

Most or all of the air vents need to be replaced whether there is another issue. I would expect if there were a leak in one of the steam pipes it would be more noticeable but I have never had to work on a steam system. There are no leaks from the radiators themselves except for the escaping air/condensate from the air valves.

One tip off to a leaking pipe is stained insulation. If you see any rust or black stains on the insulation jacket, be looking for a leak near that point.

I found the leak in the system. There is an attached shed that has a radiator in it and though it was shut off the valve is not sealing it tightly. At some point in the past the air valve was apparently removed from the radiator as it is not broken off. Since the shut off valve was not sealing tightly steam was just spraying out of 1/8" hold for the air vent. Since it is not part of the main house no leak was noticeable in the basement and the shed was just being used for storing firewood and nobody went in there to notice the radiator was on and leaking.

I have a handle on what to get for air valves but now I have an issue with one of the angle valves to a radiator. The pipe from the radiator to the angle union valve cracked. I went to get another to replace it but oddly the pipe from the furnace to the valve seems to be 1-1/4" while the pipe from radiator to the valve is 1-1/2" and I have not found a replacement valve that has those mixed dimensions.

Without being able to see what you are working on, it's hard to try to offer suggestions. Could you possibly use a 1.5" & bush down the one end to 1 1/4?
I've often seen radiators with bushings in them.

I probably can use a bushing if I can find one the right size.
I was unfamiliar with what sizes these typically came in but when I looked at the 1" I knew it was too small. All they had on hand larger than 1" was 1-1/4" so I bought that. I had to bring the radiator back to my own home to heat it up and remove the old valve as it was on there pretty solid between rust and putty. When I went to install the threaded end of the union fitting into the radiator it dropped right through the threads. I measured and found it to be 1-1/2" so I went shopping at another store and picked up a 1-1/2" valve, took it home and it fits the radiator perfectly. I brought the radiator back to the house to re-install it and when I managed to get the old valve off of the pipe in the floor found that the new valve slips right over the threads on the pipe.
So apparently the pipe in the floor is one size and the connection to the radiator another.

I took pictures with a measuring tape across the various connections but am uncertain which dimensions are used on each piece.
The pipe coming out of the floor measures 1-1/2" across.
The old valve with the union fitting and nut off measures 1-7/8" across. This is where the pipe comes out of the valve hortizontally to connect to the radiator.
These appear to be the only pictures that saved on my phone though I took measurements of each side of all fittings.
I will not be able to take more until I get back up to the house.
In all of my searching I have not seen a valve with a 1-1/4" inlet and a 1-1/2" outlet. Perhaps it was common back when the house was built?

Thanks.
Trent

Trent, The only options I can see are either to bush down the radiator & use a 1 1/4" valve or bush down the d1 1/2" valve so it fits the pipe coming out of the floor. I didn't see any combination sized valves on their site, but if anybody has them or knows where to get one, it's these folks: Patriot Supply - Industrial, Commercial & Residential HVAC Parts & Equipment
You can call them toll free. When I've talked to them before when looking for something odd, they were quite helpful.