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Hi, About domestic water heaters, I believe it was around 1960 that it became code in the US to install T&Ps on tanks. Watts filmed "Explosion Danger Lurks" in 1942 to try and convince code people to require relief valves. It's an interesting film 🤠

Yours, Larry

The original water heater n my parents' house from 1957 had a relief valve installed in a tee in to the tank. I have seen a few posts of monel and copper water heaters from around 1950 that were still in service that did not have one.

As far as pressure relief valves, they weren't needed on hot water systems until they went to closed compression tanks from open attic tanks.

Steam boilers naturally lose some amount of water and will heat possibly even after the water level is dangerously low. Steam boilers generally aren't controlled on temp so there would be nothing to stop the burner as it keeps getting hotter.

Hot water boilers don't lose water unless there is a leak and will usually stop heating because they are air bound long before they dry fire although it certainly is still possible in a few ways. Usually the aquastat will cut the burner long before they dry fire as well. I think most dry fires of hot water boilers are burner controls that get stuck on, either gas valves getting stuck open or relays on oil burners welding shut.

Or another way to look at it. A vapour system is a steam system, with a difference.

First, they are always two pipe systems, often with wet returns as well.

Second, they are designed to work at very low differential pressure between the steam mains and the dry returns — typically 8 OUNCES per square inch as a maximum.

Third, from the initial construction standpoint, as a result they don't need as much headroom from the boiler water line to dry returns — 20 inches is ample.

Fourth, as noted above, they almost always used metering valves adjusted to control the maximum amount of steam going into a radiator to what the radiator can condense. These are defeated by excess pressure differential

As a result of that, the outlet arrangements can vary widely. Some — Hoffman Equipped — always used traps. Others used various patented devices which work well at low pressure, but not at all at higher pressure differentials. Some early ones had no output device at all — just an elbow going to the dry return (but don't be fooled — some of those elbows had traps or other devices built in).

Next, many — though not all — had various patented devices to prevent that pressure differential from becoming excessive in the event the boiler got too enthusiastic. These vary from simple (Hoffman Differential Loop) to remarkably complex.

Sixth (I think) as a result of the wizardry, they almost always used crossover traps from the mains to the dry returns to vent the mains, and all venting was concentrated at the boiler where the dry returns join together.

Now having said all that… there are a number of present day advantages. Among them are that traps and vents last pretty much forever, except for the crossovers, since neither are usually called on to do anything. The crossovers are, but since the differential pressures are so low they tend to last forever, too (to give you the idea, Cedric's crossovers were installed in 1930 and are still just fine, thank you. 95 years young)..

There are a few disadvantages as well. The most critical and most common is that maximum pressure differential. They requite a vapourstat or other controller capable of managing the boiler pressure to below that 8 ounces or so. A corollary is that they misbehave rather badly if someone decides to crank up the pressure to more conventional pressures. Another is that they do not like to be kludged. Vents added where they don't belong. Traps removed or changed where they aren't needed. The differential pressure control devices removed because people don't understand them. That sort of thing. Another is that if someone does make them run on a higher than designed pressure, they can get absurdly out of balance, since the metering valves are defeated.