Living with an old mercury-based aquastat

capouch

I am the lucky man restoring the house and grounds of Sunny Crest, a beautiful old gentleman-farmer's estate in NW Indiana that was built in 1916, renovated in 1946, and then pretty much abandoned around 2015. The electric service entrance and all the copper attached to it had been removed by vandals, the pump and much of the old steel pipe was frozen and busted, and the boiler had all its supply lines cut off, some radiators were frozen and others were missing. It has been a slow but fulfilling battle, and about a month ago I began trying to get the hydronic system going again.

It has a Burnham Series 2 "209" boiler with Serial No. 00000000000, apparently a prototype/early production unit. Unlike any of the boilers in the user guide, it utilizes a 6902 gas valve. The system runs on propane.

It has a L4080B aquastat in the vestibule, and this old aquastat in a 3/4" well in the supply line, but it had been disconnected. I tested it, and it cuts in and out, so I wired it in to use as a burner control; the system is 225K BTU but about 1/3 of the radiators are missing from the system, so it heats up the water faster than the existing radiators can dissipate the heat.

My question, finally: I can't read the model number on the back, so I'm flying blind with this thing. By rotating the body, I have set it to cut out at about 200 degrees, but then it doesn't cut back in again until after a long while, at about 140. The element appears to rotate OK. Can anyone ID this for me, and/or help me understand whether the behavior it's exhibiting is to be expected? If not, can it be adjusted? Thank you in advance for any help I may receive. This is my first post to the group and I am very excited to make use of the expertise I have seen here.

Jamie Hall

That's a high limit aquastat. The limit it is set to may be adjustable -- inside the cover. I'm not sure whether the differential is adjustable.

I'm thinking that while it may be usable, you really will want to revisit the entire burner and circulator (assuming that there is a circulator -- or is this gravity hot water?) controls while you are at it -- revised modern controls will be a small part of the overall expense here.

EdTheHeaterMan

Not a fan of using old controls unless you are restoring a museum piece or you are using it for training. If this is the heating plant (or only heating system) that will be used on a regular basis, I would look into using more reliable parts and equipment. If the old boiler is not leaking and you can repipe and get it working, then use the modern-day replacement for the antique parts for reliability's sake.

Since you are currently oversized, a larger differential on the limit control will work to your advantage.

I would hate to see that old gas valve get stuck open. I had only one "run-away" gas valve in my 40+ years of working in the trade. But it does happen.

EdTheHeaterMan

Ooops... My mistake, I thought you were talking about the old mercury bulb aquastats pictured here.

L408A and L409A


The L4080 controls expanding fluid bulb technology is still used in replacement parts today. If the control is like this, then it is an acceptable limit device


You may want to look at the L4006 A2007 as a replacement part if the differential problem can't be resolved. That is an indication of a defective control and it may be unreliable as a limit control.
https://www.supplyhouse.com/Honeywell-L4006A2007-High-or-Low-Limit-Aquastat-100-240F-range-5-30F-Adj-Differential

capouch

The control is mercury-based, and seems to work fine except for the differential being so wide. I am indeed looking at a 4006 to replace it. The system was upgraded and the boiler new in 2008. We're in a mild streak here so not very good for assessing, but other than the initial "big jump" warmup the boiler doesn't run long enough with each cycle for the limit to be hit.

capouch

Is there product literature out there for the one I've got, which looks identical to the one in the image above?

EdTheHeaterMan

There are 2 pictures this is for the second picture

https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Honeywell-L4080B1352-Product-Overview.pdf
https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Honeywell-L4080B1352-Install-Instructions.pdf

if you are looking for the first picture then that literature is not easy to find online. Obsolete, too many years before the internet but not interesting enough to post (like the vintage thermostats).

I remember installing those controls early in my career, but mostly removing that control for the modern upgrade.

capouch

I was able to discern the first part of the model number but not the last four digits. I'm going to try a brighter light on it, but they appear smudged. It's a L444A switch; I have found some pieces online that would indicate, too, that such is the switch I have.

Jamie Hall

You keep saying "mercury based". Do you mean the sensing liquid in the probe? Or a mercury switch as the switching device? I it's the latter, much as I love mercury switches -- they're just about bulletproof -- they are sensitive to level, and both the trip point and the differential are affected. And they are obsolete...

capouch

It uses mercury as the switching device. I'll check the level; I see there are two almost identical for sale on eBay. It is indeed an L444A.

I understand that they're obsolete. But if they work, is there anything wrong with them?

Jamie Hall

capouch said:

It uses mercury as the switching device. I'll check the level; I see there are two almost identical for sale on eBay. It is indeed an L444A.

I understand that they're obsolete. But if they work, is there anything wrong with them?

Nope. They'll work forever. They just may not have all the modern bells and whistles -- which you probably don't need anyway.

REDD

Interesting, I gave my system a check over this year. I have the L409A on my system but it is a clamp on type. I have always been suspicious of a clamp on. Once I removed it and buffed up the contact point on the pipe and surface of the device. Not much maintenance on these since Mercury seems very reliable. Years ago I had a mercury switch which operated the alarm system on my auto--we are speaking of 1970 and the switch was typically located on the side of the driver fender--to be less conspicous. The fluid inside the switch was presumed to be mercury. In extreme cold weather (-10F), the Mercury "Sludged Up" overnight. When I operated the switch at 7AM in the morning, the Mercury did not shift position leaving the switch bulb in the "ON" position. A siren in the neighborhood at that time was most unwelcome. It took me a few times to figure this joke out. I tended not to operate the alarm in extreme cold after that. The 1970's were loaded with severe weather in my area. Some of you may remember those switches: They had a Circular type blade to insert in the switch. Don't see those much anymore.