Main Vent Testing

Mark N

There are it seems never ending posts about main venting, So this morning I had some time and therefore decided to run some tests. In order to control the variables as much as possible, this is how i conducted my testing. I ran the boiler to bring the system up to temp. On each test the boiler was off for 30 minutes, therefore the boiler and pipes should be the same temp on each test. I used a data logger to record all times. My boiler is a Burnham IN-4 with 2 parallel flow mains each 24 ft long, my normal venting is 3 Gorton #1's at the end of each main. My boiler is equipped with intermittent ignition, not a standing pilot. It takes 45 seconds to come up to full fire when the boiler is turned on. Both mains were put in the same condition on each test. What I timed was from boiler turn on to thermal switch on main vent pipe closing. The thermal switch closes at 170 degrees. On the open pipe test I observed steam venting out of the pipe about 30 seconds before switch closed. Here are my results.

Open 3/4" pipe: 5' 55"
3 #1 vents: 6' 10"
2 #1 vents: 6' 53"
1 #1 vent: 7' 44"

This is slightly different than the normal vent testing, timing from header to end of main. The header is insulated and remained hot during testing. Subtract the 45 seconds that it takes for the ignition sequence and gas valve coming to full open for a more accurate timing.

Also, boiler actively steaming 3'30' after turn on.

Mark N

Not sure of the temp, but I believe the having the vent in place slows the steam causing the pipe to heat faster.

gerry gill

I like the test ! i usually work it backwards, by timing the open pipe and putting on enough vents to eventually equal that time..but what your tests do show is that at some point it may not be economically worth it to add the last one or two vents. Great testing ! Another advancement in steam.

nicholas bonham-carter

This is great research!
For the average non-handy homeowner, using the services of a steam pro, the testing of vents on the system may use up costly labor time, and therefore it is better to err on the Side of one too many vents, than too few.
The reduced velocity of escaping air also keeps the vents orifices cleaner.--NBC

SWEI

Are these condensate sensors typically installed in thermowells, or strapped on to the outside of the pipe?

SWEI

Completely understood.

I'm getting better at finding ways to make thermowells fit.

Standard models for 1/4" probes come in a 2" minimum depth (marginal on 2" pipe, but perfect for 2-1/2" and 3" pipe.) I can get 1" versions, but they're machined out of solid stainless and cost about 3x what the 2" ones do. For 3/8" probes, Pasco makes a couple of options (one sweat-in and one thread-in.) The improvement in response time is significant.

SWEI

TT provides a 3/8" bullet probe thermistor with all of their boilers. Unfortunately, it's an oddball 12k model that is specific to the European Honeywell controls they use. Many of the residential controls sold here use the same 3/8" probe diameter (Tekmar 150 comes to mind.) You can also shim a 1/4" industrial probe using soft copper tubing and a bit of epoxy. This will allow you to use an RTD for something like an A419, or all manner of thermistor types if you have a BAC system.

Mark N

Gerry thanks very much. What I've come to understand over time is the preheat phase(bring water to boil and heat and vent mains) is very dynamic. The only part of it that is static is the amount of air contained in the system above the water line of the boiler. Depending on how hot the pipes are at the start of the cycle, the total time to get steam to the vents can vary widely.

According the way boilers are sized, using my boiler as an example. The gross output of my boiler is 87,000 btuh, and the net output is 65,000 btuh, the difference between the two, 22,000 btuh being the pickup. These rating are based upon 1 hours time. The pickup is equal to 25% of the gross output of the boiler. Therefore, in a perfect world the amount of time spent to get steam to the end of the main should not exceed 25% of the total heat cycle. From what I've been able to learn from recent downloads was my preheat cycle is running on average about 34% of the total cycle. Therefore, I would conclude that I'm still somewhat under vented. More testing is needed.

Mark N

New Year's update, downloaded my data logger this morning to look at the overnight boiler cycles. It dropped to 12 degrees overnight so the boiler was cycling more often(boiler off approx. 75 minutes in between cycles). Ran 7 heating cycles between 10:51pm last night and 8:48am this morning, The preheat phase averaged 26.3% of each cycle and the heat phase averaged 73.7%. The best cycle being 22% preheat and 78% heat and the worst being 31% preheat and 69% heat. Hopefully we'll soon have an overnight near zero to see what those numbers look like.