Forced hot water zone on steam system air bound

wrenchhappy

I recently installed a steam boiler with a forced hot water loop for some cast iron baseboard on the first floor. The boiler is located in the basement of this house. The supply for the forced hot water loop is tapped into where the boiler drain is on the lowest tapping on the right of the boiler. I then piped in a drain valve on a t for purging, then an isolation valve. Next is my circulator and then my bypass line which i adjusted to prevent circulator cavitation. The piping goes to a small room which has cast iron radiation and a 4 foot section of slantfin baseboard. My return has a spring loaded check on it before it enters the hartford loop at its lowest point on the house side of the loop. The Hartford loop enters the boiler on the left. I purge this loop the way i read in the appropriate section of "heating help" but I keep getting air in this hot water loop. Is it because I do not have a check after my circulator on the supply side of the forced hot water loop? If that is the reason, should I install a sweat check or a spring check there? Also my only current spring check is in the horizontal position on my return piping. It did not come with mounting directions, must it be mounted vertically or no? Any help is greatly appreciated. Thanks.

Steamhead

Are there

any air vents on the radiation in that loop? 

wrenchhappy

yes

on the cast iron baseboard there is a small manual bleeder. it is in the closed position. on the 4 foot section of baseboard there is a purge t with a draw off that is also in the closed position. the second time the loop got airbound I ended up isolating the forced hot water zone and pressure testing it with an air compressor. it held 20 psi for 30 mins with no drop.

Steamhead

Somehow

air is getting into that zone. What type of circ are you using? 

wrenchhappy

yes

circular is a taco 007. I just came from purging that zone at the customers house. The zone works for two weeks fine, then is totally air bound and then I have a call that there is no heat. When I show up the loop is full of air again. In the piping diagram there is a check after the circ on the supply to this zone. I do not have one. Is there a pump I can use with an internal flow check so I can fix this? Do you think the pump is pulling in or creating micro bubbles? What pump is best for this application? Thanks again and happy thanksgiving.

icesailor

Air bound:

I'm not an expert on steam but you make no mention of seperating the hydronic (wet) from the steam part of the equation. If you are just running the condensed water through the hydro loop, the water is probably boiling from differential pressures in the hydronic loop where in the steam loop it wouldn't matter. I usually understood that these loops were usually seperated somehow with a 12# fill valve and a extrol type pressure tank so the hydro loop maintained 12# pressure boosting the boil point to 244 degrees.

But it also sounds like you are taking the supply water off the bottom of the boiler and returning it to the bottom via the Hartford Loop. That sounds wrong to me. And it isn't common to mix cast iron radiators with hot water baseboards. Which is fed first? The radiator first? Does the whole radiator need to get hot first before hot water gets to the baseboard? Or the other way around? Did you use monoflows on the radiators?

If you use auto vents on the baseboard and the radiator, it would probably eliminate your air problem but would contribute to the early demise of your steam system. Because of the air.

I think that something is not right here,

But, JMO,

 

Steamhead

A couple of suggestions

First, use the circ's pressure differential for all it's worth to collapse air bubbles. Put your check valve on the return from the zone to the boiler- but before the bypass connection. The resistance in the check will cause the circ to shift up its curve and produce more static pressure, which would tend to collapse air bubbles and flush them out. We use the same pressure-differential principle in Pumping Away.



Second, I'm not clear about where your bypass connects to the zone supply. It should connect before the circ inlet to mix some cooler return water into the line to the circ inlet.



Third, do you have an aquastat in your setup that will keep the boiler from steaming if only the hot-water zone is calling?



Last but not least, the system side of the Hartford Loop is not the best place for the return connection. It should be on the boiler side.



As to the circ itself- it's best to use a bronze unit with an oil-lubed pump bearing and a mechanical seal. I know they're not cheap, but the water in a steam boiler is not very kind to wet-rotor circs.



Here are some pics of one of our installs. This one is feeding a SuperStor tank, but you can see how it comes together.

wrenchhappy

steamhead

I do have an aquastat on the hot water loop but I'm not sure if it is wired correctly. If the boiler is steaming and the hot water zone calls I'm wondering if it is wired in such a way that it allows the pump to kick on for the forced hot water loop, which would draw in air right? This would also make sense as to why it works fine for a few weeks, the whole time building up air in that zone when they both are calling.

Steve_210

click system

on the top of this page, then hot water then condensate hot water heating.

My bet is if you remove the air vent and add 2nd flow check proublem will go away.

Steve_210

also

the 007 will not last very long. anywhere from 6 months to 2 years

Steamhead

The way it works is

when the HW zone calls, it pulls in the relay. The relay has two contacts: one line-voltage which starts the circ, and the other is 24-volt which is fed from the burner primary and goes to the low-voltage contacts on the circ relay via the aquastat. If the HW zone is calling but the steam is not, the aquastat will open the 24-volt circuit from the primary and stop the burner when the boiler reaches temperature. The circ will keep going until the HW zone thermostat is satisfied.



If the steam zone is calling at the same time the HW zone is, the burner will continue to run even though the aquastat has opened. That's why we have the bypass- we don't want the circ to see steam-temperature water.



As long as the circ inlet is below the waterline, it won't suck air.



How about some pics of your problem job?

wrenchhappy

the system

only has an aquastat now not a secondary relay, but I do not understand how that could be causing a buidup of air. The pump on the supply for this loop is below the water line and is tapped into the boiler drain tapping on the right side of the boiler. Do I need to remove the manual air vent on the cast iron baseboard? If I do can I purge it at the loop at the boiler or will air get stuck because of the casting of the cast iron baseboard and how the air flows in it? Even though I pulled a pressure test on that loop of 25 psi that held for 30 mins?? I mean can it really be sucking all this air through that tapping over a two week span? I'm thinking of making the aquastat regulate the fhw pump and burner because the only thing that seems to make sense is that when the boiler is making steam and bubbling in the chamber, and the fhw loop calls, the pump is pulling in small bubbles. The pump tapping is low enough that makes me think it must be something else, but it is airtight.