Closed or open forced hot water system?


Old 09-04-09, 09:14 PM
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Closed or open forced hot water system?

My friend recently had a huge leak due to a failed expansion tank in his forcced hot water system. That got me to thinking about why my makeup water valve is open all the time. Can the system be run closed since it has an expansion tank. Is water actually consumed somehow?
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Old 09-05-09, 09:11 AM
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The system can be run with the make up water valve closed. Some are this way as they don't have a auto fill valve (pressure reducing valve). Just a valve to open to fill, then close.

Whether to run the system with the valve open or closed is always a good topic. Seems to be split 50-50 on whether to do it or not.

Pro of leaving it open: if a leak develops it will prevent the system from running dry. Which can cause major damage to the boiler. Although a LWCO will alleviate this issue.

Con of leaving it open. If there is a small leak it can go un-noticed. The constant flow of make up water will rot (corrode) the system out from the inside.

If it is a large leak there can be flooding and excess damage.

Old 09-05-09, 09:19 AM
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[edit: Al beat me to the punch! but said the same thing with far fewer words!]

First, let's define an 'open' system, vs. a 'closed' one.

A closed system is one that is pressurized, and not open to the atmosphere.

An open system IS open to the atmosphere, and generally not pressurized.

Simply opening or closing the feedwater valve does not mean you are going from an open to a closed system. You have a 'closed' system in either case.

Next, let's talk about the pros/cons of either approach:

In a perfect world, yes, you could close the feedwater valve once the system has been up and running for a while, and all the dissolved oxygen in the water has been eliminated, and the pressure should remain, and the system should not need any more water. But, as we know, this is not a perfect world. For various reasons, during operation, small amounts of water may be needed to be added to the system.

If you fill the system, and close the valve, you will have to do due diligence in observing the pressure gauge periodically. If you do happen to spring a leak somewhere, water damage will be minimized, BUT if the leak continues, and the system becomes drained, you run the risk of firing the boiler without water in it... the boiler can be damaged beyond repair, and there is a highly increased risk of fire.

On the other hand, leaving the valve open, you can pretty much forget about checking the gauge for the most part (you should check occasionally anyway), but as in your friend's case, his leak probably caused massive water damage to the home.

One possible solution that approaches the best of both cases would be to install a "Low Water Cut Off" (LWCO) device that will disable the burner in the event that there isn't enough water in the system. This will prevent the 'dry fire' scenario above. You would still want to monitor the pressure in the system if you closed the feed valve.
Old 09-05-09, 04:12 PM
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When I purchased my house, I had this really strange sloshing noise in my heating system. After a friend came over to help, we discovered that the previous owner broke the feed valve, and simply left it off -- so there was very little water in my system.

But after figuring this out, I've run my house for 4 years now with the valve closed. However, at least once every 2 weeks during the heating season, I use a vice grip to open it and top off the system. Haven't had any problems with this.

But now that I'm planning on replacing the boiler, I'll be fixing this issue as well.

If you are afraid of pipe problems causing water damage anywhere in the house, look on-line, I believe there are electric flow sensors you can get that will automatically cut off flow to an appliance (or whole house) if an unusual constant running situation is encountered.
Old 09-05-09, 04:49 PM
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Glad to hear that you are going to replace your automatic fill valve. Vise grips aren't good solution, but I guess they have worked out for you.

I haven't heard of a continuous flow detector being applied to a residential heating boiler. Might be OK in theory, but unless it's UL or boiler-manufacturer approved, I'd be leery.

The main reason to leave the make-up line open is in case the boiler water level somehow goes low, and there isn't a functional low-water cutoff. If that isn't the concern, and flooding is the main concern, then I'd just shut off the fill line like your have been doing.
Old 09-05-09, 08:53 PM
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Great comments. Thanks for the help.
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