minimum operating pressure for boiler

[sfriedri]

I'm trying to figure out different ways to set up my new EKO boiler and heat storage without the need for an expensive expansion tank. I only have one zone in the house (water to air exchanger), so to keep things simple I plan for the boiler, storage tank and zone loop to use the same water.

One option I am considering is to run a line from the storage tank to a small tank which would be located at the highest point in my system and have it open to atmosphere. So when the system water expands/contracts, the water level in this tank will rise and fall. Based on my system volume of approximately 500 gallons, a small 20 gallon tank should be sufficient to handle the expansion volume of the system water, and I would just leave a plug at the top of this tank open. Running a line to this location in my house is very easy, so there is no problem from an installation standpoint.

One concern I have with this approach is whether I will be maintaining enough pressure in the boiler. The expansion tank will be located about 15 feet above the boiler, which would give me a little more than 5 psi in the boiler. I suspect the water in close contact with the boiler exchange tubes may get close to the boiling point, so I'm wondering if 5 psi is enough to minimize the formation of bubbles at the steel/water interface, which would theoretically reduce heat transfer efficiency. Anyone with comments on this issue, or on this approach to a low pressure semi-closed system? In terms of minimum pressure on the inlet side of the circulating pump, 5psi is more than sufficient based on the specs for most pumps. I would also plan to monitor water pH etc. to make sure it minimizes corrosion.

 

[Nofossil]

I think one of the more serious long-term concerns about open or perhaps even semi-open systems is the continual supply of dissolved oxygen rather than just PH. If you want to do some long-term testing and provide us with data on corrosion rates, that would be great.

Otherwise, could you pick up a dysfunctional electric hot water tank, drain it, plug the top port(s), and connect to the bottom? This is the poor man's expansion tank. Has the virtue of being a closed system, though the tank may become waterlogged over time.

 

[Eric Johnson]

The pressure tank on my old system was (still is) a big, old riveted pressure tank that hung (hangs) above the boiler from the floor joists. It does fill with water over time, but it's just a matter of periodically draining it off and refilling. No big deal with the right valve placement.

Interestingly enough, the EKOs (and other Euro boilers) are run as open systems in Europe, as I understand it. But, like nofossil, I'd be a little nervous about doing it, especially if you have a flat plate hx somewhere in the system, as the inevitable corrosion products and other crud can clog up the tight tolerances used in flat plates. When used with open OWB setups, they need to be flushed on a regular basis.

 

[Toast]

You mean .... after spending all that money on an EKO boiler you don't have and expansion tank? They cost $60...

I use a Electric water heater for my storage... my understanding is that i still need an expansion tank.


I will say that if you use pex, copper, and a bronze pump, you probably don't have to worry about oxygen.

correct me if I'm wrong.

 

[sfriedri]

You mean .... after spending all that money on an EKO boiler you don't have and expansion tank? They cost $60...

I use a Electric water heater for my storage... my understanding is that i still need an expansion tank.


I will say that if you use pex, copper, and a bronze pump, you probably don't have to worry about oxygen.

correct me if I'm wrong.

As I indicated, I just bought the boiler and am still in the process of planning the options for the rest of the system, so nothing is hooked up yet. A properly sized expansion tank for a 500 gallons boiler/storage closed system is several hundred dollars. The typical 2-3 gallon $60 expansion tank used in domestic hot water systems is not nearly large enough.

Since the boiler is steel, you still need to worry about corrosion no matter what the rest of the system is made of.

 

[Nofossil]

As I indicated, I just bought the boiler and am still in the process of planning the options for the rest of the system, so nothing is hooked up yet. A properly sized expansion tank for a 500 gallons boiler/storage closed system is several hundred dollars. The typical 2-3 gallon $60 expansion tank used in domestic hot water systems is not nearly large enough.

Since the boiler is steel, you still need to worry about corrosion no matter what the rest of the system is made of.

Exactly. I've seen and heard some evidence that suggests that a properly configured open system can give good life, but I don't know enough to feel comfortable suggesting it.Float a layer of mineral oil on the surface to keep the oxygen out, maybe?

 

[sfriedri]

The pressure tank on my old system was (still is) a big, old riveted pressure tank that hung (hangs) above the boiler from the floor joists. It does fill with water over time, but it's just a matter of periodically draining it off and refilling. No big deal with the right valve placement.

Interestingly enough, the EKOs (and other Euro boilers) are run as open systems in Europe, as I understand it. But, like nofossil, I'd be a little nervous about doing it, especially if you have a flat plate hx somewhere in the system, as the inevitable corrosion products and other crud can clog up the tight tolerances used in flat plates. When used with open OWB setups, they need to be flushed on a regular basis.

I think one of the more serious long-term concerns about open or perhaps even semi-open systems is the continual supply of dissolved oxygen rather than just PH. If you want to do some long-term testing and provide us with data on corrosion rates, that would be great.

Otherwise, could you pick up a dysfunctional electric hot water tank, drain it, plug the top port(s), and connect to the bottom? This is the poor man's expansion tank. Has the virtue of being a closed system, though the tank may become waterlogged over time.

Thanks for the comments Eric and nofossil. My current thought is to not use any water-water heat exchangers in the system: the same water will circulate through the boiler, storage and zone loop, with separate circulation control on the boiler and zone loops. I've read through the primary/secondary loop sticky, and don't see a clear advantage to using water-water exchangers in a closed pressurized system. What is it that I am missing? I understand the need for exchangers when combining a pressurized boiler with an open heat storage tank, but not for a completely closed system.

The main reason I am thinking about the low pressure system I described in my first post is that I have access to a free 500 gallon stainless steel tank that is not a pressure vessel. I'm sure it can handle up to 5psi, thus my plan for the low pressure system. I guess one thing I could do to statisfy the control of corrosion and pressure in the boiler and still allow me to use this tank is to set up an exchanger between the boiler and tank, and have the storage tank and zone loop open to atmosphere. Any problems with this setup? Since I have only one zone, I assume I don't really need an exchanger between the storage tank and zone loop?

Thanks for the help.

 

[Eric Johnson]

The system you describe wouldn't need any heat exchangers.

However, I'm not sure how well it would link up with your existing zone, which I presume is pressurized. OWBs run basically as you describe--they're open at the top and if things start to get out of hand, the water just spills out of the system. They all require flat plate heat exchangers to connect to the house's heating system.

 

[sfriedri]

However, I'm not sure how well it would link up with your existing zone, which I presume is pressurized.

I been heating with wood for a while just using a Pacific Energy wood stove in the main part of the house. The house also has a forced hot air furnace, and I had the furnace fan on a timer to circulate the air once in a while. The house currently has no hot water heating system, and I plan to use a water-air exchanger in the furnace to heat the house with the boiler. This is why I mentioned I have only one zone and don't believe I need a water-water exchanger between the storage tank and zone loop. I put an ECM motor in the furnace last year (Ebay for $100) and it only draws 70 watts, so although the water-air system is not as good as a pure hydronic system it's all I've got to work with and the air distribution side of the system is fairly efficient.

I wish I was back in one of my old houses where I installed a mixture of heated floors, rads, and even heated towel warmers / rads in the baths.

 

[jebatty]

I agree with nofo on his response that the issue is not just pH but also dissolved O2. Any part of the system open to the air will take on O2, and every molecule of that O2 will cause corrosion. A closed, pressurized system does not have this problem once the O2 in the system is consumed, as corrosion then ends.

All boiler systems should be check for pH (range 8-11 probably OK). If pH is OK, should be no need to check again except when new water is introduced into the system. Also, systems should be checked for total alkalinity, which buffers pH changes.

As to O2, sulfite should be added in all systems to consume free O2 to prevent corrosion. Once stabilized in a closed system should not need to check again until new water introduced. In an open system, ideally should monitor sulfite. An OWB system using sulfite with pH in range should experience minimal corrosion.