Non-Pressurized Hydronics

BC

I'm deep into my Ground Source Heat Pump design (still keeping the old steam system for supplemental/backup) and I was hoping to get some thoughts on what the drawbacks of non-pressurized hydronic systems are. The reason I’m interested is that an open system would give me much more flexibility in selecting a buffer tank and indirect HW setup. The output of the heat pump is 60-70mbtu depending on the water temps, so it requires a fairly large buffer to get decent run times at part load. Also, if I make the buffer tank large enough (like 2-300 gallons), I can do much of my heating on the night rate and save even more money with the geo system.

Obviously pressurized systems are preferred, but why? Here’s what I’ve thought of or found in the archives so far:

Advantages:

- nearly infinite possibilities for buffer tank selection, components don’t have to be pressure-rated

- easy to fill/purge system, eliminates expansion tanks and air separators

Disadvantages:
- pump cavitation: the pump inlet would have to be right at the tank outlet and be unrestricted, but as long as there is a 4-5 foot head above it, it should work according to the specs for my pumps

- corrosion – Would oxygen continually enter the system? The tank would be closed, but vented. This seems like the biggest drawback I can find. Would bronze circs be required? Would the fact that my system would be limited to ~100F help?

- ????


I know non-pressurized systems on the earth loop side of Geo systems are gaining in popularity, but I don’t see much on hydronic heating at low/atmospheric pressures. The systems out there seem to be wood boilers with large tanks and solar systems.

As always, thanks for the input!

Bob Gagnon plumbing and heating

evaporation

is an issue you have to deal with when using a non pressurized tank. I just fill mine a couple of times a year. Bob

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hr

Mine grew some sort

of bacteria or algae. The whole shop had a terrible odor. I tried bleach, Rhomar nothing would handle the slime and smell.

I think a semi sealed cover with a vent to the outside would help. Better yet try not to get the grow in the first place.

I know Tarm uses a 500 gallon open tank as a buffer and DHW generstor with their wood boilers. I suspect the higher operating temperatures handle any bacteria growth.

hot rod

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LarryC

Evaporation from Vented tank

In nuclear plants I have worked in, they use water filled tanks for shielding. These tanks are vented to atmosphere and they are all marked with a sign stating that _X_ number of gallons of lube oil is on top of the water, to minimize evaporation losses.

I would believe that any clean non evaporating oil should work. Vegetable oil, 30W motor oil, fry oil, etc. Or you can use several inches of the polyethalene beads used for plastic injection machines, or similar materials.

Joe_76

I would use a good oxygen scavenger that would take care of the oxygen. I use a GE Betz or Nalco scavenger for the wood/coal fired boilers I install. I do not have any experience with heat pumps and am not sure if the use of oxygen scavengers are recommended. If you do not use any steel you can get around the oxygen problems. You are correct about oxygen always entering into the system with an open system.

Steamhead (in transit)

Or

how about an old-time open expansion tank, as the Dead Men used on gravity hot-water systems? These were vented thru pipes of 3/4-inch or so, keeping air exchange to a minimum.

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Bob Gagnon plumbing and heating

algae in tank

My tank stayed clean. It smells O.K. too, a little like boiler water. They told me if the water didn't see sunlight it couldn't grow algae. Maybe the tank didn't get hot enough? Why sould the tank have to be vented to the outside? Bob Gagnon

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Perry_2

O2 Scavanging and Slime issues

Several issues:

Oxegen scavangers work best at temperatures. Im not sure if there are any Low temperature oxygen scavangers that work well at 100 F - and are safe for use by normal people.

An alternate stratagy, corrosion inhibitors. Unfortunatly, those typically require monitoring to ensure there is enough (along with system Ph). That is fine if you are an amature chemist - or willing to pay for the analysis...

The nuclear plant mentione above would have at least a corrosion inhibitor in those tanks; and they have a fully staffed full time chemical lab to take care of it. They also would have filled those tanks with extreemly pure water to start with (distilled or de-ionized)

The biggest issue I see is slime... If the system is open to the atmosphere at least some dust and things will get in (trust me on that). Slime will grow unless you heat the system periodically to a high enough temperature to routinely kill the slime; or you use a bacteriacide treatment to prevent slime from growing.

It is unlikely that you will ever heat the exansion tank enough to kill the slime.

I would check into bacteriacides (and perhaps you can get a corrosion inhibitor and bacteriacide packaged product). However, remember to have it tested every now and then and add new chemicals as needed.

I've seen a slimed up system- even glycol water mixed "closed" system except for expansion tank. You don't want to get there - trust me.

If it were me; I'd adopt a 1 or 2 PSI system. The tanks do not have to be ASME stamped; and make sure you have an appropriate relief device (rupture disk would probalby work best). Then I'd try to flood the tank with Nitrogen gas and flush the O2 out of it.

Perry

mtfallsmikey

The unmentioned danger of doing this is

Legionella

BC_3

Legionella

Just want to clarify that this won't mix with a potable water system, that will be done via an indirect and booster tank. In fact one of the reasons I'm looking into unconventional arrangements is that the traditional method of doing DGW with GEO systems is to run an indirect at 115F or so and then feed it into a boost tank to get to 140F or whatever you need. I was concerned about Legionella growth in the first tank. I want to use a reverse indirect arrangement, but the Ergomax type heaters seem to be much more $$$ than a regular indirect. I may still go this route but I want to explore all the options first.

Thanks for the advise so far - it sounds like the biggest concern will be controlling bacteria growth somehow. I like the idea of a closed low pressure system.