Storage Tank - Freeze Protection

[jebatty]

Looking for feedback to see if my logic is correct. Gasser heats my shop. 1000 gal pressurized storage, LP tank, horizontal. During winter we may be gone for 1-3 weeks with no heat in the shop during this time. With -30F winter temps, freezing could occur. To prevent this I have installed an electric 12000 watt tankless water heater to provide freeze protection which will be activated by a tank sensor during winter absences (also will activate the circulator). Water circulation is cold return water from bottom of tank, through boiler, to tankless hot water heater, with hot water from the tankless heater into the top of tank.

Here the logic feedback inquiry. Water is most dense at 39F; as the tank cools, 39F water will fall to the bottom until the entire tank is 39F. Then colder less dense water rises to the top until freezing starts to occur, from top of tank down to bottom.

If I mount the sensor in the top of the tank to trigger the water heater "on" at about 40F, the top of tank will receive hot water. This will mix with the cold top of tank water until the differential shuts the hot water heater off, assumed about 45F. Below the 45F water will be essentially only water at 39F (colder water always rising to the top). So, if I keep the top of tank at a constant 40-45F, the entire tank should not be subject to freezing. Is this correct? And with water circulating through the system periodically as the tankless heater cycles on and off, the entire system should not freeze? I suspect circulated water will be at 39F (from bottom of tank), with any colder system/boiler water being mixed with 39F water and cycled through the tankless heater into the top of tank.

Is there a better place to mount the sensor than at the top of the tank?

My system is very simple, just the boiler, with plumbing lines to and from the tank, about 25' away. No other zones. Plumbing rises from top of boiler, along the ceiling, down to tank, and vice versa; no plumbing lower than the boiler, which is about 1' above floor level. In normal operation tank operates as a radiator to heat the shop.

 

[sdrobertson]

Your logic sounds strong. My concern would be in the piping and boiler instead of the tank. It seems to me that it would take longer than 3 weeks to get into the danger zone with a 1000 gallons of insulated pressurized water. I'd look into a sensor at the boiler or on your piping that would circulate water from your tank through these to keep them above 40 degrees or so. I think it would be much cheaper (less electricity) to heat the water in the boiler and piping than trying to heat 1000 gallons up 5 or 10 degrees. You may be able to get by with just a really small circ moving water to keep it from freezing.

 

[DaveBP]

Jim, I assume then that you have some other provision for keeping the boiler itself and the pipes from freezing? Seems they would be in danger long before the tank otherwise. I can't remember your description of how it's all insulated. You could probably just put a 250 watt light bulb on a thermostat in the afterburner with the flue blocked off to keep the boiler from freezing but the pipes are still all alone out there in the cold.
Heat tape on the pipes? Especially on the circulator.

It's a real physics quandary, isn't it? Liquid water of different temperatures will stratify because it can move and stratify quicker than the warmer water can conduct heat to the colder water. Without any cooling or heating going on the water will eventually all reach the same temperature. What puzzles me is what happens when the water cools off slowly enough that the conductivity of the water can equalize the temperature throughout the tank faster than the water can stratify? Does it just all continue along at the same temp throughout, cooling until it starts to freeze uniformly along the inside wall of the tank?

The air outside your tank but inside your fiberglass can move through the fibers with that kind of temperature differential. Will the surrounding warmer air, which will rise to the top of the tank, keep the top from cooling as fast as the bottom? Just because it might doesn't mean it will to a significant extent.

Regardless, once that almost-two-bucks-an-hour heater kicks in it will be stratified again.

If you have the drain port on the bottom of the tank piped out to a valve for draining that might be a good place to put a pressure relief valve in case it does freeze some. I think you're right, the water on the bottom is going to try to stay 39F and that might let a lot of ice form before the pressure rose too much.

Hope this gives you something to think about while you're down in the tropics getting sunburnt this winter.

Oh, and I vote for near the top of the tank.