Storage tank freeze protection

where is your tank? it should go days before you had any issues? if its in a shed it would be cheaper to find a used wall mount propane heater to heat the shed rather than heating the tank/water.

Prior poster had good questions. This is how I dealt with freeze protection. I live in a potentially very cold northern climate, winter extremes to -40F at night for several days a real possibility, also daytime highs < -10F. Radiant in-floor has anti-freeze and is isolated from the system through a plate hx. Boiler and storage just water. Storage is well insulated (R30+). A typical period of no heat during winter extremes while I take a trip somewhere would be 7-10 days on 1-2 occasions, maximum of 22 days but only once.

I have my system controlled so that it will not draw down storage below 70F. This fully protects the tank, as insulation will hold tank above freezing for weeks. I have a 240V -- 3000/5000W electric space heater for the shop that I set at 50F during long absences. Use of this last winter during one of those extremes resulted in an additional $25 on the electric bill. $25 1-2 times a winter is a small price to pay for freeze protection, and is much less than more exotic freeze protection schemes. Plus, it is as simple as flipping a switch.

I bought the heater years ago to keep my house above freezing during a winter remodel and rewiring which resulted in no heat for nearly a month. It did the job. But another very inexpensive approach would be to use a couple of 1500W space heaters, which you can buy new for $10-20 each, and locate strategically. If to be used for backup heat, I would put the heater on a piece of Durock or similar for a bit of extra safety.

I think you live in a climate much milder than mine, and I would recommend finding the easiest and least expensive solution to freeze protection, unless your situation really calls for more extreme measures.

Jim has good advice. I wrapped some heat tape around my danfoss valve and piping that doesn't circulate when stove is off. I also wired the boiler circ, primary loop circ, and circ feeding the UG loop to house to run continuous while we were gone for a week in Mexico last winter. It actually got very cold while we were gone, about -20F on some nights but fortunately the shed was still about 32F when I walked in. Probably got lucky because shed it is pretty well insulated (R19 walls, R38 ceiling). This year I will have 1000gal storage and I plan to get it hot and just circulate through the barn heaters without the fans. The house can run on the heat pump and I will actually close the valves in the house going to the HX in my air handler to prevent any ghost flows. This way the UG loop will still circulate without losing much heat (I hope). I may put one of those 1500W heaters on a low setting pointed toward the pipe that will circulate water in case my 1000 gal cools off that much during the week we are gone this year.

Last year I didn't have my tank hooked up but we went away for 7 days during our coldest stretch. I have a milk house electric heater in the boiler rm that worked fine. This year I think I will put in a electric baseboard heater.

The tank I will probably just heat to 160-180 and shut down. My underground pipe is 6' down and both ends come up in heated space.

gg

Sounds like it should work with a simple aquastat on the storage tank. I'm sure you realize heating 500 gals with a gas boiler could be very costly and I'm not sure hot you would need to get it to keep the house from freezing up. We spent last New Years eve in a hotel nearby and I let my air handler HX "steal" heat from the air put out by the heat pump heating the house by running the HX circ continously. I'm not sure how much this cost since it was only 1 night but it was very cold outside. What I did notice was the the piping in my shed was warm to the touch (about 75*) when it is usually very cold in the mornings when the fire was burning out around 3am or so. The heat transfer is obviously just as efficient in my HX working in reverse. A better long term solution for you might be a primary secondary setup that would allow the gas boiler to just heat the home and your storage tank aquastat could let your storage tank "steal" enough heat from the primary loop for freeze protection. I'm guessing you must have 3 circs now (WB,GB,house heat loop) and the primary-secondary would probably add 3 more (pri loop, tank charge, tank discharge). They extra electricity would be minimal but it would be more complex and more things to maintain. If you go the simple route that you suggested, the big question would be what should the aquastat be set at to make sure the house gets enough heat to keep it from freezing up. I suppose you could prevent house freeze up if you added another thermostat in the house that would fire the gas boiler whenever the house got below say 40* (if they go that low). That way gas boiler would run in either situation or both situations. If the wood boiler is the shed with the tank I would also be concerned with protecting it as well. While some like my EKO 40 turn on the boiler circ when their water temp gets cold enough you still have to worry about Danfoss/Tekmar valve and the piping portion that doesn't normally flow when boiler is not hot. Sorry for the rambling, but things are never as simple as they should be.