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we just drain the water heater and our pipes every time we leave during the winter months…no math required, thank heavens!
Update: It survived! Modern 2014 electric water heater (R.95 rated) with zero power for 5 days. The lines were drained so my only risk was the water heater becoming a block of ice.
Thank you for the feedback. I will work on measurement systems to further this study.
I’ll guess that the new high efficiency water heater the original poster mentioned probably does run on propane, but it needs electricity to start it (gets rid of the pilot light). The newer high efficiency products are great to reduce energy consumption but they are a pain if the power goes out.
I have a similar issue with a high efficiency furnace – it’s supposed to be drained from any residual water condensate if it will be cold and the power out. What a pain! It was much easier with older furnaces.
Back to the original poster – you are missing some information such as thermal resistance, but you could make some measurements when you are there for a time. Turn it off, take a sample out of the drain every couple of hours for some period of time and plot the result. Extend the line out and let us know what you find. That’ll be some real data.
…and if you have a propane water heater, I’ve found that it’s more than adequate to set the control to “pilot” when we leaving during the Winter…That little flame protects the tank from freezing, just fine.
Then we simply turn it to “on” when we arrive.I’ve been a service and repair plumber up here since 1979 and have never run into a frozen water heater in Big Trees that was in an enclosed sub-area. Cottage Springs and higher elevations can have extended low temps enough to cause problems.
too many missing terms to complete calculation, but 40 years of repairing split pipes; 3 days at 18 will split improperly drained lines. I would drain out hot and cold lines and ignore tank or maybe drain 25% off to allow expansion. If you are not local then stop by the hardware store for a supply of 1/2 and 3/4 elbows, several tanks for your torch, sleeves and couple of feet of pipe.
I may be low with a 25F mean estimate. The area under the cabin is enclosed so it does not get the full effect of outside temperatures. However with temperatures forecasted to drop below 20 for a couple of nights it is really going to get interesting if the power is not on soon.
Not doing your math – but some anecdotal info: our cabin was 33* last night after no power since Fri/Sat wee hours. So your 25* guess may be too cold 🙂
Any engineers out there care to take a crack at this calculation? Starting temperature of 125F, mass of 55 gallons stored in high efficiency water heater container. No wind exposure and sits under a cabin on a concrete pad with a ground temperature of 50F. Mean ambient temperature of 25F.
In the absence of electric power how many days will it take for the tank contents to freeze solid?
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