Off-season mothballing a tank-type water heater

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gthomas785

Minister of Fire
Feb 8, 2020
1,187
Central MA
So I currently run a dehumidifier in my basement during the summer. Encapsulation is poor and will never be perfect due to the age of the house and inaccessible areas, so the energy consumption from dehumidification is significant. I also have a fairly new tank-type gas water heater located in the basement.

My plan is to additionally purchase a heat pump water heater that I can run during the summer which would hopefully help with the dehumidification and provide hot water as well. I don't think I can use the heat pump full time as the basement gets pretty cold in the winter and electricity is pretty expensive to run in heating element mode. So I'm thinking to keep the gas water heater and switch back and forth seasonally.

Now to my question: Since I'll be switching water heaters twice a year, what's the best way to preserve a tank water heater when not in use? Should I drain it or just valve it off and leave it full of water? My first instinct would be to drain it but someone told me that's a bad idea because it introduces air which combined with residual water will accelerate corrosion.
Our water is from a small municipal system, it is well water but it gets treated by the town. I haven't noticed significant buildup or corrosion on our fixtures so I don't think the water is particularly hard but I have not measured TDS or chlorides to be sure.

This whole thing may be moot as I do have a newly installed rooftop solar system with net metering, but I have not seen it through a winter yet so I'm not sure how many credits I'll actually have come January so as of now I don't know if I could get totally free electric DHW all winter long. That is why I'm thinking of switching back to gas for the cold season.
 
If you know there is no chance of freezing, I'd just leave the water in and valve it off. If there is any chance of freezing I'd empty it. I empty my tank in Utah multiple times a year in winter as that can freeze, and so far so good for the past 13 years. But those are usually for 1-2 month time spans, although it was empty for over a year during COVID.
 
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i would leave the tank full and when you plan on using it dump it and refill, this way you will not introduce rust into your tank. A dehumidifier does not really take a lot of energy to operate i used to run one 24/7 and barley noticed the increase on my hydro.
 
Yup, on restart, i'd drain the tank till it runs clear.
 
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Thanks. My basement hasn't frozen yet; I keep a thermometer down there and it's usually in the low 40's during winter. There's another project - fixing the bulkhead door seal.

My dehumidifiers definitely create a noticeable bump in kWh consumed when I turn them on during the summer months - but I don't find that too surprising since I have some areas of exposed dirt in crawlspaces (which I've tried my best to cover with plastic, but some I just can't get to) and the dehumidifier seems to run a 50-60% duty cycle most of the summer. Kind of like running a window A/C constantly but without the cooling benefit. That's why it seems like a no-brainer to put a HPWH down there - at least I'll save some gas over the summer and possibly keep the house a bit cooler with no added energy cost.
 
Thanks. My basement hasn't frozen yet; I keep a thermometer down there and it's usually in the low 40's during winter. There's another project - fixing the bulkhead door seal.

My dehumidifiers definitely create a noticeable bump in kWh consumed when I turn them on during the summer months - but I don't find that too surprising since I have some areas of exposed dirt in crawlspaces (which I've tried my best to cover with plastic, but some I just can't get to) and the dehumidifier seems to run a 50-60% duty cycle most of the summer. Kind of like running a window A/C constantly but without the cooling benefit. That's why it seems like a no-brainer to put a HPWH down there - at least I'll save some gas over the summer and possibly keep the house a bit cooler with no added energy cost.
Yeah i'd like to do a similar set up. Keep us posted on the HPWH. thanks
 
Leaving a tank full of warm water on a system that has no circulation is potentially hazardous due to Legionella Disease. It breeds in stagnant warm water. Ideally valve out the tank but leave it full and then purge the system with hot water in excess of 140 deg F preferably warmer before putting it back in service.

A general recommendation is take the time to drain any sediment out of the bottom and check and replace the anode if its needed. If you keep a good anode in tank, it could last decades. many do not and wonder why they are replacing hot water heaters every 10 years or so.
 
Leaving a tank full of warm water on a system that has no circulation is potentially hazardous due to Legionella Disease. It breeds in stagnant warm water. Ideally valve out the tank but leave it full and then purge the system with hot water in excess of 140 deg F preferably warmer before putting it back in service.

A general recommendation is take the time to drain any sediment out of the bottom and check and replace the anode if its needed. If you keep a good anode in tank, it could last decades. many do not and wonder why they are replacing hot water heaters every 10 years or so.
I don't think the water would be warm, since the gas would be off... but I absolutely agree it would be wise to flush the tank before putting it back into use so that will be part of the protocol.

Great advice about the anode rod. I've had a spare one sitting on the shelf since I bought this gas unit in 2021 with a plan to check it after 3 years. We don't use that much hot water as of now... Just 2 of us in the house...
 
Anodes are not really use related as much as how agressive the water is. The city I grew up in, Portland Maine, had incredibly pure water that would leach protective coatings from hot water tanks, anodes would get eaten up in about 5 years, the local plumbers made good money on replacements. It also was leaching out lead from solder joints so at one point they started adding some sort of protective chemical to make it less agressive.

My SHW tank which has a rarely used electric backup has an anode, the tank is 20 plus years old. I did an anode around 12 years and it was still mostly there. I had to cut it to get it out so I put in hot dog type and will check it in another 12.
 
I assumed the anode lifespan would be determined by total volume times water aggressiveness but I guess it depends on the reaction rate versus how often you draw down the tank. The previous water heater in this house was 33 years old when replaced and as far as I could tell the previous owners never touched the anode rod so I am hopeful that is a good indicator of our water quality. However our water district has been adding some additional treatment steps due to the pfas contamination that is popping up everywhere so we'll have to see if that changes anything.