I’ve been told that a storage water heater can only deliver about 70% of its volume at peak temp before you begin to sense that the water is becoming less warm (i.e. a 80 gallon unit set at 120 degrees can only deliver about 56 gallons of water at that temp before the recovery mix is significantly sensed by the user).
Question:
Would two 40 gallon units connected in series deliver a larger percentage of peak temp water?
Replies
Yes, but they'd take up more room and use more power/fuel.
I tend to agree w/ Mr DanH. Not sure if you would really notice a big difference, though, because the same phenomenon is happening in both situations, really.
As you begin to use hot water, you start refilling the tank w/ cold water which instantly starts affecting the water temp ... although the incoming is at the bottom and the hot water leaves at the top, so there is a lot of stratification there.
Are you having problems running out of hot water? If so ... ease the temp up until you don't. Take care, though, you may want to take anti-scald precautions if your house doesn't already have them. Better yet consider water saving devices that use less hot water ... or reschedule your uses to not occur simultaneously.
Yeah, there's some difference based on how fast you withdraw the water, even with the heat turned off. If you withdraw it slowly then the stratification will be maintained better than if you withdraw it quickly.
And of course if you draw the water slowly then the heater has a chance to catch up. This is especially significant with an electric unit where the top element will rewarm the first contingent of mixed water, maintaining the set temp longer, but then resulting in a "cliff". A gas heater will mix the water more as it heats, due to convection, so the transition will be more gradual.
The two tanks will stay hot longer, but:
The two tanks in series, will keep the outlet temp higher longer, but the heat loss while they are standing will be higher.
The tanks radiate heat as a function of the surface area to volume ratio. The smaller tanks will have about 1.5 times as much surface area as the single large tank, for the same volume of water. I'd estimate the radiant losses would be about 1.5 times as high.
Consider putting them in parallel, as long as they can be reasonably balanced. Series works, but it puts most of the load on the first tank, causing it to fail sooner than the second one.
Ok, guys. Thanks for the feedback.
Ok, one more thought on this. According to the "textbooks", heat always migrates to an area of less heat. This puzzles me since the hot water tap is always cold until you flush it out with warmer water from the water heater. Why doesn't the significant heat from the water heater transfer to the colder standby water occupying the lines between the WH and the tap?
I didn't know that. As always, you're the man Dan. Thanks.