Water heater sizing

rjaero19473

13 years ago

My wife and I are building a house and need two water heaters. The basement is going to be a finished suite for my mother -in-law and we wanted to get a separate tank for her. We are first time builders, so we're not sure what size tank we should get. My mother-in-law likes to take long hot showers, between 30 and 40 minutes, so we need something to handle that. My wife and I need a heater that can support us taking showers at the same time. Our new shower will also have a double shower head, so it'll consume more water. I don't want tanks that are too big so that we waste exctra energy heating unused water, but i also don't want one too small. it wouldn't be a bad thing if we had to cut down on our shower times a bit, but I don't want to get stuck with a 15 minute shower limit, otherwise I'll have a very unhappy wife. Right now we have a 50 gallon Bradford White electric unit that works fine. I was thinking a 40-gallon for my mother-in-law and a 50 or 60 gallon for us. We don't have any kids yet, but plan to in a few years. Should we go ahead and get a bigger unit now in preparation, or worry about that when the time comes. Also, do you have any suggestions for good quality, efficient, electric units that are reasonably priced? I'm looking at AO Smith and Bradford White. Thanks for all your help!

Bob

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Comments (8)

vhehn
13 years ago
80 minimum for the upstairs.
chad_m
13 years ago
If a 50 is serving you fine right now I would stick with that. We have a 50 gal. A.O. Smith and haven't ran out of hot water yet. I grew up in a family of four and we also had a 50 gal. electric and it was plenty. I would either do as you had planned with the 50 and the 40 or one 80 for everything.
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The only downside of a larger tank is more space required, and initial cost differential. Usually the efficiency rating is the same. Bigger burner is usually just initial cost, and efficiency remains unchanged. I don't think there is any downside other than cost and space for going too big. In heating seasons the losses from a heater tank are going to be offset by reduced heating, so no real loss. All the ones you listed are going to put out substantially more hot water than the Lowes 50 gallon, 40,000 BTU model. Based on getting by on a 40 gallon before you should have more than enough. I would just select between them based on efficiency and cost. A 50 gallon, 60,000 BTU unit would likely work too if you can find one. Low flow shower heads are often a good investment to reduce hot water demand. You can get good ones for only a few bucks. My theory on hot water heaters is that they will all be very long life if you change the anode rod regularly. We got 32 years out of a basic 40 gallon John Wood (AO Smith). Anode rod was changed twice but need varies depending on how aggressive your water is.
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The above calculations hold true if the water heaters are connected in tandem however, if two 50gal water heaters are connected in series you begin with 100gal of water and as the dilution begins in the first water heater, the slightly cooler water enters the second tank where is is raised to the desired working temp, thus you can create a nearly continuous flow. In addition, when connected in series the first tanks temp control can be set about 10deg higher than the second tank thus under lighter loads the water will pass through the second tank without triggering the burner. In this manner under lighter loads the second tank functions solely as a storage vessel thus saving energy. On the other hand, there is a much bigger problem that generally goes unnoticed by homeowners. While most homeowners think of a shower in terms of the entire shower enclosure which is primarily intended to be a single occupant facility code must be capable of defining both the supply and drainage requirements of all showers, whether it is the single occupant residential facility or a large gang shower such as might be found in a school or factory locker room, thus for the purposes of estimating supply and drainage the code defines a shower as the shower head, thus when two or more shower heads can be operated simultaneously, we must allow the full code minimum water supply for each shower head. By example, under the IRC even though shower heads are restricted to a maximum flow rate of 2.5gpm, code requires that we size the piping for 3gpm. If we then design a shower facility with one shower head and 6 body sprays code would require the supply piping to be sufficient to supply 7 showers at 3gpm per shower for a combined total of 7x3gpm =21gpm. When using copper pipe code limitÂs the velocity of flow to a maximum of 8ft/sec thus to get the code minimum 21gpm supply, it would require an 1-Â¼" water line and 100gal water heaters. In turn, we must also compute the drain load at 2dfuÂs for each shower head, thus the combined drainage load for this shower would be 14dfuÂs, which would require a 3" drain line. I would strongly suggest you have a long talk with your local inspector before commencing this project.
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david_cary
13 years ago
If you only have electric, you probably should do 120 gallon solar. All electric elements are 100% efficient but that doesn't make it cheap. But it does mean there is little difference among water heaters regarding cost to run. They might differ in time to reheat water but not efficiency. The insulation is pretty standard - that is the minimum is pretty good.

I would tell you what I think of your family's wastefulness regarding water and electricity but that might be considered rude. Too late I guess...
srercrcr
13 years ago
For clarification, 100% efficient is misleading. That just means no energy is wasted like gas heat going up the exhaust stack. But, it takes alot of spins on the meter to heat water.
chad_m
13 years ago
Are you planning to use geothermal? If so the cost to heat the water is greatly reduced and almost eliminated while running your A/C. Also after giving it more thought I feel you would be better off with one large water heater than two. It is more efficient to heat one large tank than two small tanks.
2ajsmama
13 years ago
Even with geothermal, hot water ain't free! I haven't tried turning off the breaker to our wired tank this summer to see how the desuperheater does on its own. We have two 80-gal stonelined tanks, first is "preheated" (in the summer) by the heat pump taking heat out of the house, the second is electric and set to 120F. We're trying to get guy out for a quote on solar.

As far as what's needed, I'd say an 80gal would serve your needs now and in the future, unless you want a separate tank for MIL b/c you're going to have her suite on a separate meter. Our last house was tankless. You may want to consider that for the bathrooms and a small (40gal?) tank for the rest of the house, but I'd go with the small modern units rather than the old style oil boiler (also used for heat) that we had.

When you have kids, let's hope they don't take after your MIL. 15 min is plenty long enough for me and DH (separately - if we had a large shower with dual heads then it would still be 15-20 min so same as taking turns using 1 shower). But my kids turn it as hot as it can go and would stay in as long as your MIL if we let them! At least they only shower 2-3x/wk, but they're not teens yet...

I grew up in a family of 6 with 40gal tank and a well, my dad yelled if we took longer than 5 min *and* we were supposed to get wet, turn the water off, soap up, rinse off and get out within that time so water wasn't even running the whole 5 minutes. Not only did we often run out of hot water (lots of laundry), sometimes we ran out of water period, and my parents still do if they're not careful when my siblings and their families come to visit. DH grew up in a family of 11 with 1 bathroom (not sure how big a tank) *and* was in the Army so he takes 5-10 min showers.
manhattan42
13 years ago
Shower heads are limited in the US to producing approximately 2.5 gallons per minute.

If your mother-in-law takes 40 minute showers, she is going to waste about 100 gallons of water on each shower.

Presuming you use a storage tank type heater (maximum residential heater size is 80 gallons if I remember correctly)and presuming it is set at a usuable temperature so little cold water needs to be added to temper it...the mother-in-law will have about enough hot water with the largest tank for about 15-20 maximum minutes of hot showering.

(Remember, cold water re-fills and mixes with the remaining hot in a storage tank heater so one does not get 80 gallons worth of hot water in a single use.)

So not even 1 80 gallon tank will be enough for mother.

------------

2 shower heads in the husband and wife shower will use about twice the volume of water that mother's shower will.

This means with an 80 gallon tank, the maximum shower time is going to be about 10 minutes (45 gallons used) to 15 minutes (60 gallons used) before it runs cold.

Either way 2 80 gallon storage tank water heaters will never suffice for either gal in this family since they love to waste energy and water so much.

Only alternative is on-demand water heating if you can't break mother and daughter of their bad habits....or multiple 80 gallon heaters for each woman.

PS: Be sure to have a separate water and energy bill sent for the mother in law and wife either way....
wilkgroup_gmail_com
13 years ago
hi I have a standard 60 gal. how water tank; how many minutes of hot water do I have until it gets cold; how many minutes does a dishwasher use hot water? how many minutes to fill a 6 foot bathtub with hot water?

Water heater sizing

Comments (8)

vhehn

chad_m

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