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Tanklets > Sizing: In Parallel, In Series

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Q: Existing tanks details: 40 gal, 34K btu, 34 gal recover, 70 gal 1st hr. 50 gal, 40k btu, 42 gal recover, 82 gal 1st hr. I have 40- and 50-gallon tanks in series supplying our home (2 adults, 2 teenage daughters and one pre-teen boy). The 40 gallon tank has the cold water supply and is the oldest at 20 years.

I have no problem with hot water supply during the summer months. But as the incoming water temp goes down during the winter (to 36 degrees), the two tanks really struggle to keep up. I can easily run out of hot water in our master bathroom shower alone in 10 to 12 minutes in the dead of winter. Granted, it is a high draw shower with two shower heads and 4 body jets (8 GPM). But it should last longer than that, right?

Add the teenage daughters to the equation and we are out of hot water quickly some mornings. I have turned both thermostats up to high and it did not help. My goal: make it last longer (at least 20 minutes). Neither tank leaks, but my gut tells me that the 20 year-old tank has lasted longer than expected and I am on borrowed time.

I imagine they are both very inefficient units. I looked at replacing them with a tankless unit, but with the incomming cold water being an amazing 36 degrees and three showers running sometimes, I dismissed that idea quickly.

Question: If put a 65 or 75 gallon high recovery unit in place of the 20 year-old 40 gal unit, does anyone foresee a problem since it would be feeding a 50 gallon tank that is 9 years old. Will adding a larger tank in place of just the old 40 gal unit help or would a buying two new 40 and 50 gallon high recover tanks be better since they would be more efficent and have better recovery time?

A: Your choices basically are add more hot water generating capacity or reduce usage, or some combination. Understand that you will get about 75% of the volume of the tanks as undiluted hot water. Actually I'm surprised you even get 10 to 12 minutes with six shower heads running. My quick calculation showed about eight minutes.

I'd start by finding a way to use only the shower heads that are needed. Put shut off valves on the body sprays for instance and use them when there isn't a lineup wanting to shower. A really innovative solution to your problem might be to install a GFX shower heat exchanger http://gfxtechnology.com/ They can capture up to 60% of the lost heat so would essentially double available shower time. Once you have taken measures to use only what's needed, you can size heaters to suit. -- Larry (12/16/06)

Q: KorasMom is getting a new tub installed courtesy of KorasDad and KorasUncle. She picked out a Kohler Kathryn soaking tub. All 97 gallons of it will probably require a larger water heater than our current 18 yr/old 50 gal 40k btu/hr unit. Can I get a professional opinion on a new unit? Should I go with a single 75 or 100 gal Rheem unit or a pair of 50s in series? Thanks in advance, KorasDad

A: I'm a fan of putting tanks in series where possible. This is because they needn't match exactly to get proper flow and there can be some redundancy built in. If things are valved right so you can isolate and bypass each tank (use three ball valves each), you will always have hot water unless BOTH tanks fail at once.

Some things to consider about sizing the tanks. Normally you only get about 70 to 75% of the volume of the tank as undiluted hot water. A benefit of series plumbing is you get 100% of the first tank and 70 to 75% of the other. Other things, 97 gallons is probably full to the overflow without anybody in the tub. What fun is that?

You can take away some gallons for the person/s in the tub. You need to know how hot you want the tub and what your hot and cold temperatures are. That way you will know what the mix of hot to cold is. If, for example, you really only need 90 gallons and to reach 104 degrees you are mixing 45 gallons of 140 degree water with 45 gallons of 68 degree water. Each is 36 degrees from 104 so the ratio is 50/50. Most of us don't have such warm, cold water, so your percentage of hot water will likely be higher.

There is some figuring for you to do. Forty gallon heaters are usually the lowest price per gallon stored. So, if you put one in series with your present heater, you can get 50 plus 30 or 80 gallons of undiluted hot water. That should work nicely even if you have near freezing ground water! Note that it will be heating as you are drawing water too. When your fifty goes, a forty will replace it nicely. Do get well insulated tanks and insulate all the plumbing with thick foam, or you'll be paying monthly for the use or non use of the tub. -- Larry

A1: Two 50s in parallel for redundancy and performance. -- Parkerized

A2: Two 50-gallon in parallel give a higher flow rate if using large enough pipes, i.e., performance in the form of a faster tub fill. However, it is trickier to balance and then only provides 75 gallons. A 40-gallon adn a 50-gallon in series, costs less and provides more (80 gallons). Further, if the leading one is a high efficiency HW heater, its rated performance is realized (because it sees the large heating load it is rated at -- saving on utilities) and the standard efficiency trailing heater tops-up to full-temp any water the first one eventually is unable to get hot enough (should several showers immediately follow filling of the tub). Even if the leading unit is a 40-gallon, you're looking at 77.5 gallons (vs. 75) capacity. -- David of CA (1/22/06)

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