Properly sizing hot water storage for shower and tub.

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roger_northeast

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This is a long msg but please read it and provide me with your expertise where you can.

Thanks.
Roger
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I'm in the process of building a new house. I want to make sure that they size the hot water and domestic water storage correctly. Below is a description of all water usage

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Master Bath:

Custom shower: 3 body sprays, a rainshower head, 1 regular shower head and a hand-held shower. I'm using a Grohe Thermostatic valve 34-124 which has a maximum GPM of 18 at 45PSI. I'm putting in 4 Grohe 29274 volume controls, one for each function. All valves are 3/4". A realistic expectation is to have all going except the hand-held shower. I got a wife who likes to take long showers ~30mins.

Whirlpool tub: Tub filler 13gpm at 45psi and handheld 2.5gpm at 80PSI. Capacity: 96g

1 toilet, 1 bidet, 2 sinks
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Bath # 2: Standard Tub, 1 sink, 1 toilet
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Bath # 3: Standard shower, 2 sinks, 1 toilet
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Basement Bath: Standard Shower, 1 sink, 1 toilet
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1/2 bath #1: sink + toilet
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1/2 bath #2: sink + toilet
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Kitchen: 2 dishwashers, 1 sink
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Laundry: 1 washer + 1 sink
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Exterior: 4 sillcocks
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Hot Water: Indirect Buderus ST-200, 53g with a 2nd hour rating of 3.3gpm (I called the manufacturer and they told me they don't have the 1st hr rating, I thought this was a mandatory DOE rating, am I wrong?)
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Water source: Well, well tank (to be sized), well produces about 5gpm. Pressure can be set 40 low and 60 max.
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Irrigation system: Want to have this but don't know if system can handle it.
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Peak hour usage: Master bath in use for 30mins with body sprays, rainshower & shower head, followed by another 15 mins usage of rainshower. During this time, at least one of the other showers could be going and possibly two. At the very least 2 sinks will be used for ~5 mins each during that time.

Questions:

0) What do you estimate is the GPM of HW that I will need?

It looks to me that the provided HW indirect at 53g with a 3.3gpm production was not the right size to have provided.

1) What is your opinion?

That unit has been purchased now so I can only add to this. I was thinking adding another ST-200 and hooking both together. That would give me 106g to draw from in the first hour and a production of 6.6gpm

2) How should the tanks be hooked together, in series or parallel?

3) Could I really get 6.6gpm by putting 2 together?

A likely problem is that I may not have the BTUs in my boiler to keep two indirects going so another solution I was thinking of is to add a hot water storage tank, say 80g, just off the existing ST-200. This will require installing a re-circ pump between the ST-200 and the storage tank, but at least I get 133g before the indirect has to start producing.

4) Would this be a good solution that would provide the required GPM that I need?

5) Why do manufacturers list GPM at 80PSI? Most of the valves and other components in the system don't appear to support such high pressure.

6) What would a realistic GPM of a valve rated at 80psi be if I kept the pressure around 60PSI?

7) What's the formula for converting PSI to GPM? Is it simply a linear conversion? That is, at 40PSI the GPM would be 2.5/2?

Another problem that I have is that my well only produces 5gpm so I need to have a large reserve to supply all the above fixtures.

8) How do I properly size the well tank?

Pressure is very important. Right now in the house that we live in, if you flush a toilet or open a faucet, the shower head flow drops to a trickle. Also, the hot water goes to the faucet and as a result the person showering gets a cold shock.

9) Why do these problems happen and how do I prevent them from occuring in my new house?

10) Is it a matter of the height of the pipes? That is, is the sink always going to have a preference to the shower because it is lower?

11) Would a pressure booster pump resolve this issue?

A pressure booster pump can take my well pressure and increase to a constant 80psi (according to the literature I just read). However, it looks like most valves can only handle up to 72psi. At least the ones that I found specs for (the ones in the master bath)

12) What's the typical pressure max for contractor valves? For example, the valves he'll install in the sinks?

13) What other fixtures would limit the max pressure? For example, does the washer or ice-maker have a lower max and therefore, force me to keep the pressure below 70psi?

14) Can the copper tube handle the 70PSI day in and day out?

Thank you for all of your help.
Roger
 

Bob NH

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Hard Medicine Follows

1) What is your opinion? (Italics are applied to your text.)
This is a classic case of poor systems engineering, inadequate specifications, requirements creep, and unreasonable expectations.

YOU need to sit down and make a time chart (maybe in Excel) from wake-up to bed time, and put in the hot and cold water demand for each PERSON, and then allocate it to the fixtures. Then you need to get the peak-hour and peak-two-hour demands for the total hot and cold water, specifying the temperatures of incoming and using hot water.

It would be waste of time for people here to try to create a solution where the requirements are not well defined. Furthermore, trying to give you a design solution here risks giving you more expensive mistakes to be corrected after the tile is installed. If you can't do the requirements and specifications, you should probably pay someone to do that job. This is not a job that I would give to a plumber with oral instructions. And if you give it to a plumber to design it, you should get the design reviewed before purchase of components.

That unit has been purchased now so I can only add to this. I was thinking adding another ST-200 and hooking both together. That would give me 106g to draw from in the first hour and a production of 6.6gpm See 1 above.

2) How should the tanks be hooked together, in series or parallel? Need a system design on this system, after specifications are completed.

3) Could I really get 6.6gpm by putting 2 together? Yes.
A likely problem is that I may not have the BTUs in my boiler to keep two indirects going so another solution I was thinking of is to add a hot water storage tank, say 80g, just off the existing ST-200. This will require installing a re-circ pump between the ST-200 and the storage tank, but at least I get 133g before the indirect has to start producing. Design follows requirements and specification. See 1 above.

4) Would this be a good solution that would provide the required GPM that I need? See 1 above; then design.

5) Why do manufacturers list GPM at 80PSI? Most of the valves and other components in the system don't appear to support such high pressure. They want to sell you on a big GPM number. The idea is to fool the guy who doesn't understand his requirements.

6) What would a realistic GPM of a valve rated at 80psi be if I kept the pressure around 60PSI? The flow in a fixed orifice is proportional to the square root of the pressure differential across the orifice (AT THE FIXTURE after pipe losses); about 85%; BUT showers and some other fixtures have controls that limit the flow so the rule may not apply.

7) What's the formula for converting PSI to GPM? Is it simply a linear conversion? That is, at 40PSI the GPM would be 2.5/2? See answer above. A 2.5 GPM shower is probably 2.5 GPM for 40 psi and greater.

Another problem that I have is that my well only produces 5gpm so I need to have a large reserve to supply all the above fixtures. You probably need a large storage system. After you get your requirements, someone can do an analysis and determine the most cost effective way to meet your requirements.

8) How do I properly size the well tank?Your well pump is probably too small to meet the demands that you suggest. But first you need well-defined requirements (See 1 above).
You probably need a storage tank that will operate at atmospheric pressure, and a second pump that will deliver the demand based on your system analysis. The pressure tank will be sized to meet demand when the pump is running, and to keep the pump from cycling too frequently.

Pressure is very important. Right now in the house that we live in, if you flush a toilet or open a faucet, the shower head flow drops to a trickle. Also, the hot water goes to the faucet and as a result the person showering gets a cold shock. You need bigger pipes, sized to meet peak demand.

9) Why do these problems happen and how do I prevent them from occuring in my new house? Inadequate written specification and contract (See 1 above) and not designed to meet the requirements.

10) Is it a matter of the height of the pipes? That is, is the sink always going to have a preference to the shower because it is lower? No. Pressure head is much greater than elevation head differences. You need adequate pipes to deliver the demand.

11) Would a pressure booster pump resolve this issue? Not at the big flows you have, unless the pipes are big enough.

A pressure booster pump can take my well pressure and increase to a constant 80psi (according to the literature I just read). However, it looks like most valves can only handle up to 72psi. At least the ones that I found specs for (the ones in the master bath) Most valves will handle 100 psi. I just checked a Delta shower system and the chart goes to 90 psi.

12) What's the typical pressure max for contractor valves? For example, the valves he'll install in the sinks? Check the specs, but I would guess 100 psi.

13) What other fixtures would limit the max pressure? For example, does the washer or ice-maker have a lower max and therefore, force me to keep the pressure below 70psi? You will have to check the specs on your valve.

14) Can the copper tube handle the 70PSI day in and day out? Yes; and far more.
 

roger_northeast

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I thought my requirements were detailed enough...

Hi Bob,

Thank you for taking the time to answer.

If you could expand a little more on what other information I need to collect to properly design the system that would be great. I know you said to create a spreadsheet with each hour of the day but is that really necessary? If I choose the hour of the day when the most demand is placed on the system, then it should handle all the other hours with less demand.

The info I provided in my original post has what I consider the worst case for determining the HW design. The master shower and the kids' showers all running in the morning is the time where the most demand would be placed on the system.

As to the purchase of the storage tank, it was an oversight. When the HVAC system was designed, they missed the fact that there was a custom shower. Since the HVAC contractor provides the HW tank (connected to the boiler and the boiler sized to feed that tank), I now have to try to "fix it".

As for the plumber, it looks like most of the pipes he intends to use are 1/2" with the exception of the master shower which is 3/4". I, of course, have asked him to size the system properly, and he could tell me that he did all day long and I won't know if it is true or not, unless I ask here and learn from what you and others have to say.

Thanks again.
Roger
 

Bob NH

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The master shower and the kids' showers all running in the morning is the time where the most demand would be placed on the system.

"Custom shower: 3 body sprays, a rainshower head, 1 regular shower head and a hand-held shower. I'm using a Grohe Thermostatic valve 34-124 which has a maximum GPM of 18 at 45PSI. I'm putting in 4 Grohe 29274 volume controls, one for each function. All valves are 3/4". A realistic expectation is to have all going except the hand-held shower. I got a wife who likes to take long showers ~30mins.

Whirlpool tub: Tub filler 13gpm at 45psi and handheld 2.5gpm at 80PSI. Capacity: 96g"


Translate all the kids showers, your shower, wife's 30 minute shower, whirlpool tub, toilets, and whatever into peak hour demand and peak two hour demand for hot water and cold water.

What is your winter water supply temperature? If from a surface water in the northeast where I live it is about 33F. That has a big effect on water heater requirements.

Then identify the peak GPM for each bathroom.

For your daily well capacity, don't forget watering the grass or flowers.

You are the best one to define the requirements.
 

Jimbo

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Here are a couple of comment. ( I am not familiar with indirect units, but will offer info pertaining to "regular" gas WH, and perhaps you can correlate some of this to your situation.)

A tank water heater will deliver 70% of its gallon capacity, at a temperature 20º below thermostat set point. The recovery rate is approximately one gallon per hour, per 1000 BTU. The first HOUR delivery is the sum of the 70% plus the recovery. SO, a 50 gallon, 40,000 BTU tank will deliver in the FIRST HOUR 35 + 40 = 75 gallons. Please note that this does NOT mean you can get 75 gallons in 15 minutes. Once you use up the 35 gallons, you are limited to recovery, which is 40 gallons per hour/60 minutes = 0.66 GPM


You have described some set up where your wife will use 18 gallons per minute for 30 minutes. Do the math on this!

Now, I have omitted discussion of hot/cold mix, which does come into play. But even at a very hot setting of 140º the mix for a hot shower is still about 70% hot.


I see only two ways you could ever get the amount of hot water you need.
1) Get a very large output capacity tankless water heater. Maybe more than one.

2) Get a very large, perhaps 100 gallon 270,000 BTU commercial tank, which you can set to a very hot temperature (> 160º) and use a tempering system on the output.


I haven't even touched on the needed storage for your well pump system. Others will need to chime in..


Basically, you are woring at the outer limits, and this whole system should be designed by a very experienced plumbing contractor.
 
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roger_northeast

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Thanks for your inputs

Hi Bob and Jimbo,

Thanks for your inputs. I don't have a responsive plumber when it comes to getting these answers. I'm working with the HVAC guy who will provide the indirects.

He talked to Buderus today and they suggested something similar to Jimbo's 2nd solution. Use 2 indirect tanks: the ST-200 (53g) + the ST-300 (79g) and then raise the temp to 160% and use a mixing valve to bring down to usuable temperature. I don't particularly like this because the high temps will probably kill the lifespan of the indirects, but I don't have another choice at this point. If you have any thoughts on this let me know.

Roger
 

Jadnashua

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At least some indirects are available with stainless steel tanks...life should not be an issue. But, the boiler temperature will need to be high to get decent heat transfer, you need a decent delta T. This could affect the life of the boiler, course, it will need to run a bunch to make all of that hot water.

Keep in mind, too, that many boilers use the HWH circuit as a priority circuit...in other words, when you have a call for hot water for the tank, it inhibits the house heat.

Depending on how long all of that hot water is going down the drain, you could have the whole house cool off on a nasty, cold winter day.
 

Bob NH

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I installed my own version of an indirect heater that works great in cold NH.

I have a hydronic heating boiler with a tankless coil for supplying hot water. In the winter, with feed water at about 33 F, it was not giving me as much hot water as I needed. The boiler is oil fired and gas is not available for a hot water heater.

I installed an electric hot water heater fed by the tankless coil. During the winter heating season, the oil-fired boiler does all of the work. A small controlled pump circulates water from the water heater through the tankless coil to achieve higher temperature than coil will provide in the supply pass (the winter problem).

The advantage of this over an indirect heater with a coil in the tank is that I already have a heat exchanger in the boiler. The second advantage is cost. An electric hot water heater costs less than an indirect heater. You could use electric water heaters with a gas fired water heater. I would put them is series and circulate a small controlled flow through them all.

I found that keeping the boiler hot in the summer cost more for oil at $2.00 per gallon than it cost for electricity. Therefore, I shut off the boiler in the summer and use electricity to heat the water for two people.
 
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