Choosing size of Indirect Water Tank

[Robert Grenley]

Here in Seattle, we are replacing our oil-fired boiler for home heating (hot water, not steam, radiators) with a gas-fired boiler. At the same time, we are looking at replacing our current 75 gallon power vented gas water heater with a new indirect water tank. My issue with our current tank performance is that when I shower with our "unrestricted" shower head and 2 forceful body sprays, by the end of my 10-15 minute shower, I am running out of hot water...it is warm to lukewarm, but not hot, and this is even with reducing the body spray flow to extend the shower time. This is without other hot water usage in the house. The shower head and 2 body sprays may be using 10 Gal/min or more, though I have not done a direct measurement...We are not having any other hot water issues...just the shower as described. Also, our water heater is at about 8-9 years of age, so I would rather replace it now with a better system rather than waiting for a leak.

We are looking at an IBC Interstore indirect tank, and I am confused as to sizing, and how to interpret the ratings to make sure this tank will provide the "almost unlimited" hot water claimed. And what ratings are more important for my specific shower needs...First Hour Rating (Gal/hr), Continuous Rating (Gal/hr), Tank Capacity, or Boiler Water Flow (Gal/min). I have been told that these indirects, even if smaller capacity than what I have, will provide more water capacity and faster recovery than my current tank, but I am still confused as to how to size it for my specific needs.

Specifically, and interestingly, with 2 of the models, the 65 gallon and 80 gallon, the smaller 65 has higher ratings even with a smaller capacity, and the company said that was due to a larger coil in the tank, though it requires higher BTU from Boiler, which capacity we will have...as follows: (I only put the 55gal here for comparison)

BTI 55: 55 gallon tank, FHR 200, CR 156, Boiler water flow 8GPM, BTU required 100K
BTI65: 65 gallon tank, FHR 325, CR 269, Boiler water flow 13GPM, BTU required 172K
BTI80: 80 gallon tank, FHR 239, CR 171, Boiler water flow 10GPM, BTU required 109K

I am not sure how to interpret these ratings, in terms of what I am trying to accomplish, and which ratings are most important. I would hate to go to this expense and find that I am still running out of hot water in my shower.

Any advice would be most appreciated!
Thank you.

 

[Phog]

The number ratings you see for hot water output (continuous rating & first hour rating) assume that the boiler is supplying the indirect with "x" BTU/hr at "y" boiler loop flow rate. If your boiler BTU/hr is smaller than "x", or if your boiler circulator gal/min is lower than "y", you will get less than the rated hot water output.

FHR is the number of gallons of hot water that the tank can supply in the first hour of continuous draw. The CR is the gallons/hr for each subsequent hour of draw. Exceeding either number means the water starts to get lukewarm and then eventually turns cold. (It is completely normal to exceed the numbers and almost every home system is sized smaller than the rating for continuous "endless shower")

You can take the FHR numbers and divide by 60, and this will give you a general idea of the size GPM shower head you can install and get an hour's worth of shower time out. So, for the 3 tanks you list, you can use 3.3 GPM, 5.4 GPM, 4.0 GPM shower heads. Bigger shower heads than that equal less shower time. (Note that you can't take the GPM numbers above and play games with them, 5.4 GPM in 60 minutes based on FHR does not equal 10.8 GPM in 30 minutes, you will get much less than 30 minutes at 10.8 GPM)

Compare these numbers to a typical home water heater and you will see that the indirect has a much higher FHR. A common FHR for an electric tank might be in the 60 gal/hr range. A high recovery gas tank might be double that or even a little more.

But in your situation you possibly have a giant 10 GPM shower draw.

To cut through all the noise it may be helpful to think of things this way. When you shower you are really in the business of moving heat (BTU) from the boiler to your body, and the water is just how the heat is being transported there.

With a 10 GPM shower you are consuming a massive amount of heat, likely in excess of 325k BTU/hr. The boiler replaces some of the lost heat but only at the nominal BTU/hr rating for each tank above. Even the most "powerful" of those indirects (the 65gal 172MBTU) is only able to supply about 50% of the heat in the hot water coming out of your shower head. The other 50% comes from the hot water stored in the tank.

(Dana will probably quibble here and note that this number is actually the net heat for the rise from incoming water temps, not gross heat in the water, but you don't need to worry about that )

But long story short, you really need massively large equipment to comfortably supply a 10 GPM shower for more than a few minutes. Some combination of large hot water tank (bigger than anything you've listed) OR a really large boiler (which is not optimal from a home space heating efficiency standpoint -- oversize is bad).

The best thing you could do is look up the FHR of your existing hot water tank to get an idea of what your current equipment is outputing. You'll probably want to try to double that FHR on your next system.

Have you looked at all into drainwater heat exchangers? Your shower is the perfect application for this. This is a pipe which gets installed into your drain plumbing and recaptures heat from the hot water going down the drain, putting that heat back into your hot water system. This will increase the amount of FHR you get from any system (in shower only, not in a bath), as well as reduce your utility bills. You could get a smaller boiler/indirect and a drainwater heat exchanger and come out ahead with equal performance. Just something to think about.

 

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[Robert Grenley]

Thank you so much for your detailed and quick response.
I looked up my present tank, and it is a Rheem PDV75, and it looks like that has a UEF FHR of 120GPH (assuming they are rating this in the same or similar parameters as the IBC tanks) ...compared to the IBC BTI 55 @FHR of 200, BTI 80 @FHR of 239, and BTI 65 @FHR of 325.
So going by your advice to double the FHR, it looks like the BTI 80 will just do that, and the BTI 65 will well exceed that. Though the 65 requires a boiler output of 172K compared to the 80 requires only 109K. http://usa.ibcboiler.com/wp-content/themes/ibc/pdf/Interstor-Sales.pdf

And I would have to be sure that the boiler would output that...the Burnham Series 3 model 306 recommended has a DOE Heating Capacity of 146mbh, while the net model up, the model 307 has a DOE of 176mbh. https://files.gitshare.io/link/eAXbds2zZPA/Series 3 Product Data Sheet.pdf

Of course I don't even know what I am talking about! And whether I am mistakenly comparing apples to oranges.

I guess it is going to come down to the BTI 80 which looks like it doubles the FHR, or the BTI 65 which much more than doubles it but there is the question of whether I need to move up one model size of Boiler to give the BTI 65 sufficient BTU's.
(Assuming that a one step up in boiler models is not so "oversized" that it is not recommended in our 3600Sq.ft. house.)

Any recommendations based on that choice?
Thank you again!

 

[Dana]

With a gusher shower with side sprays you don't care about the first-hour ratings, you only really care about the first TEN MINUTES ratings.

Bucket=test the actual combined flow rate and report back.

With a luxury shower that sees daily use there is real payback in drainwater heat recover heat exchangers, and they can measurably extend the "apparent capacity" of the indirect or other tank water heater. The current best-in-class is EcoDrain's VT1000 series. At 2.5 gpm a 6' tall x 4" drain diameter VT1000-4-72 recovers 2/3 of the heat going down the drain and would "pay off" more quickly than shorter/smaller units. At 10gpm the recovery rates will only be ~2/3 of what it is at the tested 2.5 gpm, but that's still a huge payoff in fuel use, tank size requirement, and tank recovery time. But it does require access to at least 4-5' of vertical drain downstream of the shower to work. The tallest and fattest that fits is the "right" choice here.

Oversizing the boiler for purposes of better hot water performance can be a real disaster, so size the indirect for the hot water load, and the boiler for the space heating load. Even if replacing the oil-burner with a cast iron gas burner, the total thermal mass in a new gas boiler is usually a fraction of that of a traditional oil boiler, making it prone to short-cycling on zone calls if there isn't sufficient radiation. Run this napkin math on the zone (and whole house) radiation when considering replacement equipment, and run this napkin math on the oil-burner's oil use to ballpark the design load.

The "ideal" oversize factor for the DOE output of a cast iron boiler would be 1.4x the heat loss of the house at the 99% outside design temp (~27F for Seattle- yes I know it gets a lot colder than that sometimes), but there a bit more leeway for modulating boilers. Unless that shower is being used constantly a 1.4x oversized boiler will recover in enough time that the house never gets cold, even if priority is given to the water heater.

Hint: Even 146MBH would be LUDRICROUSLY oversized for an average home in Seattle, and 176MBH would be GROTESQUELY oversized.

 

[Phog]

Your understanding of how the boiler size has to match BTU ratings with the tank nameplate to provide the rated indirect tank hot water output is correct. You can still run the 65gal tank off the smaller boiler, it will just reduce the hot water output to a lower number than the nameplate rating.

Really the correct thing to do is first size the boiler to your home space heating needs (forgetting about hot water), and second then get a big enough indirect tank that stores enough gallons of hot water for your shower & bath needs. This will give you the lowest cost of operation.

This approach gets more difficult with such a high peak shower demand -- you might calculate out an 80MBH boiler with 200 gallons of standby water as your optimal scenario. Obviously not practical.

This is why you should consider a drainwater heat recover unit if there is any way to install it. You could cut the amount of hot water storage you need significantly, and still properly size the boiler to your home space heating needs.

EDIT -- I just noticed Dana's response above, and he is correct that the first hour rating is not directly relevant to your shower situation, but I still think you can use the FHR on your existing equipment to guide your selection of your next equipment and give you somewhere to start. This definitely is not a substitute for doing the calculations, but can at least give you a general idea. Hopefully you can have your installation contractor help you during the selection process, many HVAC houses have an engineer who can run numbers for you.

 

[Dana]

This approach gets more difficult with such a high peak shower demand -- you might calculate out an 80MBH boiler with 200 gallons of standby water as your optimal scenario. Obviously not practical.

A 200 gallon tank would only make sense if there were a monster sized party spa tub to fill.

If a 75 gallon powervent is almost cutting it at a storage temp on the order of ~140F, an 80 gallon indirect stored at 160F+ would probably get it all the way there (with margin.)

This is why you should consider a drainwater heat recover unit if there is any way to install it. You could cut the amount of hot water storage you need significantly, and still properly size the boiler to your home space heating needs.

Totally!

Heat recovery is often hard to pencil out financially for average showers in average homes in the age of cheap natural gas prices, but is all but a slam dunk for this type of shower, since half or more is "paid" up front in reduced sizing of tank & burner equipment, the rest in reduced fuel use.

With a decent sized drainwater heat recovery unit it might make it with margin using the existing 75 gallon powervent, or a 55-60 gallon indirect at a 160F+ storage temp.

 

[Phog]

A 200 gallon tank would only make sense if there were a monster sized party spa tub to fill.

If a 75 gallon powervent is almost cutting it at a storage temp on the order of ~140F, an 80 gallon indirect stored at 160F+ would probably get it all the way there (with margin.)

I should have made clear that this was not a guess at the actual numbers, but was rather intended as a "reductio ad absurdum"

 

[Dana]

I should have made clear that this was not a guess at the actual numbers, but was rather intended as a "reductio ad absurdum"

Does "concrete thinking" mean I (still) have rocks in my head?

 

[Robert Grenley]

As per Dana's reply, indeed...with my gusher of a shower, what I really care about is the "first 10 minute ratings", not the first hour ratings! But nowhere do I see any semi-equivalent ratings or information. And I do not understand recovery time...if it is the time between showers when the next shower will be hot, my current gas water heater seems to recover pretty quickly. But if recovery time impacts minutes 10 through 15 of my shower, THAT I care about!

I tried to do a bucket test. As best I can determine, my shower head puts out somewhere over 5 or even up to 6 GPM. (the bucket was getting heavy and I couldn't focus the spray enough to collect the water while the bucket was on the ground, so I did 30 sec and doubled.)
One body spray on full was maybe 2 or so GPM. 2 body sprays together then maybe 4 GPM.
When I turn the shower head on full (as always used), the body sprays go down a bit in flow volume.
Likewise when I turn the body sprays on full, the shower head goes down a bit in flow volume.
And I rarely have the body sprays on full as they hurt!

So, I think somewhere around 10 GPM (5-6 + 2 + 2) would be pretty close.

BTW, remembered that when my hot water heater sensor (little plastic thing that ?detects air contamination?) went out and needed to be replaced, I had to reset the temp on the tank, and I believe I set it to 130. Our hot water in the sink gets SO hot, and I couldn't remember if it was previously 130 or 140. I suppose I could benefit from turning it up to 140?? Though a bit less safe re: scalds, isn't it?

I gather what has been said about not oversizing the boiler just to reach some BTU that the BTI65 tank might require (at least to match its reported specs), and that might push me to go with the BTI 80 gallon which requires less and about same BTU as the 55 gallon but more capacity...and still double the FHR than my current heater...for what that is worth.

Do appreciate all your ongoing help!

 

[Phog]

And I do not understand recovery time...if it is the time between showers when the next shower will be hot, my current gas water heater seems to recover pretty quickly. But if recovery time impacts minutes 10 through 15 of my shower, THAT I care about!

Strictly speaking the recovery time just means how long the tank takes to recharge up to temperature after a drawdown. But in the same size tank, a unit with a higher recovery rate (quicker recovery time) will input more BTUs to the water. This means that the tank will also take longer to run out of hot water, since more heat is going back into the tank while you're showering.

One way to think of a "tankless" water heater is as simply a water heater with a big enough burner for zero recovery time, so that zero gallons of hot water need to be stored to meet demand.

Recovery time also is impacted by the amount of water stored in a tank, for example even a big 200mbh burner would take hours to heat up a 1000 gallon tank.

The FHR is the number that gives you a combination of stored hot water stored + BTU input. CR is related only to BTU input.

 

[Dana]

Codes typically require a tempering valve or thermostatic mixing valve reducing the hot water distribution temperature to sinks & bathing facilities that can be turned down to 120F, even when the storage temps are much higher. The scald risk of 140F water is pretty serious, 120F it's real, but not as dangerous. So hopefully your water heater already has a tempering or thermostatic mixing valve (?) which would allow you to crank the storage temp up. Most water heaters can take 180F storage temps, but the controls on residential gas heaters usually don't go much over 140F.

As a rough WAG you only get about 2/3 of the rated volume of the tank out at or near the set storage temp before the temp starts to fall off precipitously so think of the 75 gallon tank as 50 gallons of full temp water. With 4oF water coming into the house and a storage temp of 140F, and showerhead temp of 105F, you only get about 77 gallons of 105F water out of the showerhead before the temp starts to fade. At 9-10 gpm that's about 8-9 minutes, and by minute 12 it's pretty tepid.

At a storage temp of 160F that moves up to about 92 gallons at least a full minute more, which is good, but might not deliver as much as you would really like.

With a heat recovery unit bumping the incoming water temp from 40F up to 70F at the shower mixer (and tempering valve) and a storage temp of 140F in the tank you're good for about 128-129 gallons before it starts to fade. That's about a 67% increase in showering time/capacity before it starts to fade, and since there's 70F water in the bottom of the tank instead of 40F water the time for the tank to recover fully up to temp is cut dramatically after showers (but not after tub fills.)

With a 60 gallon indirect at 160F consider it to be about 40 gallons. With drainwater heat recovery boosting the incoming water to 70F it will deliver about 103 gallons of 105F water to the shower heads, 36% more than the standalone 75 gallon tank at 140F with no heat recovery.

Play around with this online mixer-widget tool a bit to get a feel for it.

Measure your actual incoming mid-winter water temperature.

 

[Dana]

Strictly speaking the recovery time just means how long the tank takes to recharge up to temperature after a drawdown. But in the same size tank, a unit with a higher recovery rate (quicker recovery time) will input more BTUs to the water. This means that the tank will also take longer to run out of hot water, since more heat is going back into the tank while you're showering.

That's true ut the burner size on a typical residential tank is only good for about 1 gpm (give or take) at Seattle's incoming water temp, and isn't going to make or break the gusher-shower problem. A right-sized boiler for the space heating load is usually in the same ballpark.

A heat recovery unit as big as the VT-1000-4-72 can be the equivalent "burner" to support 4-5 gpm of flow in a 10 gpm shower, assuming it's recovery efficiency drops to about 50% @ 10 gpm.

 

[Fitter30]

Two tankless water heaters 199,000 btu's that would talk to each other and would give the 10 gpm with a 75* rise. The big problem would be gas company to see what your meter would handle or if a bigger meter is possible.

 

[Dana]

Two tankless water heaters 199,000 btu's that would talk to each other and would give the 10 gpm with a 75* rise. The big problem would be gas company to see what your meter would handle or if a bigger meter is possible.

A drainwater heatexchanger + indirect off the space heating boiler would be cheaper than any tankless solution, and uses a lot less gas (just sayin'...)

 

[Robert Grenley]

I think for simplicity sake, I will decide between the IBC 80 and IBC 115 indirect tanks.
(And I apologize if I am repeating anything...just trying hard to understand, but keeping it simple...I don't want to bring up other more complex or esoteric solutions at this point to my installer.)

My current Rheem PDV75 gallon is supposedly going to deliver "EUH" First Hour delivery (assuming that is the same as FHR) of 120 GPH, and if I am using 10 GPM in my shower, I can see why I am running out of hot water at 10-12 minutes.

With the 180 degree delivered water from the boiler (which will be the case with mine) and the boiler sized for my house (at input 175 MBH, 146 MBH DOE Heating Capacity, and Net AHRI RATING Water 127 MBH) the comparative specs for the two sizes of indirect tanks are as follows:

The IBC 80 indirect (80 gallon tank) will deliver an FHR of 239 at 140 degree and 303 at 115 degree.
The IBC 115 (115 gallon tank) will deliver an FHR of 289 at 140 degrees and 360 gal/hr at 115 degree.

If our hot water needs are currently being met, except in my gusher of a shower, putting together the increased capacity and recovery figures, and if the $1000 difference is not important, which way would you go? 80 or 115 size?

Thank you!!!!

 

[Phog]

Hello Robert, you could get a rough idea of how many gallons of hot water you can draw in 15 minute time period this way:

Tank Capacity (gallons) * 0.8 + Recovery Rate (gal/hr) * 0.25 = Gallons Provided in 15 minutes

- The 0.8 comes from an 80% usable drawdown (you can't use 100% of the hot water in the tank, it gets lukewarm first)
- The 0.25 comes from 15 minutes = 0.25hrs, for the length of draw
- For the Recovery Rate you can use the "continuous rate"

Since we don't have the continuous rate for the 110 gallon, I'll just do the 80 gallon unit using the numbers from your original post.

80 gallons * 0.8 + 171 gal/hr * 0.25hrs = 106.75 gallons
106.75 gallons in 15 minutes = 7.1 gal/min.

I believe that you have given rating numbers at 140F temperature; the actual gallons delivered to your shower will be more (since it will be at a lower temperature of something like 110F). So that works in your favor. But on the other hand you may not get the full 80% drawdown that this calculation assumes before you get lukewarm water -- some calculations use 75% or even 70% drawdown. So that would tend to work against you. Finally, you can probably store the tank water at a higher temperature which would work in your favor.

Hopefully at least this can help you can get an idea of what you're dealing with. You might be able to get what you want with this 80 gallon tank but you will have to play games, increase the storage temperature and/or increase the input BTUs above the rating (which can be done but is hard on equipment). And you won't have any margin for, say, that day you are sick & want to take a long 20 minute shower.

For fun here is the 65 gallon:

65 gallons * 0.8 + 269 gal/hr * 0.25hrs = 119.25 gallons
119.25 gallons in 15 minutes = 7.95 gal/min

As you can see it is even a little better than the 80 gallon, due to the higher recovery rate. But still not delivering you 10 gal/min over 15 minutes without playing other games.

Are you sure you can't install a drainwater heat exchanger? They are not that common yet, so your contractor may be unfamiliar & therefore skeptical. But the installation is really not that difficult. The only real difficulty is if you have to tear apart finished walls to access the existing drain stack under this shower.

 

[Dana]

I think for simplicity sake, I will decide between the IBC 80 and IBC 115 indirect tanks.
(And I apologize if I am repeating anything...just trying hard to understand, but keeping it simple...I don't want to bring up other more complex or esoteric solutions at this point to my installer.)

My current Rheem PDV75 gallon is supposedly going to deliver "EUH" First Hour delivery (assuming that is the same as FHR) of 120 GPH, and if I am using 10 GPM in my shower, I can see why I am running out of hot water at 10-12 minutes.

Again, forget the first hour delivery- this is a first 15 minutes time.

With the 180 degree delivered water from the boiler (which will be the case with mine) and the boiler sized for my house (at input 175 MBH, 146 MBH DOE Heating Capacity, and Net AHRI RATING Water 127 MBH) the comparative specs for the two sizes of indirect tanks are as follows:

The IBC 80 indirect (80 gallon tank) will deliver an FHR of 239 at 140 degree and 303 at 115 degree.
The IBC 115 (115 gallon tank) will deliver an FHR of 289 at 140 degrees and 360 gal/hr at 115 degree.

Again, a 127MBH AHRI net-water boiler is ABSOLUTELY GROTEQUELY oversized for your house, unless you sleep with all the windows open or the house is more than 12,000 square feet.

Oversizing a space heating boiler that badly has a fairly significant (negative) impact on the as-used efficiency. DON'T DO IT! You probably don't even have enough radiation in the whole house to even emit that much heat at an entering water temp of 180F, which means it will be prone to short cycling into even lower efficiency. (Do you have 300 feet of 8-10" tall fin-tube baseboard, or equivalent?)

A typical 2500' house in Seattle has design heat load less than 30,000 BTU/hr (30MBH) , and a better tighter house that size can have a heat load under 20,000 BTU/hr (20 MBH). If you have a heating history on this place, run this napkin-math to get a handle on your actual heat load. Total up the amount of radiation in the house, zone by zone if multi-zoned.

If you're willing to share information on zone-by-zone radiation, exact meter reading dates & amounts, and a ZIP code (for more accurate weather data) it's possible to sketch out what boiler capacity might make sense here. Don't rule out modulating condensing boilers, which can be cheaper to install and easier to get a reasonable fit to both the radiation & heat load than Godzilla-sized cast iron.

Forget the recovery rate too- with a boiler right sized for the space heating load it's too slow to make a significant difference for the first 15 minute hot water capacity numbers.

If our hot water needs are currently being met, except in my gusher of a shower, putting together the increased capacity and recovery figures, and if the $1000 difference is not important, which way would you go? 80 or 115 size?

Thank you!!!!

The existing storage temp is about 140F, and the total volume is 75 gallons. Simply boosting the storage temp to 160F adds only about a minute to the showering time. (See response #11). If you're not installing drainwater heat recovery it's going to need the 115.

A $1000 up-charge is better spent on drainwater heat recovery, since it will increase the apparent capacity to more than that of the 115 gallon tank, has at least 3x the service life, uses half the amount of gas for showering, and won't increase the standby loss.

 

[Reach4]

The existing storage temp is about 140F, and the total volume is 75 gallons. Simply boosting the storage temp to 160F adds only about a minute to the showering time. (See response #11). If you're not installing drainwater heat recovery it's going to need the 115.

A $1000 up-charge is better spent on drainwater heat recovery, since it will increase the apparent capacity to more than that of the 115 gallon tank, has at least 3x the service life, uses half the amount of gas for showering, and won't increase the standby loss.

This all makes great sense to me, except why would a drainwater heat recovery system have a longer service life than an indirect water heating tank?

 

[Fitter30]

This all makes great sense to me, except why would a drainwater heat recovery system have a longer service life than an indirect water heating tank?

Copper drain pipe and copper tubing wrapped around the outside. Minerals are driven out of the water at a high temp 120*. Recovery you have 40* entering and 55*- 65* out not hot enought to drive out the minerals.

 

[Robert Grenley]

Wow, your formulas are very helpful to get an idea of what I would be dealing with...keeping in mind the cautions you expressed.

Here are the figures for the 80 gallon and the 115 gallon (with 180 degree delivered by boiler at boiler BTU 140K and boiler flow 14 gpm) and with FHR and CR for both 140 degree and 115 degree, coming from the manual for models IBC 80 and IBC 115:

80 gallon tank: At 140 degree: FHR 239, CR 167 At 115 degree: FHR 303, CR 231

115 gallon tank: At 140 degree: FHR 289, CR 186 At 115 degree: FHR 360, CR 257

Using your formula, (unless I made a mistake):

80 gallon tank: 7.05 gallon per minute at 140 degree, 9.2 gallon per minute at 115 degree.

115 gallon tank: 8.12 gallon per minute at 140 degree, 10.42 gallon per minute at 115 degree

So, at least by this, the 115 gallon tank will get me closer to achieving my goal.
And, assuming there is no disadvantage to going for a larger tank that absolutely needed...other than initial cost and some higher energy costs, which are of less overall concern to me...it would seem like the 115 tank would give me a bit more leeway.

Any thoughts on that?
Thanks!!