Insufficient tankless electric hot water flow

[5030tinkerer]

I just installed a Bosch AE-125 electric hot water heater. The spec's list maximum flow as 3.5gpm (which seems to contradict another spec of a 4GPM flow at a 45ｰ rise) and maximum flow at the 80ｰ temperature rise that I currently need at 2.2GPM. My Delta tub faucet is rated at 5.2GPM at my 70psi pressure and my tub filler in my master is rated at close to 20gpm (when mixing between hot and cold at least - so we can figure 10gpm for this one, I guess).

To achieve higher flows (so my wife doesn't have to start drawing a bath the day before), I am considering installing:
A. A non-insulated water storage tank or possibly insulated water heater tank upstream of the cold water inlet of the tankless to bring the present 40ｰ inlet water temperature to ambient or possibly up to 86ｰ (the maximum supported inlet temperature of the Bosch device).
B. Using the AE-125 to pre-heat the water to 107ｰ and then a 3rd party tankless hot water heater downstream of it (Stiebel Eltron's Tempra 36 - Stiebel Eltron | Tempra™ Series Technical Data) to bring it the rest of the way up (my house has 400A service).
C. Using the AE-125 to pre-heat the water to the maximum temperature possible without any flow restriction (ball valve fully open), but then having a separate hot water tank-based heater downstream of it to serve the demand.

Will any of these methods result in a flow rate equal to or greater than what I need for my tub filler? I am thinking that Option A will get me close, that option B should give me greater than 8GPM (assuming that there truly is no maximum flow specification on the AE-125 at all outside of what my 3/4" line at 70psi can deliver and it's simply a flow rating at a given temperature rise), and that option C will also work, but at the cost of having to keep a tank of hot water hot all the time (which sort of defeats the purpose to some extent). Please confirm.

Of course, there is always Option D, which may actually be the best option but one that I really don't like), of replacing the AE-125 with the Tempra 36 unit and going with option A or C. Under this plan, I'd likely just power the tanked heater off in the summer.

What do you guys think?

I have this question posed to Bosch as well, but Stiebel Eltron has already indicated that from their perspective any of the options would work. Trouble is, of course, that the Bosch warranty is 10 years and the Stiebel Eltron warranty is only three, despite the similar construction.

 

[Jimmyp5]

I installed the same unit... and do not have the problem really of flow, but then again I don't fill a bath tub, but do have two utility sinks that I fill with pure hot water at the same time. I don't get the flow issue being too slow to fill a tub...unless it's a soaker/hydro etc...

A 50 gal tub would fill in what ~14 minutes? at max flow at your tub is ~9.6 minutes for 50 gal. ?? So it it worth the extra work to gain 4 minutes?

Also, I would check the down stream second unit real close... if the water is "pre-heated" too hot the unit may not take it the rest of the way, some models need a colder supply side that the 107 plus degrees you will get from the AE-125... fyi my AE-125 kicks out at ~120 pretty constant. But I do have a lower flow rate being on a well it does bounce around a bit, the unit seems to handle it.

The PowerStar electric tankless water heater is designed to replace an electric storage tank heater. Advantages include endless hot water, reduced electricity costs and substantial space savings.

PowerStar AE125

• Provides 3.7 gallons per minute for water usage at 105ｰF* - 2 sinks simultaneously or 1 shower or bathtub.
• Provides 3.0 gallons per minute for water usage at 115ｰF* - 2 sinks simultaneously or 1 dishwasher.

 

[5030tinkerer]

I wonder if you aren't experiencing trouble as a result of your location - my incoming water temperature is right at 41 degrees. Your's is more likely in the 65 - 70 degree range. To get the water at the required temperature here, the most it can flow is somewhere in the 2.5 GPM range. So...we are talking over twice as long to fill a regular bathtub and about four times as long to fill the master's tub unless I install something else.

Since writing this post originally, I've also determined that the dealer wants a 15% restocking fee. That, with shipping charges means that changing the solution out altogether would likely cost me near $100. For that, I'll likely stick with the unit and go ahead with the installation of a hot water heater tank downstream of it.

 

[Jimmyp5]

Your right on suppy line water temp...check here and my supply in is 59 right now. They said the AE 115 would be fine here, but I went for more...no susprise there.

Looks like your into a tank....

 

[Highbeam]

20 GPM is huge. I typically measure 15 GPM as the max available through a standard city water meter at 70 psi when sizing irrigation systems. To pass 20 gpm through a 1/2" plumbing line will require some high velocities and source pressure. Have you measured the actual flow through this tub filler? Fill a 5 gallon bucket in 15 seconds.

 

[CurlyDave]

I wonder if you aren't experiencing trouble as a result of your location - my incoming water temperature is right at 41 degrees...

Since writing this post originally, I've also determined that the dealer wants a 15% restocking fee. That, with shipping charges means that changing the solution out altogether would likely cost me near $100. For that, I'll likely stick with the unit and go ahead with the installation of a hot water heater tank downstream of it.

I have often wondered if a tankless electric water heater would have enough oomph and now I know the answer.

You 41 degree water will probably not change temperature very much, if at all, in the summer if you are on a well. From Wellowner.org - Planning for a Water Well/

"Ground water below 20 feet from the surface remains at a constant temperature. The temperature of well water is equal to the average air temperature of the area. For example, in Columbus, Ohio, it's 55ｰ F and provides an excellent heat source for heat pumps."

Personally, I would go back to the dealer who sold me the unit and tell him that he sold me an undersized unit. If he would like to exchange it, I would be amenable to that, but he should not be charging a restocking fee for selling the wrong item. Print out the reference on well water temperature, and bring it with you. He knew, or should have known, that the unit was undersized. He may have a serious liability here. I don't know about your electric layout, but if you have to run a new electric feed for a higher wattage unit, he might be liable.

For a large volume of hot water in a hurry, nothing works quite as well as a good old tank, especially if you are running it on electricity. You can do a lot to mitigate the problem of heat loss from a tank by locating it in a place where you want the heat. I started out with two standard hot water tanks in series. They are in my garage/shop and what I find is that in winter they keep the temperature of the shop about 10-15 degrees higher than outside. Not quite heated, but since I am re-using waste heat, everything is OK. If your tank is inside your house, the heat it loses is going to end up inside the house. Not really a $ loss.

Haven't gone through a summer yet, but I think I will just turn the hot water temp down to 120 and since I anticipate a garage temp of 90, the delta T will be low, so the heat flow into the garage will be lower than in winter.

 

[dmccarty]

Another option is to preheat the well water with solar. We wanted a tankless water heater but given that we are on a well and only have power I figured the only way the thing would really perform is if we could preheat with a solar system. With the solar system the tankless water heater is almost a backup.

We put the money else where in the house since a 80gal water heater was $300-400 I think. And we could put in solar anytime. And the 80 gal tank could be use as part of the solar heating installation.

Maybe one day the solar water heating will get to the top of the priority list.

Later,
Dan

 

[charlz]

Another option is to preheat the well water with solar. We wanted a tankless water heater but given that we are on a well and only have power I figured the only way the thing would really perform is if we could preheat with a solar system. With the solar system the tankless water heater is almost a backup.

We put the money else where in the house since a 80gal water heater was $300-400 I think. And we could put in solar anytime. And the 80 gal tank could be use as part of the solar heating installation.

Maybe one day the solar water heating will get to the top of the priority list.

Later,
Dan

I've thought about doing the same thing. Supposedly the evacuated tube collectors are very efficient. The only down side I could see is morning showers etc. especially in the winter. Likely the solar heated water in the storage tank has cooled considerably from the previous day. I'd be interested in seeing some real-world numbers as to savings.

 

[5030tinkerer]

Thanks for all the replies, guys. I'm on rural water, not a well, so anticipate my water temp going up a bit in the summer, though not terribly much. I may hit 55 if I were guessing. The line is 5' down for most of the 800' run from the municipal supply and is closer to 9' down as it approaches the house (it comes in via 2" IPS to avoid pressure loss).

As far as holding the dealer to blame, that's sort of hard to do (though an interesting idea to try - I'd just be wanting to do a no-restocking fee return, not hold them to any sort of install or electrical reimbursement). I simply didn't research the tankless unit out enough (unusual for me). The specs clearly indicate a 2.5 or so gpm flow at an 80 degree rise. This is what I am indeed seeing. If I had to do it again, I likely would have chosen the Tempra 36 (36kw instead of 27kw) and taken the hit on the warranty. Then again, even that unit doesn't have the flow I need for my tubs, leaving me with the water tank.

I haven't called the dealer to see if they would take pity on me, though may.

I likely will just install the tank unit following the current tankless and reasonably never run out of hot water. Sigh. On the good side, I suppose I can take comfort in knowing that at the $0.031/kwh that I am paying, my standby losses will only likely cost me maybe $1-$2/mo. Not a big deal. Besides, as was already mentioned, it is not as if all is lost in the winter as it is actually helping to heat part of the house.

As far as the 20gpm flow, this is merely the spec by the manufacturer (American Standard). My tap into the rural water line is 5/8" and I am seeing about 72psi of static pressure. I'd have to run the calcs to see if that can do the 20gpm mentioned. I do know that the lines running to the fixture are 3/4" if that is any indication of what the designers intended the fixture to be able to do.

 

[MikeInEburg]

Maybe I'm misunderstanding, but I'd thing you'd want a tank before, not following the tankless unit. Use the classic tank to preheat water to 75 or 80 degrees or so - then do final heating with tankless. Your total "temp rise" on the tankless would be much lower than trying to heat from 55 degrees. This should yield the lowest total energy consumption. During the summer, the classic tank may not have to work very hard to get water to pre-heat temps.
Mike