Aqua Star 240FX temperature fluctuations

[benmiller23]

Hi all - We have an old Aquastar 240FX installed in the insulated crawlspace behind our upstairs bathroom, supplying hot water to a shower, sink, toilet, and tub. (The rest of the house is on a separate water heater.)

We've had a problem with cycling water temps in the shower since we bought the house three years ago: from scalding to cold no matter how we have the faucet handle situated. We have no fluctuating temperature problem with the tub or sink, and using the tub I've determined that the heater's burner ignites at the right minimum flow rate (from the Bosch website -- https://www.bosch-climate.us/files/201109161802030.TWH-TK-02_Activation_and_crossover_test.pdf).

In addition, if I run hot water in the sink while the shower is running, the shower does stay at a constant temperature, and it stays at a constant temp when I take the shower head off altogether, too. It seems clear that the water heater is shutting off due to the shower head's low water flow rate.

The thing is that I don't know what to do with that hypothesis: I've replaced the shower valve, gotten a new shower head (I've now used 2gpm and 2.5gpm), cleaned the AquaStar's flow sensor, and cleaned its inlet filter. I'm sure there's a solution I'm missing. Would descaling make a difference? I've been thinking not since it seems to work fine with tub & sink. Should I just buy the highest gpm shower head available and hope that'll keep the burners ignited? Many thanks.

 

[Dana]

There is a fairly high minimum modulated firing rate on that unit, (IIRC it's about 30,000 BTU/hr) so at low flow it may go into an over-temp mode and turn itself off for several seconds at a time. A low-flow showerhead will magnify the problem, as will warmer summertime incoming water temperatures and lower water pressure. Tub filling & sinks are not simlarly flow constrained, but if you run the sink at a trickle you'll see similar behavior.

Many low-flow shower heads have removable/replaceable aerators that limit flow. If yours is one of those, see if pulling it out makes a sufficient difference.

If your house has a pressure reducing valve, see if adusting the pressure up makes a difference.

 

[Sponsor]

 

[Jadnashua]

FWIW, vanity faucets are flow restricted, too, at less than a typical showerhead (I think it's 2gpm...who needs more to wash your hands or brush your teeth?). I agree that it appears the unit is hitting the high limit and shutting down momentarily. It's possible that sensor is not working properly, or there's a heat transfer issue and that might be helped by a demineralization. If you don't have a water softener and it hasn't been done in the last year, I'd try that first. If the sensor is partly covered by minerals, it can't react properly because of the insulation effect.

 

[benmiller23]

Thanks very much to both of you.
The house doesn't have a pressure reducing valve that I am aware of, since I've never noticed a gauge anywhere. Where would it be, though? Guessing somewhere between the meter and the primary water heater?
I'd looked into the recommended descaling process for the water heater, but since it was joined directly to the copper pipes (without a place to hook up the hoses -- isolation valves, I think?) I'm not sure how to proceed. Is there a way to do it without cutting the pipe first? I could do it, just hoping to avoid all that work!

 

[Dana]

According to the manual, the minimum firing rate is 37,000 BTU/hr-in, which at 85% efficiency (probably where it was when it was brand new) delivers ~31,500BTU/hr into the water. At 80% (probably closer to where it is now) that would be 29,600 BTU/hr. So for a rough cut 30,000 BTU/hr output is the right range.

A flow rate of 2 gallons per minute is close to 1000 lbs/hr, so at min-fire at 2 gpm it will be raising the water temperature 30,000/1000= 30F degrees. For a 105F shower at a true 2 gpm (bucket-test it) you should be able to keep the thing regulating temperature as long as the incoming water is below (105F- 30F =) 75F, which may be close to where it is in Arlington during the summer, but probably not during the winter. If the gas regulator is running a bit high, it may be delivering more heat than that at min-fire, but lowering the gas pressure by very much becomes dangerous.

If the bucket & stopwatch tested showerhead flow is actually 1.5 gpm (750lbs/hr) the temperature rise at minimum fire would be 30,000/750= 40F, so at incoming temps over 65F it will have this problem, which would be happening over a much larger period out of the year.

Test the pressure of your water too. Showerhead flow rates are specified at 80psi water pressure, which is on the high end of the pressure range for houses. At <30 psi a 2.5 gpm showhead is usually delivering<2 gpm:

Pressure reducing valves are usually (but not always ) placed right after the meter, and before any distribution plumbing runs inside the house. They don't usually have a gauge on them, and come a a few form factors, this being one of the most common:

 

[Jadnashua]

A nice, simple tank-type WH doesn't have these issues...hot water can be dispensed from a dribble to maximum without the temperature varying...the downside is, you can run out if it isn't sized properly. But, it will still provide some hot water if the power goes out. Standby losses on the new ones isn't as much of a factor as the older ones. Efficiency between the types can be the same. For most applications, I'm not a great fan. They can work, but not everywhere. The upfront install costs and extra preventative maintenance means more costs along the life as well. Try to get one fixed can be a hassle as there aren't as many people that can diagnose the problem, or have the parts so you may be down much longer. While you'll pay extra, you can get a tank replaced on the weekend...trying to repair a tankless system, possible, but not likely in most places in a timely manner.

 

[Dana]

But Jim, this unit is "...installed in the insulated crawlspace behind our upstairs bathroom...", which is a tough location for installing a tank solution big enough to fill a tub.

Serving only one bathroom this unit could still have 20-30 years of service life in it. The cheapest "fix" would be to install a gusher showerhead and call it a day. That's not the "greenest" solution, but from a practical point of view there is no "payback" in doing a replacement before it's time is truly up.

When that time comes there are much better tankless units out there with min-fire output under 20,000/hr (and some that can throttle back to under 10,000 BTU/hr.) Any ~150,000-180,000 BTU/hr condensing tankless would fill a tub as fast or faster than an Aquastar 240FX, and most current models would be able to dial back to under 20K-in/18K out.

 

[benmiller23]

Thanks, all.
Yes, this tankless unit was already here when we bought the house (guessing it was installed in the 90s), no doubt because there's a huge tub in the bathroom that it supplies. We would not have picked the tub or this water heater if we'd been making the decisions! At some point we'll remodel the bathroom and replace both.
But for now, I went ahead and popped out the flow restrictor from the showerhead, and that appears to have solved the problem. We'll just have to take shorter -- but less scalding! -- showers to compensate.
Thanks again. I really appreciate the help and suggestions.

 

[Jadnashua]

I'd looked into the recommended descaling process for the water heater, but since it was joined directly to the copper pipes (without a place to hook up the hoses -- isolation valves, I think?) I'm not sure how to proceed. Is there a way to do it without cutting the pipe first? I could do it, just hoping to avoid all that work!

If the thing wasn't installed to make the (often required to keep the warranty) demineralization, then, yes, you have to cut pipes to put in the required valves, or literally cut it out, do the fix, then hard pipe it back, but then that would have to be done each time...much better to add the required bits. Some places, water is quite dear, so using more than 'necessary' can be both expensive and problematic.

 

[Dana]

Thanks, all.
Yes, this tankless unit was already here when we bought the house (guessing it was installed in the 90s), no doubt because there's a huge tub in the bathroom that it supplies. We would not have picked the tub or this water heater if we'd been making the decisions! At some point we'll remodel the bathroom and replace both.
But for now, I went ahead and popped out the flow restrictor from the showerhead, and that appears to have solved the problem. We'll just have to take shorter -- but less scalding! -- showers to compensate.
Thanks again. I really appreciate the help and suggestions.

Now that you've verified the fix, to save both water and natural gas you can install a shower volume control ball-valve thingy in line with the shower head, and throttle back the flow to find the flow point at which the symptom recurs. Adjust the flow slightly higher to where it regulates temperature well enough and call it "done".

The flow rate at which becomes a problem is lower in mid-winter than it is in mid-summer, and the problem may come back again next July, but it's easy to tweak the thing if/when that occurs. Boutique-y plumbing suppliers want $25-$50 or more for them, but sub-$10 all-metal versions from a box store work just fine (I'm less sanguine about the sub-$5 plastic versions.)