Is this normal for a tankless water heater?
January 24, 2014 12:38 PM   Subscribe

Water temperature has gone way down in the past few weeks. Is this something I can diagnose myself?

I have a tankless water heater that is probably from the mid 90s (AquaStar). About a few weeks ago, I noticed that my shower was not getting as hot as it used to and it was getting cold faster. I tested the vanity sink and at full heat, I would only describe it as "mildly warm".

This indicates to me an issue with the water heater rather than the showerhead or faucet. Question: this loss of heat, is it something that an average homeowner could diagnose? Or should I call around and get a plumber out here?

If it sounds like something specific has gone wrong, how much should I expect to pay to replace it?

posted by amicamentis to Home & Garden (10 answers total) 1 user marked this as a favorite
Response by poster: Additionally: I live in the Northeast US, and it's currently like 2 degrees F here. Would that make a difference?
posted by amicamentis at 12:40 PM on January 24, 2014

Where does your water come from? What's the current temperature of the cold water?

Tankless heaters can only guarantee a certain rise in temperature from the incoming cold water. So if your cold is 20 degrees lower than last month, chances are your hot is a lot cooler as well.
posted by JoeZydeco at 12:57 PM on January 24, 2014

Yes, the ability of your heater to produce hot water depends on the temperature of the cold water feeding it. It's possible your heater isn't working right, but it's also possible that it's undersized. They are rated for a certain temperature rise at a certain flow rate. Poking around Bosch's Aquastar site, I see that they typically rate their heaters for a 35F and a 55F rise. If your water is coming in to the house at 40F, and your water heater was properly sized using the 55F number, you could end up with water at 95F, which isn't very hot.
posted by mr vino at 1:12 PM on January 24, 2014

You could check the air intake and exhaust vents. If they're plugged up, the heater won't effectively fire.

This is frequently an issue at one of my cousin's rental places, where the genius who installed it put the air intake vent right next to the dryer exhaust. The debris screen on the intake inside the unit ends up covered in lint and chokes it off.
posted by CKmtl at 1:22 PM on January 24, 2014 [1 favorite]

The one we had at our last house had a digital control on the temperature setting. Occasionally a power outage or something would cause a reset and lower the temperature from where we had set it, so you might want to check a control panel if your model has one.
posted by pappy at 1:25 PM on January 24, 2014 [1 favorite]

I had an Aquastar, and it was doing the same thing. It turned out to be a slow down in the water pressure from a restriction thing in the shower head. At first I had a plumber come and take a look at it, he said it looked fine but he flushed it out anyhow.

I took the shower head apart and pried out the restrictor, and it was like new.
posted by chocolatetiara at 1:34 PM on January 24, 2014

Older tankless units needed to be flushed. I had one that recommended once a year.

If I recall correctly, this involved (had I actually done it) a whole hassle of hooking up garden hoses and running water through it backwards or something for an hour or two.

Look up the documentation for your unit and see what it says.
posted by cmoj at 1:35 PM on January 24, 2014 [1 favorite]

My Aquastar 240FX is a re-badged Takagi TK-1. Periodically it "starts doing that" - maybe five years between fits. The issue with mine is the heater tries several times before it's able to light off, and the burner slots are found to be clogged with white fluffy ash. This seems to accumulate preferentially near the safety thermocouple. The solution is to remove the burner unit, carefully take the jets off, scrub the burner with a toothbrush and then blow the whole magilla out with compressed air.

Tankless heaters have a -minimum- flow to get moving. If turning on two taps at once brings the temperature back to normal, then one of those taps alone might be wavering near the minimum flow.
posted by jet_silver at 5:19 PM on January 24, 2014

Mine always does that in the winter, I just adjust the thermostat for a higher water temperature. When the summer comes, the water is too hot, so the thermostat is adjusted seasonally.
posted by JujuB at 5:22 PM on January 24, 2014

Are your water pipes uninsulated and running through a crawlspace? It's been brutally cold these past few weeks. Perhaps the hot water is simply cooling before reaching the taps?
posted by Thorzdad at 5:39 AM on January 25, 2014

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