Hi,

I just moved into a new house that has a Vaillant tankless water heater, model number F75-W-45 PP. The problem is that it only provides hot water for a very short period of time (5 minutes or so) until the water turns cold again. I thought that tankless water heaters were supposed to provide hot water indefinitely? I have left the water running long enough to ensure that this simply isn't a "cold water sandwich" problem.

The temperature on the water heater is not the problem I don't think, because when first turning on the shower, the water is piping hot. After about 5 minutes or so, though, the water turns cold again. I'm not sure what the problem could be.

Thanks for any help anyone can give!

 

Maybe this will help you a bit before the experts reply. http://forum.doityourself.com/boiler...less-coil.html

 

Theoretically at least, they should be capable of supplying continuous hot water... BUT ... this is highly dependent on FLOW RATE. If the water moves too quickly through the coil, it can't pick up enough heat.

One thing that happens to those tankless coils is that the get 'limed up' ... mineral deposits form inside the coil and reduce it's capacity. They can be cleaned by 'boiling out' the minerals with an acid solution... NOT a DIY project!

How is yours piped? Just one pipe in, and one out ? Is there a 'bypass' valve between the two pipes? Do you know if you have a 'mixing valve' installed at the boiler?

We could tell all these things by looking at it... if you are so inclined, snap some pics, set up a free account at Image hosting, free photo sharing & video sharing at Photobucket and upload the pics there... come back here and drop a link to your album.

 

OK, I took some pics. I wasn't exactly sure what I should be aiming for so I tried to get what seemed like the important parts, lol. If you need a better angle on something then let me know and I will take a better picture. My camera isn't the best but I think it got the job done.















Thank you!

 

An important question is when the water is cool check the piping at the boiler. We already know the pipe from the mixing valve is cool but what about the pipe from the coil to the to the hot port of the mixing valve?

 

Vail, the 'mixing valve' that rbeck is referring to is that black handle valve to the left of the oil burner.

The cold water inlet to your coil is the pipe to the right side of the gauge, and the hot water out of the coil is obviously the one to the left of the gauge.

The black handle should be marked something like "cooler --- hotter" with an arrow indicating direction...

What position is that valve in now?

What's up with those oil leaks on the floor? Shouldn't they be repaired?

 

Well, I turned the mixing valve up to the maximum temperature, and that has made a big difference! I was only able to turn it about a 1/2 rotation or so, but the water temperature is noticeably hotter now, and the hot water doesn't disappear after just a few minutes.

While running the shower, I felt the pipes; the inlet pipe is cold and the outlet pipe is warm. I assume that is the way it is supposed to be?

As for the oil on the floor, that's just residue from a past leak that has been repaired. I just moved in to this house so I can only assume the previous tenant had a leak fixed and neglected to clean up the mess. As long as I have been here, nothing has been leaking.

A question though: turning up the mixing valve has made the water temperature much better, but there are still fluctuations in water temperature which are quite noticeable. Water temperature used to vary between hot and cold, now it varies between hot and warm. It's satisfactory I guess, but it would be nice if the water temperature would remain constant for an extended period of time; having to adjust hot/cold water several times during a shower can be annoying!

I have observed the water heater while running the shower. I turn on the shower, and the water is hot. Soon after, the boiler turns on, and water remains hot. After a few minutes of running, the boiler turns off, and water temperature gradually declines for a few minutes, until the boiler turns back on, and water temperature jumps back up again. This cycle continues indefinitely. Is it normal for the boiler to shut off while I am still drawing hot water? Is there a setting I can activate to make the boiler stay on as long as hot water is being drawn?

Thank you again for all your assistance, you have been a big help.

 

No, the boiler will shut off when the temperature inside the boiler reaches a certain point...

Do this: and be careful... there is 120 VAC inside this box, so you might want to shut the boiler off before you open ... but that gray box on the front of the boiler, the cover slides straight off. There may be one screw that you need to loosen on the side... slide that cover off and tell us what the settings are on the three dials... HIGH, LOW, and DIFF.

 

Sorry for the delay, I was very busy this weekend.

Lo: 140
Hi: 190
Diff: 10

 

I would set those temps at 160º low, 180º high and 20º diff. With the settings you are using allows the water to drop to 130º and only reheat to 140º. The temp setting should be 20º higher than the setting on the mixing valve.

 

I would probably go up in steps myself... I would suggest:

High 180

Low 150

Diff 20

These settings will allow the boiler to cool to 140, and heat to 160. The settings rbeck stated will be ten degrees hotter than that.

and see how that goes. Also, remember that the higher you set the LOW, the more fuel you will be using to keep the boiler at that temp.

You might even wanna set the mixing valve back to where it was. Your valve looks like the 'fixed' mechanical variety, and not the thermostatic type. All it does is 'blend' some cold with the hot coming from the coil, before it heads to the house. If you've got it all the way on HOT now, it's possible that it will be TOO HOT! so BE CAREFUL! and when you make the change, please WARN other household members ... that the water may start out a LOT! hotter than it was! Hot water above 120 can be VERY dangerous... someone could even wind up at hospital... ESPECIALLY if there are children, or elderly in the home.

If you wanted more consistent temperatures, you could replace that mechanical mixing valve with a thermostatic one.

Better yet, abandon the coil and go with an Indirect Water Heater...

 

I changed the settings to Trooper's recommendations and the water is noticeably hotter again. Excellent!

I'll probably stick with my current mixing valve, but if I did decide to upgrade to a thermostatic valve, would installing that be a DIY project?

 

It could be a DIY if you can solder pipes. Let us know if you decide to try and we'll help if we can.