From Scalding to Cold and Back Again

[Daustinhjr]

I'm working on a buddies Bosch 940 es. As the title implies the water is going from really hot to cold over and over. This is my first time working on a tankless hot water heater; very limited experience with any water heater. Any advice is greatly appreciated.

This is what I've tried so far:

1. Checked all connections/fuses. *
2. Checked to make sure vents weren't clogged
3. Watched the flames while it was running. Looked uniform blue.
4. Lowered the temp to 122.
5. Pumped vinegar through it for 1 1/2 hours.
6. Checked the flow at the tap and it checked out.
7. Noticed one of the white blocks attached to the copper tubing had some burn marks on it. When I popped it off the copper for a closer look it broke in half like it was already fractured. After looking the part up in the manual I found it's part of the "freeze prevention kit". I ordered a replacement that will be here tomorrow. Is there any chance replacing this is going to fix the problem; or am I just replacing something that broke?

 

[Valveman]

Is this on a well pump? If so the cycling on and off from 40 to 60 PSI will cause that problem. Holding a constant 50 PSI will keep that from happening.

 

[Sponsor]

 

[Daustinhjr]

Is this on a well pump? If so the cycling on and off from 40 to 60 PSI will cause that problem. Holding a constant 50 PSI will keep that from happening.

It's on city water.

 

[Daustinhjr]

 

[Daustinhjr]

 

[Valveman]

I don't know if that burned part could make it act like that or not. But you could still have varying pressure coming in from the city, which I know will make those water heaters turn on and off.

 

[Daustinhjr]

Hmm. A regular old $10 water pressure gauge wouldn't cut it to check that would it? If not could you recommend something that will work for my purpose and doesn't cost more than the heater itself.

Any other ideas to check?

 

[Valveman]

Yes a $10 gauge is all you need to check that. A simple pressure reducing valve will hold the pressure steady, if you have enough to reduce and still have sufficient pressure. Check it first. Make sure that is the problem.

 

[Jadnashua]

At what volume does this occur? Any flow? Max? Min? It could be related to the flow rate detector. A tankless system needs some minimum flow rate to turn it on, if say at a faucet, using warm water, you may not be exceeding the required minimum flow rate, or are right on the edge. Many of them tend to need a bigger flow to turn on, but once reduced, can 'hold' the on state. That can also be a problem if it then reaches its over temp limit, and needs to shut off - this is an even bigger issue if the modulation of the heat input is not working, or the thing doesn't have that feature. At this time of the year, the incoming cold water is often quite warm, so to do something, you often don't need much hot water, which can play hell with the min flow requirement. Does it do the same thing when say filling the tub or the washing machine if you're calling for hot water?

While, in the right circumstances, these things can save some money, IMHO, some of the hassles just aren't worth it. The payback timeline is often exceedingly long, if it ever works out positive what with the cost to purchase and maintain.

 

[Dana]

Why is the thing cranked all the way up to 122F?

Most houses wouldn't need more than 110F, maybe 115F to fill an old school cast-iron tub. The higher you set the tankless temp, the more susceptible it will be to flame-out and poor temperature control, especially at lower flows. And the flow through the tankless is extra-low in the summer:

During the warmer summer months the incoming water temperatures in MS can reach 70F or higher, and the temp at the shower head is ~105F, only a 35F rise. A ~1.5 gpm low flow shower head at 35F rise is ~23,000 BTU/hr- pretty close to the minimum firing rate of that unit. If setting the output temp to 122F, the amount of flow through the tankless would be only about 1 gpm (with about 0.5 gpm of cold feed to the shower) which is only 2x it's min-flow rate. If you lowered the temp to 110F it would have an easier time modulating and controlling the temperature, with a much lower likelihood of flame-out due to either insufficient sensed flow or output that is too hot.

 

[Leon82]

that is the freeze prevention kit it should only activate if the sensor reaches 35 degrees or so