Bosch Tankless Heater & Support

[Floatflyer]

Good morning. I have recently bought a Bosch tanklesss water heater from Costco in Cabo San Lucas. This unit is to help heat a hillbilly hot tub I have built in a rural part of southern Baja, off the grid and lovely.

I have installed the heater and it is igniting for 10-20 seconds and then going out. I have measured the water flow and it is about 2 qts in 15 seconds. I tried to get help from Bosch but they are useless. They want me to be in front of the unit, to have a "professional" with tools available, etc. IF I had access to those things, I would have them!!! I am in the boonies and have to rely on myself and, hopefully forums like this for suggestions on troubleshooting my problem.

The Bosch Model is a Confort II 13. This unit does not show on the Bosch website, it is made in China and may just be distributed in Mexico, I don't know. But with it being sold by Costco, I figured I was in pretty good shape as far as being able to return it if need be. I have a few neighbors that have similar units as a sole heater for their larger hot tubs and they are pretty pleased with the results. For the record, I had an Aqua Star tankless heater down here for over 10 yrs and was basically satisfied with it, especially after I added a rv pump to boost the flow.

Anyway, what ever you can suggest will be appreciated. Thanks, Floatflyer

[Dana]

With a 10-20 second flame out it probably means it's going over-temp and the safety is tripping.

How many BTU/hr or watts or whatever is this thing alleged to put out at both max and min fire? At 2quarts/15 seconds you're talking 2gpm, or 120gph, which is ~1000lbs/hr. The short sheet only calls out a max input of 17.4kw which is ~60, 000BTU/hr. At the 84% specified efficiency that's ~50,000 BTU/hr out, so at 1000lbs/hr that would deliver a 50,000/1000= 50F rise in temp, so if your hot tub starts out at 100F, the output of the heater is 150F, and almost CERTAINLY above the anti-scald safety limit, which would cost it to flame out after several seconds.

Turn the flame modulation knob to the minimum and temperature modulation knob to the mid range and see how well it works. If it doesn't flame out, try bumping up temp modulation until you feel like the output is hot enough (feeling it with your hand- should be too hot to hold for very long), then start cranking up the flame a little bit at a time. The output temp should remain the same, but the internal flow restrictors will start to let more flow through the heat exchanger rather, but at some point the flame will be putting too MUCH heat into it even at max flow, and the temp will start rising. You can probably be able to tweak it in just by hand-feel on a copper pipe, since at 120F output it's already pretty damned hot but you'd be able to hold onto it for several seconds, but at 130F and up you have to be un muy mas macho to take the heat.

If the min-mod is ~35,000BTU/hr or leas the output is ~30,000BTU/hr, and at 1000lbs/hr that's a 30,000/1000= 30F temp rise which would probably work (or not.) At 30K the hot tub is already 100F the built-in anti-scald safety trip point would have to 130F to flame out pretty quickly. If that's the probably need more flow to get the delta-T small enough to modulate and not trip the safety.

[Floatflyer]

Thank you for your suggestions. I am not sure if we are talking about the same controls, my unit has 2 dials, one I think controls how hot the water gets and the other controls has fast the water goes thru the heater. Anyway, I dialed the temp to the lowest setting and moved the "volume" dial to a bit of a lower setting. The unit ignited and stayed on for a reasonable length of time so I am now on the right track. The water temp in the tub had reached 100 degrees after a warm and sunny day of solar production. I raised the water temp to 108 with the Bosch instant heater in about 10 minutes!! This is exactly what I was hoping for, mainly heat with solar and, as needed, top off with propane. Will keep you posted. FF

[Dana]

I thought knob on the left adjusts the flame modulation, the one on the right does the temperature (which it does by restricting flow if it's not keeping up at full-fire.)

It doesn't take much of a burner to maintain a hot-tub temp if the hot tub is at all insulated (even 1.5- 2" cedar or redwood counts as insulation) and has an insulating cover, even the Confort 6 would have worked in your application, but would have taken ~2x as long to raise the temp. In 10 minutes (1/6 of an hour) at 50,000BTU/hr output you've added ~8300BTUs to the tub for a temperature rise of 8F. That implies a mass of ~1000lbs, or volume of about 125 US gallons. Does that sound about right? With half the burner it would take 2x as long, but it would surely get there.

Sounds like you'll be able to heat up quickly without it hitting the over-temp cut out with a few optimizing adjustments though (or you may already have.)