I would like to do some "tuning" of my water heater. Here's the situation.

My heater is gas (propane) fired, and sits up on the roof in a small enclosure. It is a blend of on-demand and tank style heater, in that there are two small (5 liter? 10 liter?) holding-tanks and two burners.

Each burner/tank has its own pilot light and adjustable thermostat. The burners/tanks are connected in series (not parallel) with the first burner pre-heating the water that goes to the second. The heater is rated to provide continuous hot water to two and a half bathrooms. My limited experience with the heater supports this claim

When I shower, the water starts out very hot, with the mixing valve at about the half-way setting. After about three minutes, the water temperature drops, probably about 10-15 degrees F. After about three minutes of this diminished temperature, the water temperature climbs back up to the initial temperature and remains there for as long as I want to stay in the shower.

So, what strategy should I employ with the thermostats to avoid or at least significantly lessen that mid-shower dip in temperature? Right now both thermostats are set to their highest temperature. Perhaps the pre-heat thermostat would be left high, and the output thermostat set lower? I don't have a clear picture in my mind of just what is happening with the current settings. It appears that once the water in the second, or output tank is used, it is replaced by water from the pre-heat tank that is of lower temperature, thus the dip. But why, then, after several minutes, does the temperature in the shower come back up?

Maybe someone here can visualize how this works better than I.

tanstaafl.

Hmm...
Ever thought about a thermostatic shower valve?

Have you tried increasing the temperature setting? If you mix more cold water into the shower, then you'll use the ultra-hot water at a slower rate. Not sure if this is actually a solution to your problem.

It seems that you have an issue with one or both burners not kicking in early enough. They eventually do, but I think you need to lessen the time it takes them to turn on.

Do you know that all of the burners and thermostats are working properly?

My guess for why this is happening is that the middle tank is cooler than either the terminal tank or the on-demand supply. As you use the water in the terminal tank, the water in the cooler intermediate tank refills it. The terminal tank probably doesn't have enough heating power to warm the water quickly, so it's unable to warm the cooler water on its way through. Then the on demand system replenishes both tanks and that water doesn't get time to cool in the intermediate tank, as it just passes through, so it remains hot.

Assuming a 2.5gpm shower head, three minutes would be 7.5 gallons, which would be in the neighborhood of 30 liters. So my guess could be wrong. A 30 liter tank would be about one cubic foot. (A 10L tank would be about .35 cubic feet, so a little over eight and a half inches on a side.)

Yes.

I would prefer to avoid the expense, and the installation will be difficult. But that may be the way I have to go.

Theoretically there shouldn't be any "ultra-hot" water. Each tank is thermostatically controlled and should be at the same temperature. My thinking is that by lowering the temperature of the output tank, it will be easier for that tank to maintain its set temperature, even if the water from the pre-heat tank is temporarily at a lower setting.

I'm going to try that and see what happens.

tanstaafl.

This is probably because of the hysteresis of the thermostats.

I'm thinking that at your shower flow, you hit the lower limit of the thermostat and the flames come on, but you probably never hit the upper limit again, so they stay on.

I think you can test this by setting the shower to a really low flow. It should cycle between hot and cold if my theory is correct.

shower <<< primary tank <<<< supply tank <<<< cold water

You don't have a lot of tank-age, the inrush of cold water is chilling the supply tank. It's not recovering fast enough. The primary tank isn't triggering either until too late.

From the description, it takes both burners running to keep up with the demand of a shower.

To get both to trigger, each is going to have to see an inrush of what is relatively cold water. So the primary tank needs to be set to a higher temp than the supply tank. The tricky part is going to be figuring out how much lower the supply tank below the primary. Trial and error should work.

You may still see a drop from the initial temp, but hopefully it won't be such a fast drop as to be noticeable.