I’m sorry to resurrect a dead thread, but this is great information, I’m looking to do a tankless into a holding tank myself, the tankless will be on its own loop and will not have any impact on flow in the home, it will pull water from the bottom of the tank, circulated by an aqua stat controlled pump, through the tankless and back into the tank. The issue of really variable inlet temps for the tankless concerns me, so I’m wondering if I could utilize a thermostatic valve that mixes in cold water from the line coming into the house with water from the holding tank with hopes it can provide a narrower range of water temps for the tankless to adjust to.
With the thermostatic mixing valve in the tankless loop it cold water would/could only be mixed in when hot water was being drawn from the tank. When the cold water flow stopped the thermostatic mixer would cut back the flow to zero, limiting the temperature in the tank to whatever temp the mixer was set to.
As the temperature of the tank gets close to the temperature of the mixer output the flow will drop below the minimum flow required to keep the flame going on the tankless and it'll be prone to short cycling as the temp falls to the point where the tankless will re-light, but the pump will run continuously (and likely cavitate at the high back pressure of the restricted low flow) , never actually satisfying the aquastat if the aquastat isn't set several degrees below the mixer temp (which would render the mixer a bit useless.)
Setting the output of the tankless to it's maximum (typically ~55C for condensing tankless) and setting the temperature on the tank to ~50C would likely get you there without complicating things with the thermostatic mixing valve in the tankless loop. Install the thermostatic mixing valve between the output of the tank and the hot water distribution plumbing of the house, and be sure to select one that is good for the whole 9gpm.
The reason I want to do this is because I have a shower that flows 9gpm and while I love my old faithful AO smith Vertex 100 from back in the day, the replacement costs are too high. Our incoming water temps are 7deg C (ahhh, that is 45deg F I believe), so a tankless will never supply the volume I need on the hot side. The idea of going with a hybrid setup is reasonable IMHO, I’m just wondering if since this thread was created there has been more science done on how to make this work well. The thermostatic valve idea I understand is imprecise and suboptimal but may provide a better range of temp conditions for the tankless to operate with and maybe extend its life span a bit. My goal is to provide a solid recovery rate in the 6GPM range by feeding the tankless warmer water from the holding tank mixed with some cold water from the mains.
if I am way off base, please let me know. 95% chance I’m an idiot. I’m certainly not a plumber but I’m trying to develop this design so the plumber is presented with a well thought out plan. This is not a common configuration.
Thanks all.
Tim
You need to do some math to know what will/won't work.
A (45C-7C=)38C /100F temperature rise at 9gpm (or ~4500lbs/hr) is a heat rate of 450,000 BTU/hr more than 2x (getting on to 3x) the output of a 199K-in tankless. With a sufficiently large drainwater heat recovery heat exchanger feeding it's output to...
1 ...the cold input of the tank...
2 ... the cold feed to the thermostatic mixing valve, and...
3... the cold feed to the shower...
...can cut the heat rate needed to be supplied by the tank + tankless to about half of the 450KBTU/hr. (Again make sure the drainwater heat exchanger can handle the full 9gpm, which may require a commercial rather than a residential version.)
When being filled with ~25C water from the heat exchanger the recovery time on the tank is reduced by half, even when drawn down to near the comfort-limit of a shower (<<40C, a typical showering temp).
Do NOT set the flow on the tankless to 6gpm or you'll shorten it's life substantially. At a 5C rise (50C storage temp, when then tank is nearly fully heated, 55C output temp) and a more temperate 4 gpm (=~2000lbs/hr)t the tankless would be running it'll be burning at a comfortable ~18,000 BTU/hr, (=the minimum firing rate of some tankelss units) but when the bottom of the tank is 25C (roughly the output of a bigger drainwater heat exchanger) that's a 30C (54F) delta-T, and a burn output rate of (54F x 2000lbs/hr =) 108,000 BTU/hr, which is more than the output of a Vertex 100.
Even 4 gpm is a high enough flow rate to wreck the flow sensors on some smaller tankless units in short years, so even though you're never using it at it's max burn rate, a 180KBTU/hr - in (or bigger) tankless would be far better choice than a 150K-in (or smaller) unit.
If a drainwater heat recovery unit isn't in the budget you'll need a (roughly 2x) bigger tank in order to get reasonable showering performance out of it.
You may have to play around with the flow rate on the tankless and the storage temps to optimize hot water delivery on the equipment you choose. Raising the storage temp much higher than 5C below the output temp will cause many tankless units to flame out, or even shut down & spit out an error code.