I own a restaurant/bar that has two natural gas AO Smith 75 gallon fast recovery water heaters that can regenerate at 81 gph. Or had, one of them started leaking earlier this week and is drained and shut off. Rather than simply replacing it I'm looking into tankless options to gain more space in the storage room containing the water heaters.
I'd also like whatever I go with to use a concentric vent (direct vent or power vent, not sure of the terminology) so I won't need an opening to the attic above for combustion air - that room gets chilly in the winter and can be downright stifling in the summer. Since it houses stuff like amps for the sound system, getting that hot is not really ideal (all the wiring for audio, TV, phone and network goes there, that's not something easily fixable) I don't care if it vents out the wall (there's an outside wall available) or up through the attic like currently, I just really don't want the opening for the incoming combustion air any more if it can be avoided!
The main factor influencing the choice of water heater is a low temperature dish machine (i.e. uses sanitizer so washes at 120-140F, not 180F like high temperature machines) that uses 1.7 gallons per rack and does maybe 30 racks per hour if the dishwasher is feeding it as fast as he can. While it works at 120F (and that's what I have the water at currently because otherwise I'd need tempering valves on 13 sinks) it performs best at 140F. While it doesn't use that much water per hour, it wants a lot of quickly when it fills - I'm not sure how quickly since it isn't specified in gpm but rather PSI. Due to this (I assume) the vendor explicitly states they do not recommend use of a tankless water heater.
The problem is, I don't think a single tank water heater has a fast enough recovery rate. Between the dish machine, the prespray that 1.3 gpm, and a bit here and there for hand sinks, the 81 gph recovery of the current unit might not be enough when we get REALLY busy. A tankless water heater can deliver 3-4x that amount per hour, but I can't use it because of the dish machine.
So here's the question - is it possible to add a small storage tank to a tankless water heater? I don't need (or really want, unless there's no choice) a full sized 30 to 40 gallon tank. A small tank that recirculated back to the tankless heater to maintain temperature would be perfect. Is there such a thing? Or maybe I can use a small point of use tank heater for that storage - it wouldn't have much heating to do other than 'topping up' the temperature if the water was sitting for a while, and to account for the "cold water sandwich" of the tankless feeding it 120F water. Do they make small natural gas point of use heaters or all they all electric?
As for getting the dish machine 140F water, I'm thinking a small point of use electric heater could deliver that extra 20F. How much power would it require to heat 1.7 gallons of water by 20F in two minutes?
Perhaps what I'm proposing here way too complicated, and someone who knows this stuff better than me has a much better way to go? If so, I'm all ears.
I'd also like whatever I go with to use a concentric vent (direct vent or power vent, not sure of the terminology) so I won't need an opening to the attic above for combustion air - that room gets chilly in the winter and can be downright stifling in the summer. Since it houses stuff like amps for the sound system, getting that hot is not really ideal (all the wiring for audio, TV, phone and network goes there, that's not something easily fixable) I don't care if it vents out the wall (there's an outside wall available) or up through the attic like currently, I just really don't want the opening for the incoming combustion air any more if it can be avoided!
The main factor influencing the choice of water heater is a low temperature dish machine (i.e. uses sanitizer so washes at 120-140F, not 180F like high temperature machines) that uses 1.7 gallons per rack and does maybe 30 racks per hour if the dishwasher is feeding it as fast as he can. While it works at 120F (and that's what I have the water at currently because otherwise I'd need tempering valves on 13 sinks) it performs best at 140F. While it doesn't use that much water per hour, it wants a lot of quickly when it fills - I'm not sure how quickly since it isn't specified in gpm but rather PSI. Due to this (I assume) the vendor explicitly states they do not recommend use of a tankless water heater.
The problem is, I don't think a single tank water heater has a fast enough recovery rate. Between the dish machine, the prespray that 1.3 gpm, and a bit here and there for hand sinks, the 81 gph recovery of the current unit might not be enough when we get REALLY busy. A tankless water heater can deliver 3-4x that amount per hour, but I can't use it because of the dish machine.
So here's the question - is it possible to add a small storage tank to a tankless water heater? I don't need (or really want, unless there's no choice) a full sized 30 to 40 gallon tank. A small tank that recirculated back to the tankless heater to maintain temperature would be perfect. Is there such a thing? Or maybe I can use a small point of use tank heater for that storage - it wouldn't have much heating to do other than 'topping up' the temperature if the water was sitting for a while, and to account for the "cold water sandwich" of the tankless feeding it 120F water. Do they make small natural gas point of use heaters or all they all electric?
As for getting the dish machine 140F water, I'm thinking a small point of use electric heater could deliver that extra 20F. How much power would it require to heat 1.7 gallons of water by 20F in two minutes?
Perhaps what I'm proposing here way too complicated, and someone who knows this stuff better than me has a much better way to go? If so, I'm all ears.