Tankless water heater with a small tank?

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slice1900

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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.
 

WorthFlorida

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I own a restaurant/bar......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?

You may have already upgraded your needs but some dish machine manufactures have a built in heater that can be added or add an outboard unit. A point of use heater recovery rate could not meet the demands for the commercial environment, especially when the kitchen is going full tilt. http://www.cmadishmachines.com/node/536
 

Jadnashua

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How cold does your water get in the depth of winter in Iowa? That can make or break many tankless systems' ability to perform. Some of the tankless systems can be designed to operate in series...the first one warming the water, and the second one raising it further. The combined burners of the two (or more) can often be designed to get nearly any volume of hot water you might need. To get the desired temperature rise, it is not uncommon for a tankless system to have internal restrictions, so that the water can't pass by too fast to get to the temp you want. Think passing your hand through the flame of a candle...do it slow, and you may burn yourself, but do it faster, and there's no issue.

What do you use for your heating system for the restaurant? Do you have a boiler or is it forced air? If you have a boiler, often, that will be your cheapest way to heat your water via an indirect WH. Those wouldn't require any vents, since it doesn't have a burner - it uses your boiler's. Most of them would have way in excess of what's needed in firing capacity to provide probably any water needs you need as it can use the whole capacity of your boiler (typically, much bigger than the burner in a separate tank) when needed to provide your hot water needs.
 

Master Plumber Mark

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You will probably be disappointed if you actually install one of them in a restaurant.
Your peak demand times could be a problem . I have seen the troubles folks get themselves
into going to a tankless in a commercial setting....

Also , do you have a water conditioner in this place... you will need one with a tankless
or you will be constantly having to de-lime the unit...

Just throwing another 75 gas back in to replace the old one is the most simple solution
but I know your head is probably already spinning thinking about all that extra space you
would have in that mechanical room ...

god luck


 
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