Water heater gremlins strike in the dead of night

[OurTown]

The other night the car wash gremlins were at it again. This time they ate a hole in the bottom of our 80 gallon hot water storage tank. I'm not really sure why they like to eat rust but they have some strange habits. The 16 year old tank is leaking pretty bad so I temporarily re-plumbed the gravity tank to cold water until we fix it. It's kind of bad timing because in a few months the boiler/tank/circ pump setup would be removed for a more efficient and space saving setup to make room for more equipment. The least expensive new 80 gallon tank I found is about $1,200. It makes sense to just bite the bullet and put that money toward a new setup now. For space saving purposes we are wanting to go with two 199K BTU tankless water heaters without a storage tank. They would feed the gravity tank for three self serve bays and our presoak hydrominder tank. It is around 15 GPM at full tilt. I'm aware of the restrictive nature of tankless water heaters. In talking to Chris from Car Wash Boilers he said that the Navien NPE-240A can have the flow restrictor locked open to help with that issue. He did say that it bumps the warranty down a bit so that makes me think that it could be hard on these units. Without restricting them both units together would have a 13 GPM at 60 deg rise so it is not really far off but would be good to have them unrestricted. The other thing we would like to do is add a second fill valve with cold water set lower in the gravity feed tank. Our gravity tank has an extra bung welded into it but it is set higher than the primary one and I think it is 1 1/2". Can this be used for our cold water backup fill valve? Also would it be hard on the valve to have the float held so far under water all the time? Another concern is the presoak hydrominder chemical draw if there is a restriction in hot water flow. Are we on the right track here and is there something we are not thinking of? Also do you guys know where to get some good gremlin traps?

 

[I.B. Washincars]

I’ve been doing the cold float lower than the hot for years. My tanks are tied together at the bottom and the cold only opens if the hot gets behind or is not working for some reason. I would think that would be good for the valve because it is shut off solid instead of almost shut off like the others.

If you’re not ready to install your new setup yet, why not just get a cheap household heater at Lowe’s or HD to get you by. It doesn’t make any difference if it gets behind. Unless the hose is ice cold in the customer’s hand they’ll never know.

 

[JGinther]

So it kind of sounds like you have a pressurized hot water tank that recirculates with the boiler. If that's the case, another idea is to just plumb the recirculator and the boiler to the gravity tank. Let it fill with cold water, put the tstat in the gravity tank, and let the boiler heat up the gravity tank. It's not effiecient, but is how many washes were built to start with.

Later, when you do the tankless thing, you could use one tankless as the replacement for the recirculator, and another for the pressurized hot water feed with the higher float. The lower float would fill cold in 'holy crap it's busy' mode, then the second tankless would kick in to heat up the water... It keeps hot water at all times that way if that is a goal. I have done this several times now. The secondary tankless really hardly ever comes into play. But with this design, you never have to worry about the pitfall of running out of water during high demand, and you never have to worry about having cold water and hearing about it in an online review.

 

[MEP001]

JGinther, I looked into using a tankless heater as a recirc type in an open tank, and most of them required pressurized water. I don't remember which brands/models could operate with an open tank, but there are a few. Another option would be to have a big enough recirc pump to pressurize the heater with a restriction on the outlet - you'd probably need a tank thermostat and a control to turn off the heater and run the pump for a minute to leech the heat, but it seems doable.

I've also been thinking about just looping the pipe to the heater and turning on a recirc pump with a flow switch. The heater itself would control the water temp, and with it just running a loop in the plumbing there would be no supply issues.

 

[JGinther]

Yes, some water heaters might be too smart for their own good. But I don't really know why the tankless water heater would care when you think about it... I mean, if the water heater is in recirculation mode, I don't see how it would care for any difference between closed loop and under pressure vs open to air. The back pressure to the pump is the same either way. The only 'pressure' is the difference to atmosphere. There is no difference with respect to the water heater vs. the recirc pump. I do know you have to have a heater that supports recirculation mode though, as the inlet thermister and control has to know what to do with water already near setpoint temperature.

 

[OurTown]

If you’re not ready to install your new setup yet, why not just get a cheap household heater at Lowe’s or HD to get you by. It doesn’t make any difference if it gets behind. Unless the hose is ice cold in the customer’s hand they’ll never know.

I have thought about this.

Later, when you do the tankless thing, you could use one tankless as the replacement for the recirculator, and another for the pressurized hot water feed with the higher float. The lower float would fill cold in 'holy crap it's busy' mode, then the second tankless would kick in to heat up the water... It keeps hot water at all times that way if that is a goal. I have done this several times now. The secondary tankless really hardly ever comes into play. But with this design, you never have to worry about the pitfall of running out of water during high demand, and you never have to worry about having cold water and hearing about it in an online review.

If we were not concerned with keeping the tank hot then it still wouldn't have anymore flow than two just plumbed together to the float valve. If we were concerned with that then could we hook up the tankless heater circ pump to a copper coil in the tank to keep it hot?

I've also been thinking about just looping the pipe to the heater and turning on a recirc pump with a flow switch. The heater itself would control the water temp, and with it just running a loop in the plumbing there would be no supply issues.

Are you saying there would be no supply issues because the water is already to temp and the heater would not delay the flow?

 

[OurTown]

I forgot to ask Chris if locking open the flow restrictors would eliminate the cascading feature. That would run both at the same time every time.

 

[washnvac]

I use a single 199k btu Rinnai to feed 5 s/s bays and pre-soak on 1 auto. Rinnai feeds float in 15 gallon gravity tank. Cold water hudson valve placed at 2 inches above outflow at bottom of tank. Rinnai is capable of 8 gpm. This set up has worked great; as it is unlikely more than 2 bays at anyone time will be using hot. I use hot on hp soap and hp wax. Auto pre-soak is fed by gravity to a procon with a dema injector. Hudson valve has been buried in the water for 3 years now with no issues. I have been there on busy days, and have only seen the low water valve come on twice.

 

[MEP001]

Are you saying there would be no supply issues because the water is already to temp and the heater would not delay the flow?

No, my concern is that the heater can only flow so much water (One heater will barely do two bays), and it's restricted to almost nothing when it's not up to temp, so your tank will run dry. I've thought about a loop just so there's no supply restriction to the tank, it will just run cool or cold if the heater can't keep up.

 

[MEP001]

Yes, some water heaters might be too smart for their own good. But I don't really know why the tankless water heater would care when you think about it... I mean, if the water heater is in recirculation mode, I don't see how it would care for any difference between closed loop and under pressure vs open to air. The back pressure to the pump is the same either way. The only 'pressure' is the difference to atmosphere. There is no difference with respect to the water heater vs. the recirc pump. I do know you have to have a heater that supports recirculation mode though, as the inlet thermister and control has to know what to do with water already near setpoint temperature.

Most brands of on-demand heater state a minimum operating pressure, usually 20 PSI. I was at a wash with a Rheem and it shuts off if the pressure drops below 10 PSI. Some specifically state that they are not to be used circulating to an open-to-atmosphere tank. I'm guessing they heat at such a high rate in such a small exchanger compared to a boiler that they don't want water boiling in it.

 

[JGinther]

Some specifically state that they are not to be used circulating to an open-to-atmosphere tank.

Interesting... Never seen that. Probably because we are the only weirdo's that would want such a design that would likely flood out a house or commercial building... Each of the water heaters I have worked with haven't cared... Worked fine. AO Smith, Takagi, older Palomas, and Norritz. Each of them with the exception of old palomas use sensors to calculate flow and restrict the outflow to guaranty temperature over flow (which is what makes them not ideal for car washes). The heat exchangers use a dual-zone design in which extra hot water is mixed with not as hot water after the heat exchanger to mix the water to the desired temperature outflow. The gas amount is determined by the inlet water temperature put against the flow - which would be full open with a properly sized recirculator. The heaters each have a 'recirculator' mode in which the computer knows to drop the gas flow in a decreasing curve to make the transition from full heat to off smooth and efficiently use the heat left in the exchanger. Haven't encountered a problem yet....

 

[OurTown]

Rinnai is capable of 8 gpm.

Our hydrominder is 4.5 GPM and one bay is about 3.5 GPM. There have been times I have seen all three bays using HP soap so that is why I was thinking of two 199K units.

 

[OurTown]

No, my concern is that the heater can only flow so much water (One heater will barely do two bays), and it's restricted to almost nothing when it's not up to temp, so your tank will run dry. I've thought about a loop just so there's no supply restriction to the tank, it will just run cool or cold if the heater can't keep up.

Do you think this would be better than locking open the flow restrictors on the water heaters?

 

[2Biz]

Or you can opt for one of these instead of the on demands. It only has a 23" diameter and is 52" tall. Has a 1" NTP Flow through design capable of 40gpm. No restrictors. It is more than enough for your 3 bay and you won't have any worries of starving pumps or needing to add backup cold water supplies. BTW, Gremlins hate SS! I went through all this several years ago and opted for this one....I am glad I spent a little extra to make it as trouble free as possible. BTW, my most busiest day for my 4 bay was 4400 gallons. I heat soap and wax cycles, never once ran out of hot water. That tells me I made a pretty good choice!

http://bostonheatingsupply.com/phoenixph199-55.aspx

 

[OurTown]

Or you can opt for one of these instead of the on demands. It only has a 23" diameter and is 52" tall. Has a 1" NTP Flow through design capable of 40gpm. No restrictors. It is more than enough for your 3 bay and you won't have any worries of starving pumps or needing to add backup cold water supplies. BTW, Gremlins hate SS! I went through all this several years ago and opted for this one....I am glad I spent a little extra to make it as trouble free as possible. BTW, my most busiest day for my 4 bay was 4400 gallons. I heat soap and wax cycles, never once ran out of hot water. That tells me I made a pretty good choice!

http://bostonheatingsupply.com/phoenixph199-55.aspx

I knew you would be here with that nice water heater but with our precious floor space vs available wall space plus the price....

 

[2Biz]

I knew you would be here with that nice water heater but with our precious floor space vs available wall space plus the price....

Well, If you insist! I like the idea of having a cold low water backup supply if going the demand route. Also, If you insulate the heated gravity tank, IMHO and experience, there is no need to circulate heated water through a heat exchanger to keep the tank warm. Even after not being used all night, the gravity tank is still warm in the AM when I stop by on my way to work. It quickly heats up with the first use of the day. My gravity tank holds about 10 gallons of water. I learned that one the hard way after I designed and made the tank for a heat exchanger, made the heat exchanger, then realized I didn't have to install it! BTW, I insulated the tank with 1/2" foil faced foam board insulation, foil bubble wrap, and foil tape. Just like you'd use in heating ductwork.... Works great!

 

[OurTown]

Still questions remain. Does anyone have their flow restrictors locked open on their tankless water heaters? In times of high demand and the low level cold float valve opens up then how would we combat chemical draw issues on the presoak hydrominder?

 

[JGinther]

Still questions remain. Does anyone have their flow restrictors locked open on their tankless water heaters? In times of high demand and the low level cold float valve opens up then how would we combat chemical draw issues on the presoak hydrominder?

You wouldn't want to do that if you were to employ the second water heater as a recirculating heater to the gravity tank. If you instead want to have both feed the tank, I guess that would make more sense... Although I have never tried it... I would prefer knowing the temperature was up to what it should be, so I always elect the recirculator option for the second heater. We have 6 bays and 1 Laser 4000 working on 1 takagi tm-50 feeding hot water, and 1 of the same set up to recirculate. And the laser is using all hot water... In Colorado where inlet water is 35 degrees. Removed a rotten boiler that was 1.5m Btu and get by with 800k Btu now and its always hot and much more efficient than the original set-up... Let alone the space savings.

Regarding the hydrominder, you won't have any problem with that. The only time the hydrominder will be used is when someone isn't using a high pressure function, so the demand is always less during that time.

 

[2Biz]

Still questions remain. Does anyone have their flow restrictors locked open on their tankless water heaters? In times of high demand and the low level cold float valve opens up then how would we combat chemical draw issues on the presoak hydrominder?

I will ask? Why are you wanting to heat supply water for your presoak hydrominder? I would think if you plumbed it to the same Demand Heater Output to your float tank, it will vary PS dilution because of varying pressures created by uneven flows to the float tank?

You say its to combat chemical draw? I don't heat my PS supply water and I've never had any issues with the hydrominder drawing chemical from the 5 gallon bucket.

 

[OurTown]

Currently the presoak tank is shared between the auto and SS. The new auto will have its own heater and the SS will not be heated.