Dual water heaters

[Jm66208]

Checking for current opinions on dual water heaters. Parallel vs series.
Replacing two 40 gal n/g heaters for 4 plex. Currently plumbed in parallel. I've monitored them & one is definately doing most of the work. I'd like to plumb the new ones in series, setting the first one at about 100f and the second at 140f.
Concern is legionella in first tank. What say you?

 

[WorthFlorida]

Are these servicing four apartments?
Is this classed as commercial property?
Are both tanks right next to one another?
Do you have cold well water or city water?
How many bathrooms?
Why are there two tanks?
Any complaints of running out of hot water?

In parallel both tanks should be set to the same temperature and the plumbing should be nearly equal so water is drawn equally from both tanks. If in parallel and one tank is not putting out enough heat, as the water is drawn one tank temperature will be lower thus mixing cooler water with the hot water and lowering the temperature. Then users will turn the hot water valve higher and drawn more water through the tanks. If these tanks are servicing four units a commercial grade water heater (one unit) will be more efficient, however, they are far more costly such as Lochinvar.com. With one tank you set the temperature to 140 degrees and then use a temperature mixing valve to bring the water down to 120 degrees or lower.

In series as you suggest it will be the same as one water heater. Preheating will help in the recovery rate but you'll have just 40 or 50 gallons of heated water. Using a TMV for the 140 degree second tank will be a must.

Have you considered a tankless water heater? It may be a better option.

 

[Sponsor]

 

[Jm66208]

Are these servicing four apartments?
Is this classed as commercial property?
Are both tanks right next to one another?
Do you have cold well water or city water?
How many bathrooms?
Why are there two tanks?
Any complaints of running out of hot water?

In parallel both tanks should be set to the same temperature and the plumbing should be nearly equal so water is drawn equally from both tanks. If in parallel and one tank is not putting out enough heat, as the water is drawn one tank temperature will be lower thus mixing cooler water with the hot water and lowering the temperature. Then users will turn the hot water valve higher and drawn more water through the tanks. If these tanks are servicing four units a commercial grade water heater (one unit) will be more efficient, however, they are far more costly such as Lochinvar.com. With one tank you set the temperature to 140 degrees and then use a temperature mixing valve to bring the water down to 120 degrees or lower.

In series as you suggest it will be the same as one water heater. Preheating will help in the recovery rate but you'll have just 40 or 50 gallons of heated water. Using a TMV for the 140 degree second tank will be a must.

Have you considered a tankless water heater? It may be a better option.

 

[Jm66208]

Yes, servicing 4 condominium apartments. City water. Not commercial. Tanks sit side by side and how originally plumbed when building was renovated in 90's. 1 bath per unit. 4 washing machines in building. No complaints with current setup. I'm just not a fan of parallel setup. Not a fan of tankless either.
I could split the system up and have one 40gal for two units. And one for other two. Not sure if this is s better option as well. I hate to change what is currently working.

A single 100gal is an option as well. But means higher initial cost, and slower recovery rate.

 

[hj]

In series, with BOTH heaters set to the same temperature, will insure that ALL the water is exposed to the full heaters' input, which will be twice that of a single heater.

 

[Sylvan]

If a professional did the installation then parallel setup is the way to go BUT the piping has to be exact for cold and hot Or you can go for rapid recovery 50 gallon tanks

Or use a water heater dedicated to the laundry room (set higher temperature) so less change of running out of water for the rest of the apartments

 

[WorthFlorida]

Don’t fix what not broke is the old saying. If two tanks are working ok, keep it the same. With two tanks in parallel, should one crap,out, at least you’ll still will have some hot water.

 

[Jm66208]

In series, with BOTH heaters set to the same temperature, will insure that ALL the water is exposed to the full heaters' input, which will be twice that of a single heater.

Hj, agree. This setup is like having an 80gal setup. The first in the series does most of the work, which isn't the end of the world.
What I'd the first is set lower so it feeds the second with partially heated water, helping share the load between heaters?

 

[Jm66208]

Don’t fix what not broke is the old saying. If two tanks are working ok, keep it the same. With two tanks in parallel, should one crap,out, at least you’ll still will have some hot water.

If one craps out, I just go install a new one. I'll keep it the same if there is no "better" option.

 

[Jm66208]

If a professional did the installation then parallel setup is the way to go BUT the piping has to be exact for cold and hot Or you can go for rapid recovery 50 gallon tanks

Or use a water heater dedicated to the laundry room (set higher temperature) so less change of running out of water for the rest of the apartments

"Professional" is a loosely defined term these days. The piping looks pretty symmetrical on the hot side, but one tank still is doing most of the work, through my observation. Could be something a small as a drop of solder inside one of the pipes causing the difference.
"Exact" is like saying "if you hold the club right, swing & aim correctly, the ball should go into the hole".

 

[Dana]

Checking for current opinions on dual water heaters. Parallel vs series.
Replacing two 40 gal n/g heaters for 4 plex. Currently plumbed in parallel. I've monitored them & one is definately doing most of the work. I'd like to plumb the new ones in series, setting the first one at about 100f and the second at 140f.
Concern is legionella in first tank. What say you?

A storage temp of 100F is smack in the middle the sweet-spot temperature range for growing legionella, which is why 100F-105F hotel hot tubs are one of the primary sources of outbreaks. At 100F you're almost guaranteed to get a legionella colony growing in that tank.

A storage temp of 140F storage temp isn't enough to kill it in a reasonable amount of time, and certainly not in a quick pass-through when filling a big tub. Setting the first one to 120F won't kill legionella, but it's hot enough to keep any colonies that get established from growing. Setting both to 140F would be safer, since that will actually kill it (eventually).

 

[Jm66208]

A storage temp of 100F is smack in the middle the sweet-spot temperature range for growing legionella, which is why 100F-105F hotel hot tubs are one of the primary sources of outbreaks. At 100F you're almost guaranteed to get a legionella colony growing in that tank.

A storage temp of 140F storage temp isn't enough to kill it in a reasonable amount of time, and certainly not in a quick pass-through when filling a big tub. Setting the first one to 120F won't kill legionella, but it's hot enough to keep any colonies that get established from growing. Setting both to 140F would be safer, since that will actually kill it (eventually).

This leads me to the point where the decision seems to be between two 40 gal 40kbtu tanks, plumbed in series, or one 80gal 75kbtu tank.

 

[Dana]

This leads me to the point where the decision seems to be between two 40 gal 40kbtu tanks, plumbed in series, or one 80gal 75kbtu tank.

For a commercial application like this I'd be looking at a 80 gallon stainless steel tank with a 100-150K burner like the HTP Phoenix PH100-80. It's a chunk, o' change up front at about 4 grand, but it'll outlast a pair of glass lined 40 gallon tanks by 2x, and save quite a bit on fuel over it's lifecycle. Even the 80 gallon Phoenix Light Duty PH76-80 with the condensing 76K burner has slightly more burner capacity than a pair of 40 gallon standalones, due to the higher efficiency (95% vs 80%), which is about a grand less, but it's a bit marginal for serving 4 bathrooms. If what you have hasn't been an issue the Light Duty would be fine. (It's only Light Duty in comparison to bigger-burner commercial water heaters.)

The Westinghouse WGR080NG076 (available through the big orange box store) is exactly the same unit as the Phoenix Light Duty under the nameplate, and often a few hundred cheaper. The only difference is the warranty support network. (The technical support on the Westinghouse product is handled at HTP's headquarters- same technicians & engineers, different phone line.)

 

[Jm66208]

Man, a condensing water heater means a whole new vent through the side of a stone building, plus all that cost & gambling on the small unit carries some risk of an unsatisfied customer. Plus, I am now married to a water heater with electronics. I'd likely refer that one to the big boys, who would price a system out of reach, then end up spashing in a couple 40gal bradford whites & a new TET, and be on their merry way.

 

[Dana]

Man, a condensing water heater means a whole new vent through the side of a stone building, plus all that cost & gambling on the small unit carries some risk of an unsatisfied customer. Plus, I am now married to a water heater with electronics. I'd likely refer that one to the big boys, who would price a system out of reach, then end up spashing in a couple 40gal bradford whites & a new TET, and be on their merry way.

A condensing water heater can be vented out the roof, just not in a terra cotta lined masonry or B-vent chimney. It's usually an easy retrofit in a B-vent flue chase.

If the water heaters are currently vented into a masonry chimney or B-vent shared by other appliances the chimney can't be used as a vent chase for the water heater though.

A pair of 40,000 BTU/hr 80% burners in 40 gallon water heaters delivers heat into the water at a rate of 64,000 BTU/hr. A single 76,000 BTU/hr condensing burner delivers heat at a rate of 72,000 BTU/hr of heat to the water. This part doesn't take hard math- the Light Duty wins by 12-13% on BTU rate capacity and recovery time.

The financial math of calculating the net present value of the difference in up front cost of the doubling the lifecycle with a stainless vs. glass lined tank plus the reduced fuel use is more complicated, but it's a good investment if the building owner is paying for both fuel & equipment.

 

[Sylvan]

Parallel- Equally manifolding the inlet water pipe and outlet
water pipe in multiple heater installations. Having equal
number of fittings; as well as, lengths of pipe.
Series- Piping the outlet of one heater into the inlet of the next.
Why
shouldn’t I
pipe in
series?
Piping in series can cause the first heater to fail sooner than the
second. The reason this happens is you are using the full
capacity of the first heater and only upon an increased demand,
is the second heater cycled on. This causes the first heater to be
used more than the second, and used to its fullest capacity. This
can cause the first heater to condensate and corrode.

 

[Reach4]

A storage temp of 100F is smack in the middle the sweet-spot temperature range for growing legionella, which is why 100F-105F hotel hot tubs are one of the primary sources of outbreaks. At 100F you're almost guaranteed to get a legionella colony growing in that tank.

A storage temp of 140F storage temp isn't enough to kill it in a reasonable amount of time, and certainly not in a quick pass-through when filling a big tub. Setting the first one to 120F won't kill legionella, but it's hot enough to keep any colonies that get established from growing. Setting both to 140F would be safer, since that will actually kill it (eventually).

Do you know of any instance of legionella coming from the water heater in a single family home that uses city water, or gets the water from a deep well and has had the plumbing sanitized since well work?

Cooling towers, hot tubs, non-pressurized storage tanks, I can see getting infected, but I am trying to picture how the culture gets started in a home water tank.

 

[Dana]

Do you know of any instance of legionella coming from the water heater in a single family home that uses city water, or gets the water from a deep well and has had the plumbing sanitized since well work?

Yes.

 

[Jm66208]

A condensing water heater can be vented out the roof, just not in a terra cotta lined masonry or B-vent chimney. It's usually an easy retrofit in a B-vent flue chase.

If the water heaters are currently vented into a masonry chimney or B-vent shared by other appliances the chimney can't be used as a vent chase for the water heater though.

A pair of 40,000 BTU/hr 80% burners in 40 gallon water heaters delivers heat into the water at a rate of 64,000 BTU/hr. A single 76,000 BTU/hr condensing burner delivers heat at a rate of 72,000 BTU/hr of heat to the water. This part doesn't take hard math- the Light Duty wins by 12-13% on BTU rate capacity and recovery time.

The financial math of calculating the net present value of the difference in up front cost of the doubling the lifecycle with a stainless vs. glass lined tank plus the reduced fuel use is more complicated, but it's a good investment if the building owner is paying for both fuel & equipment.

IF the numbers are accurate, the option is intriguing. The flu is shared, so a horizontal vent is a must. That triples the cost. I just found out that my ford pickup has a computerized transmission that decided to downshift to first gear while at highway speed. The event was terrifying, and I am kind of down on computer controlled gadgetry at the moment.

 

[hj]

If one heater in a parallel installation fails, the replacement has to be EXACTLY like the surviving one, which is almost impossible given changes in design and the mineral buildup in the surviving one.