Water Heater Capapcity

RJMCTAFO

I have a customer that has a 40 Gallon direct fired water heater that is going to be replaced due to age. The home has 3 full baths and 1 .5 bath. I suggested installing a Bock 32 E set at 150* with tempering valve and expansion tank. Is there a way to figure out what that would give me in 1st hour recovery? He is worried it will not be enough when he has a house full around the holidays. I have done the same in houses with more bathrooms and the customer has never had an issue.

Don_197

Whoever you buy your Bock from should be able to tell you.

GW

I would chose my words extremely carefully if you downsize their water heater (or Hot Water Heater for all you sensitve characters). Tempering valves are a band aide unless auxiliary systems are being used (solar, wood, etc). However, 32 with mix should handle what 40 non mix did with jacked temps.

Gary

RJMCTAFO

I guess I should just not try to redesign something that doesn't need to be. Just hate installing huge water heaters for 10 days a year when I know that the 32 would most likely be enough the other 355.

icesailor

In my experience. What was there before?

Bock 32E's will give you a lot of hot water. They also stack like crazy. If you leave it set at 150 degrees or "Hot". for average residential applications, it will overheat. Its the nature of the beast. Be sure to put a quality mixer on the hot supply. You will need it. One of those numbered ones like Hot Rod always talks about.

What is the primary heat source for the home? Oil fired hot water? Was there a Tankless in the boiler?

EBEBRATT-Ed

Why not leave the temp down and only turn it up at the holidays?

icesailor

The hotter you run a Bock 32E, the faster they burn out and leak. The lower you run them, the faster they run out of hot water.

If you really have big potential loads, connect the 32E to a storage tank and let the storage tank be the mixer. The recirc pump stops the stacking and saves a ton of money.

First, they have to deal with the problem of overheated water. Then, running out of hot water. Then, the storage tank.

That's how I resolved one that was so hot, it was dangerous from scalding.

Fred

How about we give this HO a 40 gallon tank and let good enough alone? Why create a problem for him/her?

icesailor

I didn't suggest the Bock 32E to the OP. Someone proposed it to him. The OP wanted to know if it was big enough. My comments relate to my experience with them. And the pitfalls. Sort of like the OP that had a problem with his Buderus Indirect piped backwards. Bock's CAN do that on Steroids.

Some of us wondered what the old water heater was. I wondered why the change.

Jason_13

A Bock 32E will supply much more hot water than the 40 gallon gas heater could ever do.

Fred

icesailor said:

I didn't suggest the Bock 32E to the OP. Someone proposed it to him. The OP wanted to know if it was big enough. My comments relate to my experience with them. And the pitfalls. Sort of like the OP that had a problem with his Buderus Indirect piped backwards. Bock's CAN do that on Steroids.

Some of us wondered what the old water heater was. I wondered why the change.

I didn't mean to suggest that you proposed the Bock, ice. I just thought if the HO has had good service with a 40 gallon water heater, why not put another 40 gallon water heater in. How much fuel savings can you get by going to a 32 gallon Bock, and at what risk that the HO may not have the hot water needed during peak loads or may have other temperature issues requiring a mixer or additional storage tank?

icesailor

All tank type water heaters should have a mixer. It increases the theoretical size of the heater when you increase the water temperature. Bock 32E's get hot so quickly that they are almost dangerous without mixers.

I was interested in what was there for a water heater before. And what was the energy source for it. Would you have a gas water heater and a oil fired boiler? Would someone put in a LPG water heater and an oil boiler that once had a tankless but some smart person disconnected it and put in a LPG heater? If that is the case, I would put in a storage tank. They don't give out Bock 32E's as door prizes at the corner saloon on Saturday night.

We don't discuss prices here, but I wouldn't be surprised if you couldn't buy 2 - 50 gallon electrics for the cost of a Bock 32E. You'd only need one for the storage.

Fred

I guess I'm pron to keep things simple. If the HO had the water he needed with a 40 gallon tank, I'd put another 40 gallon tank back in. If the fuel source were different from the boiler/furnace and/or other appliances, I'd buy a water heater that used the same fuel (EDIT: unless there are extenuating circumstances that we don't know about) and I'd adjust the water temp so that mixing wasn't necessary. I bet that would cost 25% to 30% of the cost of the Bock and/or a storage tank, and a mixer.
If the HO says the 40 gallon isn't sufficient, when the house is full of guests, then I'd put in a mixer and let him turn the temp up at the boiler for those 5 or 6 days a year.
That's just my opinion.

hot_rod

How hot did they run the old 40 gallon tank?

Use to be most water heaters ran 140F or more. Then a rash of soft tissue lawsuits helped manufacturers decide to limit tank controls to 120F.

But now that legionella is out of the bag, tanks will probably creep back to 140F recommendations.

Some brands and models already include an ASSE 1017 thermostatic mixer in the box. I predict we will see more of that, if tank temperatures are increased.

140F is a reasonable tank operating temperature, at 160F and above the life expectancy drops quickly, according to manufacturers of glass lined vessels.

Keeping in mind the gauge of the steel, and thickness of the glass coating may be less that it was years ago

I've been told AO Smith supplies all the glass used in DHW tanks produced?? They held the original patent on the process.

Next drive to farm country notice all the blue colored AO Smith Haverstor silos. Same blue glass coating as inside WH tanks.

I remember Dave Yates writing some articles on how to calculate WH drawdown, maybe search around the P&M website.

Mark Eatherton

I know the answer.

First hour capacity of a tank style water hater is 80% of store capacity, plus the hourly output divided to reflect the rise in GPH at 100 degrees F rise.

In looking at Bock information, input can be between .4 and .6 GPH. Assuming the middle lane, .5 GPH times 138,000 = 69,000 btuH in, with a conversion efficiency factor of 80% (.8) then our net hourly output is 55,200 btuH.

55,200 divided by 8.33 divided by 100 = 66 GPH of 100 degree F rise water.

Add this to the 80% storage factor of the 50 gallon tank (50 X .8 = 40) 40 gallons and you get 106 GPH of 100 degree F rise water per hour out of your heater.

As for holding temperatures and mixing valves, if you have an issue with them, I suggest you get used to it, because I suspect it will become a code standard in the very near future. ASHRAE is working on standard 188P, which addresses Legionella Pneumophlla and that is their suggestions, plus a bunch of other stuff for commercial and health care facilities. Store at 130 to 140, and mix down to 120 or less. The lawyers hate it, the utility companies hate it, and the builders hate it, but it is in fact a serious life health safety issue that should have been addressed LONG ago. In order to compare with the original 40 gallon tank, you would have to run it through the same calculations as above.

Now you know...

ME

hot_rod

Remember for a thermostatic mix valve to operate accurately you need 25- 27 degrees difference between the hot inlet to the mixer and the mixed outlet. This allows enough delta T for the wax cartridge to respond and mix the temperature.

So if you want 122°F out which some codes suggest, you need the tank outlet at 147°F or higher.

This comes up a lot when thermostatic mix valves are used on radiant systems, the boiler supply to the mixer needs to be 25° hotter or you get a un-stable mix temperature.

Temperature out is "thready" from warm to setpoint temperature as the valve continues to "hunt" for the mix point.

Same with DHW mix applications, and you can feel that in the shower.

this is also why thermostatic mixers don't always get along with tankless style heaters. It's best to use a tankless that has a small tank so the valve has a more stable reference temperature.

Is a tankless with a tank, still considered "tankless"?

Mark Eatherton

No, it'd be "tankful"

Tanks for the updated mixer information. :-)

The only thing I dislike about raising the tank temperature is the increased stand by losses, but I guess that can be compensated for with additional insulation. Dave did do an analysis using a manufacturers information that showed the difference between keeping a tank at 140 versus 120 is negligible. Pennies per day if memory serves me right, but its been a long time ago that he did that.

ME

RJMCTAFO

Thanks for all the comments. The homeowner currently has a 40 gallon Aero oil fired. He wants new and more efficient. Will be sticking a Bock 40 in for replacement. Just wanted to ask sharper minds if there was a better way.

icesailor

hot rod said:

Remember for a thermostatic mix valve to operate accurately you need 25- 27 degrees difference between the hot inlet to the mixer and the mixed outlet. This allows enough delta T for the wax cartridge to respond and mix the temperature.

So if you want 122°F out which some codes suggest, you need the tank outlet at 147°F or higher.

This comes up a lot when thermostatic mix valves are used on radiant systems, the boiler supply to the mixer needs to be 25° hotter or you get a un-stable mix temperature.

Temperature out is "thready" from warm to setpoint temperature as the valve continues to "hunt" for the mix point.

Same with DHW mix applications, and you can feel that in the shower.

this is also why thermostatic mixers don't always get along with tankless style heaters. It's best to use a tankless that has a small tank so the valve has a more stable reference temperature.

Is a tankless with a tank, still considered "tankless"?

But But, But, wait!!!!

That would mean that you have to run the water heater way above the condensing mode!!!. Just to make shower valves to work properly?

Why I NEVER.

Every single lever shower valve made has equal sized hot and cold inlets. Newer ones have restricted hot ports. How much hot water do you need to blend with a higher flow of cold to get the temperature that makes someone comfortable.

Jean-David Beyer

I had been running my indirect with its thermostat set so 120F water came out the nearest hot water outlet when it was on full speed. Then I thought it would be smart to run the indirect at 140F for legionella reasons. So I had a tempering valve installed, and being a nut about understanding things,, I had a thermometer installed on the hot water inlet and the mixed water outlet of the mixiing valve. It was very interesting.

The valve specifies that the supply has to be 18 F hotter than the outlet, so for 120F out, I need 138F in. So with that valve, I decided to use 140 in anyway. I then ran the hot water and adjusted the aquastat on the indirect upward until I got 140F into the valve. I then ran the hot water some more until the indirect was asking for heat and let thins run until the indirect was satisfied. This went up to well over 150F (but not 160F). I have since found out that this large difference between the calling for heat (140F) and satisfied (say, 155F) is not surprising.

For a research project long ago, I got two thermostats (brass, suitable for installing in wells) that I could set with a range of only a fraction of a degree. These had to be adjusted with a screw driver and had no dials, so you used a thermometer to measure what you would get. I do not think home owners would want thermostats like that, though. Now if the temperature of an indirect can vary within a 15F range or even more, you sure do want a thermostatic mixing valve after it.

icesailor

Isn't it fun to play with our big boy toys? So educational. They never taught us things like this when we were in school.

hot_rod

icesailor said:

hot rod said:

Remember for a thermostatic mix valve to operate accurately you need 25- 27 degrees difference between the hot inlet to the mixer and the mixed outlet. This allows enough delta T for the wax cartridge to respond and mix the temperature.

So if you want 122°F out which some codes suggest, you need the tank outlet at 147°F or higher.

This comes up a lot when thermostatic mix valves are used on radiant systems, the boiler supply to the mixer needs to be 25° hotter or you get a un-stable mix temperature.

Temperature out is "thready" from warm to setpoint temperature as the valve continues to "hunt" for the mix point.

Same with DHW mix applications, and you can feel that in the shower.

this is also why thermostatic mixers don't always get along with tankless style heaters. It's best to use a tankless that has a small tank so the valve has a more stable reference temperature.

Is a tankless with a tank, still considered "tankless"?

But But, But, wait!!!!

That would mean that you have to run the water heater way above the condensing mode!!!. Just to make shower valves to work properly?

Why I NEVER.

Every single lever shower valve made has equal sized hot and cold inlets. Newer ones have restricted hot ports. How much hot water do you need to blend with a higher flow of cold to get the temperature that makes someone comfortable.

You only need to run the WH above condensing mode @ 135- 140F if you want the tank maintained at that temperature. If not run it at 125F with a 5° differential setting.

The toss up is potential legionella issues at below 140F, or potential 2nd degree burns with 140F. I think most plumbing codes state 120F or so.

So if you are in a customers home, best to follow the code, adjust the mixer, and lock the handle down. Note this on the invoice that the customer signs so everyone is aware of the setting.


That's why Caleffi designed the NSF approved temperature gauge tailpiece. Some installers use one on the mix port, and one on the hot port to see what is actually coming out of the tank. Also a press fitting version for quick and easy installation or replacement.

The allows the inspector to confirm with the same accurate gauge.

Remember to watch the minimum flow rate to provide accurate mixing. Sometimes for low flow fixtures you need an ASSE 1070 point of use valve. Or a shower mix valve with that function built in, a different ASSE number.

icesailor

@Hot Rod:

I know all that. There's nothing I like better than a good confliction.

In the 1990's, the Feds ordered that Nursing Homes not have any hot water that came in contact with human skin not ever be over 116 degrees F . Now, they don't want hot water under 140 degrees because of legionella. No residential hot water hotter than 120 degrees and shower devices that can spray hot water on bare skin not be hotter than 108 degrees or somewhere. But it takes 140 degree water to kill Legionella.

It seems that there is some bacteria that lives in shower heads and gets into the aerosol mist of shower heads. It seems that women are more susceptible to the effects of this bacteria than men. It causes pulmonary infections. They won't say what the bacteria is or how to get rid of it.

What a conflict.

Something to ponder.

SWEI

Condensing is driven mostly by inlet temperature, no?

icesailor

That's true.

But when it comes to comfort, I've learned here that there is nothing like a good comfortable steam heat system. People LOVE that nice hot "warmth" coming off a nice warm steam radiator. People like to feel warm. There's a narrow range where people put clothes on to be warm, and take them off to be cool. On a windy, cool, sunny day, people will go outside and sit in the sun, but sit in the lee, out of the wind. Because they like to feel warm. The perfectly set Delta T for a room thermostat is the one where the person sitting in a room starts to feel cool but doesn't have the ambition to get up and turn up the thermostat. Before they get the ambition to get up, the thermostat starts the equipment. It works the same as the room warms. It shuts off before it gets too warm. The warmer the outlet, the more comfortable people think they are because they feel it.

The Old (dead) Timers from as long as I can remember used to say that "Cast Iron HOLDS the heat". They didn't like convector radiation, box convectors or baseboard. One heats up slowly and gives up its heat slowly. Convector baseboard heats up quickly and cools down quickly. I've tried about every thing and way to put heat into a room. I have my own personal test subject for critical commentary. My wife. If she isn't warm and comfortable, life is miserable. When it comes to cold, she is no frozen flower. 30 years of caring for her horse(s) in any weather, cold, ice, snow, wind. When she came in. it better warm. When she turned up a thermostat, it better get warm. When she/they put their hands over the emitter, they better feel more than "Warm" air coming out. Or someone will get some serious barking. Now she complains about it being too hot in the sun in Florida to ride.

Life is driven by temperature.

hot_rod

Mark Eatherton said:

No, it'd be "tankful"

Tanks for the updated mixer information. :-)

The only thing I dislike about raising the tank temperature is the increased stand by losses, but I guess that can be compensated for with additional insulation. Dave did do an analysis using a manufacturers information that showed the difference between keeping a tank at 140 versus 120 is negligible. Pennies per day if memory serves me right, but its been a long time ago that he did that.

ME

I guess the "pennies a day" is the cost of protection from the arbitrary.

If you have concerns about legionella, or any bacteria for that matter, maybe that price is not too steep?

I'm not sure how realistic the manufacturers standby loss numbers are. That test can be tweaked to get whatever result you want.

Keep the tank at 120°F, or lower, in an 80° ambient space and the number is low. More often the tank is set higher and the room is colder, especially with outside combustion air blowing around the tank and boiler

A typical DHW tank with a sloppy HX tube running up through the hot tank to the roof? Just seems like the worse place to store hot water.

Jean-David Beyer

"A typical DHW tank with a sloppy HX tube running up through the hot tank to the roof? Just seems like the worse place to store hot water."

Sounds like a good reason to use an indirect fired hot water heater if you have a low volume boiler that can be run cold-start. Or an electric one. They do not have that heat losing HX tube running out of the house.