I have a question about gas water heaters, and I had no clue where to ask it:

Will a 75 gallon water heater with a 75K BTU heater heat 40 gallons of water faster than a 40 gallon tank with a 40K BTU heater?

My poor understanding of such things says "yes," but I don't know anything about water heaters...

Intuitively, I'd put my money on the 40/40 tank.

EDIT:
Oh, somehow I read the two choices as 75/75 and 40/40. Doh!
Yeah, no question, the 75/40 is faster than the 40/40.
/EDIT

Need to google around to discover how many BTUs are required to heat a unit of water by a given degree change though.

How do you intend to fill the 75 gallon tank to 40 gallons, and vent the expanding air above the water? What's the goal here?

A tankless water heater will heat that water "on the fly"...

What's the remaining 35 gallon in the 75 gallon heater? Air? Already hot water?

Assuming you start with full tanks of hot water in both cases and use 40 gallons (draining the 40 tank and having 35 left in the 75, and then fill both up with cold water, the 40 gallon tank will start heating cold water, but the 75 gallon tank will start with a luke warm mix.

Both heaters (ignoring differences in insulation, different volume/area ratios etc) should increase the temperature of their full volume at about the same rate (simple ratios 40 gallon/40 BTU vs 75 gallon/75 BTU. The 75 gallon tank will reach 'hot' first though since it starts halfway there already.

If air, and you're starting with equal temp water in both tanks (and you don't burn out something that should have been submerged in water in the 75 gallon tank by running it half full and can manage the air pressure) the 75 gallon tank has an almost 2x as powerful heater (75/40), so will be almost twice as fast in raising the temperature a given amount of degrees.

In heating their capacity amount of water, they will take equally long to raise the temperature 10 degrees (again ignoring the little/unknown stuff as insulation differences and that heat losses increases as water temperature goes up and that the bigger tank has an advantage in volume/area ratio for that)

Reservation - I'm assuming you by 40/75 BTU heater mean heating elements that will put 50/75 BTU of energy into the tank per hour.
(Over here we rate heaters in watts, ie a power unit, while BTU is an energy unit (power*time) )

A friend told me it didn't take long for his teens to figure out that there was a limitless hot water supply...

-jk

Wait, which door is the goat behind? And how fast is the first train going again?

I think the term you are looking for is Recovery Rate. Check the specifications for both tanks for a "water heater recovery table".

Thanks folks!

Sorry, I should have added one more bit of information to my question: assuming that each tank is filled with standard city supply water, whatever temperature that might be coming in at any given time of year.

I'm just curious because we recently replaced a 15 year old 40 gallon tank with a 75 gallon tank. I can already tell that we're getting far more hot water than we did before, but it's hard to tell just how quickly it recovers. I was just wondering if we should be expecting faster recovery than the 40 gallon tank had, at least to get back to the amount of water we were used to before the replacement.


Yeah, I think there was some confusion here. I didn't say the tanks were empty at any point. I meant after they were used. Besides, there's no real point in asking how fast a 40 gallon tank will heat 75 gallons of water


It sure will! And it'll demolish my bank account "on the fly" too!

I'd LOVE to get a tankless heater, but cost is the #1 issue. Not only do they tend to run at least twice (usually three times) what our 75 gallon heater cost, they also don't deliver enough hot water for our shower. We did go a bit all out on our master bath, with four body jets, rainshower, and handheld shower. Even with just the body jets there isn't a tankless heater on the market that can keep up, particularly in the winter.

It's possible to actually combine two tankless heaters, but then we're talking somewhere in the range of $4-6K, and that's just for parts.

Granted, some of the money I'd spend on a tankless would be made up in energy savings, but the initial investment is SO high.


Yeah, that's the part of the word problem I accidentally left out. I meant that both tanks started filled with cold water.


I'm using BTU to describe the heater the same way the companies who make them do. They're advertised as 40K and 75K BTU water heaters. They have gas heating elements. Other than that, I have no idea what I'm talking about

I got my tankless when I started working out of the house. My tank heater was lighting up about 10 times a day when no one was using it and had deteriorated over a few years to delivering less than 10 minutes of hot water.

I got a Rinnai from some local supply house, I think the largest they make. It can keep up with my dual head shower (with low flow stuff removed), the clothes washer, and dish washing.

As I recall, the Rinnai was about $1K and I put it in myself. Actually, I had installed a Sakura from Taiwan I self imported before the Rinnai. That thing was awesome, $200 and it could deliver boiling water at the tap. I had to replace it after a couple of years tho...

Starting both full of equal temperature water, I'd expect them to raise the temperature (ignoring the minor stuff) at the same rate, given that their energy input rate / water volume is the same, (1 kBTU/h / gallon)

I use a Rheem RTE 13 for two of the bathrooms in our house and have the kitchen and laundry on a traditional water heater. It's not perfect, but I'm very happy with it considering I paid $176.79.

I'd like to do that for the bathroom sink. The heater is only about 3m from the sink, yet it takes a minute for the hot water to get there. It seems a waste to fire up the big heater for 2 mins for the 15 seconds of hot water I want.

The big water heater should not 'fire up' when you draw off 15 seconds of hot water unless the water temperature inside was already on the cusp of being cold enough to trigger the next reheat cycle.

That said, the volume of water sitting in the pipes between the water heater and the bathroom faucet is a major factor in how long it takes the hot water to first reach the tap. The key aspects are;
How well insulated is every inch of the water pipe all the way back from the back of the faucet to the tank outlet?

How direct is the pipe run from tank to faucet?

What diameter is the water piping?

This last one may be a surprising factor. The smaller the pipe diameter the faster the hot water 'front' travels through the pipe. Not only is the overall volume of water inside the small pipe less, but the smaller inside diameter of the pipe reduces the mixing of cold and warm water as the hot water pushes the cold water forward.

Faster water velocity inside the pipe reduces mixing and delivers hot water faster. This must be balanced against pressure loss with very long pipe runs and high flow showers and bathtub fillers.

With a typical 1/2" copper pipe, the hot and cold water inside the 'hot' pipe mix as you run the tap, while the temperature front inside the pipe slowly progresses forward. Eventually the water coming out begins to warm up, but it still takes a while longer for the fully hot water to arrive. The metal mass of the copper pipe itself also draws some warmth from the hot water, further delaying the arrival of truly hot water.

This is one of the reasons a PEX flex tube from a faucet back to the central water distribution manifold will typically be a small inside diameter (3/8" perhaps). There is less mass of standing water inside a smaller tube to lose heat, hence less energy loss overnight. Faster water velocity inside the PEX means quicker arrival of fresh hot water when the tap is first turned on in the morning. The PEX tubing has a lower thermal mass so it does not cool the hot water flow as much.

The small inside PEX pipe diameter is not a problem for a low flow bathroom faucet, the pressure loss is modest. Since the PEX itself is flexible there can be fewer or preferably no right angle elbows so water pressure loss through the smaller diameter flex tube is similar to a larger size traditional rigid copper pipe installation. For a bathtub or shower of course a larger diameter PEX tube would be used.

Depending on your particular structure and where the existing pipes are, you might be able to add a separate flex PEX tube directly from the faucet back to the hot water tank. Properly insulate that thin flex tube and you should have rapid hot water arrival in the morning, and almost instantly even a few hours later.

A different option is to install a hot water circulation pump. These use a temp sensor and a small electric pump to circulate the hot water through the existing hot water pipe to underneath the faucet.The cooled off water is sent back to the water heater to eventually be reheated.

When you turn on the tap the hot water is always right there, inches from the faucet.

Here is an article discussing such things. Waiting for Hot Water

This one is more in depth. Hot Water Distribution Research

And this.

The big water heater is also an on-demand (tankless) type, it always fires up when the hot water runs.

So why did you say it seems like a 'waste' for it to fire up when you are drawing hot water, since that is what is was made to do?

The argument in favor of tankless water heating is to (in part) avoid the parasitic heat loss from a hot water storage tank, trading the reduced standing heat loss from not having a storage tank for the (hopefully) sufficiently efficient tankless heating system.

If the tankless system is combustion powered then the energy cost delta between a compact electric storage heater and a fuel burning tankless becomes an operating cost differentiator.

My comments regarding timely delivery of the hot water to a bathroom faucet through the piping system should apply similarly for tankless and traditional tanked hot water sources.

One option I forgot to mention is installing an inline tankless electric heater just for the bathroom faucet, located in the cabinet directly below. Might require a dedicated electric cable feed to power it.

You seem well versed in the area of plumbing, thank you for your insight and explanations.

Perhaps you can answer for me a question of strategy.

My hot water heater has two propane burners and two small (11-liter each) tanks, the tanks are connected in series so that the first burner/tank preheats the water for the second. This seems like a reasonable compromise all around, with the small tanks reducing the heat loss between usage yet enough of a buffer to handle heavier usage such as showers. It allows the use of smaller burners as well, which may (or perhaps not?) add to efficiency.

The question? How to set the thermostats (each burner/tank has its own) to minimize propane consumption and still provide enough hot water to meet demand. Both at maximum temperature? Both at the minimum temperature that satisfies? Some intermediate setting? Low on the first burner, high on the second?

There are a lot of possible permutations here.

The technicians who serviced the heater last week (new thermostats, new pilot lights, new igniters and new pressure relief valve) just cranked both thermostats to maximum hot and left it at that. Is that the best way to go?

In the next few months I will be adding a passive solar system to pre-heat the water before it goes into the water heater, so perhaps it doesn't matter too much anyway. Currently I am paying $38 a month for propane (cooking and hot water), $7 a month for water, $15 a month for telephone and $16 a month for electricity. With propane accounting for 50% of my total utilities bills, it is the one to focus on.

tanstaafl.

My first question would be, what thermostat settings does the manufactuerr of your refurbished two stage water heater recommend?

With large flow hot water consumption (shower, bathtub filling) what is important is the flow rate at which the heater system can continuously deliver additional freshly cooked 'hot' water. How much water is stored in the twin 11 liter tanks is less important than the rate at which the dual burners can make more of it as your shower duration progresses.

For small hot water draws, such as hand washing, the water in the tank(s) is enough capacity and burner heat output (aka recovery rate) is not so important.

If you need more than 11 liters of full temperature hot water at a time, then either the second stage heater can add enough heat to the incoming water flow to maintain the target outlet temperature or the hot water gets less hot after the first 11 liters are delivered. This may be why the technicians default is to set both tanks to the same high heat setting. This defers the potential temperature drop until after all 22 liters have flowed through and been delivered.

The flow rate at the point of use also affects matters. The faster the newly arrived water is moving through the tank(s) the less time the burner has to impart heat energy into the newly arrived water before that water has moved on.

I would be guessing, but I imagine the near optimal overall energy efficiency would occur with staggered thermostat settings. Preheat the water inside the first tank to some fraction of the second tank's target temperature. The thinking is that the parasitic heat loss from the standing (semi-hot) water in the first tank is lessened by the lower temperature delta relative to the surrounding air.

The second (fully hot) tank would have a (somewhat) greater hourly heat loss rate when just sitting idle, but that is true of all 'tank' type hot water heaters.

When your solar water preheating system is in place, the ideal might be to split the difference between the incoming water temperature (preheated by solar) and the target output hot water temperature and set the first stage tank to the mid-point.

Much depends on whether the propane tank stages can deliver a satisfactory water heating rate during those times you are continuously drawing hot water (more than 11 or 22 liters).

I presume the solar system will circulate water through the collectors and then into a storage tank of some significant capacity. The larger the solar storage tank, the more total energy can be stored. Of course, a larger tank takes up more space and needs to be properly insulated.

Part of the win from having preheated water on hand is to deliver that preheated water directly to the propane heating stage whenever demand for hot water use occurs. Cooled water sitting in the intervening pipes between solar tank and propane heaters is to be avoided as much as possible.

The big heater is like $1000ish and (this is just my impression) the heat cycle (cold/hot/cold) seems traumatic.

I may have formed this impression from the Sakura heater I had, it made really unhappy noises when it heated up and cooled down.

The Rinnai is quiet, but I always feel it's got a finite number of heat cycles before it explodes. If I can protect it from piddly demands, it'll live longer. Never mind it's 7 or 8 years old already and doing fine...

It also seems wasteful to run the heater for a minute to get 15 seconds of hot at the tap. A small dedicated on demand heater under the sink would give me an immediate 15 seconds of hot.

I don't think these things are that sensitive. If anything is a waste, I'd said it's putting a second heater 3 meters further up the line. That seems excessive.

Also, I wanted to address something you said further up:

According to the BBS, you live in Fullerton, CA. A quick check says that the coldest average temp it ever gets to there is a chilly January night of 48 degrees.

As I understand it (and I'm sure K447 will correct me if I'm wrong - please do) there's a trick with tankless heaters: a standard tank heater will heat up the temperature of the city water to a specific temp and hold it there, delivering a fairly constant flow and temperature of hot water until it runs out. A tankless can only raise the temp of the water coming in so much, and therefore its flow rate (GPM) is subject to the temperature of the incoming water.

If the city water is cold, most tankless heaters will bring it up to a usable temperature, but the GPM will decrease. I don't live in anywhere near the coldest part of the country, but we did get into the negatives this past winter, and there's no way that one tankless could have kept up with our shower (you did see that I have SIX showerheads, right? ).

That means we would need two tanks and the additional device that gauges when additional help from the second heater is needed. Maybe that wouldn't be $6K, but it was close when I checked it out. I could have gone for just one tankless heater, but the pressure on the showerheads would have dropped significantly to attempt to deliver the water.

That Sakura heater would probably work for you. It would literally produce steam from the tap.

When I got it, I assumed that using all hot water and regulating the heat at the heater would be most efficient, but that required running the heater at about 75%. Later I found that the slower the water flow, the hotter the water would get, so I could run the heater at about 30% and mix in cold at the shower for the same temperature. That Sakura didn't have much in the way of fancy control, just a big knob for flame (like a stove), and some basic sensors to keep it from overheating or turning on if there was no flow. Steam in the hot water pipe it was OK with.

The new Rinnai won't heat the water over 140°f, limited by the software, so you can't burn yourself. As a result, now I do use just hot and regulate it at the heater control. It is set up such that you can parallel up to 4 heaters on one control. It uses more gas than did the Sakura, but it's easier to live with.

You're right, a second heater in the bathroom would be excessive, which is why I haven't done it, but it doesn't stop me from wanting to...

My understanding, which is not all encompassing, is that tankless water heaters generally have a specified range of water flow rates they can properly heat. Below a minimum water flow rate the water heater cycles much too quickly as a very short duration of heat energy rapidly raises the water temperature since the arrival of fresh cold water is happening slowly.

Often a tankless heater will have a minimum flow sensor and will not heat the water at all if the flow rate is too slow.

Some tankless heaters may have a special heat/power management system that allows them to heat water despite rather slow water flows without excessive cycling of the heat source.

At the other extreme, if the end device (one or six shower heads, bathtub filler, or whatever) is flowing a ridiculous (from the tankless water heater's perspective) amount of water per second, then even at full tilt the heater just cannot get the passing water up to target temperature before the water has flowed out of the heater and heads towards the naked user

The hot water consuming device is the determinant of water flow rate through the tankless water heater. If the heater cannot keep up, the water just comes out cooler than desired. Slow the water flow rate down (turn the tap for less flow) and the water temperature coming out of the tankless heater will rise.

As long as the water flow rate through the tankless heater is between the manufacturer's specified upper and lower flow rates, the water temperature coming out should be at or close to the set temperature.

I recall this being one of the primary early complaints with tankless water heaters. The 'hot' water temperature would fluctuate. Slow the kitchen faucet down to a trickle and the water would become cold, increase the tap flow just enough and the heat would return. Run the water too fast and the temperature could decrease.

I expect the better designed tankless heaters can now cope with a fairly broad range of water flow rates, but at the extremes the phenomena can still occur.

There are interesting options such as using a parallel compact storage tank (with or without its own heating element) to accumulate hot water from the tankless heater, allowing the tankless burner to run for a more reasonable time period instead of rapid cycling, and also provide a source of accumulated hot water when the flow rate is very slow. Sometimes a small circulation pump is used to keep the storage tank temperature close to optimum and reheat the water as needed during non-use periods.

Part of the specification for a tankless water heater is the maximum temperature rise that is possible for a given water flow rate. If the water entering the tankless heater is very cold, a greater temperature rise is required to deliver the target hot water temperature.

Put another way, the colder the supply water the slower the maximum water flow can be and still deliver the target water temp. If you turn the tap to flow more water per second that the heater can provide given the needed temperature rise, the heater will do its best and you will get undercooked hot water.

I did live in Singapore for a spell... the water from the cold tap was about 80°f.