No, it doesn't take "days" for the water in the holding tank to cool down, at most it is hours. If you mean a holding tank with a recirculation loop, like the Naviens, that is very inefficient and actually worse than an external recirc loop. As others have noted, a tankless water heater with a tank is not a tankless water heater, just an inefficient tank heater.
Tankless water heater capacity questions
- Thread starter ironwelder85
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finn
Well-known member
You’re correct. Gas tankless with a condensing loop do qualif. Electric and non condensing gas units do not.
The unit I installed in my outdoor sauna did not, since it doesn’t have a condensing boiler. There’s no good way to deal with the condensate, since the floor drain isn’t accessible, and outside drains are problematic in winter where the boiler is installed.
I guess the other installs done have magic tanks, cause they hold heat for a few days.
No ideawhy you think it's "inefficient". Tank water heaters are in millions of homes. An electric one is near 100% efficient. Standby loss isn't much. Plus that heat isn't "wasted" for 7-8 months a year as it just heats the area it is in a tiny bit.
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dcg9381
Well-known member
Think it depends on where you are. Around here our water heaters are often in the attic or garage, good places for them if you want to heat water. I can't get the hottub below 102 right now (heat induced from the circulation pump) without taking the cover off because it's darn hot outside..
I have the Naviens. They do have a 1/2 gallon "buffer" tank. So, to some degree they are traditional tanked heater, but if I recall right you can setup the internal recirculation loop to be off/on/on-demand. The on-demand function is pretty cool, you just press a (remote) button for hot water just before you need it and the system goes to work. These versions can be setup with an external recirc loop and don't require an additional pump. We use the recirc function to save water, as we're not on city water here and it's a limited resource.
How this would be less efficient than a tank of 50-100 gallons of water held at 120 degrees or so all day long, I don't understand? Well, unless ambient air is already 120 degrees....then I get it.
I've had roof mount solar water heaters also. 80 gallons in TX would last us about 3 days (no sun) - at least ~10 months out of the year.
It looks like Navien with recirc tank qualifies:
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I have no idea what an energy credit is.
I just wanted a warm shop and to take a shower with no 34* water!
dcg9381
Well-known member
I think you are misunderstanding efficiency. I am referring to net energy consumption. An electric heater can be more than 90% efficient, in the sense that it converts nearly 90% of input power into useable heat, but it is inefficient for energy consumption, since it requires 25% to 50% more net energy to operate, due to the significant standby loss. That's why a tankless heater saves 25% to 50% of net energy over a tank heater.
It does depend on location and climate. In Phoenix Arizona they put tank heaters in the garage, where it is in the 120F range most of the year, so they rarely even turn on.
A small buffer tank won't impact energy much, but the other discussion was about larger tanks that would impact efficiency.
That depends on usage patterns. In a normal household, the standby losses are much lower than that for an electric tank, because they are well insulated and don’t have a flue. Gas powered tanks have higher losses, because of the flue. But they don’t have 50% losses unless they are vastly oversized. The only places that see losses that high are infrequently used. Like a vacation house or something.
A heat pump can be more than 100% efficient, because they are a pump, not a heat source. This outweighs standby loss for many houses.
The other concern with on demand heaters is power use. An electric on demand is about the least green way to heat water, because the high power is likely to be supplied by gas turbine, and not base load generation.
Running a min or longer of water to get hot water isn't exactly free either. It costs me money to pump that water and heat it.
I think the part I would want to see before drilling the wall is the building heat-loss number. 100k BTU sounds big against 1,200 sq ft, but northern Minnesota, a 14-foot ceiling, door area, insulation levels and slab-edge insulation can move that number a lot. Then I'd compare that load with what this particular heater can actually deliver at the water temperature you plan to run through the slab, not just its advertised domestic-water BTU rating. After that the loop lengths and pump curve tell you whether the 1/2-inch connections are really the bottleneck. Right now there are too many unknowns to know if the heater is oversized, undersized, or fine.
Yes, the tank is increasing the time to get hot water, because you are increasing the quantity of cold water you have to flush out before you get hot water. That is why it is taking "a min or longer" instead of the normal 15 to 30 seconds of a tankless heater.
No, it takes a minute or so with the tankless and 10-15 seconds with the buffer tank.
The water in the buffer tank is hot. No idea why you keep thinking otherwise.
Tankless has to detect flow, do it's usual purge/safety checks, then light, get hot, THEN the ~35* water starts getting hot. Then another ~10-15 seconds for the pipe to purge.
And if the water shuts off, it repeats that. Though in that case you get the hot water still in the line, then cold, then hot.
Look up cold water sandwich.
It's a very common setup.
The tank instructions even had plumbing diagrams that included how I set it up.
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dcg9381
Well-known member
This. The buffer tank is circulated and pre-heated. Advantage - water is already hot. Disadvantage - you're heating water in a tank (a much smaller amount) even when you're not using it.
I've got both buffer and non-buffer units. Happy with both. Where water use is a concern, I do circulation loop (on demand - push button).
Hiw to fix....
"
How to Fix or Reduce It
Like I said, washing dishes means leaving the water running.
The washer... can't do anything. It spins, fills, spins, fills based of whatever magic programs it has
so its internal water heater runs.
"
How to Fix or Reduce It
- Install a mini-tank: Add a small point-of-use buffer tank to the output of your water heater to smooth out the temperature. [1]
- Choose a built-in buffer model: Some modern tankless units include internal small storage tanks or recirculation features to prevent the drop.
- Change habits: Avoid turning the hot water tap completely off and on again during short pauses (like while washing dishes or soaping in the shower). [1]
Like I said, washing dishes means leaving the water running.
The washer... can't do anything. It spins, fills, spins, fills based of whatever magic programs it has
so its internal water heater runs.
dcg9381
Well-known member
I track water use by the gallon per day... Biggest hit is washing clothes by far and front loaders use much less water...Washing 3 loads of clothes in a big top loader uses a ton of water... Dish washer barely shows up.
Why?
dcg9381
Well-known member
Because I don't have a well or city water. I'm 100% dependent on the water I collect from rain off the shop and the house.
I started living in the shop (no house) with a single 5000 gallon water tank. I've now got 5 of them. But consider that we might go 3 months without any appreciable amount of rain, so conservation in low rain months is a thing. Biggest consumer of water is number of humans in the house!
Tankless do reduce your gas bill compared to a conventional tank WH…as long as your use stays the same. I had a fellow tell me he puts a strap with a come-along to his daughters ankle when she gets in the shower. With endless hot water you tend to relax a bit more, hence you use more gas. What is your comfort worth?
As to storage tanks on tankless I used them in commercial systems on occasion depending upon initial flow/temperature demands. You can do a lot with them. I had a system that sized out to a bank of 5 199kbtu units. The system had a tank off the side of the tankless bank. I had a mass flow for wash-down that the tankless would not cover. It took X gals at X temp in X min. Twice per day. (I can’t recall the specifics) and when the dump load was done the rest of the place ran nicely off the tankless bank. They needed the mass flow twice per day.
You can use recirc lines on tankless but I always trained contractors to walk around the place first and look at where the waterheater should be rather than where the original low cost bidder put it. Why is the water heater in the garage on the opposite end of the building from the MBR & Kitchen. The kitchen faucet is usually where delays drive me nuts. As HW delivery is a time/vol deal I’ve run dedicated 3/8” pex lines direct to the kitchen faucet. Cross sec area of 1/2” is .2sq“, 3/8=.1Sq”. 50% reduction in vol.
As to storage tanks on tankless I used them in commercial systems on occasion depending upon initial flow/temperature demands. You can do a lot with them. I had a system that sized out to a bank of 5 199kbtu units. The system had a tank off the side of the tankless bank. I had a mass flow for wash-down that the tankless would not cover. It took X gals at X temp in X min. Twice per day. (I can’t recall the specifics) and when the dump load was done the rest of the place ran nicely off the tankless bank. They needed the mass flow twice per day.
You can use recirc lines on tankless but I always trained contractors to walk around the place first and look at where the waterheater should be rather than where the original low cost bidder put it. Why is the water heater in the garage on the opposite end of the building from the MBR & Kitchen. The kitchen faucet is usually where delays drive me nuts. As HW delivery is a time/vol deal I’ve run dedicated 3/8” pex lines direct to the kitchen faucet. Cross sec area of 1/2” is .2sq“, 3/8=.1Sq”. 50% reduction in vol.
You keep saying it is "common" yet no one else has ever seen it. If your tankless without a tank is taking more than about 15 seconds to produce hot water from a cold start, something is wrong with the unit or the install.
I keep saying the tank has cold water because physics dictates it must, it's just reality. A tank will hold water hot for a few hours at most, assuming it is well insulated.
It holds the water hot for a day easy... without it turning on.
But it heats itself if needed.
this is not at all how my rinnai works. basically zero cold water comes out the "hot" port. and yes it's in "eco" mode.
i mean it does all those things just nearly instantaneously. it even stays spun up for a bit for faster relight after a draw.
So it heats water instantly?
shade
Well-known member
Water in a rinnai heats instantly. That hot water still needs time to make it to the destination.
Recirc pump make water come out hot in less than 10 seconds for me
dcg9381
Well-known member
Not sure this is a serious question. It heats water very quickly as soon as flow is detected. The only "delay" is the how much water is in the pipes if you're not circulating.
The solution to all this is a a "push button" demand system that doesn't circulate all the time. You press the button before you need the hot water. It's 2 wires... I think there was a thread here on how to do it via remote, but it's better with a circulation loop.
It doesn't have to go through all the safeties most any gas heater does? Plus come up to temp?
Ive only had 3 different tankless combi units, so dunno.
it's not gas furnace slow. pretty much it slams the gas on max then dials back once it hits temp. 199k BTU is a lotta fire.
DHW tankless units are typically way more aggressive on startup than a CH/boiler. no one cares if it takes 5 seconds to light off a boiler.
EVERYONE cares if it takes 5 seconds to light off a water heater.
Firebrick43
Well-known member
Depends on the tank heater type. Most of the condensing tank water heaters as well as HP tank water heaters meet UEF numbers to qualify for energy credits.
The also don't have the high maintenance cost of descaling and plugged heat exchangers and flame rod cleaning if the home owner is hiring the work done. I have been called out dozens and dozens of times on non op DHW and boiler naviens for no heat over the last 4 years. Some customers, because they are not that bright, multiple times when they should be calling their HVAC installer.
I highly doubt that a typical home with a DWH tankless is saving any money in overall operating cost compared to a 50 gal condensing tank WH or HP water heater unless they have jacuzzi tub and/or 3 head showers and would need a massive 100 or 120 gal tank type to cover the usage.
Not sure what a CH/boiler is, but my setup takes a good 15-20 seconds to detect flow, light, and come up to temp, and more like a minute,maybe longer to be stabilized at temp.
It was long enough I could turn the shower on, go piss, undress, clean clothes set out, towel out and I still needed to wait.
It was long enough I could turn the shower on, go piss, undress, clean clothes set out, towel out and I still needed to wait.
Firebrick43
Well-known member
CH equals central heating