Time to hot water and how to improve behavior with small volumes?

[gosko]

Hi, I just bought my first home and am doing a number of updates. It is a single-level place with 3 bedrooms and 2 bathrooms, about 1500 sq feet with easy access from above and below via attic and crawl space.

I had a new tankless hot water system installed (a Bosch T9800, 200k BTU natural gas), and once it's fired up it does a good job of pumping out tons of hot water.

But the startup time is really disappointing - starting from a cold system and cold pipes it takes about 1 min 40 seconds to start putting out hot water from a single bathroom faucet maybe 20 feet away.

If I come back 20 minutes later it starts warm and gets hot within about 20 seconds which is fine, but if I come back an hour or two later it's ice cold again and takes 1 min 40 seconds to get hot.

What's a reasonable time to expect hot water from a modern tankless system, assuming it's installed and working well? (good gas pressure, typical piping) The house and plumbing is 30 years old, and generally well-built and cared for.

My impression is that this gas-fired tankless system is great at putting out tons of hot water when needed but takes a long time to start up and shut down, so it doesn't work very well with small volumes and random usage patterns. (e.g. washing hands for 10 seconds, or getting turned on/off frequently while rinsing dishes)

Does it sound like this system is working as designed? If so, any suggestions for what I could do to get quicker access to small volumes of hot water at sinks throughout the house?

One idea I had would be to install an electric point-of-use system with a small tank (maybe 2-4 gallons) somewhere close to all the faucets so I would always have quick access to hot water, then connect that to the big gas-fired tankless system so it's able to keep up with higher volumes when needed. Does that make sense?

There are 4 sinks where I would like to have reasonably quick access to hot water and I am hoping all of them could be fed by a single point-of-use system installed in the crawl space, rather than installing a separate one for each sink.

Below is a floor plan noting the locations of the sinks, showers, and the tankless system. The lower green square is where I imagine an electric point-of-use system with a small tank might help, but I don't know if this is a good idea. Feedback or other suggestions welcome!

 

[Reach4]

Another idea is to use a hot water recirculation system. One way to do that is to have a switch that starts the recirculation pump for a timed number of seconds. That recirc rate can be high. Then when you turn the water on, the water is hot. That system seems good for a tankless WH. The downside is that you must poke the button before starting to wash your hands if you want quick hot water.

 

[Sponsor]

 

[Jadnashua]

Some tankless heaters void the warranty when you hook up a recirculation system...read your manual carefully. Those that do allow it, it must be done in a particular manner. One thing that you should do, if it's not already done, is carefully insulate all of the hot water piping. Is your crawl space heated and/or insulated? Keep in mind, insulation doesn't 'make' heat, it slows the losses. If the crawl space gets cold or is drafty, you could end up freezing things. Low volume may not trigger the burner on...each system has a minimum flow rate switch. Washing hands may not trigger it unless you turn the faucet on hotter than you'd normally want to get the flow up, then, may need to throttle it down once hot actually arrives, which may or may not keep the burner going.

Hopefully, your love of the thing will still be high in the middle of the winter when the incoming water could be 40-50 degrees F colder than it is in the summer.

After sitting, so you're starting with cold water in the pipes, turn a faucet on full, but dump the water into a bucket so you can measure how much gets wasted prior to it getting warm. In a small point of use tank, you only get to use about 75% of its volume before it starts cooling off, so you can use this to help size a tank...you need hot water arriving before it cools off much, or you'll still get a hot water sandwich.

 

[gosko]

Thanks for the great info and advice.

I don't understand how a recirculation system would work. I will try to do some reading on those. I would rather not have to advise everyone that comes over to my house that they need to push a button 2 minutes in advance of wanting to use the hot water. I have never lived or visited anywhere that didn't have hot water within a fairly short timeframe of running a tap (maybe 20-30 seconds?); 100 seconds seems like way too long to me.

The ambient temperature in the crawl space is about 16 C (60 F). The pipes are not insulated.

Our plumber said he would look into the minimum flow rate to see if it is a configurable option on the Bosch unit. Whenever I have been using the faucet so far I don't even touch the cold water tap because I know it won't be needed - I turn the hot water tap on full, then wait 1 min 40 seconds for hot water. By then I have usually finished whatever I wanted the hot water for, so I turn the tap off again and the only thing I have accomplished was heating a section of pipe in the crawl space. (and firing up the big gas tankless system for no reason)

I really feel like adding an electric point of use system with a small tank to buffer the demand would help a lot, but I don't really know anything about anything.

 

[Reach4]

I don't understand how a recirculation system would work. I will try to do some reading on those. I would rather not have to advise everyone that comes over to my house that they need to push a button 2 minutes in advance of wanting to use the hot water. I have never lived or visited anywhere that didn't have hot water within a fairly short timeframe of running a tap (maybe 20-30 seconds?); 100 seconds seems like way too long to me.

I don't have a recirc system myself. Nor do I have a tankless WH.

With a big enough pump rate, I don't think it would take anything like 2 minutes to get the hot water into the pipe. Imagine you pump 4 GPM. That recirculated water gets pumped back to the WH input. The flow is plenty high to get the WH fired up. What if you had a motion sensor start the pump?

Usually recirculation systems run continuously during certain hours or all of the time. The flow rate in that case is much slower. I think that slow continuous flow would not work so well with a tankless unit. There are semi-tankless units that are made for recirculation.

Your idea of a secondary WH near your main point of use would take a fair amount of energy maintaining the temperature. Maybe you could help that energy use with a switch and/or a timer for that WH.

A lower tech way to get hot water at the lavatory sooner is to put in a higher-flow aerator. The more flow, the quicker the cold water in the pipe gets discarded.

 

[Dana]

The minimum flow rate on that unit before it flames out is about 0.5 gpm (but varies with incoming water temperature, since it can't modulate the burner below 9000 BTU/hr, and will stop the burner if the temperature is too high) but it may take a much higher rate to reliably trigger the ignition.

Have you verified how much of the 100 seconds is ignition delay, and how much of it is pipe length delay?

Any decent tankless should fire up after ~5 seconds of flow, 10 seconds at most. There is usually a pre-purge where the exhaust blower runs a few seconds before it actually lights up (to limit the risk of a stack explosion and guarantee reasonable combustion air quality), but it's brief- maybe 3 seconds. When flow stops the burner stops nearly instantaneously, but the blower will typically post-purge the flue for a handful of seconds to push most of the water-rich exhaust out to avoid condensation/corrosion inside the unit & exhaust venting as it cools.

What is the material and diameter of the hot water distribution plumbing and total plumbing length between the heater and the sink?

Insulating the hot water distribution plumbing will extend the time period that the abandoned water in the distribution plumbing stays warm enough to be useful. Code minimum potable hot water pipe insulation for new construction in most of the US is R3. For the cheap lightweight closed cell foam R3 takes a wall thickness of about 13-14 mm, but bumping that to 19-25mm (3/4"-1") wall thickness goods is usually "worth it"on energy savings over the long term, let alone the mitigation of the inconvenience factor you're suffering. If installing a recirculation system, insulate both the supply and return plumbing of the loop with the thicker stuff. (eg Frost King P110XB/6, P111XB/6, or P112XB/6 depending on pipe diameter would be appropriate. These have ~25mm wall thickness, are rated R7.8, and relatively inexpensive, sold through box stores.)

 

[gosko]

The ignition delay seems OK, about 5-10 seconds.

I measured the output of the bathroom hot water faucet on full, it's about 1 GPM.

The pipe length between the tankless system and the bathroom faucet is much longer than I expected, about 45-50 feet total. It looks to be 3/4" copper.

I will definitely pick up some insulation, thanks for the excellent info!

We have had some newer plumbing roughed in for a kitchen reno that's underway and it's different than the main copper supply lines; some kind of flexible plastic I guess. Should I consider insulating that as well? (it looks like it would leak less heat, to my untrained eye)

I think insulating the main hot water supply line will do a lot to fix my annoyances, but I am still not sure how well the tankless system will cope with random short bursts of hot water usage. For the kitchen (not yet installed) we bought a couple Delta faucets with Touch2O feature that lets you tap them on/off by touching the faucet which I expect to be useful for quick rinsing of hands while cooking and rinsing dishes while washing, but it doesn't seem like that type of usage would ever generate enough demand for the tankless system to start up.

 

[Reach4]

If you could run a new 3/8 inch PEX line to the hot for the lavatory, your hot water would arrive much sooner than it would going thru 3/4 copper -- maybe 5x faster after the ignition delay. That assumes the same path length.

Insulation is not going to be that much help if it has been a while since you ran the hot. If you were using it on and off regularly, it would help.

 

[Dana]

With inch-wall thickness pipe insulation the hot water is still at a useful temperature after an hour of idle. It's the first draw in the AM that's the PITA.

Tankless units are notorious for not responding well to appliances that take short burst draws, such as most new high efficiency clothes washers and many dishwashers. The draw is almost done by the time the thing fires up, then it immediately shuts down.

The water volume of Type-L 3/4" copper pipe is about a gallon for every 40' of length, so if you're drawing water at 1 gpm it's always going to take awhile. With 50' of 3/4" pipe and a 1 gpm flow you have 75 seconds of raw transit time, plus the ignition delay. So if you're measuring 100 seconds total and a 10 second ignition delay it's probably a bit more than 50' total, maybe close to 60', or your gpm measurement wasn't super-precise, but there is also time-accounting for raising the thermal mass of the plumbing and fixtures up to temp. But overall it sounds about right- this is a plumbing volume problem more than a tankless water heater artifact.

The water volume of 3/8" PEX is about a gallon per 190', so for the same running length the transit time will be about 1/5 the wait. Assuming 10 seconds out of the 100 seconds was start up delay, the transit time is 90 seconds through the 3/4" copper. With 3/8" PEX the transit time is 18 seconds, plus the ignition time 10 seconds, you'd be at about 25-30 seconds of wait for hot water, if the PEX follows the same path.

If you can make a "home run" of 3/8" PEX from a manifold near the water heater, using the shortest possible paths to each low-gpm tap the wait can be further minimized. This is becoming a popular approach for hot water distribution for some architects & designers, but they usually put the water heater somewhat central to the various appliances & fixtures that need it to keep any one run pretty short.

 

[DonL]

The solution is a real old school water heater that has good insulation.

A tankless waste more water and power than it saves.

Old school is less maintenance too. Every 10 years instead of every year.

Good Luck.

 

[Jadnashua]

If your unit allows recirculation, one way to minimize some of the issues would be to have a buffer tank, which would add to the energy losses. Some tankless have one built in.

That new plumbing was done with pex...and, they wasted a lot of the advantages of using it to make it look neat by using those connectors. One of the benefits of pex is that it easily bends, eliminating most of the fittings along the way. This not only saves money, but improves the flow, but, it doesn't look as neat! It also minimizes points where it could leak from an improper fitting.

 

[gosko]

Thanks so much for these details. Super helpful.

I'm glad to hear the tankless system seems to be working properly.

Running 3/8" PEX directly to both bathroom sinks sounds like it would help a lot.

That would still leave the issue of what to do with the kitchen faucets, which I would like to work well even with very intermittent on/off cycles. (on for 3 seconds, off for 20. On for 3, off for 60...)

I guess the best thing to do there would be to add a small point-of-use system under each sink.

We will also be putting in a dishwasher in the island next to the prep sink, but it looks like it will have its own heating element (Thermador DWHD650JFP), so hopefully it'll be OK to run that off the legacy copper piping.

 

[gosko]

After considering this a bit more I still think my original plan as stated above is a good one (add a mini tank point-of-use system in the crawl space somewhere close to all the sinks). This would give me the best of both worlds: endless volumes of hot water from the main gas-fired tankless system, and hot water within seconds at each of the sinks.

The mini tank system I have in mind would be something like the Bosch Tronic 3000T ES4,
https://www.bosch-climate.us/produc...-heaters/point-of-use-electric-mini-tank.html

(I have no particular allegiance to Bosch but might as well use the same brand as the main tankless system already installed so Bosch can't blame someone else when problems arise)

Based on the info in this thread I would probably connect the output of the electric mini tank to a 4-way manifold, then run 3/8" PEX from there to each sink faucet, to minimize my time-to-hot-water. (the kitchen faucets are Delta Essa, 1.8 gpm at 60 PSI; 3/8" O.D. straight, staggered pex supply lines)

Does this plan make sense? Anything to watch out for? Should I hook it up to the showers as well, to avoid cold water sandwiches?

 

[Jadnashua]

Did you measure how much water was required before it got hot? That's a small tank that may not be big enough on a larger draw that would start out hot from the small tank, then run out of hot water before hot arrived from the tankless. It needs to be big enough to stay hot until hot arrives on the input without cooling off too much and that is less than the tank actually holds.

 

[Galen Smith]

Hi, I just bought my first home and am doing a number of updates. It is a single-level place with 3 bedrooms and 2 bathrooms, about 1500 sq feet with easy access from above and below via attic and crawl space.

I had a new tankless hot water system installed (a Bosch T9800, 200k BTU natural gas), and once it's fired up it does a good job of pumping out tons of hot water.

But the startup time is really disappointing - starting from a cold system and cold pipes it takes about 1 min 40 seconds to start putting out hot water from a single bathroom faucet maybe 20 feet away.

If I come back 20 minutes later it starts warm and gets hot within about 20 seconds which is fine, but if I come back an hour or two later it's ice cold again and takes 1 min 40 seconds to get hot.

What's a reasonable time to expect hot water from a modern tankless system, assuming it's installed and working well? (good gas pressure, typical piping) The house and plumbing is 30 years old, and generally well-built and cared for.

My impression is that this gas-fired tankless system is great at putting out tons of hot water when needed but takes a long time to start up and shut down, so it doesn't work very well with small volumes and random usage patterns. (e.g. washing hands for 10 seconds, or getting turned on/off frequently while rinsing dishes)

Does it sound like this system is working as designed? If so, any suggestions for what I could do to get quicker access to small volumes of hot water at sinks throughout the house?

One idea I had would be to install an electric point-of-use system with a small tank (maybe 2-4 gallons) somewhere close to all the faucets so I would always have quick access to hot water, then connect that to the big gas-fired tankless system so it's able to keep up with higher volumes when needed. Does that make sense?

There are 4 sinks where I would like to have reasonably quick access to hot water and I am hoping all of them could be fed by a single point-of-use system installed in the crawl space, rather than installing a separate one for each sink.

Below is a floor plan noting the locations of the sinks, showers, and the tankless system. The lower green square is where I imagine an electric point-of-use system with a small tank might help, but I don't know if this is a good idea. Feedback or other suggestions welcome!

Your first idea is a good one; install a small (e.g., 2 gallon electric) tank close to where it feeds the sinks. We did exactly that in our (too big of a) house, even though we have a tank system (2 large gas water heaters in the basement). This small heater uses almost no energy - much less heat loss than in a recirculation system, I suspect, unless you go back and insulate all the piping which is impossible unless you tear out walls, etc. A small point of use system would work too, i guess, but it's more expensive and may not have enough juice for the shower anyway (we did not hook up the shower to the 2-gal tank, just the sinks). Just my 2 cents. When we need to replace our large water heaters we'll install what they call hybrid systems - they still have a tank (but it's smaller than a full tank system) which is then replenished with a point-of-use system (which again is smaller than a full point-of-use system). The hybrid is the "best of both worlds."

 

[gosko]

Did you measure how much water was required before it got hot?

Probably about 1.5 gallons.

That's a small tank that may not be big enough on a larger draw that would start out hot from the small tank, then run out of hot water before hot arrived from the tankless. It needs to be big enough to stay hot until hot arrives on the input without cooling off too much and that is less than the tank actually holds.

Wouldn't the electric heater be capable of heating the incoming 1.5 gallons of cold water more or less in real time?

I understand it would struggle to keep up with demand from all 4 sinks at once, but if the house is that busy the main tankless system would be constantly active, preheating the input to the mini tank.

 

[gosko]

When we need to replace our large water heaters we'll install what they call hybrid systems - they still have a tank (but it's smaller than a full tank system) which is then replenished with a point-of-use system (which again is smaller than a full point-of-use system). The hybrid is the "best of both worlds."

I wish I had researched things a bit more before having this tankless system installed - I kind of assumed it would have a mini tank built in to buffer the demand.

 

[Reach4]

Wouldn't the electric heater be capable of heating the incoming 1.5 gallons of cold water more or less in real time?

No. That is what the 4 gallon storage tank is for.

"Recovery Rate @ 90°F Rise 6.8 GPH "
That is 0.113 gallons per minute. Since you don't need 90 degree rise, that is .34 GPM for 30 degree rise. Still a very small flow rate. So you are relying on the tankless to deliver water before the 4 gallons are gone. There could be a cool sandwich as the cold water that was in the pipe makes its way through the WH. There will be some mixing, so it will not be a very cold sandwich.

 

[gosko]

"Recovery Rate @ 90°F Rise 6.8 GPH "
That is 0.113 gallons per minute. Since you don't need 90 degree rise, that is .34 GPM for 30 degree rise. Still a very small flow rate. So you are relying on the tankless to deliver water before the 4 gallons are gone. There could be a cool sandwich as the cold water that was in the pipe makes its way through the WH. There will be some mixing, so it will not be a very cold sandwich.

OK, thanks. Practically speaking, I think this should be good enough to keep up with my intended use (a few seconds of low-flow hot water to wash hands)

I guess if someone gets up first thing in the morning and turns on a faucet on hot at 1 gpm, after a minute or two they'll get a temperature drop as the 1.5 gallons of cold water that was in the pipes arrives and dilutes the 4 gallons of preheated water in the mini tank. But I think that would happen very rarely.

In typical use when we're active in the house (cooking, cleaning, entertaining) the 1.5 gallons that sits in the pipe wouldn't ever be allowed to cool down very much so the temperature in the mini tank would stay more stable.

 

[DonL]

Wouldn't the electric heater be capable of heating the incoming 1.5 gallons of cold water more or less in real time?

It takes a lot of electric power to heat water on the fly.

Make sure you have enough electric power available.

Good Luck.