DHW system for new large home

[Mikesm]

Hi. Sorry if this isn't the right forum for this post, but moderators feel free to move it if it's not.

So we are building a new large house, 8000 sq ft, here in the SF Bay Area. It's come time to make some decisions about the hot water heating supply. I am hoping you guys could give me some advice on home to proceed.

The house has 1 master bedroom, and 4 bedrooms for the kids, and two guest rooms for family and friends to stay with us which happens pretty often. Two of the kids share a bathroom, and the a couple of the kids bathrooms have shower/bath combos (the girls like baths), and two are just showers, no baths. The master bath has a large tub.

There are also 3 dishwashers and 3 sets of washer dryers (we do a TON of laundry) that also create lots of demand for hot water. 2 dishwashers being active at the same time is pretty common when the kids are showering/bathing at night, and while the washers (all front loaders) don't usually run when people are showering or taking a bath, we do do a lot of laundry so one of the washers could be active at hot water peak demand.

My GC (who is great) and the plumbing contractor we are looking at using both advise us to stay away from tankless units (because of reliability issues they've experienced) , and to make sure we don't run out out of hot water, are proposing 2 large gas fueled tank units, and a recirculating pump system. This creates a bit of space problem for the mechanical room which has to accommodate two large air handlers as well.

I looked at this extremely useful thread: https://terrylove.com/forums/index.php?threads/water-heating-problems.63277/ on how to deal with the cold water sandwich problem for tankless units,. but this seems a rather complicated system to install, and I'd have to do some convincing on going that approach given my GC's experiences.

I was wondering about a different approach, where we used something like a HTP PH76-80, which is high efficiency, and requires less maintenance than a traditional tank unit, and doesn't have the cold water sandwich problem.

It has good amount of capacity, but to make sure we don't run out of hot water, to feed it with a tankless hot water unit, that would only get powered up if the water temp out of the water heater started falling several degrees below the setpoint temp. This would provide added hot water heat, but only kick in at peak periods so the tankless unit would have comparatively light duty, and not need as much maintenance, and would hopefully run for longer periods of time and not be short cycling due to the recirculation being active. It would take a lot less space than a 2nd 80 gal tank unit, and I think be more efficient than having two tanks being heated all the time feeding the recirculating system.

What do folks think about this approach? Is there a thermostat that could mounted to the HTP's outlet that could trigger a relay to energize the tankless unit? Is there an issue with using the tankless unit in this way?

Additionally, because we live in an area which can have significant earthquakes, and because we have a big generator for backup power, I'd like to also have an electric water heat for backup in case the gas supply was shut off. I was thinking a electric tankless unit for that that would not kick in unless the water temp hit a really low temp, and could be sized for only a limited amount of DHW production (if we had a big earthquake that would take the gas out, I doubt we'd all be going places and need to shower at the same time!).

Could an electric tankless unit be wired in series after the outlet from the HTP water heater, or should we put in valves that manually cut off the water from the gas heater and open the outlet to the electric heater? A cold water sandwich is OK in the backup mode.

Thanks again for all the good info here, and I'd love to hear your reactions to this idea, or another approach. We have a large 2" feed from PG&E for gas, so gas supply in the mechanical room is not a problem, and we are using a Mitsubishi heat pump system for heating, so no gas needed for that.

And because everything is still under construction, almost any venting and distribution choices are doable now.

Thanks in advance!
mike

 

[Dana]

One of the bigger-burner HTP Phoenix units (not the PH76-80 , with only 76,000 BTU/hr of burner) is the right choice here.

Forget the tankless- you're trying to re-engineer an already well-engineered system. The HTPs have modulating burners as big as any residential tankless- you won't run out of hot water.

The PH199-80 has 80 gallons of storage, 199,000 BTU/hr of burner(that modulates down to 40,000 BTU/hr at minimum fire).

The PH199-119 has the same burner but on a 119 gallon tank.

No cold water sandwiches. Pick one. (Take the PH199-119 if it fits in the available space.)

An electric tankless would be a truly insane NON-solution. For the kind of hot water needs you're looking at you'd need at least 40-50,000 watts of electric tankles , and even that would come up shy. (50,000 watts = 170,600 BTU/hr- enough to run 2-3 normal showers simultaneously- not more.) That would probably triple or quadruple the size of the generator needed to run the critical circuits in the house. A standard 50 gallon electric tank would be only a tenth the instantaneous power draw (4500 watts), and would give you at least SOME hot water at a decent flow rate in a disaster situation, even if it isn't big enough to fill your deluxe tubbie.

 

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[Mikesm]

Dana, thanks for the note. I did see the bigger Phoenix units, and they looked like they might be the answer. Are HTP units reliable? I was thinking if the HTP failed to start (I have seen complaints about them online), the tankless unit could heat up the water in the tank and the tank could just act as a reservoir. But complexity adds it's own sets of failures, so a single system is simpler.

I see AO Smith has a similar line compared to the HTP, the vertex series. We have a conventional AO Smith unit that has been a good performer, but I am not in the trades and have no experience with any of these manufacturers on their high end units. Is there a better unit for this sort of thing than the HTP?

So if we went with a conventional electric unit there is the space challenge, but there are smaller size tanks that might be adequate in a pinch. How many gallons does a conventional shower usually take? The appeal of the electric tankless was not that it could supply the houses needs, but provide some residual capability in a small space.

Would running the HTP at a higher temperature and using a tempering valve to mix with with cold water to get to the target temp be a good idea? It would enable the hot water supply to last longer, but I it seems like that would increase heat losses from the tank a bit.

Thanks,
mike

 

[Dana]

The HTP Phoenix is a lot more expensive than a Vertex, but it's also a lot more heater by more than one measure.

The Vertex series are glass lined, which won't last as long as HTP's stainless tanks, the Vertex burners on non-modulating, and the biggest one is 100,000 BTU/hr, which isn't big enough to sustain 2 full flow showers.

Kluding together a tankless to a tank adds complexity, making it LESS reliable rather than more reliable. The Phoenix series has been around awhile. If you find an installer who will support them (as opposed to an online buyer who DIYs it and botches the installation or a hack installer clueless about big burner commercial water heaters), they're fine (really best in class. )

Running at a dramatically higher temperature reduces the efficiency in at least two was, the most important of which is the raw combustion efficiency (less condensing efficiency), not just the increased standby loss. The 80 gallon tank with a 199K burner will probably be adequate- it really depends on the size of your tub. The 119 gallon tank will definitely make it without having to play around with storage temps higher than 140F.

All new water heaters are required to have a tempering valve or thermostatic mixing valvel limiting the temperature of hot water distributed to sinks & showers (but untempered water can be used for laundry, etc.)

You're biggest standby/distribution loss will be the recirculation loop. Insulating those pipes to something more than Title 24 requirements isn't insane.

For showering families you'll get a bigger apparent-capacity boost out of a drainwater heat exchanger than raising the storage temp. It can double the apparent capacity in showering mode, but it'll do nothing for tub filling capacity- you still need a tank big enough to fill the tub with a bit left over..

 

[Mikesm]

The HTP Phoenix is a lot more expensive than a Vertex, but it's also a lot more heater by more than one measure.

The Vertex series are glass lined, which won't last as long as HTP's stainless tanks, the Vertex burners on non-modulating, and the biggest one is 100,000 BTU/hr, which isn't big enough to sustain 2 full flow showers.

Kluding together a tankless to a tank adds complexity, making it LESS reliable rather than more reliable. The Phoenix series has been around awhile. If you find an installer who will support them (as opposed to an online buyer who DIYs it and botches the installation or a hack installer clueless about big burner commercial water heaters), they're fine (really best in class. )

Running at a dramatically higher temperature reduces the efficiency in at least two was, the most important of which is the raw combustion efficiency (less condensing efficiency), not just the increased standby loss. The 80 gallon tank with a 199K burner will probably be adequate- it really depends on the size of your tub. The 119 gallon tank will definitely make it without having to play around with storage temps higher than 140F.

All new water heaters are required to have a tempering valve or thermostatic mixing valvel limiting the temperature of hot water distributed to sinks & showers (but untempered water can be used for laundry, etc.)

You're biggest standby/distribution loss will be the recirculation loop. Insulating those pipes to something more than Title 24 requirements isn't insane.

For showering families you'll get a bigger apparent-capacity boost out of a drainwater heat exchanger than raising the storage temp. It can double the apparent capacity in showering mode, but it'll do nothing for tub filling capacity- you still need a tank big enough to fill the tub with a bit left over..

Thanks, this is super helpful. The HTP Phoenix definitely sounds like the way to go. I have heard some bad stories about them, but I think many of them do appear to be DIY jobs or a plumber not used to installing them. Is there a difference between the HTP banded ones and the Westinghouse branded ones?

On the electric backup heater, would a compact 30 gallon one do? We have a space problem, but if it's short, we could mount things above it since it doesn't need any venting. How would you hook it up?

On the drain heat exchanger, that doesn't heat the return of the recirculating loop, but the inlet cold water feed to the hot water heater? I guess if nothing is draining from a shower or the dishwasher, etc..., it doesn't help at all, but won't be getting colder than what is already coming in from the city water. And if a long shower is going on, then you are recovering some of that heat to preheat the water? Doesn't help with a bath I guess.

Are the drain heat exchangers reliable? The main drain for the house does come through the mechanical room, so it wouldn't be hard to install one there. Is there a brand or model you like? Is it worth it to install insulation on the drain lines from the showers if we are doing that?

What is the best way to insulate the hot water loop? I assume you don't want it spray foam or the like so you can still getto the pipe to do a repeair if needed right?

thanks again. This will help me have a useful conversation with the plumber...

thx
mike

 

[Breplum]

I happen to be a huge fan of Navien tankless water heaters. NPE 240A series is as good as it gets. Well designed, easy to service because of the large box, and very reliable.
They are as reliable as the HTP, well supported here in the bay area (I am based in the East Bay).
I have quite a few of the HTPs installed as well.
I just do not see any benefit.
The Navien's buffer tank and built in recirc pump solve every complaint.
My guess is lack of familiarity on the part of your GC and PC.

 

[Dana]

Thanks, this is super helpful. The HTP Phoenix definitely sounds like the way to go. I have heard some bad stories about them, but I think many of them do appear to be DIY jobs or a plumber not used to installing them. Is there a difference between the HTP banded ones and the Westinghouse branded ones?

On the electric backup heater, would a compact 30 gallon one do? We have a space problem, but if it's short, we could mount things above it since it doesn't need any venting. How would you hook it up?

On the drain heat exchanger, that doesn't heat the return of the recirculating loop, but the inlet cold water feed to the hot water heater? I guess if nothing is draining from a shower or the dishwasher, etc..., it doesn't help at all, but won't be getting colder than what is already coming in from the city water. And if a long shower is going on, then you are recovering some of that heat to preheat the water? Doesn't help with a bath I guess.

Are the drain heat exchangers reliable? The main drain for the house does come through the mechanical room, so it wouldn't be hard to install one there. Is there a brand or model you like? Is it worth it to install insulation on the drain lines from the showers if we are doing that?

What is the best way to insulate the hot water loop? I assume you don't want it spray foam or the like so you can still getto the pipe to do a repeair if needed right?

thanks again. This will help me have a useful conversation with the plumber...

thx
mike

Westinghouse versions are exactly the same under the nameplate as the HTPs, but the support network is Westinghouse, not HTP.

For a backup water heater any size will do, given that it's just the emergency backup. Just assume you won't be taking long showers or very many back-to-back showers in the days after the big 'un hits.

The drainwater heat exchanger needs to feed both the cold inlet to the water heater AND the cold side to the shower's mixer(s) to get the full benefit. To serve multiple showers with one heat exchanger it's usually easier to set it up to feed the entire cold water distribution to the house. While the output temperature during showers is well above the normal incoming cold water temps, it's tepid, not hot, or even very warm, usually no warmer than a few degrees above room temperature.

The heat exchangers are VERY reliable. It's just a section of copper drain pipe tightly wrapped in potable copper pipe, and is reliable as drain pipes and potable copper plumbing. The fattest and tallest that fits is the "right" one. The installation labor is about the same for a 2" x 30" as it is for a 4" x 96", but the heat recovered is dramatically different. The marginal cost of the taller fatter unit is more than made up for by the increased efficiency, yielding a shorter payback time on the fuel savings.

When serving multiple showers it's important to choose a unit that doesn't insert a large pressure drop when the flow rates are high. Renewability's PowerPipe series held the best-in-class for that for a number of years, but has been recently bested by the EcoDrain V1000 series:

< Ecodrain V1000

For apples-to-apples comparisons between models Natural Resources Canada (NRCAN)developed a testing protocol testing the return efficiency at 2.5gpm under specific incoming/outgoing flow conditions. NRCAN maintains a searchable third party tested database of models here, and have a downloadable spreadsheet of all tested models here.

 

[Reach4]

The heat exchangers are VERY reliable. It's just a section of copper drain pipe tightly wrapped in potable copper pipe, and is reliable as drain pipes and potable copper plumbing. The fattest and tallest that fits is the "right" one.

No worry about a large part of the warm drain water molecules falling down the middle of the 4 inch copper pipe without contacting the sides?

Also, "insane" is considered a complement/positive by many. On TV, that is the predominant use. That is the situation. Insane to mean insane is almost not permitted on tv, and insane as a positive is.

 

[Mikesm]

Westinghouse versions are exactly the same under the nameplate as the HTPs, but the support network is Westinghouse, not HTP.

For a backup water heater any size will do, given that it's just the emergency backup. Just assume you won't be taking long showers or very many back-to-back showers in the days after the big 'un hits.

The drainwater heat exchanger needs to feed both the cold inlet to the water heater AND the cold side to the shower's mixer(s) to get the full benefit. To serve multiple showers with one heat exchanger it's usually easier to set it up to feed the entire cold water distribution to the house. While the output temperature during showers is well above the normal incoming cold water temps, it's tepid, not hot, or even very warm, usually no warmer than a few degrees above room temperature.

/QUOTE]

Dana, thanks again. Will definitely ask about the drainwater heat exchanger, which would help even in the backup case as well.

One question about the backup electric hot water heater. What is the best way to hook it up? As a parallel feed to the hot water distribution, with valves that would shut off the feed from the HTP, and open to the electric heater, or in series? In parallel, wouldn't the tank have a lot of water sitting there for a months at a time? Would stuff start growing in there?

If it was in series, wouldn't that hit the efficiency of the the HTP's superior tank and insulation, and create a single point of failure with a cheap tank unit?

Maybe you'd run in on the cold water inlet side of the HTP, but then it would take forever for the hot water to get through the cold 80 gal tank of the HTP, etc...

My plumber had to take an action to research how to set this up.

Thx
mike

 

[Dana]

No worry about a large part of the warm drain water molecules falling down the middle of the 4 inch copper pipe without contacting the sides?

Also, "insane" is considered a complement/positive by many. On TV, that is the predominant use. That is the situation. Insane to mean insane is almost not permitted on tv, and insane as a positive is.

The surface tension of the water causes it to cling and spread out onto the sides of vertical pipes. Once a droplet of water touches the sides even a little bit it doesn't fall off- it' sticks.

Language is a social contract- words mean what the people who use them think they mean. The term "insane" has many meanings in common usage in different contexts. This is a web forum, not TV, and I'm using the most common most direct definition, here, as should be obvious to the reader (even those in the TV biz.) In the Netherlands the idiomatic use of "te gek!" (= too crazy), is also commonly used as as a superlative, but only in context. It sometimes really means "too crazy".

Mike: The electric tank should be plumbed in parallel with isolating ball valves between the two systems, and leave it off until needed. At room temperature it won't grow stuff rapidly, but flushing it to purge the stagnant water prior to powering it up should be part of the emergency protocol.

 

[Mikesm]

...

Mike: The electric tank should be plumbed in parallel with isolating ball valves between the two systems, and leave it off until needed. At room temperature it won't grow stuff rapidly, but flushing it to purge the stagnant water prior to powering it up should be part of the emergency protocol.

Ok, that is not hard to do. But what about the approach of plumbing it in series with the HTP heater inlet, and use a bypass valve there and at the HTP outlet to bypass the HTP gas heater? The power would normally be switched off, and when going to emergency operation, you'd flip the bypass valves and turn the power on. That would avoid stagnant water ever forming in heater, and ever worrying about stuff growing in there.

Would that not be a good idea?

Thx
Mike

 

[Dana]

Fresh water is corrosive. The rate at which the electric water heater deteriorates is mostly a function of how much water you're moving through it, so even un-powered it would fail well before the active stainless HTP does. The stagnating water finds it's chemical balance pretty quickly (days, weeks) after which it no longer degrades the heating elements or sacrificial anode. Storing it full of cool "dead" stagnant water is almost as easy on the water heater as storing it dry. It doesn't take more than 15 minutes of flush to purge the stagnant water- it'll take longer than that for the tank to come up to temp.

Not many pathogens will grow in water stagnating at room temperature, and without being periodically replenished with new water anything that does get started will likely die off. With an undersized electric water heater you'll want to be cranking the storage temp will above a typical 120-140F on up to the max (for more apparent-capacity) anyway, which will rapidly kill off anything live that remained in the tank after the purge.

 

[Mikesm]

Fresh water is corrosive. The rate at which the electric water heater deteriorates is mostly a function of how much water you're moving through it, so even un-powered it would fail well before the active stainless HTP does. The stagnating water finds it's chemical balance pretty quickly (days, weeks) after which it no longer degrades the heating elements or sacrificial anode. Storing it full of cool "dead" stagnant water is almost as easy on the water heater as storing it dry. It doesn't take more than 15 minutes of flush to purge the stagnant water- it'll take longer than that for the tank to come up to temp.

Not many pathogens will grow in water stagnating at room temperature, and without being periodically replenished with new water anything that does get started will likely die off. With an undersized electric water heater you'll want to be cranking the storage temp will above a typical 120-140F on up to the max (for more apparent-capacity) anyway, which will rapidly kill off anything live that remained in the tank after the purge.

Oh, interesting - I would have thought the fresh water was not corrosive, but the heated water would be more so. If it's the opposite, then I can see why it would be a good idea to not have it in series, and why that little tank would wear out much faster.

And if it's not any harder on the unit to store it wet as opposed to dry, that would seem to make more sense, as there would be less of a risk of turning it on with a dry tank that would burnout the element.

Would it make sense to drain the tank first before filling it with fresh water, or just drain with the fresh coming in? Either way it seems pretty straightforward to do.

Thanks again for the info - it's super helpful.

thx
mike

 

[Dana]

The corrosivness of the water isn't a matter of whether it's heated or not, but rather the total amount of dissolved oxygen. If the water is being constantly replenished, the oxygen is replenished too, which keeps the chemical reactions with metals going.

Draining the tank risks burning out the elements if somebody throws the breaker too early.

At 10 gpm it theoretically takes 5 minutes to purge a 50 gallon tank, but give it 15 to ensure the dilution factor with the 2 tablespoons of stagnant water that somehow remain in the tank is super-high. How much water would still be clinging to the innards of the tank if you drained it? Not much, but not zero either.