Which water heating method

[bikeboy]

For a gas water heater I would be looking at installing a single 75 gallon high recovery gas water heater coupled with a thermostatic mixing valve. For example the Honeywell mixing valve that this forum links at the top, plus a water heater similar to:

https://www.supplyhouse.com/AO-Smit...dential-Gas-Water-Heater-w-Side-Connection-NG

Can you tell us more about your heat pumps? Any spare heating capacity there?

My heat pumps are sized directly for the square foot of the house. Would that 75 gallon gas high recovery water heater, be more efficient than my existing 2 electric water heaters, and could a circulator loop be installed w/ that gas unit as spoken about previously ? Howie...

 

[Phog]

My heat pumps are sized directly for the square foot of the house. Would that 75 gallon gas high recovery water heater, be more efficient than my existing 2 electric water heaters, and could a circulator loop be installed w/ that gas unit as spoken about previously ? Howie...

Yes to the circulator loop. (You can do that with any type of water heater).

As for the operational cost efficiency, you would have to use your local electricity and gas rates to do a cost comparison to know for sure. But for a large majority of US households gas is much cheaper per gallon of hot water produced (25-50%) when compared to standard electric tanks. You'd also want to factor in the cost of installing a gas line & flue exhaust vent though -- an upfront cost you won't have with the electric.

There are other options as well. Such as hybrid heat pump electric tanks; however they take a long time to charge back up. I'm not sure that fits your needs with that big tub. There is also the modulating/condensing gas tank type such as HTP Phoenix Light Duty, which has a very long lasting stainless steel tank & is super efficient (around 90%, nearly as efficient as tankless and much more efficient than the Rheem I linked to which is about 70% efficient). But of course significantly more expensive upfront.

 

[Sponsor]

 

[Phog]

Also just want to be crystal clear with the 75 gallon Rheem tank -- you'd absolutely need the mixing valve & to have the temperature cranked up, to have just enough hot water to fill your big tub with very little left over in reserve. With the high recovery 76,000 btu/hr burner, the tank will heat back up quite quickly though.

 

[bikeboy]

Unless you end up with a commercial WH with a huge burner, a 50-gallon one won't fill your 84-gallon tub...it just can't produce enough hot water and the tank size is too small. Say you have a fairly common 1/2" fill valve at the tub, that's about 5-gallons of hot and one of cold for a typical fill or so. At 6gpm, it will take 16-minutes to fill the tub. A larger fill valve will be better, or worse, depending on how you look at it. That's not much time to reheat the incoming water, and even less if you have a bigger fill valve.

Thanks for replies
I have 2 zones; 4 ton(main) and a 2.5 ton(lower level) for hvac. Would just a 75 or 80 gallon electric replacement be adequate for master bath heater ? With the additional capacity, why would I still need a mixing valve when the 50 gallon was just short of filling it ? Again ; gas or electric; I'm still heating more water for minimum use of the tub. The advantage would be less loss when recirculating hot water. After I fill tub, recovery is not that important as it's only the 2 of us. That's probably why i thought of the Tankless unit. Here in Tennessee, My average ground water temp is 50-55 deg. In New Jersey; I'd fill whole house and a 93 gallon tub 110 deg. with a Navien 210-18K btu heater.

Thank You; Howie...

 

[Phog]

My understanding is that you are getting the tub almost full with your 50gal tank cranked all the way up to its highest setting, probably in the neighborhood of 140-150F.

This is against code and not really very safe. I know it's not something likely to happen, but imagine falling down and disabling yourself in the shower with 140F water coming down on you. And being unable to move yourself out of the way. You can get extensive, life altering 3rd degree burns in a short time. Any time you're setting your thermostat above approximately 120F, as you'd have to do with any tank smaller than the tub, you're going to want the mixing valve.

Your incoming water is a little higher temperature than I'd imagined. So the numbers Dana calculated for you earlier are a little conservative. But It will still take 12-14 minutes to fill the big tub with a 190 MBtu tankless at 50-55F incoming. It's not my call to tell you if that's right for your household, if it's acceptable to you then that's great.

By the way some of the Navien tankless units come with a built in recirculation pump & a small internal reserve tank (to help mitigate the cold water sandwich effect).

 

[Dana]

OK; i'm using 2 heat pumps for hvac. Will a 50 gallon gas water heater be able to keep up with that 84 gallon demand for the mast bath zone(including tub), and if so; could that alone be used for what I'm trying to do w/ a circulator ?
Also; using the 2 electric water heaters together as we discussed, wont wh1 be over worked and have a shorter life span and wh2 be passive since it's being fed w/ hot water to the system ?
Howard...

The faster recovery rate of the gas water heater buys you next to nothing from a tub-fill perspective- it's all about the BTUs stored in the tank.

At a storage temp of 170-180F you'd be able to fill an 84 gallon tub just fine from a single 50 gallon tank (gas or electric). That would require custom controls and low-head thermostatic mixing valve to deliver the high flow and safer water temperatures. The max storage temperature rating for most tanks is 180F. You may or may not be able to find retrofit controls for your current water heater to raise it that high.

A bigger water heater would allow for lower storage temperatures. A 75 gallon tank at 140F storage temp would fill an 84 gallon tub without issues.

Tank longevity is more about the total water volume that flows through it, not about being "over worked". The heating element life on the water heater doing the bulk of the heating will be shorter than on the other, but so what? They're replaceable at reasonable cost, and don't burn out quickly.

Most water heaters are plumbed with 3/4", and both sides of the mixer on an 84 gallon tub should be 3/4" too. (That's not to say that yours is.)

 

[Dana]

Thanks for replies
I have 2 zones; 4 ton(main) and a 2.5 ton(lower level) for hvac.
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My heat pumps are sized directly for the square foot of the house.

Way off topic but...

Heating and cooling loads are NOT a function of the square footage of the house- it's about exterior surface areas & types. Sizing it by some BTU/square foot usually oversizes by a WAY suboptimal amount, with lower than nameplate efficiency and lower comfort to boot. At 6.5 tons it sounds about 2x oversized for most "normal" sized houses, houses that have niceties such as glass in the windows, doors that actually shut etc. Most insulated reasonably tight houses in NJ would be heated & cooled at about a ton per 1000-1500' of conditioned space, with a few outliers on both sides of that range. For 6.5 tons of heat to be right-sized you'd be looking at at 6500'-9500' house. (Unfortunately that level of oversizing pretty common.)

With heat pumps it's usually more efficient to UNDERSIZE it by a small amount and cover the shortfall at the seasonal extreme temperatures with a bit of auxiliary heat strip, to keep the heat pump operating at an efficient duty cycle. A 2x oversized system will heat/cool the place faster during a recovery ramp, but it will have lousier latent load control during the cooling season and bigger indoor temperature swings during the heating season. A right-sized system will seem like it's "struggling to keep up", running constantly for hours when its hot or cold outside, but not actually lose ground.

If you're curious about your oversize factors, try data logging the duty cycle on the system(s) on days/nights when the outdoor temps are near the 1%/99% outside design temperatures. With that information you'd be able to intelligently right-size the replacement equipment when the time comes.

Meanwhile, back on-topic...

A 75-80 gallon heat pump water heater could be a good solution for the tub fill if you have space for it. A heat pump water heater has a slower recovery rate than a standard electric , but it will lower the latent cooling loads on the house in summer, and use only about 1/3 the amount of electricity of a standard water heater, and add a small amount to the heating load for the heat pump to work with, improving it's duty cycle & as-used efficiency.

 

[Dana]

A graphic to go along with the heat pump sizing issue- this is a graphic plotting square feet per ton against house size using a few dozen real Manual-J calculations done by an outfit in Decatur GA:

Most of these houses were in the Gulf Coast states, with higher 1% design temps and higher latent loads than coastal NJ. The cluster between 200-4000 square feet is in the ton per 1400' range, which is at least possible in NJ. The worst performing home in the 2000-4000' size came it at about a ton per 800'. So if your house is as lousy as the worst of that cluster, 6.5 tons at a ton per 800' would imply a 5200' house?

The 99% outside design temps for coastal NJ are about +15F give or take (Atlantic City's is 13F). A reasonably tight 2x4 framed house with clear glass double panes (or clear storms over wood sash single panes) and no foundation insulation will usually come in between 15-20 BTU/hr per square foot @ +15F outside, +70F inside. With an insulated foundation it'll be between 10-15 BTU/hr per square foot. The AHRI output of most 1-2 stage heat pumps at the AHRI test temperaure of +17F is about the same as it's rated cooling load. That's pretty close to your 99% design temp,, so you're looking at about 6.5 x 12,000 BTU/hr= 78,000 BTU/hr of heating capacity, would imply a house no smaller than 78K/20BTU per foot = 3900 square feet to be right sized in a house with no foundation insulation, and more likely to be somewhere around 78K/15 BTU per foot= 5200'.

There are of course outliers on both sides of those heating BTU per square foot ranges too, which is why it's never a good idea to just use a BTU per square foot rule of thumb for sizing the equipment, even if it's useful for sanity-checking your load calculations.

 

[bikeboy]

See Dana's post above, he does the math on how long it will take to fill the big tub using the biggest tankless on the market.

If you're thinking of spending a lot of money on new equipment, and gas is an option, then you should be including other options in your decision making process as well. Such as a gas-fired tank water heater, or even integrating your hot water & home heating needs into a single appliance.

But still by far your cheapest option is to just reconfigure your existing tanks as we've discussed.

I'm realizing that the two tanks together is probably the best way for me. I see a chart that there are several piping methods, both having advantages and disadvantages. So far series loop w/circulator and check valve seems to be recommended over(parallel) and the rest, and for different reasons. it seems wh1 could be shut off and used as a tempering tank. Would that temp rise from wh1 if given enough time while power off to that tank, entering energized wh2 be adequate to fill tub w/ hot water or wh1 always needing to be on ?

Thanks, Howie...

 

[Dana]

I'm realizing that the two tanks together is probably the best way for me. I see a chart that there are several piping methods, both having advantages and disadvantages. So far series loop w/circulator and check valve seems to be recommended over(parallel) and the rest, and for different reasons. it seems wh1 could be shut off and used as a tempering tank. Would that temp rise from wh1 if given enough time while power off to that tank, entering energized wh2 be adequate to fill tub w/ hot water or wh1 always needing to be on ?

Thanks, Howie...

Using the first water heater as a tempering tank doesn't cut it. The insulation makes the temperature rise very slow, but stripping the insulation would result in "sweating" condensation in the summer. Even if the water in the first water heater was 70F instead of 40F in winter, it would not improve the ability of the system to fill the 84 gallon tub. Even with 4500W (15,342 BTU/hr) of heater element running on the finishing tank it can't heat the water from 70F to 110F fast enough to matter at tub fill rates.

The standby losses of electric water heaters are quite low (and can be made lower by adding insulation on the tank and near-tank plumbing)- it's not going to appreciably change the total amount of electricity used, or the longevity of the water heater(s) to keep them both powered. If keeping the tank at 130-140F adding insulation to the tank may not be "worth it" , but if the storage temperature is raised to 170F+ it is definitely worthwhile to retrofit some tank insulation. But insulating the near tank plumbing is always worth it (and required by code in many jurisdictions.)

[edited to add]

The 15,342 BTU/hr delivered by the heating element of an electric water is only good for another three quarts per minute (less than 1 gpm) when raising water from 70F (in a tempering tank) to 110F (a decent tub-filling temperature). Putting that in perspective, over the course of 10 minutes it's still delivering less than 10% of the heat needed for the tub fill, even when starting from 70F.

 

[Phog]

I agree with everything Dana says above, and to reinforce his point: It doesn't matter how many tanks you have or how big the tanks are, you still only use energy to heat up hot water that you *actually consume*. Bigger / multiple tanks just means you have more sitting in reserve. Nothing is working any harder.

To use a simplified analogy. Imagine adding a 2nd gas tank to your car. You wouldn't use extra gas by having two tanks. Your miles per gallon would stay the same. You would just be able to drive farther between fill ups.

 

[Dana]

[QUOTE="phog, post: 588076, member: 79549"

To use a simplified analogy. Imagine adding a 2nd gas tank to your car. You wouldn't use extra gas by having two tanks. Your miles per gallon would stay the same. You would just be able to drive farther between fill ups.[/QUOTE]

Actually the additional weight in the extra gas tank analogy lowers your net miles per gallon efficiency by some small amount, which is also the case with the dual water heaters (twice the surface area = twice the standby loss), but it's not a large hit in efficiency.

 

[DIYorBust]

Could you just tee off the hot side of the WH1 and plumb that directly into the cold inlet on on WH2 leaving the plumbing on WH1 otherwise intact? A check valve could prevent reverse flow when drawing from the piping near WH1, and no need for re-circulation if there isn't now.

 

[bikeboy]

I'm realizing that the two tanks together is probably the best way for me. I see a chart that there are several piping methods, both having advantages and disadvantages. So far series loop w/circulator and check valve seems to be recommended over(parallel) and the rest, and for different reasons. it seems wh1 could be shut off and used as a tempering tank. Would that temp rise from wh1 if given enough time while power off to that tank, entering energized wh2 be adequate to fill tub w/ hot water or wh1 always needing to be on ?

Thanks, Howie...

I have another idea that may be easier; Could a separate line from water heater #2 just be run, and tied to either the hot side of WH1, or the tub valve w/ a check valve . Since the water pressure's the same at both points, would that hot water from WH2 need to be at higher pressure, or need of a circulator ; or could they work together w/o a circulator and when tub demand is required, both tanks will fill tub ? I wouldn't be using any other appliance for hot water @that time, if I mustn't.

Please reply, thank you.. Howard..

 

[Phog]

If I understand correctly, it sounds like you're proposing to run the tanks in parallel instead of in series. In theory you can do this, but in practice it's very difficult to get the hot water flow to balance 50/50 between the two parallel paths. And so you end up with one water heater tank draining all its hot water out much sooner than the other, followed by lukewarm water coming out the tap. You can try it & see what happens, it might work ok, but as they say "shortcuts often lead to long delays".

 

[WorthFlorida]

bikebot; What are you trying to accomplish? Save on fuel cost, need more hot water because your running out or just want one water heater?

 

[bikeboy]

bikebot; What are you trying to accomplish? Save on fuel cost, need more hot water because your running out or just want one water heater?