I like the vertex , whats the space heating heat load? what type of radiation? any big whirlpool tubs or multi head/ body spray showers?

The calculated space heating load is 21K BTU at the design temp of -10. The radiant is in concrete slab, two loops of 7/8" pex, covering 560 square feet.  No body sprays, 2 gal/min showerheads. The bath tub is 6 foot x 3 foot outside dimension air bubble tub with expected usage of once per week.

You can't fill that tub with a 50 gallon WH.

Posted By BadgerBoilerMN on 04 Nov 2011 07:40 PM
You can't fill that tub with a 50 gallon WH.

I concur completely with Badger. The 50 gallon can not meet the recovery rate and first hour fill required of it for the bathtub not to mention the space heating.

What are you using to fill the tub now? I have seen people use 50 gal wh on tubs that size and be happy and others who were not. It depends on expectations.

This is a new install so the tub hasn't been used. The tub capacity without anyone in it is 69 gallons. The tub manufacturer recommends that the DHW system be able to provide 70% of that total, or 48 gallons. This may be the way the manufacturer accounts for a tub never being filled to its capacity and the displacement of a person. I do agree that factoring in the space heating requirement results in the system being undersized on paper. The expected infrequent use of the tub minimizes the amount of time that the capacity of the system would be stressed. Certainly the first hour capacity and recovery rate would further minimize that time.

I'm interested in thoughts about the Vertex 50 gallon in the 76K BTU versus the 100K BTU, as well as experience with various flat plate collectors to support the space heating load.

You'll have a better shot at filling the tub with a 100K burner behind it than with the 76K burner, but it's still cutting it close. First hour gallon performance is one thing, but first 8 minute performance (6gpm for your 48 gallons) is another.

If you cut the flow and don't mind it take 12 minutes to fill a 100K burner can almost keep up:

Assuming 40F water from the street and 110F tub-fill temp, for a 12 minute fill you get 70F rise x 4gpm fill rate =140 KBTU/hr (1.5x the burner output)

...whereas for the 8 minute fill you get 70F rise x 6gpm fill rate = 210 KBTU/hr (more than 2x the burner output.)

At 2.5gpm shower flows you could run the shower all day during low space heating loads, and at 2 gpm in the shower you'd have enough left over for 100% of your space heating load. With the smaller burner you'd likely run short at times, whereas with the bigger burner it would only become an issue with tub fills, and a much-mitigated issue at that.

You also need to take into account washing machines, dishwashers and other faucets. A total fixture unit count is needed to accurately figure the sizing of the water heater.

Without doing the calculations in a real world installation it will work fine. You dont fill or shower with 100% hot water so your actual temp rise is 45 to 55. The amount of time that it would be off line to heat the building would not be noticed. When I have used these I install an aquastat to cut out the HX to DWH pump when supply temp drops bellow 110*. You could also set the tank at 140 in the winter time if you install an anti scald mixing valve. I see no reason you would not be happy with this. I have used the Vertex in several applications and have been pleased its with performance and reliability. I am using one in the house I am building with 2.5 baths and 65 gal to fill tub. It will also provide back up to geothermal supplying radiant floors thru a gea flat plate HX.

Without doing the calculations in a real world installation it will work fine. You dont fill or shower with 100% hot water so your actual temp rise is 45 to 55. The amount of time that it would be off line to heat the building would not be noticed. When I have used these I install an aquastat to cut out the HX to DWH pump when supply temp drops bellow 110*. You could also set the tank at 140 in the winter time if you install an anti scald mixing valve. I see no reason you would not be happy with this. I have used the Vertex in several applications and have been pleased its with performance and reliability. I am using one in the house I am building with 2.5 baths and 65 gal to fill tub. It will also provide back up to geothermal supplying radiant floors thru a gea flat plate HX.

110F is a fairly typical tub-fill temp (about 5F warmer than a typical shower-head exit temp). It doesn't matter much what storage temp the tank is, only the fill volume and the temp of the incoming water. It absolutely IS the delta-T between the incoming water and the tub-faucet exit temp- only in a place as warm as FL would you only be looking at 45-55F delta-T in the winter months. This is pretty straightforward high-school algebra- do the math. You have a bare margin with a 48gallon 110F tub fill with a 120F 50 gallons storage temp and 40F incoming water, but nothing to write home about.

And you won't get 65 gallons of 110F water out of it with it with 40F incoming water, without signficant burner input even at 130F storage temps. Solving for the minimum incoming water temp (Ti):

65g x 110F = (50g x 130F) + (15g x Ti)

7150gF= 6500gF + (15g x Ti)

650gf= 15g x Ti

Ti=43.3F

You'll barely make it at the full 65 gallons with 43.3F incoming water, and would fall short at 40F. With 40F incoming water you'd need to cover the 3.3 shortfall with the burner during the fill period. That's 3.3F x (65g x 8.34)= ~1800 BTU out of the burner in 6 minutes, or ~18KBTU/hr- the burner can handle that so long as you have no other loads of substance during the fill period, but you're definitely relying on a significant fraction of an active burner to get there.

If you keep the tank at 120F and need only 48 gallons in the tub you'd make it with a modest margin, but nothing to write home about, eh? But for the 65 gallons of soaker tub you're screwed at 120F storage temps unless you slow the flow rate and bump up to the bigger burner.

65g x 110F= (50g x 120F) + (15g x Ti)

7150gF=6000gF + (15g x Ti)

1150=15g x Ti

Ti=77.6F

To fill the 65 gallon tub and the 50 gallon tank @ 120F , the 65 gallons (77.6-40)= 37.6 degree shortfall is (65 x 8.34) x 37.7= 20,437 BTUs in only 6 minutes, which is 204,370 BTU/hr, more than 2x the rate of even the 100K burner. Even stretching the fill time to 8 minute isn't gonna cut it. You can still fill 'er up in 12 minutes though.

Raising the storage temp above 130F cuts in to condensing efficiency, and the difference in standby loss between 120F and 130F is real. Going with the bigger burner buys a bit of margin and allows you a lower standby temp. It's worth it for big tub-fill situations.

One can get around the other-appliance accounting to shoot for the minimum sizing on the HW heater simply by planning & scheduling, which isn't too tough in a 3-4 person household of cooperative & reasonable people.

...and mind you, the above is an idealized/simplified best-case scenario, with minimal to no mixing of the incoming cold into the tank. In practice the last 1/4-1/3 of hot water in the tank will have mixed substantially with the incoming water, and at 40F incoming water temps would be well below an acceptable fill temp for the big tub unless you have the bigger burner going.

"do the math"

Now that's a thought...hehehehehehehee

Matching the stored water volume a large tub capacity is the safe way to size any water heater. If you are heating space with the same water heater an experienced professional should be involved.

Why would anybody want to do the math when wishful thinking is more fun? Math is boring (sometimes depressing when it doesn't match your desired outcome), whereas wishful thinking is exciting, inspiring, where all things are possible! :-)

The spa & soaking-tub fill problem with undersized hot water capacity is incredibly common mistake, distantly followed by the six side-spray dual master-bath showers, etc. Luxury bathing facilities usually require an upgrade on DHW capacity, and it's dead-obvious even with 2 minutes of napkin-math. There are a limited number of ways to get that capacity, bigger tanks, bigger burners, (or in the high-flow shower case), drainwater heat recovery, each with their own limitations & advantages and sometimes it takes all of the above to achieve satisfactory performance.

Even with 21K (the design condition load) of LOW mass radiation hanging on it the 76K burner wouldn't cut on a 48 gallon tub fill without inhibiting the space heating calls during the fill period, but the 100K would. With the slab radiation you're taking your chances- if a cold slab with bang-bang pump controls starts calling for heat during a tub fill odds are the radiation alone will be pulling more heat out of the tank faster than even the 100K burner would keep up with for the first couple of minutes, diluting the tank with a big slug of tepid room temp water in addition to the flow 40F or colder potable water. With constant flow on the radiation with an outdoor reset mixing valve it'd probably be fine with the 100K burner, but could fall short with the 76K burner when it's cold out.

$20,000 for the new bath, but not one nickel for engineering...it makes me crazy...or was I already crazy...OK, I am crazy.

But, Dana is right...99.97% of the time.

I saw one of these luxury bath upgrades recently with over $5K in the platinum plated plumbing fixtures alone, let alone the custom spa and marble showers. (A friend of mine was their tile contractor.) Dont' know what the total bill for the bathroom was even without the cost of the mechanicals required to support the hot water load. I'm glad somebody has that kind of money to spend on construction these days.

(Morgan you flatter me, but I can't be right more than 90% of the time- I try to look it up when I'm not so sure, like when the lipstick-on-napkin math is too smeary around the edges. ;-) Many people much smarter than I have looked at these issues in much greater depth for decades.)

I have found the "looking" is the smartest part.

My point is always, that there are people who have the answer if you are will to look for them AND pay them. The people who install the equipment may be very fine mechanics but, if they don't what to put in, or how to make it work, a professional designer or contractor that does know the why, should be retained before the first hammer stroke.

Where space and domestic hot water are concerned the field is fairly narrow and the competent contractor (rarely a General) must be sought out.

You and your "lipstick"....