Question about condensing gas water heater

[uscpsycho]

Please see the attached chart of water heaters by AO Smith. All these water heaters have either 34 or 50 gallon tanks but each with different specs.

My question is regarding operating cost. Assuming water usage is well below the capacity of the heaters, would the lowest rated 50 gallon heater (GTP-130) be cheaper to operate than the highest rated 50 gallon heater (GTP-199)? Or would it be the same?

Based on my limited knowledge, what I think is that they will cost the same to operate. Since both have 50 gallon tanks the standby loss is the same for both but the 199 can heat more water more rapidly when the usage is higher. So you have the power there when you need it but if you don't use it, it doesn't cost you anything to have it.

Unless the GTP-199 is costing more to run even when water usage is low, could that be the case?

Thanks in advance guys.

 

[uscpsycho]

Also, what if you compared the GSP-150 to the GTP-150?

Or compare GSP-150 to GTP-130?

In both cases the recovery of the 34 gallon GSP-150 is the same as or better than the 50 gallon models. So isn't the performance of the GSP-150 equivalent to, or better than, GTP-130 and GRP-150?

 

[Sponsor]

 

[Dana]

Any of them have enough burner to support a 24/365 shower at 2.5 gpm.

Size the tank for the biggest tub you have to fill. A bigger tank has more surface area, which adds up to more standby loss, independent of the steady state thermal efficiency. But operational costs are really all about the same. Raising or lowering the storage temperature a few degrees can tip it either way.

The installation costs are NOT the same however.

There is often an up-charge in the installation cost going from 100K burner to 150K or 199K, since it takes fatter gas plumbing to support those bigger burners. In most houses you can run a 100K burner off 3/4" gas plumbing. In most houses you would need 1.25" gas plumbing to support a 199K burner if there are any other burners sharing the line.

Note- the pipe lengths need to add in the "equivalent length" of all the tees & ells on the path, not just the straight pieces, and any lenghts that are shared, teed off to another burner need to be assessed. Even at 100 equivalent feet of run to a 100 K burner you'd be fine with 3/4" gas plumbing, but with a 199K burner you'd be limited to about 35 equivalent feet of 3/4", which isn't very much.

 

[uscpsycho]

Any of them have enough burner to support a 24/365 shower at 2.5 gpm.

Size the tank for the biggest tub you have to fill. A bigger tank has more surface area, which adds up to more standby loss, independent of the steady state thermal efficiency. But operational costs are really all about the same. Raising or lowering the storage temperature a few degrees can tip it either way.

You mentioned my largest tub which is a concern of mine. I have a 120 gallon tub from Jacuzzi. They specifically recommend at least a 75 (or maybe it's 80) gallon TANK water heater (not tankless). The tub has built in heating so standby loss isn't an issue. It's made for two people so with one or two occupants the tub will never be filled to its 120 gallon capacity.

These condensing water heaters have impressive first hour and recovery specs given their size. The GTP-199 is probably around 300 gallons in the first hour. But are any of these 34/50 gallon water heaters going to be able to fill my Jacuzzi tub without running out of water?

Thank you.

 

[Breplum]

You only get 65% of a tank water heater's output at full hot, then it becomes cooler, depending on the btus.
You will never get your big tub filled with a 50 gal WH.
Consider upgrading to a tankless WH. Navien NPE240A is the best IMHO.

 

[uscpsycho]

For whatever reason Jacuzzi specifically recommends a tank rather than tankless heater to fill this tub. Also, every time I've done the analysis I've determined that I prefer a tank heater to tankless. And while I can't recite the reasons now, because it's been a while, I think part of it is more maintenance for tankless and I despise things that require maintenance. Or maybe it was sold water sandwich.

One thing I definitely recall is that it is best to have a tankless water heater somewhere central to the home and my water heater might not be central enough.

I don't even want to use a condensate neutralizer that will require annual replacement, I just want to drain the condensate in a way that I never have to think about again.

 

[Dana]

You would have NO problem filling a 120 gallon tub with 110F water from a 119 gallon tank with a 199K burner (such as the HTP' s PH199-119) with water stored at 140F.

To do it with an 80 gallon tank would require throttling back the flow a bit for the final 15-25 gallons, even with a 199K burner.

With only 50 gallons of stored water it'll be almost as bad as doing it with a tankless.

The biggest residential tankless has a 199K burner, and (of course) no stored water, and would take longer to fill the tub.

 

[uscpsycho]

Keep in mind that I will never fill it to 120 gallons. It's made for two people so with 1-2 adults in the tub it sounds like a 75-80 gallon tank should do the job? But will I need a 199K heater?

I can't go any larger than 80 gallons because I don't have the space. And even if I did, I suspect I'll be using this tub so infrequently that it won't be worth getting such a big water heater just for the tub.

 

[Dana]

A couple of mid-sized adults would displace 30-35 gallons if floating, but probably only 20-25 gallons. Figure on needing at least 100 gallons of 110F water to fill the tub.

If your incoming water is 50F (could be cooler or warmer than that depending on your location) and the storage temp is 140F it will use 67gallons of water out of the tank to deliver 100 gallons of 110F water into the Jacuzzi.

If your incoming water temp is 40F it'll be 70 gallons of 140F water to deliver the 100 gallons of 110F water to the tub.

So, yes it looks like you can get there (just barely) with an 80 gallon tank without needing a big burner.

 

[uscpsycho]

Dana - Thank you for working that out for me! I feel much better going forward with a traditional water heater now.

But I am still curious about condensing water heaters. They recover so much better than regular gas water heaters, but there don't seem to be a lot of condensing options in the 70-80 gallon range. Perhaps this is a subject for a new thread but does anyone have thoughts on this? I know HTP and American Water Heaters make large tank condensing water heaters but the reviews for both are rather horrid. Do any other companies make larger condensing water heaters?

 

[uscpsycho]

I found another large tank condensing water heater one made by AO Smith but it is sold as a commercial water heater, whatever that means - https://s3.amazonaws.com/s3.supplyhouse.com/product_files/AO Smith-BTXL-100-Product-Overview.pdf

 

[Dana]

Commercial water heaters aren't tested or rated in the same way for efficiency the way residential water heaters are. The EF test presumes a standard ~64 gallon daily water use, which is clearly not appropriate for commercial water heaters or homes with tubs that take more than 64 gallons. Commercial water heaters and water heaters with a burner larger than 75,000 BTU/hr only test the steady-state thermal efficiency.

The commercial BTXL-100 series tanks are glass lined mild steel, unlike the HTPs & American Water Heater units with their stainless tanks. In a residential application the stainless tanks will outlast glass lined tanks by a factor of about 2x. (10-12 years for glass lined, 20+ years for stainless.)

There are 80 gallon heat pump water heaters out there that can fill the tub. The Stiebel Eltron Accelera 300E or Rheem Performance Platinum 80 gallon unit would do it. The recovery time and first hour gallons are a lot lower than big-burner gas water heaters but the operating costs aren't too bad at ~EF 2.5-2.7. There are probably others (I haven't kept track of that market too closely.) During the cooling season there is the benefit of over half the heat being drawn from the room air (it has to be a sufficiently sized room to support the load). During the heating season your heating system is supplying that fraction of the heat. Operating cost can be lower or higher than a condensing gas water heater depending on your actual utility rates and how much hot water is actually used.

There are 80 gallon residential & commercial resistance heater electric tanks that will do it too but at ~3 x the operating cost of a heat pump water heater.