replacement system question

first post here, have been browsing for years ... appreciate the input from all contributors

anyway, purchased a new to me home, bigger than we need but it is what it is. System has two NG tank heaters- one 75g one 50g. 50g is newer. 75g is leaking, plumbed in parallel . Home has 6 1/2 baths. 1 soaking tub with recirculating heater. House is plumbed with a home run manifold in utility rm with runs to each fixture.

My current home has 3 1/2 baths and 1 soaking tub with recirc heater. 40 gal NG( 40kbtu) heater with recirculating loop and traditional branch piping from WH

currently 4 people in home two teenagers, in current home we seem to have adequate hot water. No complaints by anyone even with long duration showers, or my wife using the soaking tub.

New home plan is to remove both tanks and install a high input (75kbtu) 50 g NG heater with a tempering valve running the WH at 150-160. Which I think would have the effect of a 75g tank with the physical size of the 50g to reduce standby losses. Thinking that the tempering of the hotter water and the high BTU recovery will keep up with demand for 4 people, and be reasonable with guests.



Questions;
does this sound like a reasonable plan?

Is the high input a good idea?

Should I just get a 65 or 75 gal( cheaper than the 65) tank to cover the occasional use of guest baths ? High input or no?

Thanks

Steve

In most houses a single tank sized for the biggest tub-fill is enough, and for your family of four that's probably going to be fine too. Depends on how big that soaker-tub really is. I suspect you could do just fine keeping the existing 50 gallon unit as long as it can fill the soaker tub. The only time you'd need more is if all four of you are showering at the same time, taking full advantage of the 6 baths. Keep it, add the (code required) tempering valve, and crank up the storage temp if it doesn't quite fill the tubbie at 140F or whatever.

If you find you are running short of showering capacity with the existing unit, you can probably still keep it. Installing a drainwater heat recovery heat exchanger rated at 50% or better return efficiency would provide more total showering time than a 50 gallon tank with a high storage temp and a 75K burner, and would cost less to install. For showering loads it's like adding to the burner size, but uses no fuel. EFI is probably still cheapest place to get them in the US. (Home Depot has them online, at a full retail kind of price, as does the manufacturer's buy-direct page.) Comparative efficiency rating lists from third-party testing of different models are maintained by Natural Resources Canada to be able to compare apples-to-apples. At lower flow the efficiency is a bit higher, at higher flow a bit lower, but the BTU-rate increases. (40% energy return on a 10gpm flow is a lot more BTU/hr than a 55% return on a 2.5gpm flow.)



But it's only good for increased showering capacity, since the drain has to flow at the same time as the hot water to get the exchange. They do nothing for tub filling capacity.

When it's time to replace the 50 gallon unit, you'll still have options. The 50 gallon condensing Vertex has a 76KBTU/hr ~95% efficiency burner and puts out considerably more heat than a 75K burner on a typical tank (~80% steady state combustion efficiency), and you wouldn't even have to crank up the storage temp. But if the house is heated with a boiler, an "indirect" tank running as a zone off the boiler is often the best overall solution.

Seem like a reasonably staged "see how it goes" plan? That way the expense is deferred until you KNOW you need an upgrade, and you're not scrapping equipment and replacing it with something else that may or may not cover your actual loads. And adding the drainwater heat exchanger buys your more showering capacity for less money than a bigger tank or bigger burner, for roughly the same installed cost of a plain-Jane 50 gallon hot water heater.

FWIW: A 76K condensing burner like the Vertex is enough to supply a continuous 24/365 two gpm shower with margin to spare at D.C.'s incoming water temps, but a 40K/80% burner in conjunction with a drainwater heat exchanger (which uses less fuel) would keep up too.

Dana said:

In most houses a single tank sized for the biggest tub-fill is enough, and for your family of four that's probably going to be fine too. ..... Installing a drainwater heat recovery heat exchanger rated at 50% or better return efficiency would provide more total showering time than a 50 gallon tank with a high storage temp and a 75K burner, and would cost less to install................Seem like a reasonably staged "see how it goes" plan? That way the expense is deferred until you KNOW you need an upgrade, and you're not scrapping equipment and replacing it with something else that may or may not cover your actual loads. And adding the drainwater heat exchanger buys your more showering capacity for less money than a bigger tank or bigger burner, for roughly the same installed cost of a plain-Jane 50 gallon hot water heater.

Click to expand...

Thanks for the quick response.

In talking to the neighbors no one else has two tanks... no one else has cut lines near the WH and the air handlers either. My HVAC guy thinks the house had hot water to air heat exchangers run off of the WH system at one point. The Existing furnace is 3 years newer than the house which is kind of odd, but makes sense if they had a water to air system they changed out to gas furnace. So my thinking is that the 125 gal storage was for space heating purposes rather than hot water supply.

The 75 g is leaking and I am having the plumber over to cut it out, I am getting slugs of rusty water so we will check the 50 to see if it is servicable. If so I was thinking along what you outline- add the tempering valve see how we do- I didn't think of the heat recovery.
This is a foreclosure that's been sitting far a while. It is on well and they took the treatment equipment. Well water here is on the acidic side and there is rust in the toilets etc I believe from the untreated water sitting. So I am not hopeful that the 50 g tank is in very good shape. Rust could be WH's, maybe the pump screen or WH anode other items ?? PH was 6.2 if I remember right. Plumber has a couple days of going through everything, cleaning, replacing some fixtures etc. installing filter and treatment equipment.

On the heat recovery , I forgot all about those, saw them in JLC yeas ago. I'll do some more research on it. I assume they work best on verticals where the water would flow around the whole pipe circumference rather than ride on the bottom of a horizontal run. That would be a good solution for showers. I have a nice exposed horizontal run but no large accessible vertical runs. Septic exits almost 4 feet off of basement floor level.

Unless you have a vertical drop you can install one, the heat recovery units are probably not worth considering. They do come in various diameters and lengths, but if your waste line goes out that high, and most of the showers are on the first floor, it's probably not a viable option.

Gravity film heat exchangers absolutely need to be mounted vertically, so that the surface tension/wetting forces don't have to overcome gravity, and will spread the water in a thin film onto the surface of the pipe for maximal surface area (==higher heat exchange rates.) They make them as short as 24", at which point you get a ~30% return out of a four incher, but only when mounted vertically.

Your guess is probably right- they were probably using one of the water heaters for a hydro-air. When that didn't work I'm surprised they opted to replace the air handlers with the coils with a hot air furnace, when for about the same money they could have installed a boiler with better output characteristics than the water heater (either temp or total BTUs). If the hydro-air coils were returning water cooler than 125F to the hot water heater there is a good chance it's life was shortened by the condensing in the heat exchanger, which may be why one of the water heaters is newer, and the other one is toast(?).