water heaters....tank or on-demand?

[nhmaster]

This is the thread that never ends... It just goes on and on my friends.........

[Master Brian]

As for ducting a new tank heater. I'd love to find a high efficiency one, if they are truly cost effective, especially if I can vent it out a side wall. I need to check cost as well, but I think a 50 or 60 gallon tank is what I'd like to try to go with. Maybe it is overkill, but if cost isn't a HUGE factor, why not. Especially if I try to install some supplemental radiant heat at a later date.

The good news, if you can call it that, is that my current chimney isn't lined. I've had several people out to quote me the job, and they all tell me to just do it myself no harder than it would be. So, it is on my project list for the spring. I say it is good news, because if the new heater would require a different flue than what I have now it shouldn't be any more work to install a larger diameter liner. Of course it would be nice, if it could be tied directly into my furnace flue, so I would only have to run one flue pipe up. That way, I'd definately have room for a gas fireplace later.

And from what I understand, as long as the system is allowed to flow through and not sit and stagnate, it would be fine to use on the same heater as the drinking water.

[chris8796]

Another thing you might want to look into is a drain heat recovery unit, especially if you take more showers than baths. I installed one because I have 2 teenagers and a 40 gal tank. It it extended out capacity a lot. In my tests, we recover about 1/3 of the original heat used to heat the water for a shower. I also like that they are essentially idiot proof with no moving parts. Now is a good time to consider one, since copper is back to normal levels. Mine is like 30 lbs of copper.

If your motivated, I would also consider looking at solar water heating. I plan on taking that step next and plan to incorporate radiant heat for a workshop.

[Master Brian]

Those drain heat recovery units seem like a good idea, but why are they so expensive? Maybe, I'm missing something, but they just look like coiled up copper tubing.

On another note, regarding the Bradford White hot water tanks are there different models if I want a 50gal natural gas unit? I can get one locally from a plumbing supply for approx $450 - 15%. Is that a decent price? I just don't know if that is a bottom fed, if it's high efficiency, etc. I was in a hurry and didn't get a chance to ask, besides the guy helping me on another item didn't seem to know much.

[leejosepho]

Is that drawing a setup that is legal to use? I thought that the domestic hot water and heating water had to be kept seperate.

That drawing comes from here: http://www.radiantec.com/index.php, but I have no idea how "legal" such a system might be in any one location or structure or another. However, what you see there is actually nothing other than a recirculation line that happens to secondarily double as a cold-water feed for the water heater. Other than the cold-water-feed part, I have the same kind of recirculation line supplying a small kickspace heater under the kitchen sink ... and the water involved is simply "heated water" for domestic use and not "heating water", as such.

[chris8796]

Those drain heat recovery units seem like a good idea, but why are they so expensive? Maybe, I'm missing something, but they just look like coiled up copper tubing.

They are more expensive than I think the should be, I think it is just patent issues that prevent them from being cheaper. I paid $430 for mine and DIY it. I would estimate the payback to be 4-5 yrs in my case and get the benefit of greater capacity in the mean time. We have take showers back to back to back in the morning so its a good setup for us and we can easily see the benefit. I considered trying to make my own, but once you start pricing 3-4" copper and soft 3/4" its still pretty expensive at retail prices. The tubing is flatten out somewhat to get better contact so it would be hard to make an identical one.

A few reports I've seen say a DHRU increases the efficiency factor of a water heater about 0.22. That is about the average difference between tank (0.62) and demand units (0.80-0.85)(non-condensing). So for the price difference, DHRU seem to be a safe bet to bridge the difference. They don't have any maintenance issues either.

They are just an interesting way of adding capacity and saving some energy that most people don't consider.

[Dana]

And from what I understand, as long as the system is allowed to flow through and not sit and stagnate, it would be fine to use on the same heater as the drinking water.

In MA there is a specific requirement on non-isolated combi systms to run the circulation pumps X-minutes out of every Y-hours to mitigate stagnation issues. (I don't recall the ratio)

For space efficient (and reasonably efficient) combi systems, this guy's reverse-indirect HW heater as buffer approach is about right, and has a high-capacity heat-exchanger:

http://www.heatpro.us/designtree/doc...anklesssys.htm

The other advantage with the above system is that the tankless only heats (and the tank only stores) "dead" boiler water, cutting down on scaling & corrosion issues in both. It's not as efficient as a mod-con + indirect, but it's not bad- better in small heat-load scenarios than McMansions. If the domesting HW flow is detected with a flow switch to inhibit heating system calls the HW delivery has significant buffer to work down before the tankless needs to fire at full modulation- the best of both the tank/tankless worlds from a HW heating point of view.

Used as a boiler, an on-demand will run a few percent more efficient than the EF numbers imply since it'll have guaranteed longer duty cycles. eliminating efficiency-robbing short cycles like half-gallon handwashing draws. It generally takes a couple of minutes for even low-mass burners like a tankless to hit their efficiency strides. Draws of less than a gallon or so results in rather poor relative performance (under 60% efficiency), draws of a half gallon or can even be less than 50% efficient.

I tend to believe people who actually measure stuff: These folks did a comparison using a un-buffered Rinnai tankless (82EF) to heat HW & run an air-handler and it tested out in the mid-80s for combustion efficiency vs. high-70s for a pretty-good forced-draft tank heater in a similar system & load. It's shoulder & summer efficiency numbers were favorable by comparison as well.

http://dsp-psd.pwgsc.gc.ca/collectio...2-106-108E.pdf

If going combi-system, almost any tankless (buffered or otherwise) will likely outperform any conventional tank heater with some margin. But buffering the tankless with a reverse-indirect will squeeze a bit more performance out while removing all the tankless quirks in the process, and avoids any stagnation issues. Heatpro's combi probably beats the eKoComfort design's efficiency, during the heating season, and possibly even during the summer.