Tankless with small tank heater...

[single_digit]

I have a bit of an unconventional dilemma. I have an older gas Takagi tankless HW heater in my upstairs. It has had issues for years and I've concluded that the gas line does not deliver sufficient volume under high demand. So I need to replace it. When I did some other plumbing work years ago I added a HW line that could be run from my basement Rinnai tankless to the upstairs. However, it's a long circuit and would take quite a while to deliver hot water to the upstairs bathroom. Further I didn't plumb in for a recirculating loop. To solve my issues I was thinking I could have had a small gas tank heater where the Takagi was. I would then run hot water from the basement tankless heater up to the small tank heater with the idea that initially I deplete the tank while hot water comes in from the basement to recharge it. Note that I don't have room for a large tank heater upstairs and I don't think I can deliver sufficient gas (long run with several turns on 3/4 line to supply a large tankless heater. Is my proposed solution going to create unforeseen problems for me?

 

[single_digit]

So, I didn't really look and assumed there were small volume NG tank heaters, but it seems all small volume (<20 Gal) are electric. I don't think that changes my fundamental question though. Is it okay to put a small tank heater (or for that matter tankless as long as it doesn't pull too many BTU of gas) in series after my primary tankless model?

 

[Sponsor]

 

[Phog]

Yes, it's fine to use the Rinai tankless as a pre-heater for an electric unit as you describe. But does your upstairs Takagi have user settings where you could lower down the upper limit on the burner modulation range? Some models do. If so you could limit its burner output down to some lower level, say 50%, so it will not attempt to draw more fuel flow than your gas pipe can safely supply. Then put it in series with the Rinai so the Takagi is getting pre-heated water from the Rinai. A little unconventional (the Takagi will only be on for a minute while it's "waiting" for hot incoming water), but hey it's already installed so why not try it out.

 

[single_digit]

Yes, it's fine to use the Rinai tankless as a pre-heater for an electric unit as you describe. But does your upstairs Takagi have user settings where you could lower down the upper limit on the burner modulation range? Some models do. If so you could limit its burner output down to some lower level, say 50%, so it will not attempt to draw more fuel flow than your gas pipe can safely supply. Then put it in series with the Rinai so the Takagi is getting pre-heated water from the Rinai. A little unconventional (the Takagi will only be on for a minute while it's "waiting" for hot incoming water), but hey it's already installed so why not try it out.

It's a TKD-20 from around 2005. Here's the manual, but I have no idea if what you are saying is possible for it. It's running on Nat Gas.

 

[Phog]

Looks like it's not an option on this model. You could still try lowering the temperature setting (set the dip switches as described in your manual) to lowest temperature & pipe it in series with the Rinai. Which would have a similar (even if not exactly the same) effect of lowering the flame. That, combined with the warmer incoming water, might work acceptably. And if it doesn't work then you can just proceed with your original plan & rip it out for the tank. Good luck and let us know what you end up doing.

 

[single_digit]

Thanks for the advice! Will do. If I go with an electric option I'm a tad concerned about electrical load (old house, small panel). Do you have any suggestions there? It's also a small closet space. I was looking at a small electric tank heater that was around 12 gal and drew 1500 watts. Would this be what you would suggest if I can't make my current model work? Or would you opt for a smaller gas tankless model to replace the Takagi that pulls
less gas? If so any suggestions?

 

[Bannerman]

I've concluded that the gas line does not deliver sufficient volume under high demand. So I need to replace it.

Have you considered replacing the gas line with a larger diameter pipe instead of replacing the existing tankless?

If the existing line is concealed, perhaps it could be abandoned and replaced with a new line following an alternate path.

 

[single_digit]

Yes. I considered it, but it's a long run from the meter no matter what. I'm already running 3/4" and I believe the meter is only 1". The only possibility seems to be to run 1" from the meter (which would require several elbows to get around the corner and up the wall to the eve near the current location. But I'd still be looking at no fewer than 4 or 5 elbows and a run of probably 50 feet. I'm not sure I'd solve my problem. The basement Rinnai unit is on a pretty straight shot through the house with 3/4" and has had no problems.

 

[single_digit]

Follow up question about the two heater arrangement... Would I likely be dealing with weird changes in temperature as the system transitions from the small volume of HW in the upstairs tank to the hot water coming in from the basement (that initially will be flowing cold)? I'd hate to do all of this and have a blast of cold or scalding water 90 seconds into a shower.

 

[Phog]

You could see weird temperature fluctuations, or not -- no guarantees on that front, you would just have to try it and see. A small electric tank would not have that issue. As far as tank electrical supply, only the wattage matters, not the gallons of the tank. 1500W is small for a normal electric water heater (typically is 3500-4500W) but should still be fine for your application. (Since the gas tankless will be doing most of the work heating the water, the electric element can be small).

 

[Jadnashua]

THe actual amount of volume of water you need to run to get the tankless' hot to your new, small electric unit, is probably less than 2 gallons, and maybe a little more as it heats up the line after purging the cold out. That colder water will mix with the already hot water in the tank, and the element will likely turn on. If the tank is large enough, you should never notice.

FWIW, it's not a 'hot' water heater...if it was hot, you wouldn't need to heat it...you're heating 'cold' water, so it's really a water heater.

 

[single_digit]

Just as a follow-up on this thread for anyone facing the same issues... I ended up purchasing a Takagi remote for the Tankless heater so I could manually adjust the temperature. It does seem to help to an extent. Unfortunately, my wife wants the setting around 108 for her showers and that causes the unit to follow it's old behavior if we aren't very careful turning on the water. I may yet go through with adding a tanked heater inline with our basement unit.

 

[Bannerman]

A gas tankless will typically 'fire' at maximum capacity until the out flowing water acheives the set temperature and then the flame will be modulated down to maintain that temperature. The maximum capacity for your unit is 199,000 BTU.

Although it will attempt to utilize 199,000 BTU (~199 ft3 gas) only briefly during startup, the gas pressure within the extended length of 3/4" pipe will rapidly drop to cause the unit to error and shut down.

You may want to contact Takagi technical support to inquire into a possible settings adjustment or the possibility of exchanging the current gas orifice with a lower capacity orifice to reduce the maximum fuel consumption rate during startup. Reducing the maximum output to 120,000 - 130,000 BTU maybe sufficient to maintain adequate gas pressure during startup. Although the maximum heating capacity would be reduced, because the water is fed from another water heater, the capacity actually needed will be low. If the temperature of the incoming water from the Rinnai is hotter than the Takagi temp setting, once hot water arrives from the Rinnai, then no additional heat will be required from the Takagi so it can then shut-down for any remaining hot water use.

With a temperature setting of only 108℉, I anticipate the shower control will be almost fully open for almost 100% hot water to flow to the shower head. This will result in the greatest water flow through the heater so the Takagi will likely consume more fuel compared to using a higher temperature setting. With a higher temp setting, to achieve 108℉ to the shower head, the shower control will blend some cold water with the hot so the amount of water flowing through the Tagaki will be less while there will be equal flow to the shower head.

As you are currently experiencing some success by opening the shower control very slowly, perhaps partially closing the water supply valve to the Tagaki will be sufficient to reduce the maximum flow rate so it possibly might use less gas during startup.