Installing a tankless water heater
 

Well, it seems my ~14 year old water heater has sprung a small leak. There is some water puddling around the bottom of it. Nothing horrible, but it needs to be replaced soon. Thankfully, a while back, I picked up a tankless water heater for a good price.

So, I'm looking to install the new water heater very soon. Any hints/tips would be appreciated. I'm not expecting it to be difficult though. I just have to read through the install manual.

Gas? Electric? Oil?

I've had a Bosch tankless for about 17 years, as I recall.

Mine uses D-cells and a step-up transformer to make it's spark. I had the transformer go out in the first year. They replaced it VERY FAST and I have had no problem with the electrics since then.

Things to keep in mind:

• You need massive gas flow when it runs. Check out the BTU rating of your unit, and make sure that the gas line is of sufficient diameter to supply that BTU rate. That was important for Nat. Gas... I suppose you're using propane, don't know if that's an issue for you.
  
• Your unit comes with a wire grid 'filter' to keep out chunks. I would advise a pre-filter, in other words, dedicate a 'whole house' water filter to your tankless. You shouldn't need to change the filter element very often because the water volume will be fairly low. A whole house filter shouldn't give you much pressure drop... which is good. Keep the tankless innards free of crud and your unit will last you a really long time.
  
• A tankless is NOT an 'instant on' water heater. some sales folk will call it instant on, but it does require a brief warm up. You'll just have to get used to it, and plan for it by...
  
• Mount your tankless as close as possible to the most frequently used water source, not the largest volume water source (For me the most frequent was the kitchen sink). This will greatly reduce the wait times and water volume while you wait. You might also want to run 3/8 PEX or similar to the most frequent hot source, to reduce the water volume from warm up. In many of the foreign films I have seen, the tankless is mounted right above the kitchen sink, right where it belongs.
  
• Consider the possibility that you Bosch tankless (which really is very nice) is a whole house water heater and is actually too large, and that a better solution might be to have several smaller cheaper tankless units at each point of use. I have not... I'm still waiting for my Bosch to die... I've been waiting 17 years now.
  
• Be sure to build in flushing connectors (I used CPVC hose bibs) and isolation valves I used (CPVC ball valves) when you install the unit. Flushing with white vinegar should be done yearly and takes about a half hour, if you have installed the valves & connectors.
  
• When you use a DHW tank, the philosophy is to keep the storage temp hot, and mix in cold for a larger effective hot volume. Since your tankless will never run out, make your hot temp lower... equal to the highest temp you will use. For me that's shower temperature, I like about 103F (no cold water in the mix). Tankless units build up scale faster, the hotter they are used, so it is to your benefit to run your unit cooler... don't deny your self of hot water, just tweak the temp, and you'll be happy and so will your tankless. I have found that I need to make occasional, modest seasonal temp tweaks... maybe once in the early fall (hotter just a tad), once in mid-spring (cooler just a tad).

Here's the flushing procedure... the guy's kind of a dweeb, but he knows how to flush.

YouTube - ‪Tankless water heater flushing cleaning‬‏

My tankless is 17 years old... it's still working, but I need to replace a $35 valve... should be good for another 17 years.

... and did I say I've saved a heap of money in the meantime?

-AC_Hacker

The water heater is a natural gas Bosch AquaStar 125B.

Well I'm not sure there's much more I can say the AC_Hacker hasn't already! If you are familiar with the basic procedure of connecting a gas stove to the line you should be all set. Just use pipe dope. If the new unit is located remotely from the old unit can you use a flex line to bridge the gap? Just trying to make thing a little less complicated for you as soldering gas lines always gives me the heebie jeebies.

I've had tankless gas-powered water heaters for the last 18 years and can say that AC_Hacker pretty much covered it:thumbup:

One thing that I noticed on one of the heaters is that if I turn the hot water off, then on about 3-5 minutes later, the water runs VERY hot for a few seconds. This is probably because when water is standing in hot pipes it soaks up heat, but whether this is heat from the water heater itself, or from the metal plumbing between the heater and shower - I don't know. I've never seen this happen in hot water tank system, so it may just be the heater.

Also, tankless water heaters (at least the gas powered kind) have a minimum flow to keep them on - you can't use only a tiny stream of hot water. Plus some water is wasted while waiting for the heater to fire up.

Pretty similar to my model.

Here's a place where you can get parts for your model.

-AC_Hacker

I'm reading through the install manual and it says that I need a minimum of 6 vertical feet of flue. I'm guessing this is to create a proper draft?

I'm considering just adding a power vent to it so I can reuse the 4" flue (vs 5" the instant water heater requires without the power vent) I have going up the chimney and through the roof. This would avoid any cutting extra holes in the house.

I've also looked over the specs on Bosch's site and have a question I think I know the answer to, but want to confirm. They rate the energy factor of the unit at .64 for natural gas, but the thermal efficiency is rated at 80%. I'm guessing the energy factor takes into account that this unit has a pilot light (booo for pilot lights :))?

Very sorry I didn't include exhaust considerations, as they are very important...

Yes, proper draft. The amount of fuel the unit consumes when it fires is very large. If you cut corners on the minimum draft specs, you can get water vapor and carbon monoxide entering the house... both are obviously undesirable.

You also need to observe minimum stack height as it relates to the top of the roof of your house and the potential for wind swirls causing positive pressure that would impact the exiting of exhaust from your unit. Similar considerations apply to properly venting a wood stove.

An additional consideration is that the unit can create a negative pressure in the house and would pull in cold replacement air when it runs...

I picked up a more modern Bosch (LPG) at a garage sale for $35. The sizes of both the exhaust vent and built in combustion air vent on it is three inches in diameter... so a power vent can make do with smaller diameter, but passive venting really does need large diameter, careful attention to the number of elbows, etc.

If your house is still pretty leaky, you shouldn't have any problems with combustion air, but if you tighten your house up dramatically, you will need to supply external combustion air.

I don't mean to be discouraging... I'm just trying to share the lessons I have personally learned, by having done almost everything wrong initially.

Tankless heaters are a great concept, well built and very reliable. Yours should provide you with many years of reliable, economical service.

Best Regards,

-AC_Hacker

P.S.: Tankless heaters have continued to evolve since your unit and my unit were built. Two big features have greatly improved the efficiency of tankless DHW heaters, condensing combustion gases (which recovers the heat of vaporization, about 15%) and also modulating fuel consumption to match demand (very similar in effect to inverter technology that is being used in mini-split heat pumps). These features are currently found in tankless hydronic boilers, but are sure to enter the DHW market eventually.