How to add a hybrid-electric hot water heater to my existing oil-fired boiler/tankless coil setup

Jamie Hall

Yes you are. if valves 1 and 2 and 4 are closed, you have trapped water in the coil. If you then fire the boiler, that water is going to expand, or try to -- and something, somewhere, will break.

Maestro232

Oh...gotcha! I understand now. So I can just put a second one on the left side of 4 on the new crossover piping. Perfect!

Jamie Hall

Got it. Great!

Maestro232

I still have to confirm that I can run hot into the hybrid tank. Finding an answer is elusive. But assuming I can, I'll give this a go and post my results

hot_rod

If there is no check valve, back flow preventer or PRV on the incoming line, then neither heater needs an expansion tank.

If there is no way to expand back into the main line, then both tanks would need an expansion tank if they are valved off from one another.

It's a good idea to have thermal expansion tanks, the water provider could install a back flow device if you have meter pits, then you will have excessive pressure problems.

Maestro232

So I'm kind of re-thinking this and want to either cut the tankless coil out altogether, preferably in a non-destructive way so I can revert to it in the event of tank failure.

For either options I need to get a bit more detailed about exactly what I need where. I think the attached drawing is a pretty accurate representation of things with the proposed tank placement. The street water doesn't go straight to the boiler for some reason, even though they are like 5 feet away. It goes up and out to a T and back around to the boiler for some weird reason. On the bright side, you can see it makes for a nice tap in with the tank just to the right of the boiler since the cold and hot lines pass right over on their way in/out.

If I do cut (literally) the tankless line out, I don't know exactly what and where I need to cut and possibly cap. I'm also not sure where various drains would need to be. I don't see a way to drain the coil water currently. On the cold intake, maybe change the T after valve A to an elbow so I just feed the boiler water itself. Then cut right before valve B and cap under the valve...or make it a faucet drain for the hot line...or take all that copper on the hot coming out and just elbow or cap where the hot comes out of the new tank?

Trying for just isolating the tankless loop, which would be my preference if it's possible, I can't shut off existing valve A because that feeds both the boiler water and the DHW coil, and we want to run the boiler for a few months in the winter. But it has me wondering if I could just shut off existing valve B and isolate the whole loop, cutting off hot water going from the tankless to the house. It seems like there would be water trapping and pressure and I don't know if these are issues. When the boiler is off, cold water would be in the line from the cold water T to the tank all the way back to the boiler and through the coil on up to valve B. It's hard to imagine issues with cold just chilling out in all that piping, but you would know better. Again, the cold needs to be let past valve A at least to feed the boiler itself. Now imagining the boiler's running and the water sitting in that coil is heating up, with valve B closed it has nowhere to go.. I'm guessing it would expand back along the cold line toward the tank? So maybe an expansion tank between valve A and new valve X would actually kill two birds with one stone, both letting back-flow expansion from the the coil water heating up as well as back flow from the new tank (Rheem suggests putting one on the cold line for the tank).

Jamie Hall

If you take the tankless coil out of the picture -- probably not a bad idea -- you really don't have to do anything much to it except to make sure at least one end of it is open to the atmosphere. That old expansion thing again. I think I'd be inclined to disconnect the line from the T which your sketch shows going one way to the boiler and the other way to the tankless coil and cap the leg of the T which went to the tankless coil. Leave the tankless coil outlet line open to air and close valve B.

Now... is the pressure relief valve shown on the sketch the only one on the hot water system? If so, you will need to put one near the hybrid heater on the hot water line, and you will need an expansion tank there, too, anyway.

pecmsg

hot_rod said:

@hot_rod
In your diagram you show a anti scold valve on the boiler, (The boiler being 1st stage of domestic water heating) New guidelines recommend running domestic water at 140°F for at least 1hr to kill legionella bacteria so a anti scold Is required for 120°F supply. How is that going to happen in the summer. The geo heater will at best be 110 - 120°F and that's after hours of running.

hot_rod

pecmsg said:

hot_rod said:

@hot_rod
In your diagram you show a anti scold valve on the boiler, (The boiler being 1st stage of domestic water heating) New guidelines recommend running domestic water at 140°F for at least 1hr to kill legionella bacteria so a anti scold Is required for 120°F supply. How is that going to happen in the summer. The geo heater will at best be 110 - 120°F and that's after hours of running.

Good point, if that is all a HPWH can produce, 120F, then you have an ideal temperature for legionella breeding. Not ideal in todays time.

STEVEusaPA

If you change one of the shutoffs to a Webstone shut off with drain, and put it in the proper orientation, then closing that valve and open it's drain, opens the coil to the atmosphere. I'd still follow Mass. code (or original Mass. code) and put the relief valve in there.

Maestro232

Jamie,

I think I'd be inclined to disconnect the line from the T which your sketch shows going one way to the boiler and the other way to the tankless coil and cap the leg of the T which went to the tankless coil. Leave the tankless coil outlet line open to air and close valve B.

This makes sense. Thank you.

Now... is the pressure relief valve shown on the sketch the only one on the hot water system? If so, you will need to put one near the hybrid heater on the hot water line, and you will need an expansion tank there, too, anyway.

I think it is the only one, but I should note that there is a pressure relief valve installed on the condensate line of the new heater tank. Having said that, the documentation says the expansion tank should be installed on the cold water line before it goes into the heater tank (to reduce burden and failure on the relief valve on the condensate valve and tank components according to the documentation). So if I put a tank on the cold line, do I also need one on the hot line?

Jamie Hall

No -- one tank is enough, provided it is connected to a pipe connected to the water heater.

Maestro232

pecmsg,

New guidelines recommend running domestic water at 140°F for at least 1hr to kill legionella bacteria so a anti scold Is required for 120°F supply. How is that going to happen in the summer. The geo heater will at best be 110 - 120°F and that's after hours of running.

Argg...why does this all have to be so complicated! It looks like from the charts that legionella can't form at 122F, it's just not hot enough to kill what's existing. So can I just have a point of entry water filter and keep the heat tank at 125-ish?

Jamie Hall

As a general rule, filters -- except some special purpose ones -- will not remove bacteria from water. In fact some, if not maintained properly (particularly activated carbon types), may actually add bacteria. Heat, protection of supply, and disinfection (heat or chemical) are the ways to go. Public water supplies are disinfected as is, although you should still keep your hot water hot enough -- unless you use some sort of filter to remove the chlorine/chloramines which the public water people use to protect you. Then you certainly have to keep your water hot enough!

pecmsg

Maestro232 said:

pecmsg,

New guidelines recommend running domestic water at 140°F for at least 1hr to kill legionella bacteria so a anti scold Is required for 120°F supply. How is that going to happen in the summer. The geo heater will at best be 110 - 120°F and that's after hours of running.

Argg...why does this all have to be so complicated! It looks like from the charts that legionella can't form at 122F, it's just not hot enough to kill what's existing. So can I just have a point of entry water filter and keep the heat tank at 125-ish?

ASHRAE recommends 140°F for 1 hour to Kill the bacteria. Reduced to no higher then 120°F point of use. Im not sure what the high end of that heater is but 120°F is pushing the limit for the average refrigeration system, again that's while heat is available and plenty of run time.

Solution …...Geo for 1st stage of heating or as I prefer to call it warming the water from 55 °F to 100+. then a small storage water heater elec, oil, gas set at 140 - 160°F to completely kill the bacteria. A mixing valve can then reduce to 110 - 120°F for point of use.

You'll still have substantial savings over standard heaters.

Maestro232

Jamie Hall said:

As a general rule, filters -- except some special purpose ones -- will not remove bacteria from water. In fact some, if not maintained properly (particularly activated carbon types), may actually add bacteria. Heat, protection of supply, and disinfection (heat or chemical) are the ways to go. Public water supplies are disinfected as is, although you should still keep your hot water hot enough -- unless you use some sort of filter to remove the chlorine/chloramines which the public water people use to protect you. Then you certainly have to keep your water hot enough!

It sounds like you are saying I just need it hot enough to prevent growth which is 122F. 140F is to kill anything existing.

hot_rod

It really comes down to what you are comfortable with for legionella protection. There are no codes for residential protection.

It’s not so easy to get every line in the home up to kill temperature, you really would need a recirculating loop to assure all the system is contacted.

Elevating the tank give you some piece of mind, it may or not be a priority for you and yours?

Maestro232

pecmsg,

ASHRAE recommends 140°F for 1 hour to Kill the bacteria. Reduced to no higher then 120°F point of use. Im not sure what the high end of that heater is but 120°F is pushing the limit for the average refrigeration system, again that's while heat is available and plenty of run time.

Solution …...Geo for 1st stage of heating or as I prefer to call it warming the water from 55 °F to 100+. then a small storage water heater elec, oil, gas set at 140 - 160°F to completely kill the bacteria. A mixing valve can then reduce to 110 - 120°F for point of use.

You'll still have substantial savings over standard heaters.

This is a hybrid tank which uses a combination of heat pump and electric. It can heat to 140 and higher no problem, but I think if I set it to 130 or so it prevents potential growth and should be in the 120's at the faucets.

Jamie Hall

Your 130 or so should do it.

If you're on public water, and there is a chlorine residual at your house (you can ask them to check), you should not have a problem. Where problems arise is more in private water supplies -- no chlorine -- or where the residual is too low (usually towards the outer parts of a public system) -- or where someone decides that they don't like chlorine and they remove it, such as with an activated carbon filter.

Maestro232

OK, good. I am comfortable with 130-ish as a balance between legionella risk and too hot water at the tap.

I really like the suggestion to use a drain valve. Now it has me trying to figure out if I can shut out the tankless coil "non-destructively" so I can revert easily if necessary.

Let me run this one by you all... what if instead of destructively cutting and capping the T right after valve A so it only feeds the boiler water I replace it instead with a 3-way T valve and set it to only let water to the boiler cutting off the supply to the coil, with the option to feed both again with the turn of a valve. Then I swap out valve B from my original drawing with the drain valve hot_rod suggested. I close that off to take the rest of the coil out of the DHW loop but open it's drain to allow for thermal expansion as suggested. Again, with the easy option of closing the drain and opening the valve to get the coil back in the DHW loop.


Does this make sense? Can/should I do it this way?

pecmsg

> @hot_rod said:
> It really comes down to what you are comfortable with for legionella protection. There are no codes for residential protection.
>
> It’s not so easy to get every line in the home up to kill temperature, you really would need a recirculating loop to assure all the system is contacted.
>
> Elevating the tank give you some piece of mind, it may or not be a priority for you and yours?

Those are ASHRAE Recommendations

Local codes differ until the next outbreak

Maestro232

One of my friends is trying to convince me to just plumb it in series by taking the hot line out of the tankless coil into the cold intlet of the tank. In the winter boiler is down and cold will just circulate through coil into the tank. In winter the coil will do some pre-heat. What's wrong with this?

Jamie Hall

Nothing... so long as the water heater doesn't object to having hot water as an input, which is a question the heater manufacturer would have to answer.

You'd still need at least one expansion tank...

hot_rod

And as @rick in Alaska suggested, you may see some condensation in the boiler from cold water inlet against a warm (ambient) boiler block?

Maestro232

But putting the expansion tank between coil out and tank in should do it, correct? To check my water pressure so I can charge the expansion tank, is it fine to check pressure coming in from the street, or do I need to check it somehow coming out of the coil? Not sure how I'd do that.

SuperTech

You can put it on the cold water line before it enters any equipment. It can be anywhere between the backflow preventer/pressure reducing valve and the first boiler or water heater.

Jamie Hall

SuperTech said:

You can put it on the cold water line before it enters any equipment. It can be anywhere between the backflow preventer/pressure reducing valve and the first boiler or water heater.

So long as I've noted before that it is always -- except when being tested -- connected to all heating sources.

Speaking of testing. It is very helpful to have it valved in such a way that you can isolate it (temporarily!) from the system and then drain it. Then you can check -- and adjust if needed -- the air pressure through the Schrader (tire) valve on one end. It should be set at, but not below, the static pressure which it will be subject to with everything off and cold.