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

Hello All!
I have an oil-fired boiler (Dunkirk Empire II) which provides hydronic baseboard heating and domestic hot water with a tankless coil. After a couple years in our house we had mini-splits installed in our bedrooms (thus heating one side of the house) and use a wood stove for the other side, so we are not using the oil to heat the house (except on rare occasions for an hour on a lazy morning), and are therefore wasting quite a bit of fuel to have hot water. We have also since gotten solar panels installed, thus making electricity a good value for us. I ended up purchasing a 50 gallon Rheem hybrid-electric hot water heater, and now I'm trying to figure out the best way to actually connect this in to my existing system. In my research I've seen the following (and related questions/concerns):
>> Shut down and remove the boiler and oil tank. OK, but I'm concerned about resale with no hydronic system at all. We love heating our kitchen/living room with the wood stove, but others may not.
>> Cut the tankless coil system out and shut the boiler down when not in use. (I am told that boilers with cast iron plates are prone to leaking when they go cold).
>> Cut out the tankless and get your boiler running on minimum year round so it doesn't break and you can have working hydronic heat on the rare occassions you use it. (OK...maybe the best. what should I set my Aquastat to for HI/LOW/DIFF?)
>> Plumb the DHW line coming hot out of the boiler into the cold of the new tank so you are at least getting some value out of that oil. This sounds cool, but I couldn't figure out if there would be plumbing/pressure/seal issues with this.
>> Plumb the tank in parallel with the tankless system and use shut off valves to provide an "either/or" solution. Even if you keep the boiler at the lowest possible idle and never use the tankless, at least you can switch over if the tank fails or you just need a fall back for any reason. This sounds ok to me too, though I couldn't tell if there were going to be some plumbing issues with having the water in the coils sitting there heating up from the boiler but not going anywhere.

So I guess I'm looking for professional advice on how to approach this. I don't think I'll be attempting the plumbing on my own, but I'd like to be armed with the best solution. Thanks!
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Comments

  • Jamie Hall
    Jamie Hall Member Posts: 15,741
    Can't really tell you what to do -- that has to be your decision. But... a couple of thoughts.

    First, unless the boiler has been sadly mistreated, it won't suddenly start to leak if you shut it down and leave it shut off. Many boilers don't run over the summer at all, for instance, and they're fine.

    Second, your comment on resale value is completely valid. Unless there is some form of more or less automatic heating system, you'd get very few buyers -- and they'll get even fewer mortgage acceptances. You might get lucky... There might also be a certificate of occupancy problem, depending on local codes.

    I think what I might do is disconnect the tankless coil (leave at least one end open) completely, and depend on the hybrid unit -- if I'd already bought the hybrid unit. Keeping in mind two related characteristics of hybrids: if they are running solely on their heat pumps, recovery is painfully slow. If they engage the resistance elements, they aren't cheap to run. Depends a lot on how you use hot water.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 8,851
    Pipe the dhw coming from the boiler tankless to the cold on the hybrid. Disconnect the boiler aqustat so it doesn't maintain temp. Pipe a cold water bypass in so you ca bypass the tankless coil if you want to. Make sure the tankless coil has a pressure relief valve on it if you isolate it with water in it
  • Maestro232
    Maestro232 Member Posts: 69
    Thank you both for the replies.

    Jaimie Hall, I must confess that I really want to be able to shut the boiler off for ~9 months out of the year, so I was happy to hear that it might be possible, but I really do see conflicting info out on the interwebs in regards to some of the cast iron sectioned/push-nipple type boilers being prone to leaks when they go cold. Perhaps some others with experience could also weigh in and verify Jaimie's comments in that regard? It's a Dunkirk Empire ii. Thank you also for your other thoughts. You confirmed what I was thinking about resale. I can probably take the idea of removing the oil system off the table. In regard to the hybrid tank itself, I am generating cheap electricity with the solar, so I think this will work out in my favor, and I normally have to run a dehumidifier in the basement, so there will be some offset there with the heat pump taking over that job. I guess we'll see.

    EBEBRATT-Ed, I also really love the idea of piping the coil into the tank, but a few clarifying questions if I may:
    >> In the scenario where the boiler is running to heat the house (sure it's rare), are there concerns about HOT water going into the tank. It seems like such a great idea, but I just am not sure if there will be weird issues from temperature differentials, etc...
    >> The tank installation guide suggests a compression tank when there are check valves at the street source (which there aren't in my case from what I've been told), but the reason has to do with, again, wearing out certain parts on the tank that have to engage to deal with backward hot water expansion. Would connecting to the coil possibly be effecting this further?
    >> You mention disconnecting the aquastat so the boiler doesn't maintain temp. What about when the boiler is on at a low idle for a couple months in the dead of winter? The boiler won't have any controls then (like a high shutoff), right? Wouldn't that be dangerous? It seems like Jaimie's suggestion, if it really is ok to do, to just turn on the boiler for a couple months out of the year in dead winter and maybe turning the low/diff way down would be better?

    Sigh...I wish I understood all of this better. I'd love to see some other confirmations of some of these comments. Thanks again for responding! You guys are awesome.
  • pecmsg
    pecmsg Member Posts: 1,963
    edited April 2020
    I would do the opposite. Hybrid up stream of the Boiler. Let the hybrid do whatever it can, say temper the incoming water if It hits temp fine, now if its not the boiler will pick up the slack.

    The idea of the hybrid is fine but the recovery time is the biggest drawback.

    OP where are you planning on putting the condensing unit?

    Also with heat pumps a backup source of heat is needed.
  • Maestro232
    Maestro232 Member Posts: 69
    pecmsg, I didn't quite understand your question about where I was putting the condensing unit. The hybrid tank is for domestic hot water only. Am I missing something? One question about plumbing the tank upstream of the tankless coil is whether I am doing more harm than good. If the boiler is off for 9 months out of the year, that hot water from the tank is going through the coil in boiler which is now full of cold water, so I'll be losing all of that heat before the DHW gets to my house, right?
  • STEVEusaPA
    STEVEusaPA Member Posts: 4,925
    edited April 2020
    I'd just go simple. Make the boiler cold start, use your hybrid water heater for domestic hot water. Disconnect (proper/code way) to disconnect your coil.
    Actually, without running any numbers, I'd just put an indirect on the oil-fired boiler, and forget the hybrid water heater. Indirect is much simpler.
    steve
    SuperTechHVACNUTZman
  • Jamie Hall
    Jamie Hall Member Posts: 15,741
    A further comment on boiler leaks: if a boiler (or radiator) with push nipples were going to leak when it went cold -- no matter for how long (time is not a factor here) -- it would do so every time it went cold. They don't. I really don't think that's a problem.

    On the cheap electricity -- well, perhaps. Unless you have a very big array, though, when the hybrid is running you will be dependent on the grid anyway. Also, unless you have a good big storage battery bank, if you take a shower in the evening... in terms of cost -- either running or capital -- it doesn't pencil out. In terms of nice and green, maybe.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Maestro232
    Maestro232 Member Posts: 69
    STEVE,
    I do like your thinking, but when you say make it a cold start, what does that entail besides just literally turning the boiler off and on? Also, to both you and Jamie, why do so many say you can't cold start these kinds of boilers?

    Jamie,
    I am not using batteries. I am feeding the grid for credits for any electricity generated and not used, so the grid is my storage essentially.
  • Jamie Hall
    Jamie Hall Member Posts: 15,741
    Perhaps the best reason I can think of for avoiding cold starting -- and it's not a very good one, in my view -- is to reduce the amount of condensation and related corrosion on the fire side of the boiler.

    A less important reason is that it does take fuel to warm the boiler from a cold start to where it makes useful heat -- and that results in a complex tradeoff between energy used to maintain the boiler warm enough to avoid condensation (around 100 to 140 or so) vs. how much fuel it takes to warm it from 50 or 60 to that temperature. The tradeoff involves just how often does the boiler sit idle, and for how long.

    It's actually a very similar tradeoff to the question of shutting off an automobile engine at a stoplight vs. letting it idle, although the variables are a little different. Which, as you know, the automotive folks have resolved -- resoundingly -- to shutting off the engine.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • HVACNUT
    HVACNUT Member Posts: 3,908
    The concern about the boiler leaking while cold is justified with Dunkirk. Cast iron push nipples and quantity vs quality with them IMO. I don't see them too often anymore in the field. It seems like a DIY Home Depot boiler. It might or might not leak. I'd be more concerned with gaskets and seals. Circulator, coil, drain valves, relief valve, etc. It seems there's no rhyme or reason. Some do, some don't.

    I've never seen a hybrid water heater in action so I don't have any personal advice, but have heard similar things mentioned by @Jamie Hall .

    Also location of the water heater. I hear they throw off a lot of heat to the surrounding area, so that might or might not be an issue.

    Have you thought about eliminating the coil and installing an indirect water heater in lieu of the hybrid? Very efficient. And properly sized and installed, will supply all the hot water you need. Depending on the age of the boiler, the supplied aquastat might convert to cold start. If not, it can be easily replaced.

    Where are you located? Is there the potential of frozen heat pipes if the boiler isn't being used?
  • Maestro232
    Maestro232 Member Posts: 69
    Jamie,
    Assuming for a moment that cold start will not damage the boiler, We are not talking about cold starting it year round, but keeping it off altogether for most of the year. So I would be bringing it up from 50-60 to 100-120 just once a year and let it idle there for the possibility of heat only in the dead of winter before turning it off again. So maybe that works in terms of "idling at a stop light" for those couple of months. I am curious how low I can push the LOW temp when it is on, and I would still love to have others weigh in on cold starting and possible damage. I don't care if I shave one or two years off the life of the boiler doing this, but If I am going to create a leak in two years that's a different story.
  • SuperTech
    SuperTech Member Posts: 1,634
    I agree with Steve. Indirect tank and cold start. Condensation issues can be avoided with the use of a properly setup Aquastat. Boiler piping can altered for condensate protection as well.
    STEVEusaPA
  • Maestro232
    Maestro232 Member Posts: 69
    OH boy, a million opinions. haha. HVACNUT, just a point about the hybrid heater: It does not put out a bunch of heat. It does the opposite, taking heat out of the air and putting it into the water, providing both cooling and dehumidifying. The electric is the backup when demand is higher than the heat pump can keep up with. Here's the little teaser doc if interested: https://s3.amazonaws.com/WebPartners/ProductGroupDocuments/F920BE95-1A4E-4AB9-B625-E9617C883E84.pdf
  • Maestro232
    Maestro232 Member Posts: 69
    SuperTech,
    Just an honest question...doesn't an indirect rely on heating the water in the tank via circulating the hot water in the boiler through piping in the tank? If that's the case, cold start seems like it doesn't make sense. If the boiler water is cold, how would it heat the tank water? I'm probably missing something.
  • SuperTech
    SuperTech Member Posts: 1,634
    > @Maestro232 said:
    > SuperTech,
    > Just an honest question...doesn't an indirect rely on heating the water in the tank via circulating the hot water in the boiler through piping in the tank? If that's the case, cold start seems like it doesn't make sense. If the boiler water is cold, how would it heat the tank water? I'm probably missing something.

    The boiler water heats up a coil inside of the tank, a water to water heat exchanger. The tank maintains temperature, typically 120-140 degrees. The boiler goes cold unless a call for space heating or hot water begins. The burner might only fire a few times a day when the boiler is not used for space heating. Much less than a boiler with a tankless coil keeping the boiler 160 degrees 24/7 just so you can have enough hot water to wash your hands before the burner fires again.
  • pecmsg
    pecmsg Member Posts: 1,963

    pecmsg, I didn't quite understand your question about where I was putting the condensing unit.

    A hybrid heater has 2 parts, the actual water heater and a condensing unit that captures the heat from another source (in your case air) and transfer it to the water. If seen these done in a basement with limited results. Another option is an Outside condenser but like a heat pump they slow down as the outdoor temperatures drop.

    they work fine 1/2 - 3/4 of the year but loose capacity as it gets cold. That's why I say use the Hybrid as 1st stage the boiler as 2nd stage.
  • Jamie Hall
    Jamie Hall Member Posts: 15,741
    I think the Rheem heater is all of one piece. Goes in the basement -- the compressor and evaporator coil is on top.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Maestro232
    Maestro232 Member Posts: 69
    Jamie, yes, you are correct. And just a reminder that it is a hybrid, so it uses heat pump as it can and supplements with electric coils if it must.
  • Jamie Hall
    Jamie Hall Member Posts: 15,741

    Jamie, yes, you are correct. And just a reminder that it is a hybrid, so it uses heat pump as it can and supplements with electric coils if it must.

    That is the beauty of them -- they do have the resistance heat if needed to make the recovery if the heat pump isn't adequate. But there's no free lunch -- those resistance heat units use every bit as much power as a regular electric water heater, so you want to be careful about how you use hot water to avoid turning them on!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • pecmsg
    pecmsg Member Posts: 1,963
    So what are you going to do as this hybrid cools the basement?
  • Maestro232
    Maestro232 Member Posts: 69
    pecmsg,
    It's already cold down there. So it will be colder.
  • pecmsg
    pecmsg Member Posts: 1,963

    pecmsg,
    It's already cold down there. So it will be colder.

    Basements are generally 55°F but to answer your question, Yes.....The hybrid WILL absorb heat from the ambient and place it in the water tank. That in turn will lower the ambient air. How much depends on several factors but that's why I asked...…………. inside or out side. During the Summer, Late spring and early fall not an issue. The other 1/3 - 1/4 times of the year YES It Can be an issue!
  • Maestro232
    Maestro232 Member Posts: 69
    The boiler room is warm, so I can either intake from there or install it in there. I think the big outstanding questions that I see a couple conflicting answers to is:
    >> Can I turn off the Dunkirk for 9 months out of the year without destroying it.
    >> If not, can I plumb the tank inlet from the coil coming out of the boiler.
  • Jamie Hall
    Jamie Hall Member Posts: 15,741
    In answer to your first question -- it doesn't matter. 9 months or 9 weeks. If taking it out of service is going to make it leak, it's going to leak when you take it out of service. It's not wine...

    But I doubt that it will leak. Could be wrong.

    Whether you can plumb the tank inlet from the coil out of the boiler depends on how the Rheem's control strategy works. It may -- or may not -- like being fed hot water. A standard water heater doesn't object, but you'd have to ask your Rheem rep. about that one.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • rick in Alaska
    rick in Alaska Member Posts: 1,178
    I shut down a lot of boilers during the winter as we have a lot of snowbirds here who are gone all winter, and I never have had a boiler leak. Now gate valves and dielectric unions, they usually do.
    I would be concerned about feeding your hybrid from the domestic coil on the boiler if you are planning on shutting it down, as I would not want that cold water flowing in and out of the coil, and possibly causing some kind of condensation in the boiler. Most likely it won't , but it seems like if you ran it long enough it could.
    Rick
  • Maestro232
    Maestro232 Member Posts: 69
    Thank you Rick. This might help clarify my options a bit...
    1. Shut down boiler during warm/moderate season, and just use the tank for DHW.
    2. Plumb the coil into the tank but then leave boiler on year round at lowest idle possible.
  • Maestro232
    Maestro232 Member Posts: 69
    Jamie,
    Very helpful! It looks like I should test whether a leek will occur as as starting point.
  • STEVEusaPA
    STEVEusaPA Member Posts: 4,925

    STEVE,
    I do like your thinking, but when you say make it a cold start, what does that entail besides just literally turning the boiler off and on? Also, to both you and Jamie, why do so many say you can't cold start these kinds of boilers?...

    You change or re-program the aquastat to not maintain temperature.
    As far as condensing, you can pipe the boiler with a bypass and 3-way mixing valve to protect it.
    It reality, you are making a warm start boiler, as even with no one using domestic hot water, and the indirect and piping all well insulated, your boiler is probably going to fire 2x a day for 10 minutes. With the building occupied, more, so the boiler will rarely get down below 100° and with the bypass, will heat up quickly and out of condensing range when it runs.
    Another, additional strategy is to use an aquastat with a 'circulator hold off' feature, which will turn of the boiler loop circulator when the temperature drops.

    The only problem with cold start is when there is no protection for the boiler with low return water temperatures. Then the heat exchanger will foul rather quickly and without annual proper maintenance, your boiler won't last long.
    There are hundreds of cold start oil fired boilers in my customer base working as intended.
    steve
  • hot_rod
    hot_rod Member Posts: 14,753
    You could valve it so it has 3 options. HPWH by itself, cold feed in hot water out.

    Valve it off in winter and run just from the boiler.

    Of valve it to feed from the tankless, although I don't see any advantage and as @STEVEusaPA mentioned, you could have some condensation issue with the cold boiler and cold supply water?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Maestro232
    Maestro232 Member Posts: 69
    hot_rod,
    That could work. If I can kill the boiler for 9 months without a leak I could definitely just close the HPWH off for the few months I run the boiler and revert to the tankless for hot water.
  • hot_rod
    hot_rod Member Posts: 14,753
    In the heating season let the cold run thru the tankless coil to the HP. Keep the HP plugged in so you always have hot, as you pull down the tankless runs into the HP. So really the HP would just run in winter to maintain that tank, cover jacket loss

    Summer months, valve off the tankless run in and out of HP.

    You could try running thru the tankless in the summer also , watch for condensation in the boiler sections. Water would maybe get pre-heated to ambient room temperature before the HP?

    costs you 4 valves.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Maestro232
    Maestro232 Member Posts: 69
    hot_rod, this drawing is a gold mine for me. Thank you! One additional concern I'm curious if you have specific knowledge of for these hybrid tanks...It is recommended to install an expansion tank pre-intake if there are check valves from the street. My understanding is that there aren't, but plumbing this way makes me wonder if this will be able to expand properly. Thoughts?
  • Jamie Hall
    Jamie Hall Member Posts: 15,741
    If there is any doubt in your mind at all about whether or not there are check valves... put in an expansion tank. They aren't expensive. They're easy to pipe in. Just do it.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    SuperTech
  • Maestro232
    Maestro232 Member Posts: 69
    Better safe than sorry, huh? I notice there is one for the hydronic part of the system.
  • Jamie Hall
    Jamie Hall Member Posts: 15,741

    Better safe than sorry, huh? I notice there is one for the hydronic part of the system.

    Something like that. This is a different one, of course -- but the reason is the same: when you heat water, the stuff expands, and you really don't want it to find a weak spot if it can't...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Maestro232
    Maestro232 Member Posts: 69
    hot_rod (or other),
    How would I integrate an expansion tank into hot_rod's 4-valve plumbing drawing? Would I need two expansion tanks?
  • Jamie Hall
    Jamie Hall Member Posts: 15,741
    My understanding is that the hybrid heater is NOT a tankless design. Nor, technically, are the coils in the boiler.

    Ok. Having said that, the reason one adds an expansion tank to a boiler or a hot water tank is that when water is heated it tries to expand. If it is in a confined volume, such as a hydronic heating system, this expansion creates high pressures -- sometimes extraordinarily high pressures (hundreds of psi). So an expansion tank is a an obvious requirement.

    But what about hot water coils in a boiler for domestic hot water? Or a water heater? Then the answer is both obvious -- and not obvious. The question which must be asked first is simple, however: where are valves (check or otherwise)? Then it becomes fairly simple: if there is a heat source -- such as the coils, or the water heater, and there are valves which do (e.g. check valves preventing back flow to an open source) or could (such as isolating valves) prevent the water in the heat source from expanding, then there must be an expansion tank directly connected to the piping connected to the heat source. The tank might have an isolating valve for maintenance, but used only when the heat sources is disabled.

    Therefore in @hot_rod 's sketch, there must be an expansion tank ("tank A")for the hot water coils connected somewhere between valves 1 and 2 and 4 -- it doesn't matter which side of the coils it is in. There must also be an expansion tank (tank B") for the water heater somewhere downstream (that is on the hot water heater side) of valves 3 and 4.

    Tank A is required to protect the boiler piping if valves 1, 2 and 4 are closed at the same time ("Oh that will never happen" is one of the most lethal last words around). Tank B is required to protect the water heater and the house domestic hot water piping in the event that valves 3 and 4 are closed, or if there is any sort of check valve or device (such as a pressure reducing valve) which acts as a check valve anywhere on the cold water feed to the house.

    To summarize: an expansion tank is required if there is a heat source and if there is, or could be, nowhere for the water to freely expand.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 14,753
    would this work? One tank for both DHW sources?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Jamie Hall
    Jamie Hall Member Posts: 15,741
    No, no, no. That is a cross connection which completely defeats valve 4. Please...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Maestro232
    Maestro232 Member Posts: 69
    Jamie and/or hot_rod,
    Given that valves 1 and 2 already exist with the existing tankless system with no expansion tank, it is able to expand to the street. So I'm actually wondering if I just need one tank where indicated to cover all the possibilities
    . Am I missing something?
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