Replacing a water heater connected to a boilers tankless coil

JanS48

Greetings everyone - newbie here…

I have a rental property - 1000 sq ft 3 beds 1 bath, no dish washer with a washing machine. It has a 6 year old boiler with a tankless coil, the coil is feeding a 50 gal electric hot water heater that is failing. This installation was performed by a professional heating contractor.

Heating system is a single zone circulator feeding a one-pipe [black-iron] system connecting to 6 small tube cast iron radiators. Consumption wise the house is fairly economical to heat.

I'm a fairly good plumber although not up on all the latest 'tech' regarding boilers but I'm a quick study… and have a background in electronics.
My choices seem to be:
1) Bypass the electric hot water heater and run the house from the tankless coil - this is the least expensive.
2) Add an indirect water tank like an Amtrol (40 gal) or something similar. This brings up a few questions: I assume it will need it's own circulator even if connected to the boilers tankless coil - Is this even an acceptable installation method? Also I see references to 'cold start' can a tankless boiler's control be modified for that? The boiler installer told my wife that it was necessary to have the boiler maintain (which I think is a crock) - Is that something that is required by some manufactures?
3) Replace the electric hot water heater with a new one - this seems to me to be the most expensive option (for the renter).
4) Install a new electric hot water heater and not hook up the electrical connection, instead install a circulator to maintain the water in the tank from the boiler's tankless coil. I actually did this on a system a number of years ago which worked ok - it eventually got replaced by a solar system.

Regarding option 2 would it be better to supply the indirect water tank to the boilers hot water vs the water heated in the tankless coil? If disconnecting a tankless coil is it best to leave empty?

Thanks in advance to anyone that can offer advice.
Jan

HVACNUT

You don't pipe an indirect to a tankless coil on a hydronic boiler.

What you currently have is an Aqua Booster

Why would that be the most expensive for the tenant? Up front, or long term?

The tankless alone would be more expensive longterm for the tenant if they're paying for the fuel. The indirect more expensive upfront (for the LL) but more efficient and less expensive for the tenant longterm.

Cold start is fine with the proper controls to prevent condensing of flue gasses.

ethicalpaul

1) Bypass the electric hot water heater and run the house from the tankless coil - this is the least expensive.

I don't think that is anywhere near the least expensive

2) Add an indirect water tank like an Amtrol (40 gal) or something similar.

That is by far the most expensive, and complicated with lots of things to fail.

3) Replace the electric hot water heater with a new one - this seems to me to be the most expensive option (for the renter).

This is the least expensive, unless you think that running an inefficient boiler all year is free. Even with electrical resistance water heating, it's not very much. It's by far the simplest plumbing/installation, you can turn off your boiler all summer, and you can choose a heat pump water heater if it's advantageous.

STEAM DOCTOR

Just to clarify. When you talk about more or less expensive, are you talking about the installation or the cost for producing the hot water (i.e. electric, gas, oil bills)?

EBEBRATT-Ed

install another electric water heater and use it with the tankless you know have. EWH are fairly cheap. You can use the tankless for in the winter and shut the electric elements off or keep them as a back up

Larry Weingarten

Hi @JanS48, What clues is the water heater giving you that it's failing? There is a lot that can be done to keep tank-type heaters going if the tank itself isn't leaking. That maintenance would probably be your lowest up front cost.

Is the boiler room warm? If so, a heat pump water heater could make good sense. You might be able to get rebates for it as well. Check at https://www.dsireusa.org/ to see if you have incentives waiting. 🤑

Yours, Larry

JanS48

Thanks to all for the comments. When I talk of expenses you are correct two distinct categories - 1) my expenses as a plumber and LL, and 2) the cost to the renter. I'm not terribly concerned with my costs as an LL frankly.

Electric water heater: It's a GE approx 10 years old. It's currently leaking around the elements but more importantly it's emitting rusty hot water (cold water is fine - no rust). My next step will be to drain the tank, remove the elements an take a look inside with a bore-scope. It would be great to just seal the gaskets on the elements or replace them if they are rusted - but… given the age I think a new water heater is needed.

And a question is it acceptable to run a hot water heater the way this one was installed ? After the tankless coil.

Judging from the comments just replacing the EHW will be one of the easiest remedies.

Again, thanks in advance for all the info - and if there are more opinions out there lets' hear it.

Jan

EdTheHeaterMan

I have done this before for a customer on a budget.

1. first off, you have an electric water heater that is failing. You need t oget rid of it
2. You have a boiler with a tankless coil. If you are not using the tankless coil for the primary source of the hot water, then you are most likely wasting fuel by maintaining the oil fired boiler temperature at any temperature other than a "cold start" (to be explained later)
3. You need to select the most energy efficient water heater to replace what you have.

My customer had a 15+ year old boiler with an abandoned tankless coil with the boiler aquastat maintaining 160° in the boiler. The old tankless coil was partially clogged due to lack of maintenance. They had an electric water heater. and they asked me how to save on the oil bill. This is what I did:

1. Cleaned the tankless coil
2. Set the boiler aquastat to operate as a cold start This means that the boiler can get as cold as the basement without a call for heat or hot water.
3. Connect the cold water in to the tankless water heater COLD inlet.
4. Connect the tankless water heater HOT outlet to the electric water heater inlet.
5. Connect the hot water heater HOT outlet to the home's hot water system
6. Install a stainless steel circulator and check valve on a return pipe from the water heater HOT out to the tankless coil COLD in.

This way, the boiler can get cold in the summer and only operate when there is a call for hot water from the water heater's thermostat. Basically it was a indirect fired water heater, the difference is the heat exchanger coil was in the boiler, not in the water tank. This diagram from the John Wood manual on the upper left is what it looks like.

The customer told me the following year that both their oil bill was lower and the electric bill was lower. (My first time oil burner tune ups are magic)

@JanS48, If you had a working electric water heater then I would suggest this option. If you want the same efficiency, you can just get an inexpensive electric water heater, or you can opt for a stainless steel or plastic indirect tank with a longer tank life expectancy. It will cost more up front, but you will not need a new water heater for a long long time.

As far as the oil burner being less efficient, energy cost wise? If it is only 5 years old, I can't see it being any lest efficient than one you may purchase today. Just keep it maintained and properly adjusted for peak operating efficiency.

john123

@EdTheHeaterMan : it looks like your design brought the recirc loop off the HOT out but the design on line suggests a different design. Can you explain the advantages in your design.

Is the OP's fuel gas or oil?

What about a gas hot water tank instead of an electric one if you are using gas for the main boiler?

EdTheHeaterMan

https://forum.heatinghelp.com/discussion/comment/1802309#Comment_1802309

The numbers are in favor of an oil fired boiler having a tankless coil. Although there are many gas fired boilers with tankless coils. Folks that do not have access to natural gas, and where LP gas in expensive, will opt for oil heat. When the tankless coil DHW output is reduced as a result of lack of maintenance over a 20+ year life of that coil, The plumber called to "fix the hot water" problem has the tried and true fix of the electric water heater.

So my guess is Oil Heat

Mostly because Plumbers on the whole don't like to mess with "Oil Heat". Even though the tankless coil problem has nothing to do with the oil burner. The next mistake they make is that a boiler with a tankless coil does not know if the tankless coil is in use or not in use, so the control still maintains temperature to make hot water that can not possibly be used since the coil is usually disconnected. that means the fuel oil bill will not reduce that much and for the first year or two, some customers do no know to turn off the boiler service switch, and leave that boiler on all summer generating heat for no reason.

The diagram from the JW water heater instruction paperwork is designed with the circulating pump connected from the cold in on the water heater tank to the cold in on the on the coil has merits because the pump will not draw the hot water from the top of the tank to feed the coil in the boiler. But the hot out of the Coil does not feed at the cold in location, but uses the bottom drain tapping for the Hot out from the coil to the bottom of the tank. This may be a good idea or not.

When you start using hot water from the tank and the cold replacement water is going thru the tankless coil before it passes to the tank, before the burner is activated, the cold temperature water will reach the thermostat location and turn on the burner and the pump, which will mix the hot water from the tank with the cold water from the cold water service from the house. This mixing action may be a good thing to get the coil water to be hotter a little faster. with my description.

With the JW illustration the cold water from the coil will go in to the bottom of the water tank and draw from the bottom of the tank leaving that colder water at the bottom of the tank until the coil temperature is hot enough to heat that water. Then the hotter water will be at the bottom of the tank with a layer of colder water in the middle of the tank and the hot water at the top of the tank that was there before the hot water tap was opened.

In theory, that stacking of different temperatures will mix together by the time the colder stack section reaches the outlet. That may be a better idea.

But in actual practice, I used the hot out of the tank or the relief valve opening with a tee fitting, to actively cause tank circulation from bottom to top, mixing all the water to prevent stacking.

If you like we can spend a lot more time overthinking this, but I have only used this idea once in my 40+years of doing boiler work. And the only customer feedback was that it saved $$$ in operating cost on both the oil tank fill up, and the electric bill.

EDIT: If that sounds snarky, sorry, I was trying to be funny.

🤪😈😮

john123

@EdTheHeaterMan When you are talking about the "burner" I guess you mean the oil/gas burner. Except for the summer when the "burner" and redirc pump may be turned off—-how, in your opinion, in the winter, could/does the electric heating element play into all this (if at all) when the boiler and DHW coil are in operation providing the initial DHW?

Is the DHW coil always kept hot (in the winter) by way of recirc loop from the boiler? or one inside the boiler itself? Otherwise where would be the control sensor/thermostat to ——turn it on as you say "the cold water will go through the tankless coil before the burner is activated."

Can you get away without the recirc loop by just piping the water from the DHW coil directly to the "in" port of the electric HW tank? or do you need to worry about legionnaires disease in the part of the water which may be cooling down after the coil but before entering the HW tank and sitting there for days/weeks of non-use.

I see an an advantage in your design in that the JW design may have a tendency to pick up the presumably very hot water that has come from the tankless coil and recycle it right back to the tankless coil! (because the cold "in" pipe of the tank extends down quite far inside the tank) —— whereas your design picks up the supposedly "hottest" water from the tank (at the top of it) but allows the new perhaps even hotter? water from the coil to mix freely with the rest of the tank water. ????

Thanks for your interest

john123

@EdTheHeaterMan : I like it! I am guessing here a bit so please comment freely:

Electric tank with a Thermostat near the bottom of the tank and electric heating element at the bottom of tank. The HOT "out" is a short pipe inside the tank and takes the DHW from the top of the tank. The COLD "in" is a longer pipe inside the tank ending near the middle or middle bottom of the tank. When the DHW tap is opened, the new replacement water, wherever it comes from, if cold enough activates the electric element which of course is slow to heat but will helpfully stay on until the water temp meets the Thermostat setting and you have a whole tank of water (but no more than that) ready for your next shower.

In the summer,

when the Delta T is smaller than in the winter, you might get by without using the boiler—just use the electric tank—- taking the COLD water which still has been routed through the boiler HX but now moves through without being heated at all. New heat from the water at the bottom percolates up, no Legionnaires issues, all water is cold until it reaches the electric tank and then heated to 130F—Just like normal. Boiler is off, check valve in place, circulation pump off (Is it a better idea to have a circulating pump with an internal check valve—what do you think?) Does the check valve always go "after " the circ pump as in JW design or possibly before?

No that much different from the JW design. (why do you think MFG's don't just provide the COLD water "in" port at the bottom or middle bottom of the tank instead of using an internal pipe fed from the top?

In the winter,

boiler is on and providing radiator heat and along with it heat to the DHW coil and the coil heats the DHW when it passes through.

When the DHW tap is opened, the Cold water comes in, gets heated in the DHW coil which is always hot (due to a recirc pump on that loop from the boiler?). The heated water now goes to the COLD "in" on the DHW tank, feeds to the bottom through a downtube and comes out there without necessarily triggering the electric element (unless it is not hot enough) and the heat from the DHW percolates to the top.—-great—once again not too different from the JW design.

Should we put a mixing valve on the HOT "out" port and then we wouldn't have to worry about overheating the water in the Electric tank? Or is there a control to stop the boiler water heating the DHW HX. Perhaps we should add a boiler protection loop to assist with cold start on the boiler? Will the DHW coil loop actually need a circulator pump or is that automatic when you have a DHW coil in the boiler? I guess when we speak about a "DHW coil" we are referring to a coil inside the boiler instead of a coil in some sort of external tank..?

SO what triggers the recirc pump from the tank back to the Cold water in from the street? Is it always on? Does it turn off when the tap is opened? If always on, does it decrease the normal water pressure coming out of the DHW tap? Does it assist with the legionnaires problem? Can we do without the recirc loop?

So I think that the best feature of your design is that you take the water from the top of the electric tank at whatever temp and reheat it in the HX and send it back to the bottom of the tank. Super! The JW design takes the water from very close to where the heated water from the HX comes in and then basically sends IT back to the HX.

Thanks for your interest

EdTheHeaterMan

First step in my design is to disconnect the 240 VAC supply to the electric elements. Those elements are only there to serve as a plug to seal the hole in the tank. That is where the electric savings starts.

The next step is to use the bottom electric element's thermostat as the Tank thermostat to operate the DHW zone to the boiler. May not be the best thermostat on the market compared to the standard indirect thermostats, but it is the only real option for type tank.

More to follow after I read the rest.

EdTheHeaterMan

To address why tanks have "dip tubes" on the cold inlet, as opposed to a side tapping near the bottom… That seems to be an industry standard for vertical water heater tanks. By having all the water connections on the top, you can squeeze water tanks in between stuff without worrying how you are going to get to the pipe connections.

Addressing the mixing valve issue. If you don't mind the added expense of heating water in the tank 10°F hotter than you need it for showering, and you like the germ killing ability of storing you water at 130°F, then you MUST add a mixing valve to avoid the scalding hazard of hotter water reaching you and your children or elderly family members.

To better understand what I have designed here, Just think of the electric water heater as if it were a indirect water tank with a coil inside it, and there were no electric elements. In a standard indirect the boiler water gets pumped into a coil inside the tank. That hot coil transfers the boiler water heat @ say 180°F to the potable water in the tank. The difference is that the heat exchange coil is in the boiler, not the water tank. The circulator pump is in the potable water side of the DHW system, not on the closed loop boiler water system. The boiler water still heats the potable water. It is just that the boiler water never leaves the boiler because the heat exchanging coil is located in the boiler.

So the control system that you would employ on an indirect water heater is identical. Tank thermostat, operates the burner and the DHW pump. when the tank temperature reaches the thermostat set point, the burner and the circulator stops and the boiler is allowed to go cold. If no hot water us used (because no one opens a hot water tap) then the non vented water tank can stay warmer for a longer time than a vented tank you might have with a Gas or Oil fired water heater.

john123

@EdTheHeaterMan : When all else fails, read the instructions! my bad, I should have done that rather than trying to figure it out from first principles. Thanks for the patient explanation. I mean JW is calling a "booster"—-where is the "booster" in a storage tank.

However,—is there not always a however?, I would really like to explore the idea of using an electric tank as a water heater along with the tankless coil. The OP has the electrics in place too. If you could turn off the boiler for the summer or when the boiler was turned off and just use the electric tank, I think that would be a good thing to do. Do you think that would reduce the life expectancy a lot by using it in that way— 1/2 a year— as opposed to just using it 100% as a storage tank?

The electric tank is not vented, as you pointed out—and if you were really worried about heat loss from the tank, you could cover it with one of those specially made fiberglass "blankets" etc.—or two of them! If you are using the tank twice a day for some reason morning and night, for showers in the winter, you would mostly be heating the water with the tankless coil during that period. And if you could save even 20% on the maintenance and repairs on the boiler over 10 years by not using it for 4, 5 or 6 months in the year—perhaps it might be a wash in terms of cost savings.

I think the piping is the same—the problem I worry about, is the legionnaires in the water after the coil cooling slowly in the pipe leading to the tank and perhaps no DHW use for a few days or weeks.

Hmmm

EdTheHeaterMan

Actually @john123, in my area of the world that I used to work in, an electric water heater would cost a minimum of 3 times more to make hot water than a gas or oil fired water heater. And the recovery rate of the electric elements in a 40 gallon tank were as long as 4 hours in comparison to the 30 minutes for gas and the 18 minutes for oil. Electric for the summer did not make sense.

The oil or gas heater that was high efficiency, large enough top heat a home, often times made the recovery time of a lot shorter, compare to the 30,000 to 40,000 BTU burners often found in gas automatic water heaters. And more often, those big burners operated more efficiently than those water heater burners.

That is why Indirect water heater systems always outweigh the tanks with burners that are vented. The down time heat loss out the vent pipe does not happen with an indirect or electric tank because there is not connection to a vent. The appliance that is vented is allowed to cool to room temperature for as long as 12 to 20 hours, if there is no demand for hot water (like taking an overnight trip or a weekend getaway). The is where you get the greatest efficiency bump. When you operate any water heater and use the heated water, there is going to be a burner operating to recover the lost hot water. And if your oil burner or your gas boiler is operating at say 80 to 85% efficiency for oil and even higher for condensing gas boilers, you are using a more efficient flame source than say an atmospheric 40,000 BTU burner at the bottom of a vented tank that may top out no higher than 78% efficiency. (the reason water heaters are not rated in AFUE is because they are really inefficient and the industry does not want you to know those numbers)

I have actually sold an indirect water heater to a customer that just had an electric water heater installed a year and a half before, by showing that the cost of operating that electric water heater over the next ten years would cost him more than 3 times the price of installing an indirect on his new oil fired boiler plus the additional $50.00 of fuel oil per year for that same time period. I also explained that if he put that 1.5 year old electric water heater on Craig's List he would get more that he would if the water heater was 5 years old. Which he did and that added a few extra dollars the the savings.

Lots of folks thing that operating a big oil or gas fired boiler in the middle of the summer is going to cause then to have a gas or oil bill that looks like a February bill. That is not so. Just ask anyone that has a tankless coil. They have about a $50.00 to $80 per summer (4 months) fuel usage on average.

In summary, your idea of turning off the BIG BAD Boiler in the summer in favor of the itsy bitsy water heater, may not be a great idea. It all has to do with the cost of different utilities. Near Niagara Falls in Canada and New York, I understand that there are lots of homes heated with electricity. I think that has a lot to do with the fuel cost of operating those generators with free water flow.

You can do the math or, you can test this in the real world with real operating methods by opening and closing valves and turning on or off the electricity.

Or you can take it from someone that has already figured it out. (That would be me). But you don't learn unless you ask the right questions, or make the right mistakes.

Yours truly, Mr. Ed

hot_rod

this free calculator will show you the cost differences between energy sources.

https://coalpail.com/fuel-comparison-calculator-home-heating

You would probably need to be 8 cents or below for electricity to make it the least expensive.

Or consider a heat pump water heater

Screenshot 2024-05-24 at 7.16.50 PM.png

JanS48

Great info… The boiler is oil fired - seems to be a couple of options:

1. Replace the failing electric water header leaving the piping as-is - that is tankless feeding the DHW with the electric elements still hooked up.
2. Add a SS circ as described and use the lower thermostat as a trigger to the circ. With this option the 240 v connection to the water heater would not be used. All hot water heading would be via the boiler. Costs: electricity in Middletown RI is a whopping .23 per KWH. Oil is averaging 3.20 per gal. And a question regarding the heating of the boiler - is it being assumed that the boiler temp and the tankless temp just be the low-limit of the boiler? Otherwise how would it trigger to use the high side ?
3. Some years ago I installed a DHW system with these components in play - it worked ok but was eventually replaced by a solar system.
4. Thoughts on cost of electricity vs oil for a 100% oil fired solution?