# Indirect vs tankless coil dhw



## joecool85 (Mar 7, 2012)

So, really, how much more efficient is an indirect hot water heater vs a tankless coil?  I understand that with an indirect you could have the boiler fully shutdown between "calls" for heat, but if it has to keep the indirect hot water tank hot, wouldn't it have to kick on just as often as keeping itself hot?  The other thing is that some people say you can't use an outdoor reset for a tankless system, but there is a Honeywell that allows for keeping the "Low" limit, then the outdoor reset just changes the "High" for when the system has a "call" from one of the zones.

So, whats the difference between say a Weil McLain w/tankless set up with a Honeywell outdoor reset, Low set to 130F to maintain dhw vs the same boiler with a tankless and the tankless is set to maintain 130F (don't want lower than that due to Legionares Disease risk, obviously use a mixing valve)?


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## maple1 (Mar 7, 2012)

I can only speak from experience with my in-boiler coil, but it is a real pig keeping it warm all summer. Next to no insulation, and with the big flue open all the time there is a constant (even if small ) flow of air thru it sucking the heat out of it. I think the Buderus indirect tanks (parents had one installed this past summer) say they only lose 1/4 degree an hour at steady state. I know my boiler loses heat way faster than that just sitting there - has to be something like 5 degrees an hour, although I've never timed how often the burner cuts in.


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## tom in maine (Mar 7, 2012)

I can tell you that an indirect setup is about 33% efficient in the summertime.
Obviously, a tankless coil is a fair bit worse.


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## JP11 (Mar 7, 2012)

I have a 80 gal buderus indirect.  Just my wife and I.  I had an hourmeter on the boiler this summer.

I can tell you that my boiler ran about 1.2 to 1.4 hours per day in summer.

Wife works at home.  We do a fair amount of dishes.

I'm sure it will be more this year, as I just turned on the hot water recirc as I am building the master bath at the far end of the house now.

JP


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## velvetfoot (Mar 8, 2012)

I have an indirect with outdoor reset, but have it set up so that dhw is on priority and has the same 190F or so as the main aquastat.
I've been told that dhw recovery would be real slow if the circulating water temp. is too low.
I haven't checked it myself so I don't know for sure-maybe I will one day.


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## SmokeEater (Mar 8, 2012)

joecool85 said:
			
		

> So, really, how much more efficient is an indirect hot water heater vs a tankless coil?  I understand that with an indirect you could have the boiler fully shutdown between "calls" for heat, but if it has to keep the indirect hot water tank hot, wouldn't it have to kick on just as often as keeping itself hot?  The other thing is that some people say you can't use an outdoor reset for a tankless system, but there is a Honeywell that allows for keeping the "Low" limit, then the outdoor reset just changes the "High" for when the system has a "call" from one of the zones.
> 
> So, whats the difference between say a Weil McLain w/tankless set up with a Honeywell outdoor reset, Low set to 130F to maintain dhw vs the same boiler with a tankless and the tankless is set to maintain 130F (don't want lower than that due to Legionares Disease risk, obviously use a mixing valve)?



Wish I had more data to send your way, but I only thought a short time on which type to put in and upon discovering that a tankless coil might give you 4 or 5 gal per minute, I opted for the indirect.  Only had it hooked up for a few weeks, but here's how.  Have a 100 gal TT Smart tank that I raise to 185 (higher if I can) degrees and then take the hot water off through a type 1170 mixing valve.  I'm heating the house with that same water put thru a plate HX.  In the summer (this is the part I don't have any data on), I think I can heat the tank to max with about 15 to 20 lbs of pellets and that should be enough hot water to DHW for about 8 days.  That would burn about 2 bags of pellets per month.  I could live with that!


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## joecool85 (Mar 8, 2012)

maple1 said:
			
		

> I can only speak from experience with my in-boiler coil, but it is a real pig keeping it warm all summer. Next to no insulation, and with the big flue open all the time there is a constant (even if small ) flow of air thru it sucking the heat out of it. I think the Buderus indirect tanks (parents had one installed this past summer) say they only lose 1/4 degree an hour at steady state. I know my boiler loses heat way faster than that just sitting there - has to be something like 5 degrees an hour, although I've never timed how often the burner cuts in.



Most indirect heaters lose 0.5 degrees/hr.  My 1987 HB Smith boiler w/tankless coil runs twice a day in the summer, once right after showers and normally sometime in the evening to bring itself back up to temp after people washing hands etc all day.  The temp must drop 20 degrees for the boiler to kick on (Set at 140 w/20 differential, it goes from 150 down to 130).  I can't count the first run because it's just running due to us taking showers in the morning.  It only runs once other than that.  So that means it is only losing 20 degrees in 24 hours.  Even call it 24 degrees in 24 hours thats only 1 degree per hour.  It holds 16 gallons of water.  Compare that to a 30 gallon indirect losing 0.5 degrees/hr and it's the same efficiency!  Compare it with larger tanks and it's MORE efficient to use the tankless coil.

I honestly thing it's only important to have an indirect if you use large quantities of really hot water.  For the two of us (now three counting my little boy), the tankless coil works just fine.

As for installation cost, it's only $500-$750 cheaper to do the tankless in most cases, but it is cheaper.  Replacement/repair cost is about the same.  Most coils of both types are good for 10 years on average and both types of coils cost $400-$600.

I think we're going to save our money and our space (small basement) and go with another tankless coil when we replace our boiler.


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## goosegunner (Mar 8, 2012)

I have slightly aggressive DHW are there coils that stand up better than copper?  I get copper leaching in my hot soft water.

I would like to buy some type of coil to but in the top of a buffer tank.

gg


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## TRUTH (Oct 26, 2014)

The most efficient and least expensive thing is to STOP heating hot water all day....  The last of our kids went off to college 8 years ago.  Just the wife and I are using hot water and heat....  Our house is 1 1/2 stories and about 1700sqft. - most sqft on the first floor.  We bought a really efficient woodstove and put it in the centrally located LR.   We have two fires a day in the winter - one in the morning and one in the early evening.  We STOPPED heating hot water all day 8 years ago   We run the oil boiler for 5 minutes in the morning and 15 minutes at night during weekdays.  On Saturdays - clothes washing day - we turn the boiler ON for 2 hours.  The boiler is a cast iron 25yo Utica Starfire with an internal coil.  This cast iron boiler (as with most cast iron boilers) will heat and then hold the heat and the hot water in the boiler for over 8 hours...  If you have a similar cast-iron boiler, try it !!  Steel boilers likely won't last as long....  Our oil consumption plummeted in the first year ( and every year since) from 1200 gal/yr to 80 to 150 gal/yr - depending on how many days we are gone during the winter months. During those periods, we just set the LO on the Triple Aquastat to 110F and the HI to 160F.  We bought a digital timer with 6 present times - and replaced the Emergency switch on the basement stairs.  That is how we turn on the oil boiler - ONLY as we need it and it automatically shuts off - unless we are on vacation.  BTW. we use about 3 cords of hardwood/yr.  The $4000 ++ that we have been saving each year helps to fund our summer vacations!


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