Why is an indirect advantageous?

RascalOrnery

I'm struggling to understand how an indirect system has 'efficiency' advantage over a tankless coil. Maybe I have a poor understanding of how the two interface: Here's my understanding, please tell me what I'm missing or misunderstanding.

Indirect is a treated as a 'zone' to the boiler.
When indirect drops below setpoint (like 120 for example) circulation begins between boiler and indirect until temperature is restored.
This means the boiler must be held at a higher temper, like a traditional 160.
This also means that devices with heat losing potentials are two, unlike a tankless coil system in which there is ONLY the boiler.
This also means that the boiler has to hold it's low temperature setting, just like a tankless coil system.
There will also be some heat loss between piping (although potentially very little)
Now there are two systems which have a lifespan < forever

To me it seems like the indirect is just an extension of the boiler, basically increasing the volume of the boiler only....aiming for longer length and longer in between combustion calls.

So maybe I'm misunderstanding the whole point of an indirect? Do they only match up with cold start or condensing boilers as efficient and other reasons would be water quality or something? Is efficiency NOT the main reason for having them?

mattmia2

The boiler needs to be kept hot 24/7 for the tankless coil, the controls are set up as what is called "warm start". The coil has no control of the boiler, it just heats water by being in the hot boiler water.

With the indirect the boiler is set up cold start, it cools to ambient between calls from the indirect so there are only losses from the boiler when the indirect is calling for heat. This is called "cold start". Indirects can be well insulated so they have little standby loss. You would want to size the indirect so it mostly only calls during large demands so you don't have the loss of the boiler heating and cooling for small loads like handwashing and most kitchen tasks.

jumper

Direct fired supposedly burns out sooner. Sellers sold a direct fired horizontal tube water heater that is supposed to last longer. Anybody see a forty year old one still chugging? Anybody see a forty year old steam fired Aerco water heater still operating? Those were supposed to last indefinitely as well.

EdTheHeaterMan

The indirect is not connected to the flue. so there is no loss up the chimney from the indirect. A tankless coil in a boiler has heat loss up the chimney 24/7.

Since it has been observed that DHW causes a burner to operate for only 2 hours per 24 hour period (on average) and that many of those indirect tanks are so well insulated that if unused for a 24 hour period, and the reheating of that tank from the boiler may only take about 7 to 10 minutes to increase the temperature of the tank from the low setting of the thermostat to the differential of the thermostat (usually about 10°F) in any given 18 to 10 hour period of non use, then there is much less burner operation over and above what DHW is actually used.

On the same DHW non use in a 24 hour period, the tankless coil in the boiler may loose heat up the chimney at a rate that will cause the burner to operate ever 3 to 4 hours, for 7 to 10 minutes even if there is no DHW usage. That means the burner will operate as much as 500% more for down time loss that that of a burner that is operating to charge an indirect.

YOU DO NOT NEED TO MAINTAIN THE BOILER TEMPERATURE AT ANYTHING ABOVE ROOM TEMPERATURE WHEN YOU HAVE THE THERMOSTAT IN A ROOM SET TO 68° IN YOUR HOME.

YOU DO NOT NEED TO MAINTAIN THE BOILER TEMPERATURE AT ANYTHING ABOVE ROOM TEMPERATURE WHEN YOU HAVE THE indirect thermostat set to 120° in the tank.

Because, as you said:

Indirect is a treated as a 'zone' to the boiler. TRUE
When indirect drops below setpoint (like 120 for example) circulation begins between boiler and indirect until temperature is restored. TRUE
This means the boiler must be held at a higher temper, like a traditional 160. FALSE
This also means that devices with heat losing potentials are two, unlike a tankless coil system in which there is ONLY the boiler. mostly FALSE
This also means that the boiler has to hold it's low temperature setting, just like a tankless coil system. FALSE
There will also be some heat loss between piping (although potentially very little). The VERY LITTLE is important here

The thermostat will call for heat in a room or in the tank and the burner will operate and the circulator will operate. Once the room or tank is satisfied, the burner will go off and the circulator will stop. Just like a zone in your home. Also many indirect tanks have lifetime warranties. Since there is no fire touching the metal parts of the tank, they last much longer.

RascalOrnery

Ok, so how is that better than a designated hot water heater? (if it's insulation than we're talking about a design issue, not a layout issue?)

So does this mean that indirects are only designed to work with non ferrous boilers and ones that do not use fuel oil? (like NG or propane or electric) ? Oxygen dissociation (spelling?) and flue gas condensation would seem to be increased by letting a fuel oil boiler 'return to ambient temperature'.

So what temperatures are indirects held at? (not output, storage temp) 140? That means if it's a 30 gallon tank and I'm pulling 120 to my fixtures I only have a 20' difference, in the case of a tankless coil where the boiler is held at say 160 then I have a 40 degree difference which means in the case of the same 30 gallons the boiler is going to have a longer wait, do indirects have a much larger resevoir? I don't understand how this is advantageous, when that indirect calls there's a whole lot more water at <140 -diff than in the case of a tankless coil.

Also, do gas boilers even need chimneys? don't they just have 'vents' since they're so clean burning?

I'm not saying I don't believe you, it just seems like the gain kinda small compared to just have ONE unit to do it all (whether a boiler or designated hot water heater)

mattmia2

https://www.heatinghelp.com/store/detail/modern-hydronic-heating-for-residential-and-light-commercial-buildings

Mad Dog_2

Simply put..an Indirect water heater is a very well insulated Thermos.  Mad Dog 🐕 

Jamie Hall

I'll try to go point by point here, @RascalOrnery

Are indirects better than a well designed direct fired water heater? Not necessarily, although due to the generally shorter gas path in the water heater they are usually slightly more efficient overall.

So does this mean that indirects are only designed to work with non ferrous boilers... Indirects will work with any boiler, with any fuel source (oil, gas, LP, electric, whatever). The fuel source is irrelevant, and the boiler design is also -- all they need is a source of hot water.

So what temperatures are indirects held at? For safety, any hot water heater or storage vessel should be kept at about 140 F. Higher is acceptable, although by code most can't do that. Indirects have a reservoir of hot water, like stand alone hot water heaters, so it is not necessary to fire the boiler for every hot water draw. Tankless hot water heaters and tankless coils have no reservoir. Both of those are flow limited -- that is, over the design flow they cannot provide hot water, whereas an indirect or direct fired water heater are volume limited. This is a design consideration.

Also, do gas boilers even need chimneys? don't they just have 'vents' since they're so clean burning? Gas boilers need either chimneys or powered vents. The notion that they are more clean burning is, to a great extent, a fiction. A properly adjusted oil fired boiler has much the same list of combustion products as a gas fired boiler, although often the gas fired boiler exhaust, if allowed to condense, is considerably more acidic and corrosive.

As to your last point, there is no one "right" solution for every application. In some applications a tankless coild works fine. In others, a tankless water heater. Sometimes a combi works well. In some an indirect is just the thing. In others, a direct fired water heater will be the best.

Hot_water_fan

Ok, so how is that better than a designated hot water heater? (if it's insulation than we're talking about a design issue, not a layout issue?)

Not much benefit to an indirect vs an equivalent dedicated tank. Most all Americans don’t have an indirect, but if you have a boiler already, they can be fine. 

RascalOrnery

Thanks everyone for your patience and sharing of info!

mattmia2 said:

https://www.heatinghelp.com/store/detail/modern-hydronic-heating-for-residential-and-light-commercial-buildings

I have this book, although it is a slightly older edition ($100+ for a leisure reading book was a little much). It is a very interesting read!

Jamie Hall said:

I'll try to go point by point here, @RascalOrnery


So does this mean that indirects are only designed to work with non ferrous boilers... Indirects will work with any boiler, with any fuel source (oil, gas, LP, electric, whatever). The fuel source is irrelevant, and the boiler design is also -- all they need is a source of hot water.



***So is my understanding of a boiler returning to ambient temperature wrong? I've heard horror stories of rotted out boilers because people set the low TOO low, isn't this the same thing but worse?***

So what temperatures are indirects held at? Tankless hot water heaters and tankless coils have no reservoir.



***How do you mean? On ergomax's website the starting unit has 28 gallons, this isn't much of a reservoir compared against some older oil boilers is it? Maybe my whole error is that I improperly assume newer boilers are in the 20+ gallons range. I was just peeking on supplyhouse and from what I could tell most new boilers are well under that...in that case, I fully understand how just a quick shower would force the boiler to short cycle!! I have a boiler in my basement that's 55 gallons!***

Jamie Hall

Two slight -- but quite understandable -- misunderstandings there. First, on boilers returning to ambient -- also usually referred to as "cold start" operation. This is actually the normal way boilers are operated -- unless they have a tankless coil in them for domestic hot water. It does very little harm -- while the boiler is operating, all the metal and certainly all the flue gas is above the dew point of the flue gas, and so there is no condensation on the fire side of the boiler -- and hence very little corrosion. The problem arises when a boiler is operated so that it is continuously, or nearly so, with portions of it or the flue gas below the dew point. Then the water in the flue gas does condense on parts of the boiler with dire results -- the stuff is very corrosive -- unless the boiler is specifically built to handle the condensate (which is the "con" half of the "mod/con" boiler terminology).

Now the other gets a little confusing. A tankless coil in a boiler is just that -- a coil of pipe submerged in the water in the boiler. There is no reserve in that coil, but the boiler water on the other side of the coil has to be maintained warm enough to heat the water flowing through that coil when there is a demand for hot water. This is a much older technology, and was fine in the day, but... A tankless water heater, on the other hand, also has no reservoir -- but has a very powerful burner (or electric heating element) and a very efficient heat exchanger, so that when there is a demand for hot water the burner fires and the water flowing through the heat exchanger is heated to the desired temperature very quickly ("instantly"... well... almost anyway). These can be excellent, provided two things: first, that the maximum hot water flow is respected and second, that you have a big enough gas or electric service to manage them. To give you an idea on the latter point, 2 gallons per minute -- a nice shower -- takes over 70,000 BTUh in a tankless -- as much as the average house takes to heat.

On the short cycle point -- indeed. a boiler with a relatively low volume combined with a tankless coil is indeed going to short cycle. One very good reason for an indirect...

EdTheHeaterMan

Well Done @Jamie Hall with the explanation of different ways to heat water and why the tankless coil in a 160° boiler will provide 120° water at the tap.

I also remember living in a home with a 30 gallon gas automatic water heater when I first got married. This was a pretty standard water heater in the 1950s thru to today. It needed to be connected to a chimney or power vent in order to get the exhaust out of the home. I currently live in a home with an electric water heater that has elements that directly touch the water in the tank. The home I raised my family in had a direct fired oil water heater for about 12 years. Eventually I installed a Boiler with a Reillo burner that was very efficient. I connected an indirect to that boiler. The tank is still there over 25 years later.

These first 3 water heaters, I would call direct fired. The last one (on the right) is the indirect. It is more efficient when it is connected to a more efficient boiler. So connecting that indirect tank to a 1940's coal conversion boiler that is operating at 50% efficiency or less, is not the best use of an indirect. If however you have a 96% efficient space heating boiler to connect your indirect to, then that is probably the best combination for long lasting hot water.

The two tanks in the center must be connected to a chimney or vent of some type. So a tank with 120° to 140° water in it with a exhaust pipe connected to some vent (like a chimney) that is constantly sucking air that is less than the stored water temperature in the tank will, by design, cause off cycle heat loss out that vent. The electric and the indirect do not have a vent connection, so they are somewhat equal in the down time loss. The savings happens with the cost of the fuel that makes the water hot. In most cases Gas or Oil will cost less to make that water get hot in the first place.

That has been my experience. But times, they are a changing!

mattmia2

EdTheHeaterMan said:

If however you have a 96% efficient space heating boiler to connect your indirect to, then that is probably the best combination for long lasting hot water.

An indirect water heater in most cases isn't going to get the return water temp low enough to make a mod con run in its condensing range so the efficiency will be in the 80's somewhere, the same as with a modernish conventional boiler. That is still better efficiency than the tank with a hole in the middle with a burner under it of a direct fired water heater, but you would have to design it to give a lower return water temp to get the advertised efficiency of a mod con. You would have to make the tank bigger, run it at a lower temp and probably use a commercial/solar version of the tank with a larger or 2 heat exchangers.

EdTheHeaterMan

mattmia2 said:

An indirect water heater in most cases isn't going to get the return water temp low enough to make a mod con run in its condensing range so the efficiency will be in the 80's somewhere, the same as with a modernish conventional boiler. That is still better efficiency than the tank with a hole in the middle with a burner under it of a direct fired water heater, but you would have to design it to give a lower return water temp to get the advertised efficiency of a mod con. You would have to make the tank bigger, run it at a lower temp and probably use a commercial/solar version of the tank with a larger or 2 heat exchangers.

I must disagree to a point. The 96+% boiler will get to 96% based on a cooler return temperature like from a radiant floor system. BUT even if the return water is 140° or more, from the indirect, that boiler is still operating at a low enough temperature to be able to vent with plastic pipe. That is telling me that it is more efficient than a cast iron non condensing boiler that requires a metal or masonry chimney with a liner. So maybe 89% or 91% but no where 80%. or it would require a different venting system. Don't you agree?

ModCons work well on high temperature radiators. They work better on lower temperature systems but they work well on hot water baseboard. Just sayin'

That variable input and outdoor reset makes for a lower fuel bill compared to the same size cast iron boiler that is vented with B-vent. However, I do believe that the cast iron boilers are a better choice sometimes when you look at a high temp system because of the longevity of CI Boilers.

mattmia2

EdTheHeaterMan said:

mattmia2 said:

An indirect water heater in most cases isn't going to get the return water temp low enough to make a mod con run in its condensing range so the efficiency will be in the 80's somewhere, the same as with a modernish conventional boiler. That is still better efficiency than the tank with a hole in the middle with a burner under it of a direct fired water heater, but you would have to design it to give a lower return water temp to get the advertised efficiency of a mod con. You would have to make the tank bigger, run it at a lower temp and probably use a commercial/solar version of the tank with a larger or 2 heat exchangers.

I must disagree to a point. The 96+% boiler will get to 96% based on a cooler return temperature like from a radiant floor system. BUT even if the return water is 140° or more, from the indirect, that boiler is still operating at a low enough temperature to be able to vent with plastic pipe. That is telling me that it is more efficient than a cast iron non condensing boiler that requires a metal or masonry chimney with a liner. So maybe 89% or 91% but no where 80%. or it would require a different venting system. Don't you agree?

ModCons work well on high temperature radiators. They work better on lower temperature systems but they work well on hot water baseboard. Just sayin'

That variable input and outdoor reset makes for a lower fuel bill compared to the same size cast iron boiler that is vented with B-vent. However, I do believe that the cast iron boilers are a better choice sometimes when you look at a high temp system because of the longevity of CI Boilers.

I said 80's.

I don't think you're going to get over 90% without removing the heat of vaporization from the water in the products of combustion.

EdTheHeaterMan

mattmia2 said:

I said 80's.

I don't think you're going to get over 90% without removing the heat of vaporization from the water in the products of combustion.

How many ModCons are having overheated venting issues based on your opinion? If they are not condensing at all, then I believe that there would be hundreds if not thousands of overheated plastic vents in the news. I have not heard of many recalls on this problem. And I have personally installed indirects on mod con boilers, and I have observed condensation from said ModCon boilers in the summer when only the indirect was calling for heat.

Have you any facts to counter this? I would like to know what you observed.

Just talking from experience. I'm not a scientist or engineer with a slide rule. Just a dumb ole' wrench turner.

GGross

EdTheHeaterMan said:

mattmia2 said:

I said 80's.

I don't think you're going to get over 90% without removing the heat of vaporization from the water in the products of combustion.

How many ModCons are having overheated venting issues based on your opinion? If they are not condensing at all, then I believe that there would be hundreds if not thousands of overheated plastic vents in the news. I have not heard of many recalls on this problem. And I have personally installed indirects on mod con boilers, and I have observed condensation from said ModCon boilers in the summer when only the indirect was calling for heat.

Have you any facts to counter this? I would like to know what you observed.

Just talking from experience. I'm not a scientist or engineer with a slide rule. Just a dumb ole' wrench turner.

Most modcons do have overheating vents on PVC, that is why places like the east coast of the US, and Europe have pretty much done away with those PVC vents and moved to UL listed and tested plastic vent materials that can withstand the heat better.

That being said you were likely installing equipment in the best way you knew how at the time, and most likely did a better job with setting up temperatures to condense more than a lot of installers, most just set a modcon to a target of 180 and walk away.

ChrisJ

GGross said:

EdTheHeaterMan said:

mattmia2 said:

I said 80's.

I don't think you're going to get over 90% without removing the heat of vaporization from the water in the products of combustion.

How many ModCons are having overheated venting issues based on your opinion? If they are not condensing at all, then I believe that there would be hundreds if not thousands of overheated plastic vents in the news. I have not heard of many recalls on this problem. And I have personally installed indirects on mod con boilers, and I have observed condensation from said ModCon boilers in the summer when only the indirect was calling for heat.

Have you any facts to counter this? I would like to know what you observed.

Just talking from experience. I'm not a scientist or engineer with a slide rule. Just a dumb ole' wrench turner.

Most modcons do have overheating vents on PVC, that is why places like the east coast of the US, and Europe have pretty much done away with those PVC vents and moved to UL listed and tested plastic vent materials that can withstand the heat better.

I still see plenty of new PVC installs.

Also...........just because a burner isn't achieving 90%+ efficiency doesn't mean the exhaust temperature needs to be high. They can blend cool air in with the exhaust to drop the temperature. This is exactly how power venting water heaters work. They're far from high efficiency and yet still use PVC venting;

EdTheHeaterMan

GGross said:

Most modcons do have overheating vents on PVC, that is why places like the east coast of the US, and Europe have pretty much done away with those PVC vents and moved to UL listed and tested plastic vent materials that can withstand the heat better.

Did not say PVC. My exact quote is "I believe that there would be hundreds if not thousands of overheated plastic vents in the news"

Read the venting instructions if you are going to do the work!

GGross

EdTheHeaterMan said:

GGross said:

Most modcons do have overheating vents on PVC, that is why places like the east coast of the US, and Europe have pretty much done away with those PVC vents and moved to UL listed and tested plastic vent materials that can withstand the heat better.

Did not say PVC. My exact quote is "I believe that there would be hundreds if not thousands of overheated plastic vents in the news"

Read the venting instructions if you are going to do the work!

Yes I think I jumped into the middle of that conversation without understanding the context. I would agree with your original statement
"The 96+% boiler will get to 96% based on a cooler return temperature like from a radiant floor system. BUT even if the return water is 140° or more, from the indirect, that boiler is still operating at a low enough temperature to be able to vent with plastic pipe. That is telling me that it is more efficient than a cast iron non condensing boiler that requires a metal or masonry chimney with a liner. So maybe 89% or 91% but no where 80%. or it would require a different venting system. Don't you agree?"

ChrisJ

PEEK can be used continuously at 480°F.
It's plastic.

PTFE can be used at 500F. Also plastic.


As I said previously, plastic venting does not indicate an appliance's efficiency.
Direct vent water heaters blend cool air in with the exhaust and use PVC venting. What's their typical efficiency, 60%? I see absolutely no reason direct vent appliances cannot do the same trick.

Jamie Hall

Wait wait... the difference between the potential efficiency of a hgher temperature boiler -- an ordinary one -- and a condensing boiler is exactly the difference between the lower heating value of the fuel and the higher heating value. For natural gas -- methane -- the difference is around 10% (that is, the lower heating value is about 91% of the higher value. For diesel, which creates much less water vapour, the difference is less: only 3%, which is one reason why no one has spent much time on developing oil fired condensing boilers.

Now that's the potential difference. The actual in service difference will be less -- probably much less -- as the higher heating value is measured assuming, among other things, that the exhaust is cooled to 25 Celsius (77 F) which simply doesn't happen, as the return water to a boiler is almost always a good deal warmer than that.

The difference in values comes from condensing the water vapour created in combustion, so the exhaust gas must be cooled below its dewpoint, which for natural gas is usually around 140 F. The cooler the boiler can get the exhaust gas, the more water vapour will be condensed, and more of the potential heat will be extracted. For a boiler, it will never be able to get the exhaust temperature below the return water temperature -- although one should be able to get close with a well-designed system. Hot air furnaces generally have lower return air temperature, but air to air heat exchangers start to get pretty large for small temperature differencees, so in practice they don't do much better.

Now all this is independent of efficiency losses due to poor setup of the burners, or mismatch of the burners to the boiler, or any number of other things. Such as overall poor system design or control.

ChrisJ

@Jamie Hall 5 point penalty for using "Vapour" instead of "Vapor.

EdTheHeaterMan

I substitute Query for Question often in many of my posts @ChrisJ, does that merit the 5 point penalty. And what if we were talking to someone from UK where Petrol is substituted for Gasoline. If it is a liquid, Why do we call it GAS?. That should be at least a 10 point penalty.

ChrisJ

EdTheHeaterMan said:

I substitute Query for Question often in many of my posts @ChrisJ, does that merit the 5 point penalty. And what if we were talking to someone from UK where Petrol is substituted for Gasoline. If it is a liquid, Why do we call it GAS?. That should be at least a 10 point penalty.

Sometimes...... sometimes.

Honestly, it is kind of annoying because we have gas engines like LPG and natural gas. And then we have gas engines, for gasoline. because of that we have to say gaseous.

But, such is life.

hot_rod

Direct fired water heaters have a very inefficient HX. On purpose so they can deal with cold water their entire life without corroding to death.

Tankless manufacturers will tell you 60% efficiency range Somewhere in the 70's is maybe realistic.

With a dual coil indirect you can keep a mod con condensing longer and also run lower SWT, as you have more HX surface area. With enough HX surface you get what is called close approach, where the boiler side temperature is a few degrees above the DWH supply.

A good reason to use plate HXers to size for lowest boiler temperature.
On my last solar system I could shower with solar supply to the HX as low as 120F! A generously sized plate HX allowed for that.

Application #4 at heat-Flo

https://www.heat-flo.com/documents/product-applications/applications/dual-coil-applications.pdf

Wrightlb

As a homeowner, I faced a similar dilemma last winter when replacing my boiler/DHW system. In the end, the decision to use an indirect was driven by a need to satisfy a large dump of HW for large soaking tub and multiple baths which would have required a 199BTU combi. However my home heating load is relatively low and it was a close call on that even satisfying the dump. On top of that, my home heating load is relatively modest and the minimum modulation was too high at times. Given the on/off required combi, and the annual maintenance involved, I elected to go with a separate indirect and a simple boiler. It's worked out very well. Good luck with your decision.

RascalOrnery

Does an indirect have some sort of bladder to compensate water usage or as you use hot water from it, it is refilled via tap? I can see the HUGE disadvantage of a tankless coil in that you're heating 55-60ish degree water up to 120, where as if the indirect water tank is ALL at 140 and your not heating incoming water but instead just using up what you've heated then it would make a lot of sense to me...but if it's continually receiving cold water to refill as the rest of the water is being used it seems like it would be calling for heat very quickly....even at 40 gallons at 140degrees, a draw of 10 gallons would bring that temperature roughly to (140+140+140+60 / 4 120 and demand a heat call which if the boiler has fallen to ambient temperature of 70 degrees then once circulation begins (say the boiler is 18 gallons with another 2 gallons in the line) that really drops the temperature... does that make sense?

mattmia2

10 gallons is kind of a big draw if you're not running a washing machine or showering or filling a tub. The cold water inlet is at the bottom of the tank and the hot water outlet is at the top of the tank so the cold water tends to stay at the bottom of the tank. The coil from the boiler is also at the bottom. The well for the aquastat is usually near the lower center so it takes a fair it of a draw to cool it enough to call to heat the tank again, the 2 or 3 gallons you use to wash your hands won't do it until you've washed your hands several times. This is also part of choosing the size of the tank.

You can't use ratios like that to figure out the water temp because absolute zero is -459 f. The 90 f degree difference in water temp is a relatively small fraction of the energy contained in the water.

RascalOrnery

mattmia2 said:

You can't use ratios like that to figure out the water temp because absolute zero is -459 f. The 90 f degree difference in water temp is a relatively small fraction of the energy contained in the water.

Great to know! Thanks!

mattmia2 said:

1 gallons is kind of a big draw if you're not running a washing machine or showering or filling a tub. The cold water inlet is at the bottom of the tank and the hot water outlet is at the top of the tank so the cold water tends to stay at the bottom of the tank. The coil from the boiler is also at the bottom.

I appreciate all the input! thanks everyone!