View Full Version : questions about adding indirect

10-04-2010, 03:38 PM
I have a 30yr old peerless oil fired boiler, single zone hydronic heat, 5gpm tankless coil setup in NY. We've been in the house for 2 yrs now and so far so good. We are looking into adding a 30-40 gallon indirect in order to improve the efficiency of our system. I having trouble getting answers to a couple questions I have.....

-It seems that an indirect should save about 20% on oil compared to the tankless. I'm estimating that over the past 2 years we used about 350 gallons/yr of oil for DHW. So the indirect might cut that down by 70 gallons? From actual experience does this sound high or low?

-The contractor said that we'll use the same aquastat that's currently installed in the tankless coil to control the boiler temp. It just has a hi & lo setting. I read that the indirect temp setting should be at 140F to prevent bacteria....so doesn't that mean that I have to keep the boiler's aquastat at a minimum of 140 on the LO side? Right now I run 150/120F on the aquastat until the weather outside really drops, then it's at 180/155. If I have to keep the boiler at a higher temp all year round, wouldn't this negate the benefits of the indirect?

-The plan is to leave the tankless in place and add valves so that it could be shut down but available in case we needed it. Sounds like shutting both the inlet & outlet is a bad idea....so if we close off the outlet and leave the inlet open we're okay?

Thanks in advance for your replies....

10-04-2010, 04:22 PM
Some boilers don't like return water lower than 130-degrees, so what you have may be a problem in the first place. Your manual should say. It depends on the type of heat exchanger in the boiler. If the return water is too cool, you end up with condensation, and ruins it by rusting things out. Most of the useage of an indirect is based on the storage water being 140. Where I live, you MUST have a tempering valve on the outlet of the WH. You can set that lower (often 119-120 is recommended for a home). The response time to reheat and get the first hour draw is based on the incoming water generally at 180.

You'd normally set the WH indirect up as a priority zone. It depends on what your boiler does when a thermostat calls for heat. Often, that triggers it to go to the high setting. A priority zone means that when the WH is calling for heat, it stops trying to heat the house, so you get all of it for the indirect. This normally doesn't take all that long, and when finished, it returns to maintaining the heat for the house.

10-05-2010, 06:38 AM
Thanks Jim. I turned up the aquastat to 130/160 until I could locate a manual for the boiler online. The estimate I got for adding the indirect included a mixing valve. Right now the tankless coil has a gate (or globe?) valve between the inlet and outlet which I think is supposed to act like a mixing valve but it doesn't work.
I'll check to see what the priority is for the heating zone.

10-05-2010, 09:14 AM
130F is too low for an oil fired boiler that's not designed as "cold start" boiler. It's at the hairy edge of condensing acidic oil exhaust on the heat exchangers themselves, and will be almost guaranteed to condense some in the flue.

If you haven't already, installing a Beckett Heat Manager or Intellicon HW+ economizer will very likely save you more than 70 gallons/year on an oil fired system. Depending on how oversized the boiler is for the system, you may get another signficant benefit out of using a "reverse indirect" as both a system buffer and heat exchanger for DHW. A pretty good online calculator here (http://nora-oilheat.org/site20/fsa/FSACalculator_1_1_0_8.zip)for determining whether (and by how much) your boiler is for the load, and the net effect on efficiency. See also: http://www.nora-oilheat.org/site20/index.mv?screen=home. If you're 3+ oversized, an your radiation is fin-tube baseboard rather than high-volume radiators, using a reverse idirect to add mass to the system will boost efficiency significantly. Ergomax (http://www.ergomax.com/New-Tanks.htm) & TurboMax (http://www.thermo2000.com/content/en-US/s2_produits/optimizer.aspx) have similar lineups (Ergomax is uglier to look at, but is a cleaner install IMHO ) and the Everhot EA series (http://www.tfi-everhot.com/pdfs/TFI_EAseries.pdf) can also be plumbed as a system buffer. When going this route indirect tank is the ONLY zone as far as the boiler is concerned, and is essentially the hydraulic seperator making the heating system a primary/secondary type configuration. The aquastat on the indirect makes the heat calls from the boiler (keep it at 140 minimum), and the circulator for the heating system is conrolled by the room thermostat(s). It's more expensive than a standard indirect, but when the system is signficantly oversized, the benefits are larger. During the heating season you'd need to bump the temp of the tank up a bit if the radation can't deliver it to the rooms at 140F, but bump it up 5F at a time, to limit standby losses. (You can also insulate the heck out of the tank to reduce standby loss.)

[Edited to add] For a better understanding of how using a buffer type indirect system approach in conjunction with a boiler-heat purging economizer control boosts the performance at partial load (which is all the time when the boiler is oversized) see the results of a Brookhaven Nat'l Labs series of reports here: Performance of Integrated Hydronic Systems (http://www.nora-oilheat.org/site20/uploads/BNL%20Integrated%20Hydronic%20System%20Report.pdf)

Purging boiler heat in to the buffer at the end of burn results in something similar to the steel boiler in system 3. Look at the regression curve for partial-load performance on page 3 of Appendix 3, and compare it to the similar curves for other systems. As a retrofit system using off the shelf components it won't be tweaked as perfectly as Unit 3, but you can get pretty close.

Also, by keeping the tank at 140F (min) you can cold-start even hot-start boilers without fear of condensation damage, and lose the summertime standby losses.

You'll note in that study that a pretty-good internal coil can be as efficent in hot-water-only mode as a less-well-controlled indirect, less well insulated boiler with an indirect. Compare summertime efficiency of units 1 & 2 in the table 2, p.7 of the main document. The key difference is the amount of standby loss, which is a function of the boiler temp and insulation level. (lower temp, higher insulation==less loss.)

Also look at Table 3 p.9 to see why an oversizing factor of 3 or greater makes adding the buffer and economizer control worthwhile, whereas if it's only 2x oversized or less, maybe not so much.

With a buffer-tank type of hydraulic separator the system becomes less sensitive to burner sizing, but also the mass of the boiler- any heat source will do, and will run pretty much as-efficiently at perfect-sizing as for 4x oversizing. If at some point you replaced the boiler with a smaller boiler or even a low-mass modulating condensing boiler, the rest would continue to work with reasonable efficiency. (If you had a gas-fired low-mass boiler you could lower the temp on the tank to whatever you needed just for hot water, as long as your radiators could still deliver enough heat at the lower temp.) With a low-mass boiler, the amount of heat "abandoned" in the boiler at the end of the burn is a fraction of that of a cast-iron beast, further enhancing system efficiency.

Insulating all of the near-boiler plumbing (and all of the plumbing hooked up to the indirect) to at least R4 also reduces the efficiency robbing standby losses, flattening out those regression curves at part load. Keeping a boiler at 140-160F all summer with the plumbing sapping heat 24/7 just to have hot water can easily add up to 70 gallons worth of standby over the course of a year, even if some of that heat loss goes toward space heating in the winter since the boiler is inside the thermal envelope of the building.

10-05-2010, 09:17 PM
Thanks Dana. You really threw a wrench into the mix there. I'm trying to get a good understanding of what you're suggesting, starting with the HW+ controller. This looks like something that I could do myself, and doing some research online it seems this thing isn't a gimmick. Coincidentally the company is around the block from my house. Now I'm trying to wrap my head around how my aquastat operates. It is a Honeywell L4081B. Please let me know if my understanding is correct....

-With the HI setting the L4081B will fire up the boiler until it hits the HI temp setting and then shut it down until it hits the HI setting - DIFF. It will operate this way regardless of the thermostat setting or anything else. Basically the HI setting will determine the boiler temp all year long, and that temp should hover around the HI to HI-DIFF range.

-The LO setting only controls the circulator. If the thermostat is calling for heat the L4081B will turn on the circulator as long as the water temp is above the LO setting or LO-DIFF setting (when falling)

Am I right so far? If so, I have some questions....

-There is no way to set the DIFF on this aquastat. Looks like it is 10F by default....is that correct? I would need to know this in order to set the HW+

-Whenever I don't need the boiler for heating the house, the HI setting should be as low as I could get away with, which sounds like 140F, and the LO setting wouldn't matter at all (so long as it is 20F lower than HI)?

10-06-2010, 11:44 AM
The low, not the high, should be set to 140F- the boiler needs to be at least up to 140F before pulling any heat out of it with the circulator or you'll have condensation issues. The high can be set for whatever the radiation needs to deliver the heat, (which will vary with weather conditions.) "Outdoor reset" controls basically reduce the high-temp based on the outdoor temp. The Beckett & Intellicon units perform a similar function based on their algorithms for sensing the true heat load, but try to maintain some minimum burn-time to minimize the number of burns. If the burner isn't running at least 5-10 minutes at a time, you're losing efficiency to "short-cycling", which the economizer attempts to limit, while keeping the average temperature as low as possible.

I'd have to look up the specs on the L4081B to figure out if your understanding of how it works is correct. It's probably available online somewhere. The Intellicon would basically replace some/all of it's functionality.

[edited to add]

Ignore the above...

See p.8:


The low limit switch & adjustable differential determines when the circulator turns on/off. The low-limit setting has a fixed 10F between the setting number, and the temp at which it turns the circulator off, so if you set it to 140F, it could run as low as 130F without kicking back off. The DIFF setting sets how high the turn-on is above the (10F below lo-limit setpoint) the circulator kicks on. Dropping the low limit to it's lowest setting would keep it from pumping heat into the radiation during the summertime hot-water-only season.

The high-limit controls the burner, with a fixed 10F hysteresis, turning the burner on when it's 10F below the setpoint. So when you're using the internal coil for hot water heating , so if you set the HI to 140F it'll only fire up when it's dropped to 130F, which has some amount of condensing danger. Setting it to 150F would better protect the boiler, but increase standby losses.

Setting the system up to run an indirect we want to inhibit the burner until the indirect is actually calling for heat to save on standbylosses from at hot but idle boiler. With a reverse-indirect tank set to 140F, starting the circulator at the same time as the burner works, because the ~140F water from the reverse-indirect pre-heats the boiler up to the right range more quickly from room temp than the burner would on it's own, minimizing the condensing time. Then after the tank is up to temp you want the circulator to keep going until it cools the boiler down to just above the temp of the tank. This is approximately the strategy used with the Energy Kinetics System 2000, with some differences in system detail. (The Sys2K purges heat to the indirect at the end of heating cycles that are boiler direct, not buffered by the indirect itself, and will run water-heating only burns to satisfy min-temp on the indirect.)

Using a reverse-indirect as the hydraulic separator, the heating system calls only turn on the circulator pump (via a zone relay triggered by the thermostat) for the heating system, and the boiler controls remain ignorant of the state of that system. The thermostat for the boiler loop & controls is the tank's aquastat. But to minimize standby losses, the boiler's burner needs to be inhibited until the tank is actually calling for heat, and turn off as soon as the call for heat has been satisfied, yet the circulator needs to continue to run post-burn until the temp has dropped to near the tank's temp, which IIRC is a function that can be reasonably provided by a Beckett &/or Intellicon. Operated this way you can run the boiler up to 170F during the burn, but it'll cool off to 140-150F for standby, and can get down to room temp (zero standby loss) if there's no call for heat from the reverse-indirect.

10-06-2010, 03:47 PM
When your boiler contains an internal domestic HW coil, it must maintain some minimum temperature all year. So, it will cycle between the high and low. WHen it hits the low setting, it will run until it hits the high setting. If a thermostat is calling for heat, it will cycle between the high and the differential settings. The indirect could be one of the things that tripped the thermostat (but controls its own circulator).

What you really want is one with different logic to run an indirect and no internal coil. You want one that would maintain some minimum temperature to preclude damage from condensation, but not try to maintain the high setting until some thermostat calls for heat, whether it was the indirect or the house. Thus, it would be sitting at nominally, 140 degrees or so until something needed heat, or it dropped enough so it had to turn on to maintain it above that minimum.

Assuming I understand what Dana was suggesting with regards to the reverse indirect, if that indirect tank was maintained at say 140, the heating coil from the boiler feeding it would be at least that temperature (that coil or jacket is inside the indirect). So, when the indirect called for heat (someone taking a shower for example), the boiler would turn on, and instead of maybe room temperature water coming back and creating a condensation situation, it would be then preheated by the indirect until the boiler could heat it further and replenish the indirect. this might allow you to let the boiler low temp go lower. It might not work well in the winter when you may need faster response to the house, plus, unless the indirect was also calling for heat, its circulator wouldn't run, and there would be no preheated water to warm the incoming feed to the boiler (cold water from the radiators), and damage is likely.

10-07-2010, 08:11 AM
Jim- The reverse indirect would be fed on a boiler loop, and the internal coil would be abandoned. A reverse indirect has a mass of boiler water, with only a quart or so of potable inside a heat exchanger (simlar to an embedded coil, but with 5-10x the thermal mass surrounding it, and 3-5x the insulation value of a typical boiler.) The heating radiation would be hooked up to the reverse-indirect, configured as a hydraulic separator- the boiler never gets the slug of cold water coming back from radiation undiluted, but only when the mix of return water and tank water tripps the aquatstat (presumably set to trip at ~140F, with some amount of hysteresis):


When that happens the boiler loop turbulence into the tank thoroughly mixes the thermal mass of the tank with the incoming radiation return (and increases the heat exchange efficiency to the DHW coil), breaking any stratifcation that might have developed. (Here the Ergomax has some design advantages over the TurboMax and Everhot EA, which would need external Tees to plumb it as a hyraulic separator.) Response to the house is pretty quick, since it's drawing from asubstantial pre-heated thermal mass- it's as-quick or quicker than running it direct off the boiler. From the heating sytem's point of view it's simply a classic primary/secondary configuration, but with a massive hydraulic separator (that only incidentially has a heat exchanger for DHW.):


In winter you'd likely be raising the temp of the tank a bit above 140F DHW if the radiation can't keep up with the heat load at that temp, but bumping it up only 5F at a time until you know what temp is actually NEEDED would minimize the standby & distribution losses. Most oil-fired systems I've seen are 3x oversized for the heat load- response time issues would come into play if the boiler was too small for the DHW load (which it might be, were the smallest ~75KBTU/h oil burner in coldwater country during a high spaceheating load.) Reverse-indirects tend to "share nicely" with space heating loads, because as the temp in the tank drops during large DHW draws, the radiation output also drops.

10-15-2010, 10:04 AM
If you are adding an Auxiliary tank to a non condensing boiler, wouldn't you just run the cold water inlet or "make up" Feed into the auxiliary tank and circulate only heated water into the non-condensing boiler?


10-15-2010, 11:32 AM
If you are adding an Auxiliary tank to a non condensing boiler, wouldn't you just run the cold water inlet or "make up" Feed into the auxiliary tank and circulate only heated water into the non-condensing boiler?


In a word, no.

What your picture shows is an embedded coil in the boiler with a DHW buffer, which typically has a lower heat-transfer capacity than an indirect-fired hot water tank and if it requires that the boiler be kept hot when the only load is DHW, has much higher standby loss. In a typical indirect installation the coil is in the tank, not the boiler (mentally move it to the left, in your picture), and is far larger, with boiler water pumped inside the coil which is surrounded by the potable.

A "reverse indirect" like the TurboMax/Ergomax has boiler water as the bulk water in the tank, making it suitable to use as a heating systembuffer &/or as a hydralic separator for a primary/secondary hydronic heating system (as shown in the Ergomax conceptual drawing.) This has great advantages to multi &micro-zoned heating systems, since the thermal mass of the buffer participates in EVERY burn, and defines a minimum burn time independent of the heat load of the zone(s) or DHW load calling for heat. It both reduces the number of burns and lengthens them, minimizing cycling losses (and wear on the boiler) with substantial enhancments in average efficiency at partial-load. In a multi-zoned system EVERY call for heat is a partial-load, by definition, but with a massive buffer as the hydraulic separator (as shown in the lower picture of my last post) the size of the heat load of the calling zone is of very little consequence- since the whole mass is heated, it extends the short-cycle of a micro-load call. The Ergomax conceptual drawing is exactly the buffer-as-hydraulic separator configuration, but with a heat exchanger for the DHW inside the buffer.

And even if the boiler is stone-cold at the beginning of a burn, it is pre-heated quite rapidly with the water being pumped in from the tank- it's the nicest-ever way to cold-fire a cast iron boiler.

This approach works- the more oversized the boiler and the greater number of zones, the more it will improve system efficiency and reduce wear on the boiler. Even the smallest oil-burners out there have outputs 2x or more the design day heat load for many homes, let alone the average heat load of one zone of a home. Giving it a DHW load to work on (as with a standard indirect operated as a separate zone) reduces the oversizing factor somewhat, but giving it the mass of the tank to work on independent of which load is being served is HUGE by comparison, when using bang-bang non-moduating boilers & multiple zones, even if the boiler right-sized for the peak load, but especially when it's several times oversized for any single zone. Then, heat-purging the boiler at the end of the burn to reduce it close to tank-temp becomes the icing on the efficiency cake.

Most oil boilers out there are oversized and running 70% efficiency as-installed, but can be increased to 80%+ using these methods. Typical combusion-efficiency (when freshly tuned) might be 84-86%, but the oversizing, standby & cycling losses dump an inordinate amount of heat into the boiler room (and idle flues). With an efficiently configured & controlled system, the boiler room won't be dramatically warmer than the rest of the house. (If your boiler room is the warmest place in the house, especially if it's in an unheated uninsulated basement, you really SHOULD take the bull by the horns and deal with it- it'll pay back on both fuel & maintenance..)

[end soapbox discussion]

10-15-2010, 12:00 PM
^^don't look at the water heater per se, however, just the way the feed water is routed/blended in the expansion tank. This addresses the condensing issue with the boiler and cold start ups.....wouldn't it?

10-15-2010, 02:47 PM
^^don't look at the water heater per se, however, just the way the feed water is routed/blended in the expansion tank. This addresses the condensing issue with the boiler and cold start ups.....wouldn't it?

Methinks we've both been misunderstanding some stuff in the Ajax diagram you posted, according to this page where that image appears (http://www.ajaxboiler.com/?page_id=109)

It's a hot water tank full of potable water, not an expansion tank, and the arrows indicate water flow into the coil heat exchanger inside the boiler which is the ONLY heat exchanger in this boiler). The cold feed is not the makeup water for the boiler it's the cold water feed to the water heating system. There is no separate boiler water- only his particular series of Ajax water heaters have nothing potable water in them. It's a copper-fin water-tube boiler, which is quite a different beast from a cast-iron fire-tube/ plate heat exchanger boiler like those used for space heating in homes:


It's somewhat like the tank HW heater in most homes, but with the burner & heat exchanger off to the side, pumping water through it to heat the tank up to temp. With no center-flue heat exchanger, the standby losses of the tank are miniscule by comparison to the typical home hote water heating system.

The "RETURN WARM WATER LINE" indicated in the schematic is somewhat obscure. That would seem to indicate that it can be used either as a potable hot water heater or as a combi-system with potable hot water in the radiation, or as a heating only system where the tank is both a buffer and the point of hydraulic separation between the boiler loop & radiation loop. The primary difference between it and a reverse-indirect HW system is the absence of the heat exchanger and that the heat is stored with potable, rather than separate boiler water.

The only way that helps in a cold start situation is when there's an aquastat controlling the pump on that loop maintaining a minimum temp at or above condensing temp. Water tube heat exchangers are usually fairly low-mass, and would come up to temp very quickly. But if the tank is allowed to cool in a heating-only application there would be a long stretch of condensing-mode burn as it brings the full mass of the tank up to temp. Many water-tube boilers are somewhat condensing-tolerant (didn't see much in the way of min-temp specs.) If the tank is maintained at or above condensing temp, the boiler's cold-start condition is somewhat like that of a reverse-indirect, but we're really talking apples and pears here. An Ajax Ace high-flow commercial potable-water heater isn't very much like the heating boilers you find in homes- it has much less mass to heat up on a cold start in the first place. It's much more akin to a tankless hot water heater than a cast iron boiler. (Some people use tankless HW heaters ponied onto buffer tanks as space heating boilers too.)