Condensing Boiler Efficiency

Dr Pepper

my old furnace getting older and discussions at work about implementation of the new energy codes for commercial heating and A/C. If you don't have the completed permit before the March/April start date, the design must comply with ASHRAE 90 and/or local & state energy codes. This is building envelope and HVAC design and construction. One bank we do work for has mandated Geothermal on the next building. The learning curve for us started last year and even the code officials are still not up to speed on what is or isn't required. "Standard efficiency" is no longer the standard, and R-22 will be history in a few years.

Hi BP, been a while since I stuck my $.02 in here. There is going to be a compromise between coil size, fan HP, and boiler efficiency. But, "off the shelf" sizing doesn't sound like it has happened yet, and I wonder if the manufacturers are thinking beyond the lowest price.

Dr Pepper

Condensing Boiler Efficiency

Here's the situation...Your replacing a furnace with a condensing boiler and hydroair unit. The ducting and fan were designed for 68-70 return and 120 - 140F Supply Air Temp., below condensing, but the coil is listed/designed by the Manufacturer for a water input of 180F, out at 170F. (Most of them don't have enough info I've found) Reset with OAT will lower the water temp, but not much.
A condensing boiler running at 190F out and 20T/D (170F return) isn't much more efficient than a standard 80+ boiler. I had one condensing boiler mfg. rep. tell me that I couldn't find a condensing boiler that would operate with a greater than 20F T/D. OK...is that true?
What if the air handler had multiple coils and you could design for a temperature cross (think plate HX), and design output air temp. was achieved with 140F water input and 30F or 40F T/D (100F return, 30F approach).
So here's the question, does anyone give data for this type of installation?
Would any of your installations be better if these boilers were operating at lower water temps? Or, how do you get your installations down to condensing operation on a design degree day?
The new Energy Codes start in another month....are we ready?

tim smith

hydro air & temps

The last one i did i sized the coil for the heatloss with a 150 degree coil design temp, made the coil larger but now I can stay in condensing conditions much more often. I had the coil built to my spec. to do this. Good luck, Tim.

Unknown

Most condensing boilers are also modulating, so I would say that it would, in fact, be more efficient than a standard boiler.

Think of how they are rated... near steady state, right?

How often does a conventional boiler actually get near steady state, even with OR? Not often.

A modulating/condensing boiler? As long as you're over minimum modulation.. much better, right?

The ratings may not be much higher, but IMHO you're actually AT the ratings much more often with a modulator.

Course I maybe all wet.

Brad White_12

Hydro-Air and Condensing

Coils simply have to be selected for (what I use as optimum) 140 EWT and 110 LWT which generally means more rows and counterflow (leaving air sees entering water). I find 4 rows to be ideal, 6 rows if I have to. Custom coils sometimes and specifically selected.

Standard unit coils generally are 2-row at best and derating tables for lower temperatures typically do not place you where you want to be. They often stop at maybe 150 degrees and with a 20 degree drop.

Sure, you can select coils for higher or lower water temperatures and wider delta-T's. The loss is that capacity drops off rapidly with wider temperature drops.

Typically when using hydroair systems and condensing boilers you will have cooler air (of course! That is the idea, not scorched air.) So your air handler, ducting and registers have to be selected for higher airflows.

As you said, little point in running a condensing boiler at high temperatures.

Brad

hr

You should be able

to find an "off shelf" air coil to meet that spec?? If not coil manufactures, and there are dozens of them, would be able to build a coil to your exact spec.

I shopped the McQuay site once for special purpose coils for outdoor furnace installs. The math gets a little above my head but there is a lot of info that goes into air coil design. It looks like they could build just about anything by manuplitating coil sixa nad spacing, number of loops of copper through the coil, etc.

Also just paging through my Lochinvar Knight program manual, their control allows a wide range of temperatue adjustment. Min. setpoint from 32- 190, offset SH 0- 86, same for differential 0-86 degree with a 20 default value.

hot rod

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Brad White_12

Hot Rod- he be right

I personally like TSI coils (Division of RAE Corp. in Pryor, Oklahoma.)

McQuay, Heatcraft and others can get you what you need.
Generally, rows are added to get where you want to go, then greater face area. I like to keep fins less than 10 per inch; easier to clean.

Boilerpro_3

Just remember

even if you design for 180F supply at design day, you are down around 140F on the typical day. So for 90% of the heating season, you are still in the condensing range. Add some internal gains, like sunshine, appliances, and you can drop the supply temp even further. I have found it extremely rare that any system needs 180F supply on even the coldest day... there is just too much conservativeness in typical load calculations to ever be that close to the edge. Besides, after a load calc, I bet you won't need much heat, so the ductwork will be plenty big.

Sounds like you're finally getting rid of the scorched air, eh, Art.

Boilerpro

EBEBRATT-Ed

I'll go with the other guys on this. Design for a larger coil and lower water temps. the coil manufacturers can make anything you want (within reason). it will cost more but you will get the payback. jmho.


ed

Keith_8

fan HP

As I'm reading about 4 and 6 row coils to allow lower water temps the 1st thought that comes to mind is the large pressure drop across that coil.

You guy's that deal with a lot of air would know:
In sizing for lower water temps does it make sense to install a larger external coil and blow out the duct work?

Or do you prefer to order an air handler with a larger coil and blower motor?

Keith

Constantin

Hmmmm....

I think the trick is to take a look at the specs that the manufacturers publish on their coils and then see if the fan blower can handle the extra head or not. Depending on the spacing of the fins, the cleanliness of the coil, etc. it may add very little to a lot of head. I would try to keep the box as standard as possible, i.e. try and fit the largest coil that will fit w/o modifications. With any luck, it'll give you the flow and heat transfer you were looking for.

Brad White_9

When designing a Condensing Hydro-Air System

with a custom coil (OK, mine tend to be that way), I look at each system from a number of angles.

Firstly, I want the hot water coil to promote maximum heat transfer (duh! ) to meet the load with the lowest entering water temperature and maximum delta-T to meet the load. This often is a 4-row and sometimes a 6-row coil. Again, 120 to 130 entering water and 90 to 100 degree leaving water seems about ideal. When selecting toward the lower of these numbers, I tend to get more rows required. Why?

Key to the design is to have a warm enough discharge air temperature. Anything less than 100-110 near occupants feels cool. Moving air especially in a drier environment is not a good thing. 6-row coils give me flexibility.

Secondly, the balancing act of air pressure drop, fan HP, total energy input is critical. Sorry state of air systems is that you are moving an insulator (air) to heat. Low specific heat means you move a lot of it compared to water. At higher elevations even more so.

That said, the next factor is to increase the face area of the coil (after adding rows) to reduce pressure drop. Good news is, pressure drop decreases by the square of velocity.

Say I have a given coil, (6-Row, 8 fins/inch) at 500 FPM.
Air pressure drop is about 0.32 inches. System is 1000 cfm and the fan efficiency is 70 percent.

This coil as a component will impose a Wattage load on the system of about 54 Watts (0.072 Brake HP) which adds up over time.

Take the same coil and parameters at 400 FPM. Air pressure drop decreases to 0.20 inches (much more in line, IMHO). Wattage drops to 34 Watts (0.046 Brake HP).

Keep in mind this is fractional compared to other ever-present airstream obstructions: The cooling coil if any will have at least a half inch SP dry and double that when wet in summer, filters, ductwork, grilles, all will add resistance until the system sees probably 1.25 to 1.75 inches WG total pressure across the fan, more in summer than in winter. So we are talking a reduction of 0.12 inches 1/8th of an inch WC, but better heat transfer by longer air retention time across the coil.

Thirdly and Lastly: The use of ECM and variable speed motors is a nice albeit expensive touch that allows optimization of air delivery to temperature and the ultimate goal, comfort.


My $0.02,

Brad

Boilerpro_3

Cool air discharge solution

I worked on a high end hydro air system from the 1950s in a very nice home (back when they put the extra money in quality, rather than sqaure footage). The heart of the system was a huge commercial air handler and a proportinal thermostat driving a three way valve that supplied fully reset water temps to the coil. The ductwork was set up for high volume, low velocity, as was the case back then, based on a wet heads limited knowledge of hot air. However, all the supply registers were located high on the walls. I suspect this way the cooler discharge air would not blow on occupants. On a typical winter day the discharge air was about 80F and comfort excellent, for non radiant. The returns were located at the floor. I believe the design concept used in this system is the same used for the air turnover systems now marketed for heating warehouses.... these also discharge at the ceiling. This places no lower limits on the discharge temp, and, therefore, the supply water temp to the coils. With a condensing boiler, you now have maximum boiler efficiency, and greatly improved home efficiency due to the elimination of hot air stacking and its related increased heat loss problems.

BTW, the system also had an air style TRV on nearly every supply register.

If someone ever wanted forced air instead of a more conventional hot water system, this is how I would do it.

Boilerpro

Brad White_9

Wow

WAY ahead of the times, that was! Thanks for sharing.

The low discharge temperatures and reverse displacement (high to low) make abundant sense.

The paradox of heating from above has always been that when folks are too cold, the raise the supply air temperature which of course causes buoyancy and the hot air to remain near the ceiling.

I think there are great lessons from what you posted.

Brad