modcon or not ?

[dukehayduk]

Hi, I am brand new to this forum. I am looking to replace my 40 yr old oil fired boiler with a new boiler. My choices are fuel oil or propane. The old boiler dates from the original 900 sq ft slab on grade with double stack fin tube baseboar heating (circa 1965). I have increased the original footprint with radiant floor and a second floor with slant fin 30 baseboard. The house is now 2300 sq ft and I use the same amount of fuel oil as I did before the add-on (1000 gals fuel oil). Highly insulated and very tight.
My heat load calculations come out to 42-45,000 btu/hr on a 0 degree day (I live in southern michigan, use Lansing HDD). Total fin tube emitters equals 102 lineal feet. Radiant floor heat equals 450 sq. ft with tubing on 6 inch centers.

My contractor is quoting a Lochinvar 080 Knight ModCon propane. He is basing his boiler size on my potential radiation, says heat load is secondary. I think the boiler is oversized. Am I wrong in thinking that this will negate the ModCon efficiency? I have heard good things about Lochinvar, especially concerning parts availability inMichigan. I initially was going to stick with oil ( I have no problems with oil). Was looking at American Std, Utica and crown. Looking for comments. I am big on efficiency and am very handy. What about Indirect DHW? I currently use electric DHW tank.

[Dana]

Boiler sizing based on total radiation is right for steam systems, but for hydronics, not so much. It's too much boiler, and too high a min-modulation. To get the most out of a mod con, look at the MINIMUM modulation number more than the max- the lower the min, the more time it'll spend in a modulating mode=fewer, longer cycles, more time spent in condensing mode, less wear & tear on the system. The system design also has to minimize the return-water temp to the boiler to wring out that 90% + efficiency. This is (most likely) a dual water temp system, with lower water temp for the radiant than for the fin-tube(?).



With only 100' of baseboard (and another 100-150' of distribution plumbing) with a low mass boiler with a 50k min modulation it'll short cycle on the high-temp zone. (Even a smaller mod con could have issues). I'm assuming the radiant is in a slab, and already has sufficient mass to preclude short-cycling on that zone? You can add mass to the high-temp zones with a buffer tank, but what makes more sense is to buffer the high temp zone with a "reverse indirect" hot water heater (Everhot EA series, Ergomax, Turbomax) maintained at whatever temp that zone requires, with a tempering valve on the DHW heat exchanger's output roughly this:



This would make the return water to the boiler a bit high for good condensing, but even an oversized boiler would now have a minimum burn cycle. If you ran the tank as one zone of a 2 zone system (from the boiler's point of view), running all high temp zones off the tank, and ran the radiant as it's own zone (not running off the tank) you can maximize the condensing benefit.

But go with a smaller mod con with a sub-20kbtu/h min-mod (Burnham Alpine 80, Triangle Tube Prestige Solo 60, Peerless Pinnacle T50/T80, etc.) and you'll do a lot better efficiency-wise. You basically want something that'll run 100% duty cycle whenever it's below 30F, but with enough of a high end to deliver the design-day heat load, with a bit left over for the hot water. With a design-day heat load of ~45K, a 60-80K max modcon should do it (a 50K will if you have a big enough indirect.)

As a side note: If there's wall space for it, with ~50-75% more baseboard you can usually get the heat out of it at low temps too. If you can get your design temps down to ~125-130F or so you can run the whole shebang off a reverse-indirect maintained at 130F and still get sub 120F return water for 90%+ efficiency.

If you stick with oil, the Energy Kinetics System 2000 uses a medium mass boiler and smart controls to purge heat from the boiler into an indirect at the end of each burn cycle to minimize cycling losses, and it would DEFINITELY beat an oversized mod-con on efficiency:

http://www.energykinetics.com/produc...shtml#ek1front

This is similar in effect to the reverse-indirect buffering with a low mass boiler, but since the thermal mass of the boiler itself is higher, they need the purge cycles to keep it from falling off an efficiency cliff with heat abandoned in the boiler. With a properly sized low mass mod-con and a buffer it'll modulate down under part load rather than cycle, and when the load is lower than the min-mod, the amount of heat abandoned in the boiler each cycle is lower- a tiny fraction of that delivered to the buffer tank, and net efficiency stays high.

There are even some decently efficient designs using this approach using tankless HW heaters instead of mod-cons. You're limited to about 83-84% efficiency with standard tankless HW heater, but unlike bang-bang cast iron boilers, it'll DELIVER that efficiency in the real world even in during the shoulder-seasons:

http://www.heatpro.us/designtree/doc...anklesssys.htm

Even if you went with a lower mass oil-fired boiler, you'll maximize the efficiency of the boiler if you buffer the fin-tube, and using a reverse-indirect kills 2 birds with one stone. There's a lot to be said for integrating the heat/hot water even WITHOUT the buffering effect (running a standard indirect as a "priority zone") but the efficiency gained with the buffering is substantial, especially with heat loads much lower than the boiler's output. See:

http://www.nora-oilheat.org/site20/u...ciencyTest.pdf

(Unit number 3 is a system 2000, system #11 is an oversized mod-con. Skim the whole thing, but note the degradation of oversizing in Table 4, and peek at the part load regression curves in the appendices for a clearer picture.)

For a comparison caluculator based on the test data from the above summary, see:

http://nora-oilheat.org/site20/fsa/F...or_1_1_0_8.zip

[Dana]

Some more bedtime reading for system efficiency issues spelling out the limits of modulation and the relative importance of not oversizing for the load (or the partial-load), and how buffers ease that pain:

http://www.patkelco.com/uploads/file...e962d0ef7d.pdf

http://www.raypak.com/module.htm

[dukehayduk]

Thank you thank you, phenomenal info. Your reply and the links confirmed that my contractor is still not sold on modcon's or completely understands them. A couple additional notes on my system ( I have done the construction myself fyi, as well as all the associated plumbing, minus the original 1965 loops).

I have two circulator pumps (grundfos, one for the radiant concrete slab over 2"styro, one for the 3 baseboard zones) and 3 erie zone valves for the baseboard zones (2 thermostat zones upstairs, 1 thermostat zone in the original 1965 loops, I could combine the upstairs zone onto one thermostat{open floor plan}). The radiant zone has a manual mixing loop to moderate the temp of heating water. I try to keep the water around 120 for that loop.

I like your idea of the reverse indirect acting as a buffer tank. Makes complete sense to me to get rid of short cycling. I am not beholden to a condensing boiler, or propane. I am now beholden to fully modulating, though.

So, I need to narrow down my choices of boiler. 0 degree design day w/ delta 72 degrees will occur here in Ann Arbor about 2-5 nights per year on average. The 2300 sq ft house is actually built into a hillside. With my foam in place foundation/2x6 wall construction/blown in cellolose, my design heat load comes in between 40-45,000 btu/hr. (did calcs three different methods, all in the same ballpark). My The radiant floor is the lower floor built into the hillside. It hardly ever calls for heat (january/february for the most part). Seems like the lochnivar 080 is too big. System 2000 is unavailable in Mich and too expensive. Buderus is not well serviced in Mich. Utica and Crown is available. Unfortunately, I am not made of money. 8-9k is tops for me to spend.

Any additional insight would be helpful. I guess I am looking for boiler size range that would be appropriate along with other helpful hints to look for in a boiler considering my unique situation. Thanks in advance.

[Peter Griffin]

If the choice it between propane and fuel oil, fuel oil is always going be be less expensive to operate. There are 90,000 btu's in a gallon of propane and 140,000 in the same amount of #2 fuel oil. System efficiencies for propane (gas) fired equipment can approach 97% but even at that rate the cost differental still favors fuel oil. I would recommend a Buderus G115 series with the logamatic boiler re-set control

www.buderus.net/

www.nora-oilheat.org/

[Dana]

If the choice it between propane and fuel oil, fuel oil is always going be be less expensive to operate. There are 90,000 btu's in a gallon of propane and 140,000 in the same amount of #2 fuel oil. System efficiencies for propane (gas) fired equipment can approach 97% but even at that rate the cost differental still favors fuel oil. I would recommend a Buderus G115 series with the logamatic boiler re-set control

www.buderus.net/

www.nora-oilheat.org/

That's true today but don't let history be your guide on that. Going forward, fuel oil is competing ever more directly with a competing market share of diesel as an automotive fuel, and will for the forseeable future be largely determined by oil commodity prices & automotive fuel demand.

Propane has multiple sources (produced both from natural gas & oil), and isn't poised to become a significant fraction of automotive fuel any time soon. Spot prices will vary. While propane has traditionally tracked crude oil prices, that will be changing with the massive exploitation of shale-gas in the US currently under development. I expect fuel oil prices only have room to go up relative to crude oil prices, but with propane maybe yes, maybe not...

Also, barring rapid development of new technology, the minimum output of oil burners is quite oversized to the heat load of well insulated (not just super-insulated) homes in most of the US, which leads to cycling loss inefficiencies. Propane mod-cons can go down to well under 20KBTU/h out and can make true AFUE 95% efficiency in well designed low-temp hydronic systems.

The delivered-to the house BTUs/$ is what you're after, and that's always a moving target, but the long term prospects are for higher oil inflation than propane inflation. Currently in my neighborhood oil is averaging $2.45/gallon, , or ~58fuel-KBTU/$, burned at 86% efficiency in a Buderus G115 (assuming it's less than 2x oversized for the load and properly buffered to never short-cycle) gives you ~50kbtu/$ delivered to the house.

In the same neighborhood propane is currently $2.60/gallon or ~35fuel-KBTU/$. Burned in a mod-con at 95% efficiency (similar assumptions about sizing & system design) thats ~33KBTU/$, significantly more expensive.

Flipping it over, oil heat is ~$2/100kbtu vs. propane heat ~$3/100kbtu, delivered as heat to the floor. But that isn't a fixed ratio- the market pressures are higher on oil than propane.

With the crash of natural gas prices with all the newly on-line production, that may shift. Currently in my neighborhood NG is under $1/KBTU, even in a 78% efficient system. As the Allegheny shale deposits come online in a big way in another half-decade or so it may drop even more, and pull propane prices down with it at about the time oil goes stratospheric again.

It takes a better crystal ball than mine to predict what fuel prices will be. Prices are volatile, and often very location-specific. Two years ago (many months before the oil price spike of '08) I was betting I'd see NG prices hit 3x my current retail-New England price by 2010, but I wouldn't make that bet today, eh? Prices now are less than half what they were then, but I expect that'll change (but in which direction?)

It's a crap shoot, but for now oil is significantly cheaper.

But back to the system in question: The smallest Buderus G115 is about 2x your design-day heat load (the smallest oil burners are all about the 75-90KBTU range- your peak heat load is half that, your average heat load about 1/4 that or less), and if your radiant is in a slab the LAST thing you want to do is control the water temp via a boiler's outdoor reset (there will be a serious lag between load and slab temp- it'll over & undershoot the setpoint significantly. PID thermostat control will deliver the best comfort. You might get mid to high single-digit percentage savings from outdoor reset on the baseboard zone, but at 2x oversizing you'll get double-digit savings out of buffering. The setpoint of the buffer should be no higher than the most you'd need on design day (and if you've tightened up the place a lot, that might be 140F). The high limit on the boiler should be based on the tank temp, and the boiler loop needs to be designed to stay within the delta-T limits of the boiler manufacturer, while protecting it from return water temps below 140F.

Another key aspect to boiler choice: Go with a direct vent/sealed combustion version. Combustion air drawn from conditioned space is a 3-5% cost of operation that doesn't appear on an AFUE test.

And if you have good fuel use & degree-day data, DO use the FSA calculator:

http://nora-oilheat.org/site20/fsa/F...or_1_1_0_8.zip

It will deliver a far more accurate estimate of the true design-day heat load than any sort of heat-loss estimate. (It's an actual MEASUREMENT, not a best-esstimate the way manual-J, etc are.)

[dukehayduk]

Thanks for all the help. I am pretty much decided to get quotes for the following system install. As a reminder, my heat load calculations come out to 40-45k btu/hr on a 0 degree day with delta T=72, I have a 100 feet of fin tube baseboard and 450 sq. ft. of radiant floor slab(lower level into hillside). Very tight, well insulated2300 sq. ft. house set into a hillside.

1. Triangle Tube prestige Solo60 (I like the exchanger design, less maintenance supposedly) (converting from oil to propane)

2. Reverse Indirect tank (still undecide concerning brand, Ergomax or Everhot EA. I like the Everhot removeable exchanger for cleaning potential scale down the road vs. piping in shutoffs and drains to flush the Ergomax coils).

Looking for comments on the following questions.

A. Would it be more efficient and cost effective to run my current electric hot water heater and shutting down the boiler for the 3-5 summer months? Current energy costs in southern michigan = 12 cents a kwh total for electric , $1.75/gal propane cost, $2.50 fuel oil cost.

B. If boiler reverse indirect is better than conventional electric DHW during the summer, anything I can do with the electric tank to make it useful? Additional buffer storage, solar storage, etc. or garbage?

C. Which is preferred for boiler makeup water: softened or hard well water ?

D. I have a fireplace flue 2 feet from my proposed concentric air/vent for the boiler (both flues in the same chase, concentric is going up the existing oil boiler vent which is a 6 inch SS fuel oil rated flue, about 20 lineal feet total). Should I place another section onto the fireplace flue to raise it a foot or so above the concentric air/vent for the Solo 60?

Thanks for the advice and comments in advance!!

duke

[Dana]

Thanks for all the help. I am pretty much decided to get quotes for the following system install. As a reminder, my heat load calculations come out to 40-45k btu/hr on a 0 degree day with delta T=72, I have a 100 feet of fin tube baseboard and 450 sq. ft. of radiant floor slab(lower level into hillside). Very tight, well insulated2300 sq. ft. house set into a hillside.

1. Triangle Tube prestige Solo60 (I like the exchanger design, less maintenance supposedly) (converting from oil to propane)

2. Reverse Indirect tank (still undecide concerning brand, Ergomax or Everhot EA. I like the Everhot removeable exchanger for cleaning potential scale down the road vs. piping in shutoffs and drains to flush the Ergomax coils).

Looking for comments on the following questions.

A. Would it be more efficient and cost effective to run my current electric hot water heater and shutting down the boiler for the 3-5 summer months? Current energy costs in southern michigan = 12 cents a kwh total for electric , $1.75/gal propane cost, $2.50 fuel oil cost.

B. If boiler reverse indirect is better than conventional electric DHW during the summer, anything I can do with the electric tank to make it useful? Additional buffer storage, solar storage, etc. or garbage?

C. Which is preferred for boiler makeup water: softened or hard well water ?

D. I have a fireplace flue 2 feet from my proposed concentric air/vent for the boiler (both flues in the same chase, concentric is going up the existing oil boiler vent which is a 6 inch SS fuel oil rated flue, about 20 lineal feet total). Should I place another section onto the fireplace flue to raise it a foot or so above the concentric air/vent for the Solo 60?

Thanks for the advice and comments in advance!!

duke

For maximum condensing efficiency run the radiant slab as a separate zone directly off the boiler (not the indirect), run the baseboard off the reverse indirect set to the lowest temp that actually keeps the place warm (this may take some tweaking of the aquastat). If the baseboard can deliver the heat with 140F water or lower (and it might) you'll get some condensing efficiency out of that side too (but about 90%, not 95%) The designer may tweak some of the details.

Summer for summer hot water you can turn down the tank to 125F or so and get about 85-90% efficiency out of the TT Solo (some losses from cycling, some from standby). At $1.75/gallon propane burned with 85% average efficiency that's about $2.30/therm ($1.75/0.9therms/0.85 ) delivered to the water. At $0.12/kwh in a 0.90EF tank it's more like $4.00/therm. Looks like your electric tank becomes a lawn ornament until propane prices double. You can download a handy li'l spreadsheet and play "what if" games with future fuel pricing & efficiencies if you like:

http://www.eia.doe.gov/neic/experts/heatcalc.xls

It'll take quite a shift to make propane more expensive than electricity, but unlike electricity it's an unregulated market, and prices are volatile.

The boiler water should be softened, and in some places pre-treated with buffering agents, the kept at a 12psi or so to keep oxygen from infusing in and corroding the pumps, etc. The installer/designer should know what treatment (if any) is appropriate for your area. If there are no leaks, the system should stay pressurized without having to add water for years.

The Solo 60 can be vented, directly out the side- no need for sending it up a flue, which might complicate the installation (I doubt the flue-in-flue approach would pass code inspection) The venting can go laterally quite some distance, if the location most appropriate for side venting isn't close to where you want to install the boiler. It's a tiny boiler- can be wall hung lots of places.

Last (and probably least), if yours is primarily a showering household with at least 2 people, you can cut your summertime fuel use (and all hot water fuel use) by 20-25% using a drainwater heat exchanger downstream of the main shower:



The output of the heat exchanger should feed the cold supply to both the Everhot and the shower (or the whole house) for maximum benefit. Unless you get an 8 footer the output is going to be just a bit above room-temp, so it's not a big deal if someone else needs to run cold water when the shower is in use- it'll be tepid, but not hot.

At 12cents/kwh it's quite cost-effective, and still decently so at $1.75/gallon propane. See the 7.5cents/kwh present-value analysis done by the folks in MN. Also note, the value added of going with a bigger heat exchanger more than makes up for the additional cost, so get the biggest one that fits if you go this route.

[dukehayduk]

Thank you. I have another question about what part of the heating load to pipe to the buffer/boiler. My radiant floor heat load is only 1/8 (if that) of the total house load. My baseboard represents the other 7/8ths. I did another heat load calculation (heatpro1 Flash estimator). This one came out to 38k total load on a 0 degree day, 22k on a 30 degree day, and 11k on a 50 degree day. So now I have 4 differently sourced heat load calculations all in the same ballpark (38-45k total)

With my 102 feet of baseboard, I can get 32k btus out at 140 degree water, 26k btus at 130 degree water, and 21k btus at 120 degree water, according to Slantfin. I run my current DHW electric tank at about 120 degrees.

With the radiant piped directly off the boiler, and the baseboard coming off the buffer/DHW indiret, how should the ODR be set to get the most efficient boiler burns? I guess I am asking should we set the parameters to maximize the baseboard efficiency or am I not quite getting it? I have not been able to find any condensing/temperature curves for the solo 60, ie what temps does it condense at to get the 90-95%burns

Sorry for all the ?s. Trying to save the most, since I am on a tight budget from year to year since energy costs went up the last few years. Thank you for all your help so far.

[chris24]

Still can't decide if mod/con is worth it even with the $1500 t-credit after install. I'm replacing a way-oversized 173Kbtu Crown OF boiler with tankless DHW with a gas unit and am still shoping around. Most contractors are pushing the hi-eff units but after reviewing alot of websites and forums posts here and there, I'm still not sure it is the way to go.

One contractor is sugesting the Buderus GB-142/24 (84Kbtu) with an indirect hw tank after taking room measurements. Another is suggesting a W-McL Ultra at 105Kbtu after taking exterior measurements. I have done my own simple heatload calcs using an on-line site that seem reasonable for a 1965 2100sf 2S colonial in philadelphia area. Based on 0* ODT (low for philly) I came up with ~40 Kbtu/hr loss. Tried a few other website's calcs. and they all came in close. After reading up alot on different units, I think I will stay away from aluminum heat exchangers (listed above). I am looking into the USB/Burnham Alpine or the Triangle Tube Prestige Solo with stainless heat exchangers for mod/con.

I understand that these units run best when not oversized for the system load and are better suited for cooler running radiant systems. Some acceptances are larger volume systems. What is considered a large volume system? I have ~125 LF of 3/4"baseboard convectors (Edwards 2.5"- 72 fins/ft) being fed off a 1.25" supply and return that run 30 ft down the center of the basement. Would adding a large boiler water capacity idirect tank (Tringle tube "Smart" tank-in-tank IDHW with 6 gals of boiler water) work as a buffer to the furnace.

I was hoping the system could be zoned seperately (68ft d'stairs loop-57ft upstairs) at the existing take-offs but now not so sure after seeing how proper plumbing is so critical. Am I getting close to the large volume system preffered for a mod/con? Will it even get to condensing conditions?

The tax credit expires soon, might end up going to cast-iron at 86% AFUE anyway.Any feedback is appreciated.

[Dana]

Both Burnham and Triangle Tube have a good reputation, and the support network is good for both in your area. It boils down to the competence of the system designer and THEIR level of support that ultimately determine which is a better choice. In general, the smaller the min-modulation, the longer the burns, and the higher efficiency you can tweak out of the system. Bot modulation AND condensing count for as-used efficiency, and you'd be looking at the smallest of either the Solo or Alpine at your heat loads.

Breaking it up into zones increases the number of cycles and shortens them lowering overall efficiency, but may be necessary/desirable for balancing the system. Whether or not 6 gallons of buffer will be necessary or not may depend on your minimum operating temp.

To gain condensing effiency the return water coming back from radiation to a mod-con has to be under 125F, and 115F is signficantly better. But the practical limit for getting predictable performance out of most fin-tube is ~120F average water temps. With radiators or cast baseboard or radiant ceiling/floor it's fairly linear down to 100F or lower, which is what you'd need to get 95% operation out of it. Look at the manufacturers specs for 125F (or fudge-factor it from the 140F and 180F numbers most will be on the order of 200-250BTU/foot @ 125F), and compare your lowest-emitting zone's output to the min-modulation of the boiler. If the fin-tube's output at that temp is comparable to the boiler's min-mod you can probabably tweak it into never short-cycling without resorting to a buffer. If it's less than half the boiler's min-output at 125F you'll probabaly need to buffer & tweak some more.

With 125' of fin-tube at 200btu/foot you're pretty much going to be OK with the smallest of these boilers if you set the lower limit of the outdoor reset to something like 120-125F when you set it up, and bump the high-temp at design conditions to what it needs to be from the manufacturers' spec (or guess, and bump it down until it won't quite keep up.) Don't even think about trying to run fin-tube at 100F though- it just won't cut it from a control perspective, and you'll short-cycle the boiler into an early grave. If it has nice long (10minutes or more) burns at low heat load you can try lowering the output temp 5F or so and see if it short-cycles. Often you can do OK with 120F output water with fin-tube but rarely 110F.

Since most of the heating season you'll be at less than half your design conditions, you'll easily hit the low-90s for efficiency with the reset curve dialed in, since the boiler will be running pretty much at its lowest modulation and lowest practical output temp for the radiation the vast majority of hours. Don't use setback thermostats- it'll rob you of some tweakable condensing efficiency if you do. Any time it's running above 140-145F output (or more precisely, above 122F return water) you're pretty much at the 86-88% efficiency range with a mod-con.

[chris24]

I've been leaning towards the modcons and have made my decision to go this way. Thanks for helping me fall on the correct side. Now I need to talk my contractor into the "correct" size, the smaller one. He keeps saying the output should be closer to the needs of the IDHW heater. If I understand how this works, I just wont get its full rated 1st hour and peak capacities. With a household of 4 on a tankless system, we have learned how to cope with limited HW, Drawing baths are the hardest.

I found your reply on another thread here about managing micro-loaded zones (10/20/10; http://www.terrylove.com/forums/show...ight=zone+loop). I was thinking that the large mass (6 gals) of boiler water including another 36g of DHW would help buffer things as oppsed to other same-sized IDHW heaters that have less than a gallon of boiler water in them. I guess it would be set up as the p/s example you provided (I saw another post of yours on greenbuildingtalk.com that addresses the piping cofiguration.) Any other sites that you recommend? I would like to know what my contractor needs to do for the near boiler piping.

I like the TT Smart also because of the lower pressure drop achieved with tank-in-tank reducing pump requirements. I just can't seem to find buffer tanks on any supply house site for sizing issues. I am leary of a "reverse indirect" with DHW coils being able to provide enough HW at the lower boiler operating temps. to get the most out of the modcon. I saw the prices of Caleffi hydraulic separators (wow) and they do not fit into my budget ($10K).

You suggest not running with a setback thermostat at least when operating at/near condensing temps. Does this still hold true when/if I find the boiler temps. need to be above 140*F?

[chris24]

Another scenario I am considering. Assume I forego zoning which was suggested to allieviate the upstairs from going cold when I build a fire in the living room (2-4/mnth). The upstairs and downstairs convecting loops otherwise seem to be well balanced. On average, the upstairs is about 2*F cooler then what the prog. thermostat is reporting. (This could probably be corrected using the existing balancing valves on the system although I'm afraid to touch them as they are original installs). With my system size, I think should be able to to run in condensing mode for a fair pecentage of the heating season. This could alleviate the need for a buffer and the possible IDHW. I would also relocate the thermostat to a more central location although closer to the front door in the centrl hallway.

The IDHW would be replaced with a high-eff. gas-fired direct-vent unit that could be located closer to where the demand is and save a run of @40ft of 3/4" insulated pipe run through unconditioned space. It currently takes about a minute to even start seeing a sensable temp rise in the master bathroom shower or faucets. Other bathrooms, laundry, dishwasher would also be served better. Would the benefits of this more centralized location for HW supply outweigh the efficiencies lost in going away from IDHW. Any recommendations on stand alone hi-eff HW heaters. Is there away to compare the EF value to the efficiencies of an IDHW?

[Dana]

If you go the reverse-indirect route with a mod-con in order to micro-zone it you end up limiting your condensing efficiency somewhat in exchange for a high minimum burn. If your smallest zone has over 100' of fin-tube you'd be better off with a standard indirect running as a priority zone, since it would give you more hot water and a few percent better AFUE on the space heating. As long as the radiation of the smallest zone can deliver ALL of the heat of the mod-con's lowest fire the flow and reset curve can be set up to run in condensing mode most of the time, and won't need buffering to keep from short-cycling. A 100' loop of typical fin-tube typically delivers ~20KBTU/hr @ 120F average water temp, which means the return water will be in the 110-115F range, with boiler output in the 125-135F range, depending on how it's set up .

The min-mod of the smallest Alpine or Solo are well under 20K, so this is totally possible. Only if you micro-zoned it with a really stubby zone would you absolutely need to buffer it. If I understand it correctly your smallest zone is 57', which would deliver ~11.5K at 120F average water temp, and ~14K @ 130F average water temp. The min-mod input of both the Alpine 80 and Solo 60 is 16K, so at 90% efficiency it's output right in there with the 130F average water temp number- you can almost certainly tweak the flow & delta-T to get it to run mid-mod at temps under 130F without short-cycling (or buffering), and almost always be running better than 90% efficiency. If there's room to add length (or cast-iron baseboard) to the radiation in the coolest rooms you can most likely get it to run 125F or lower much of the time without having to add mass. (I'm a fan of replacing fin-tube with recycled cast-iron when you can get it cheap, since the output is better at low temp and FEELS better, and is fully specified down to 100F, where it's still putting out 125BTU/foot, a temp where fin-tube just doesn't operate reliably. It's over $40/foot new, but can be had for half that or less scrounging on Craigslist or scrapyards. Weil McLain's cast baseboard specs are nearly identical. If you lenthen or replace radiation, consider it, especially for rooms where you actually hang out.)

You won't necessarily need to go with a hydraulic seperator on a system your size either, but if you go that route, using the smallest Ergomax (~$1.2K) as both the hydraulic seperator adds over 25gallons of thermal mass to the heating system, and uses as pre-heat to a small indirect would mean that most of your water heating BTUs could also be had in fully condensing mode (during the heating season, anyway.) Otherwise, fabricating a low-mass hydraulic seperator out of copper fittings & pipe isn't rocket science. But I'm thinking your zones are big enough yet low-head enough that this could be done with a single pump + zone valves.''

BTW: The ASHRAE 99% design temp for Philly is 10F, the 97.5% design temp is 14F. Yes, it sometimes gets colder than that as an absolute low, but it never STAYS colder than that long enough to matter. The 99% design temp is based on binned hourly weather data over 25 heating seasons- fewer than 1% of the hours in a heating season will be lower than that number. If you're designing to 0F in Philly you're starting out with an 18% oversized assumption, (then typically adding onto that for some margin.).

If you run deep setbacks AND run outdoor reset, your recovery times are long unless you bump up the temp on the reset curve. With a condensing burner it's almost always a net-negative to do that. Even with 140F average water temp you're getting some condensing, hitting around 89-90%, and getting ~300BTU/foot out of your baseboard. You may NEVER have to exceed 140F on your system in Philly if you tweak the curves optimally. (Depends on the heat loss & baseboard length on your worst-case room, and how critical the temp in that room is.)

You can't reliably place an EF number on an indirect, since it depends a lot on the installation, and the BTU rating, combustion efficiency and mass of the boiler. With a tiny mod-con driving it and you insulate all the plumbing it'll be over 0.80, even in summer. If the indirect is set up to a low enough temp with a decent hysteresis a mod-con might hit the high-80s or even 0.90 EF in water-heating only mode. With a mid-sized cast-iron beastie behind it it'll be pretty close to the AFUE of the boiler if the boiler is less than 2x oversized for the peak load, but in summer the mass and standby loss of the boiler can take it below 0.40. On an annualize average it's still an efficiency improvement compared to a separate tank-heater + boiler though.

With 40' of 3/4" pipe you're abandoning about a gallon of hot water at the end of a draw. If you insulated those pipes with 3/4" walled pipe insulation (not the 3/8" stuff availabe at box stores), it's not a complete loss- you'll still have usably-hot water even 20-30minutes later. If it's faster hot-water response you're after, you could arguably install the boiler & indirect where you're proposing to put the tank, since a wall-hung mod-con is easily side vented, and takes little space. You may have to reconfigure the heating loops a bit to minimize the run-lengths of heating system plumbing in unconditioned space. From an overall cost and efficiency point of view I can't imagine goint with a separate tank would be the right thing in most situtations.

You might want to price out what a 55 gallon smallest-burner Phoenix would run you. You can run similar efficiencies as a mod-con + indirect with one of these, and it's inherently self-buffered. They're dead-simple to design around, and has plenty of burner for your needs- your design-day heat load is about min-mod on the thing, and it'll automatically run min-mod all the time except during heavy hot-water usage. I'll bet you can hit 93% without doing the math on the heating system design, just setting it to 125-130F and bumping it up 5F at a time only if it doesn't keep up in the coldest room. Without low-temp radiation, only baseboard you'd never hit 96% in your wildest dreams anyway.



Uh, sorry, I meant the Pioneer, (basically a Phoenix with a space-heating heat exchanger.)

[chris24]

That Pheonix unit looks nice and simple, I just can't seem to find a price anywhere for comparison to see if it falls into my budget. I think I will go with the USB Alpine80 & TT Smart 40 IDHW combination. Still not sure if it is worth the price and effort to zone out the upstairs seperately using zone valves if large ( is 6*F large?) night-time setbacks. Today it took about 4hrs. to drop 4* (35*F OD and fairly windy). As you describe, it seems better to just let the whole house drop, lets say 3* (68-> 65) for the night/day setbacks, if at all, then optimize the running temps. It's a whole new mind set vs. on/off control of oversized CI boilers. During the milder months I'll determine another running temp (flow?) that keeps me in condensing-mode longer. I'll relocate the indoor t'stat as good measure against the fireplace issue. I can't say how much I appreciate your feedback/knowledge here and on other posts except for thank you.