Ok I have  questions about  gas fired boilers.  This is for a new construction, house in Connecticut

~1050 ft^ house  Walls are 2X6 filled with cellulose + 1” isocyanate insulation ~ R30 clear wall value.  Basement is ICFwall construction

Ceiling is ~ R50, widows are either high solar gain R3.5 (south side) or R 5.2 other 3 sides.  There is a solar DHW with an 80 gallon tank.   There is only 1 bathroom (shower only)

Heating will be via a 3 fold approach,  1.  Passive solar, 2 small wood stove, 3 propane fired boiler.  I would also like to use the boiler as the secondary for DHW.  House also has a HRV.

Given the design temp for this area the max heat load as calculated by heating contractor is 21.2 KBTU/hr.  When I do the numbers, I get around 13 kbtu/hr.  

This house will be occupied by at most 2 people.    

If you account for solar gain, an 85% efficient boiler burns around 100 gallons of propane per year for house heating,  to my mind this makes the difference between 85 and 95% not so important.  The system will be side vented.   You can’t get a boiler this small anyway so any system will be oversized for heating. 

In the summer the solar DHW will hopefully mostly  take care of the hot water usage but I don’t want to take cold showers on cloudy days and all winter so some backup will be necessary year round.  Does anyone have suggestions as to a good boiler and indirect tank combo for low usage?  I have a friend who installs HVAC stuff who will install it.  He is suggesting a Utica boiler and a DHW indirect tank or a Crown Boiler Maui  which is a tankless combi boiler?

Any suggestions as to specific models/ brands? That would work well together?

Thanks,
Eric Anderson

You're overestimating the clear wall R value by quite a bit. The inch of iso is worth about 6.5, figure maybe another R1.0 for interior & exterior surface sheathing, but the 5.5" of cellulose by itself is only ~R21, and that's WITHOUT the thermal bridging of the studs. Depending on stud spacing, with bridging accounted for the clear wall R value (before iso + sheathing) of the 2x6 & cellulose part is between R14 and R17. The final clear-wall R-value finished is going to be ~R21-R24, not R30. (Could be your contractor's numbers are closer to reality.)

But even so, it's still a fairly small heat load, one that really calls out for a combi system.

The Maui's minimum modulation output is still 3-4x for your heat load, which means it won't actually hit it's AFUE numbers- it'll be lower than that due to cycling & standby losses. The Baxi Luna 310 comes closer (1.5-2x your heat load at minimum fire) and will give you higher annual efficiency:

http://www.wallhungboilers.com/pdf/salesgen_broch_BaxiLuna3ComfortBrochure.pdf

Most boilers don't have outputs anywhere near that low, so with cycling losses even mod-cons end up running about where a right-sized 85% efficiency boiler would run anyway. But some of the tiniest mod-cons will modulate down to under ~20K, (eg. Munchkin MC50, Peerless Pinnacle T50, Triangle Tube Solo 60), and may be price-competitive with bigger ~85% efficient combis. They'll a bit slower on DHW recovery than the bigger 85% boilers though.

The more adventurous price-conscious cheapskates will kludge something together with a tankless Takagi T-K3 or Navien water heater. ;-) The T-K3 modulates down to ~9k out- if done right, it can work, and will be more efficient than any oversized combi or tank HW heater. If you design your DHW solar storage to work as your heating system buffer tank as well there can be significant cost savings and some efficiency boost. Avoid running DHW through your heating radiation if you can or you'll be fighting scaling and stagnation issues- keeping the heating water isolated and in a loop avoids both (even if there's still some scaling potential on the DHW heat exchanger).

Against all warnings I'm going with an Ergomax reverse-indirect type heat exchanger/buffer tank and a tankless Takagi KD20 (modulates down to ~19K out) but that's probably not sufficient solar storage capacity for your application.

For space efficiency total system price and overall efficiency you may make out pretty well with the Phoenix Solar PHS130-80

http://www.stssolar.com/pages/STS_Phoenix.html

One tank for heating system buffering, solar panel input coil, DHW output coil, with an internal 96% combustion-efficiency heat exchanger all in a tight package. Price it out compared to separate systems- it may be right in there (or not). Even with an oversized burner it's standby losses and cycling losses will be pretty small, since the burner is inside the well-insulated tank.

there is nothing optimal, IMHO, heating a tank used for solar with fossil fuels.

the baxi is a problematic unit.. I would stay away. the small mod/cons are decent though.

the arguements dana and I have had on the setup he's doing are now quite extensive. suffice to say, I think his arrangement only makes sense if you have a very large DHW demand, and a very small heating demand, when you need a lot of BTU output, sometimes, for not much money.

but if your DHW is small and your heating is small, I think the extra cost, maintenance, and consideration for on demands are wasted. small mod/con if cost is no object, tank water heater otherwise, or if you really think the heating system is 'marginal', you could use electric on demands to boost, and keep a cold tank for solar.

DOH!! If there was ever perfect example of when a low-efficiency low-output tank HW heater makes sense, this might be it!

Even though fuel to support the standby losses of a tank HW heater might even exceed the fuel used to support his actual heat & DHW use it'll still be wicked-cheap to install and "cheap enough" to run. At some low load even a 40-60% AFUE doesn't much matter- so WHAT if his 100gallons/year of propane use becomes 150-200gallons? If the difference in system cost for the alternatives doesn't become present-value-positive on fuel savings in several decades it doesn't much matter that it's as-used efficiency will be pretty low. (It'll be reasonably efficient in the darker weeks of winter, but nothing but standby loss the rest of the time.)

With a woodstove and solar DHW, the tank HW heater isn't the primary source of heat in an already low-load situation. Somehow when I got to thinking about the Maui or other combis I was forgetting that this was the 3rd tier supplemental backup, not the primary thermal system as in my situation.

I haven't heard any Baxi horror stories yet, but your comment has me googlin'...

for the record, I'm still not accepting (nor seeing in the field) your 40 to 60% AFUE presumption for tank heaters. maybe the top end, but not 40%. Not for heating.

not sure what's on google for baxi's, but they got dropped up here in maine for reasons any of us who used to work with them would be aware of.

I don't doubt that HW tanks when used as the primary heat source would do way better than 40-60% in-situ (as opposed to lab-tested) AFUE- mid to high 70s would be my presumption in most cases- figure on about combustion-efficiency minus 3, if the burner matches the design-day heat load.

But here the design day heat load is lower- about half the burner output.  Were that the primary heat source you'd take off about another 2-3% for being 100% oversized, but it's not the primary heat source.  The larger fraction of the heating load is carried by a wood stove in winter, the solar array in the summer.  The 40-60% annualized was a WAG for Eric's situation.  He was estimating 100 gallons of propane a year for heating with low standby loss mid-to high efficiency equipment.  The standby losses of most tanks is going to be well over 50 gallons/year (WAY more if it has a standing pilot, which will burn 6-8 gallons/month x 12).  If it has an EF of 60  or better it may beat 65% for him overall, but don't count on it- meeting the performance level of it's EF test calls for heating 60 gallons/day every day, whereas for 200 days out of the year I expect he'll be using it only for very modest supplemental shoulder-season heating that gets mostly covered by passive solar & a stick or three in the wood stove- hot water, not so much, space heating even less. Depends on how big his anticipated solar fraction really is, and whether he lets the fire in the stove go out, but it'll be more standby loss than actual use a lot of time.

Mid winter it'll probably do better than 60%, but again, it depends on whether he keeps the wood stove stoked, in which case it'll do no better than it's rated EF, probably less on sunny days when he can squeak something out of his solar panels for DHW.  This isn't a yor usual space-heating with tank HW heater scenario- the tank is the hail-Mary backup, and as such, it's standby losses will be a large fraction of the (very small) total.  This means a dismal in-situ AFUE, but a happy situation since it's cheap, low maintenance and still doesn't use much fuel.

Are you seriously seeing in the field anybody who is burning less than 200 gallons of propane/180 NG therms annually in their tank-based heating/combi system?  If so, how are you measuring it's efficiency?  200 therms is lower than the number on the EnergyGuide label pasted on most tanks, a number based on EF test loads. Eric is apparently anticipating an even lower load.

well, the mid to high estimate you're giving now is about what I expected, but you're right in this case of course. This is almost like just doing DHW. except, you seem to skip over the fact that he is still doing DHW. maybe he whacks off 50% of his DHW with solar: but he's got a raw need of about 400 therms/years at 75%.. no passive for auxiliary. I hope for the guy too, but I doubt he's going to knock of 75% of that with passive solar and wood in CT. but if he did, he's right at the EF number, which is still sliiightly over your top range estimate earlier ;)

But really... is anyone doing standing pilot these days?

I am obviously not measuring efficiency: I am calculating energy use estimates though, and I am not horribly surprised by any results I've seen from tanks (except one, when the return water was "spoofing" the tank sensor, causing it to overfire like crazy), and 40-60% efficiencies would stand out... I assume. It's not scientifically rigorous, of course, but then, we don't live in labs either ;)

Shockingly, standing pilots on tank heaters are still sold widely!

40-60% efficiencies WOULD stand out, if you're burning 400+ therms/gallons a year, but under 200 it would be pretty murky. If his estimate of 100gallons/year for heating was right (and I suspect it's not, but it's probably the right order of magnitude), and his DHW use is small (likely with only 1-2 people living there) the solar fraction will be large- probably larger than 50%, and the total annual burn less than 200 gallons. But it's all just a WAG.

I think we agree: If he burns more gallons/year than the number on the EnergyGuide tag he'll have better efficiency than the EF number. Conversely if he burns less, he'll have worse efficiency. With tank HW heaters getting the efficiency is all about total volume. Even a highest-efficiency 0.66 EF electronic ignition forced draft NG tank drops below 50% efficiency when only 28 gallons/day are drawn. (See Heater 3, page 9: http://aceee.org/conf/08whforum/presentations/1a_davis.pdf )

With tankless HW units getting the efficiency is all about minimum burn lengths. Those sub-2 gallon handwashing draws just KILL the operational efficiency of a tankless. The EF test somewhat overstates as-used tankless efficiencies, since it depends on the actual hot water use profile- the draws in the EF test are all 10 gallons at a time- no short draws. Even at very low annual use as long as draws are never shorter than 6 gallons a tankless will meet or beat it's EF numbers, independent of the total daily/annual volume. The previously referenced document also tested a condensing tankless, and it too dropped to ~75% efficiency in the low-use profile, but that's easily explained by the percentage of the total that short draws represent in the low-use profile used. If minimum burns are maintained (as would be the case in space heating use), they'll do better than that- see fig 5.1 (p48 of the pdf) here- http://www.energystarpartners.net/ia/Water_heaters/Documents/WHPAGette_Final.pdf

A Takagi Jr costs about the same as a typical power-vented, LP tank heater, and for a hundred or two more it can be set up as a sealed combustion unit using a standard kit. Even at use levels of 200-250gallons of LP/year, @ $2.50/gallon + there may be an economic argument for yet another Rube-Goldberg tankless combi heating contraption, since it'll save more than 50 gallons/year... (As Morgan rolls his eyes...;-) ) I'm not recommending, just sayin'... The installation costs would be more than with a tank, but since his buddy is doing it shouldn't be too painful- not any worse than the oversized Maui combi he was considering. There's probably even a rebate available in CT for installing a tankless, which would offset some of that.

But I'd only recommend going that route if his buddy the HVAC guy was very comfortable designing around it's quirks.

I spoke incorrectly when I talked about a clear wall value.  I was referring to the part of the wall that does not include studs, headers etc.  The house is built using OVE with warm corners, ridged foam insulated headers,  raised truss etc.  From outside to inside, the wall is 1” pine shiplap, ¾” airspace, 1” isocyanate insulation, zip wall panel, cellulose insulation, drywall air sealed with acoustical sealant with all penetrations calked, and or foamed, for heat loss purposes I am assuming  this wall assembly will be ~R24 overall.  I figure the 13 Kbtu/hr I quoted is the lower limit on what I need and 23 is the upper limit on what the average couple living there would need.  I sized the baseboard to deliver 24 KBTU/hr at 175 deg water.  I figure if actual demand is lower, I just decrease the max water temp.  I would assume I will use an outdoor reset anyway. 

Thank you both for your suggestions.  One of the things that worries me about any system is that I am gone for long periods In the winter.  I race iceboats so I am gone in the coldest part of the winter for 2-3 weeks and almost every weekend.  When I am at home the boiler is used primarily for DHW, when I am away in the winter the DHW is off.    One of the reasons that I did not look at the tank type heaters was that generally they seemed to have lower lifespans then boilers and Indirects.  Is this an incorrect assumption on my part?    I hope the basement will not freeze if we lose power based on this winters results.  Even with no heat in the house and no real insulation upstairs the basement temp never went below 32 this winter.  The upstairs would definitely freeze unless there was significant sun.  I am assuming when I leave for several weeks I will drain the water out of the plumbing system, and dump antifreeze in the traps,  but the Hydronics will still be screwed because the baseboard is still upstairs unless I use glycol In the loops. 

The solar hot water heater is an 40 ft^ flat plate collector with an rheam 80 gal storage tank.

The house has 70 ft^2 of south facing windows ~7% of floor area.  In addition it has a 65 Ft~2 solar air heater that delivers heat to the basement so I have the slab as a heat sink. 

http://s45.photobucket.com/albums/f66/eric_anderson5193/?action=view¤t=housepic.jpg

I am building this house to live in hopefully long into retirement so I am willing to pay to do it as well as I can

I am a bit of a freak when it comes to temperature.  I keep my bedroom at 50°F, the rest of the house at 55 and the bathroom seems to end up at around  62.  It is kind of nice to warm the house up to 62 in the morning for an hour or so when I get up.  In the evenings  I always figure if I want it warmer than 55 I have to light the woodstove.  Wood is an unlimited resource for me other then the time to cut and split it.   I can tolerate a heat range of ~50-70 degrees in the winter so the fluctuations from passive solar and the woodstove don’t bother me. Typically I try to tough it out and not turn on the heat at all until thanksgiving.  The heat gets turned back off about now in the spring.   When I am away, the temp is set to 50°.  I think that if I did not have indoor plumbing, I could happily get away with only a woodstove and passive solar.    I don’t think my ex girlfriend was too happy with my heating habits though.   

50F? What, are you BRITISH or something? :-)

With R24 clear-wall and a slab as your thermal-mass buffer you don't need to much worry about the 23KBTU/h peak load, since it generally won't stay that cold long enough for the indoor temperature to drop much even if you're flat-out with a 15K burner. Design-day temps are designed with more conventional R8-R12 clear wall insulation and low interior thermal mass in mind. You can probably do just fine designing for for the AVERAGE temp of the typical annual coldest day of the year, minus 10F, or something like the 80th-85th percentile of binned hourly temperature data (the ACCA design criterion is the 97th percentile of binned hourly temperature.)

This means you can probably COULD run your radiation at 130F and even "normal people" wouldn't get cold, even on the colder days, except during extreme & lengthy once in a decade cold snaps (in which case, turn it up!) but with an oversized burner it'll never run very continuously anyway- let the high-limit be "whatever" and forget about it. Also forget about modulating the output of an oversized bang-bang on/off boiler with an outdoor reset- the net savings will be under 10% and the net cost of the controls high relative to those savings. The way you're operating it your only significant burns will be the AM warmup, and after you use a big slug o' hot water, so just set the high-limit as high as reasonable and let it run until the thermostat is satisfied. If you were trying to maintain the place at 68F 24/7 and this was your only heat source an outdoor reset might buy you something, but you're not going to get much if anything out of heating it up more slowly for your hour of 62F bliss. (I'd guess that much of the time the radiation temp will never hit 175F before the thermostat is satisfied anyway, unless you're keeping windows open to get your room down to 50F for sleeping.)

Tank heaters will last a decade or so, cast-iron boilers & indirects can last indefinitely (50 years?). Better modulating/condensing boilers will probably go a coupla decades on minimal maintenance, but then so will most tankless HW heaters. (At your use levels, even if used as a boiler a tankless could go on, and on, and on... Your annual duty cycle will be about what a family of 4 would get using it for hot-water only.)

If system longevity is your primary issue, can't argue much with getting the tiniest 2-plate cast-iron beastie you can find that comes with electronic ignition (there are still quite a few standing-pilot version being sold),say, something about the size of a Burnham P202, which has an output of ~30K. (You don't say which model Utica he was recommending, but the smallest MGB series has a 42KBTU output- go smaller, MUCH smaller if you can) It'll be on the small side for both indirect HW and space heating, but it'll do the job adequately 99% of the time, and will have higher efficiency than a tank heater of equivalent burner size (due to the very low standby loss of the indirect). In space heating mode it'll probably fire once/day for 15-30 minutes for your AM warmup, then never again until the next day (or next week?) with the thermostat programmed to YOUR standards.

By going with the smallest in the line, if it's less than 200% of your peak load you'll probably still get over 80% out of it even with intermittent use as long as you kill the indirect heater whenever you leave, so that it isn't doing a bunch of temp maintenance cycles on the indirect for no good reason. It'll never truly short-cycle, but the boiler will cool down completely after every fire. But as long as your burns are always over 15 minutes it'll likely hit around it's AFUE test numbers. The only likely time it'll short-cycle is when serving up a maintenance temp burn on an idle indirect tank, (which wouldl be a pretty lossy burn, since it has to bring the boiler up to temp first), but still better than you'd do with a tank HW heater.

With the solar HW as pre-heat under-sizing of the boiler for the indirect will be less of an issue, but DO make the indirect a "priority zone" or your girlfriend will be screamin' atcha from the shower if you take one first! ;-) Most of the time with tepid-to-warm solar pre-heated water coming into it 30KBTU should keep up just fine until you've depleted the 80 gallons of the solar tank. But on darker winter weeks you may want to monitor the solar tank temp if you have overnight guests... If it's under 50F in the solar tank, best to let the guest shower first.

OTOH, I keep forgetting- you're almost never using it- a 50KBTU cast-iron boiler won't break the bank on operating efficiency either, and it'l ALWAYS keep up with your hot water loads (even your girlfriends! :-) ) and it'll still be more efficient than a tank heater, somewhat less efficient but more durable than a tankless combi. It's all "in the range", and none of it much matters if you're heating with wood, passive solar air, and have solar HW. The duty cycle on the equipment is low.

It is kind of overkill, but in the end I went with a munchkin 50 KBTU mod con boiler and a superstor  ultra 45 stainless indirect tank.  The difference in cost was about 800$ over a  non mod con  boiler.  Becasue the boiler won't be able to keep up with DHW fully under high demand in the winter, I am going to connect power to the backup electric coil in the solar tank and just flip the breaker  when I have guests in the winter.   

Cheers,
Eric

50kBTU boilers may not always keep up with domestic demands, to contradict dana on that (and also on the longevity assumption, I just replaced a 12 year old cast iron boiler in my own home recently... don't assume 50 years on modern cast iron!).

eric, maintain the 140 degree recommended temp in your indirect (with a tempering valve for safety) and just set the electric aquastat to a lower temp like 125. then leave it on, presto, automatic backup.

Excellent choice.

I think the Munchkin and indirect will keep up (you could get a bigger one and store more). The Combi 1 is 82% efficient! The average CI boiler lasts 35 yrs. (of low-efficiency).

Sealed combustion, direct vent, outdoor reset; the ModCon boiler has it all and will not short-cycle (less than 5 minutes is considered a short-cycle for a Mod/Con) on a typical slab panel.

Rob,
The cold water goes into a 80 gallon rheam solar tank that has a backup electric coil.  The hot water output from that goes to the cold inlet on the superstor tank. 
What I was planning on doing is setting the tstat in the su 45 to 120, so any time the solar tank is over 120deg, a call for water  won't triger the tstat in the superstor tank.  If the water in the Rheam tank is say 95 deg, the boiler can keep up with the demand untill the solar tank cools off, which should net me ~ 100 gallons of water before ths superstore can't keep up and the temp drops.  

I have a tempering valve after the superstor tank.  
My worry is only in the winter when  the solar tank is 50 deg because it has been snowing for a couple fo cays and the collectors are covered and I have multiple guests when the munchkin can't keep up.  Then I throw the breaker for the rheam electric coil.  Mayby I use it once or twice a winter.

Cheers,
Eric 

gotcha.. missed the solar part, sorry about that.