Questions to Ask Boiler Contractor

[larryleveen]

I live in the west-side of WA state, where it rarely gets very cold. I am replacing aging gas forced-air furnace and gas tank-HWH. We have a small 1200sqft house that has been retrofit with insulation (walls, floors and attic). We are pretty good about not keeping the thermostat ~66 when we are home/awake, and are good at conserving water in showers. We are looking into getting a high-efficiency gas boiler and hydronic radiator (not in-floor radiant) heating AND hot water system:

Scenario #1: a Triangle Tube Solo 110 with SuperStor 60 tank with two heat exchangers AND a solar DHW pre-heat Veissman flat plate collector closed-loop.

Scenario #2: If I decide not to do solar, a Triangle Tube Prestige Excellence boiler (has a smaller internal tank, so no external is needed). Our house is small and space is precious, so that is attractive.

I am getting a quote from an experienced, licensed contractor. I would like to know what questions I should ask him. Also any thoughts you have about these scenarios (solar compatible vs. not) are appreciated. Thanks.

[Dana]

First off, the Solo 110 is probably at least 2x oversized for your loads- a Solo 60 would do. (I live in a bigger not-so-well insulated house in MA with design temps 15-20F colder than Seattle's, and the modulated output of my system never exceeds ~38KBTU/hr, and that's when all zones are calling for heat, when somebody is taking a shower.)

Solar would almost never be cost-effective, but a drainwater heat exchanger downstream of the shower would be (if you shower rather than tub-bathe.) See: http://www.efi.org/wholesale/pdfs/power_pipe.pdf That would also eliminate any question of the a 60MBH boiler being too small. I can link you to more info on this stuff if you like. The biggest/longest/fattest that fit the available will be the most-cost-effective. They do nothing for tub-filling capacity, but you can get better than 50% return of shower use energy (which is like adding 20-30KBTU to your boiler output while showering, depending on your flow rate.) Size the indirect for your tub fills, and you're done.

Alternatively the Excellence would PROBABLY work, but you might have flow issues if you have high peak-demands for hot water. It's minimum modulation is probably 1.5-2x your heating load, but it's max might come up shy if you're trying to fill a tub and take a shower at the same time in January. My WAG would be that your design-day heat load is less than 25KBTU/hour, probably even under 20K if it's a reasonably tight house with reasonable amounts of window area, and all windows double-paned or at least fitted with storm windows.

Demand a computer generated room-by-room heat loss calc from contractor for both radiator & boiler sizing. Those that offer that offer it up without you haveing to ask move to the head of the line.

Ask them for cheapest/nicest/best for radiator types. There's quite a range out there- it need not cost an arm and a leg, but you may want nicer-looking versions in some rooms rather than purely-fucntional versions in others. Cheap fin-tube baseboard can work with condensing boilers, but the practical limit for most is ~120F water, below which it's tough to design for output. Taller or cast iron baseboards work better, but can be pretty pricey. Low-temp panel radiators are probably a better choice for most rooms.

[larryleveen]

Thanks, Dana. I do not know anything about "sizing" the boiler. If there are any online resources that a layperson would find useful, please send them along (and any other relevant info, as you kindly offered). The Solo comes in a 60, but the Excellence only comes in a 110. Am I to understand that there is a _problem_ with oversizing? Is it like running an 18-wheeler to carry a six-pack of beer -- i.e. inefficient? Was there a reason that you described the Solo 110 as 2x oversized, but the Excellence 110 1.5-2x oversized? Sorry if that sounds picky or rude, I'm just trying to understand what accounts for the slight (?) difference there. Here is some more context on my situation:

- We have one bathroom. Therefore we cannot shower and take a bath simultaneously. We are a couple and can easily ensure that no laundry or dishes are done during a shower. It's not an inconvenience to us.

- The bathroom is on the first floor. We have no basement -- just a crawlspace. IIRC, the vertical drainpipe in the crawlspace is VERY short. I'd have to check to make sure I could even _use_ a Power-pipe!

- The house was built in 1937. I would guess it is decidedly not tight. We've done some insulation (mentioned above), but I don't have too much faith in the stuff blown into the walls -- seems like it is hard to get a good consistent coverage, and is likely to settle anyway.

- We replaced most of the windows when we bought it ~5 years ago. There are two older picture windows -- right near two new picture windows in the front rooms. I'd guess that for the size of these rooms, they have significantly more than average glazing, though at 1200 sqft, the house is small.

I hear ya on the radiators -- thanks. I know they are not all the same _visually/stylistically_ and have asked for specific make/model info so I could look at them online, but I didn't know about the operating temp differences of fin-type versus cast type. Tell me more about this. What do you mean by "the practical limit for most is ~120F water, below which it's tough to design for output". What is tough about it? I will spend more for better/nicer/properly sized/matched stuff. The problem is, I don't know know what that is! Any help is greatly appreciated.

Thanks again!

[Dana]

Oversizing a boiler mean it cycles on/off more than is optimal, affecting both efficiency and longevity. If your burns are shorter than 10 minutes at a time it'll have an effect. At extremely low heat loads (say Seattle in April) you're relying on the thermal mass of the system to keep it from short-cycling. Modulating boilders are designed to do load-tracking based the outdoor temp for very long burns, but if your design-day heat load is less than the lowest-fire output it doesn't have much advantage over a bang-bang on/off non-modulating control. The internal buffer tank on the Excellence probably keeps the minimum burn time over 30 seconds, but the heating system may need a buffer-tank with 30-50 gallons to keep it from short-cycling if it's a baseboard system. Radiators will have add more water-volume==thermal mass, but with 30K as a min-output you may still need some buffering. (An electric water heater not wired up is often the cheapest buffer solution.)

When looking at a modulating boiler it's the minimum, not the maximum firing level that ultimately determines how oversized. Lower output at min-modulation is always better for efficiency and maintenance, since there are fewer on/off cycles, and longer burn times. The miniimum modulation of the Excellence or Solo 110 is ~30KBTU/hr, whereas with a Solo-60 you're at about half that at ~16K- you'll get at least SOME modulating benefit out of it in space-heating mode, but the min-mod on the bigger burners is already likely to be over your peak heat loads.

Sorry about the confusion about calling one "at least 2x oversized" and the other "1.5-2x oversized"- it's the same burner, different packaging & features. They're both oversized for your space heating loads. The Excellence has enough output for your hot water flow needs, but you'd want to use a high-mass (== high water volume, not necessarily heavy-iron) radiation or a buffer tank to keep it truly happy in space-heating mode.

To take a 2.5gpm shower with 45F incoming water from the street and 105F water coming out at the shower head takes 75KBTU/hr, which is more than the Solo-60 would deliver, which means you either need an indirect-tank &/or a drainwater heat recovery heat exchanger. Even a 24" x 4" stubby will deliver something close to 25% recovery, which may be enough to give you the "endless shower" experience using the Solo-60 without a tank, but it would be cutting it close. And filling a 30 gallon cast-iron tub with 110F water could take awhile without at least SOME hot water storage going with the smaller boiler. But efficency & longevity would be somewhat higher with a Solo 60 than it's bigger-burner siblings/cousins.

The tough part about designing fin-tube baseboard for water temps below 120F is that it's response with temperature is no longer a linear or predicable funciton- the convection currents induced by the difference between the room air and the water temp begin to fall off rapidly, and can be adversely affected by things like dinged/bent fins or even dust-kittens that accumulate between vaccuming of the baseboards. Radiators and radiant-type cast-iron baseboards continue to have predicable BTU output even at 85-90F water temps. With fin tube you're still getting something, but it's not consistent- there's no way to reliably specify how many feet of it you need to deliver a particular BTU/hour rate to the room with temps lower than 120F.

Triangle-Tube boilers really are great & all, but there are many good condensing boilers out there. NONE are any good without the right system design and technical support though, so be flexible about manufacturers if heating contractors suggest others, but look up the specs yourself, and be prepared to discuss what the minimum-modulation vs. peak & average heat load of the house is. Even the tiniest mod-con can support your hot water load with an indirect-fired tank, but even the nicest boiler is a dog if it's minimum modulation is well above your peak heat load, which is why you really need a good heat load calculation for the whole house to pick a boiler, or to design the system to work efficiently around any particular boiler. You need the room-by-room heat load numbers both to get a good balance, and to be able to deliver the heat at water temps low enough that the boiler works in condensing mode most of the time. When the water temps returning to the boiler exceed 120F by very much the efficiency drops pretty rapidly to the ~ 87% mark, then more slowly with rising return temps. If it's set up with radiation that requires 150F water even when it's only 40F out, it'll almost never condense, and you'd only get about the same efficiency out of it as a much cheaper tiny 30-50KBTU/hr 2-plate cast-iron beast.

Flat-panel radiators that might be appropriate look kind of like this:



But there are also things like heated towel-rack radiators, or designer radiators for the tres-chic designer types that'll set you back a heluva lot more per BTU delivered:








So, there are alot of options- the thin steel panel radiators are probably the least expensive unless you're dropping all the way down to fin-tube baseboard. Cast baseboard is going to run you $40-60/foot, which adds up. (Those can be had used as well, for a half price or less.) Euro-modern & designer panel radiators and towel racks turn into real money real fast too. I personally like to go with recycled Burnham Sunrad or Radiant or similar 5" deep cast iron in classy older houses (they can be sandblasted & repainted quite easily with auto-body type paints- no need for high-temp paints since you're not running steam here), but the sizes you need might not always be readily available on the local used-market. (But they are from the Burnham distributor for only half a leg and one arm. :-) ) But you'll know better what YOU want, and where.

As for your insulation issues, blown cellulose and some of the super-fine fiberglass (JM Spider, or Certainteed Optima) insulation can be "dense packed" to deliver extremely low infiltration rates an ZERO settling. Low density cellulose (2-hole method) will settle ~5% in wall cavities over a couple of decades, at which point it's behaving about as well as R13 fiberglass batts would have on day-1. Blown insulation almost ALWAYS outperforms batts, since it is inherently gap & compression free the day it's installed, and if dense-packed to the right density, it never settles. Even low-density cellulose retards air movement better than standard density fiberglass though. Only the new-school superfine fiberglass blowing wools match that aspect, and only when installed at 1.8lbs/ft^3 density (or higher), not the more common/cheaper 1.0lbs density. Rock wool, fiberglass, cellulose, whatever it is that you installed, feel good about it- it's probably outperforming typical 1950-1990 2x4 construction with batts and will continue to do so for some time.

[Dana]

(I hit the number limit for image posting on the previous post so I clipped this from the middle to repost)

Cast-iron baseboard looks like this, and will deliver ~210 BTUs per linear foot @ 120F water temps, 125 BTU/ft @ 100F:



And classic radiant-type convecting cast-iron radiators that work well at low temp and "architecturally appropriate" for a '30s vintage house look like this:







Burnham still makes the "Radiant" series, but they're often available used at lower cost from recyclers and antique home outfitters, etc.

[hj]

I doubt that solar water heating would be cost effective in Washington. Drain water heat collectors "sound green" but given the amount of water used, the final temperatue when it enters the drain, and the limited amount of "actual heat transfer" area they are usually more of a waste of money, than an effective money saver.

[Dana]

I doubt that solar water heating would be cost effective in Washington. Drain water heat collectors "sound green" but given the amount of water used, the final temperatue when it enters the drain, and the limited amount of "actual heat transfer" area they are usually more of a waste of money, than an effective money saver.

When it allows you to down-size the output of the boiler &/or indirect, drainwater heat recovery is cost effective up-front since it's close to cost-neutral, and allows the space heating boiler to operate at a more favorable point for efficiency. Whether it's cost effective in other scenarios is a function of the volume of water going through the shower head & heat-exchanger, the temperature of your incoming water, your water heating efficiency, and your utility costs. For most 3-4 person families heating water in a 0.90 EF electric hot water heating @ 7cents/kwh there's a financial argument for it at $1000-1500 installed-cost, but clearly not all: http://www.renewability.com/uploads/..._minnesota.pdf

But in ANY situation it has a much higher ROI than solar hot-water.

"...effective money saver" doesnt' mean much when you're talking $200/year in total hot water heating costs, but something else if you're spending $500 on hot water. In showering households it WILL save 20-25% of the hot-water heating bill, but that's not a huge number for 2-person families in low-utility cost areas. While 20-25% isn't a heluva lot, it's getting onto a third to half of what typical solar hot water installations return, at about 10% or less of the up-front cost. For showering households it's a better bang per buck than buying into a condensing on-demand tankless HW heater (for fewer bucks), and edges out the efficiency boost of an indirect water heater on a high-mass hydronic boiler at similar or slightly lower cost.

Methinks you're underestimating the effective heat transfer area and heat transfer efficiencies in gravity-film type drainwater heat exchangers. This isn't some krap-huckster-sales new-fangled thing. They've been in use for nearly 30 years, have a well-measured well-known transfer efficiency at realistic shower flows & drain temps. In cool-water Canada and many US states they're subsidized at rates based on their actual performance data (usually using Natural Resources Canada's test data for particular makes & models), in much the same way that indirects, tankless HW heaters, condensing furnaces & boilers are.

Crudely measuring mine I'm getting ~20KBTU/hr out of it during the warmest months, and well over 25K during the months when the incoming water water is colder, and in my family's water use patterns it gets ~30 minutes of use per day.

At the buck-a-therm I'm currently paying & ~75% water heating efficiency (annualized, possibly a best-case estimate for actual efficiency) it's not exactly a huge payback on the $600 I paid, but it does OK in a 25 year NPV financial analysis even at 0% fuel-inflation (better than most $500-1500 low-risk 25 year investment instruments do after taxes.) The sub-$50 annualized cost reduction isn't something that I'll be jumping up and down with excitement about, but it's real, it's in after-tax dollars, it's better than a 5% ROI (simple analysis), so WTF? It does much better at the $1.50/therm I was paying when the decision was made to install it, and at $2/therm (should we hit that lofty retail price after carbon taxes, economic recovery, or both) it would look downright excellent as an investment, if not big money overall.

But for me it paid of instantly on spousal-satsifaction, being able to set up the burner near it's min-modulation to squeak out a couple percent boost in space heating efficiency, all without experiencing the cold-shower effect, eh? :-) (Clearly YMMV.) With the shower running and all zones calling for heat the burner output never even ramps to 45K, and spends most of its run-time between it's 15K min-mod and 30K.

[larryleveen]

Interesting stuff Dana. Oddly, since we are a couple who use water efficiently the payback might be even slower. I wonder how well PowerPipe maintains its effectiveness as slimy stuff grows on the inside of the waste pipe. I can only assume that when the water loses direct contact with the metal pipe, heat wont transfer as efficiently. I expect slime to conduct heat poorly.

I'm still stuck on the boiler sizing issue. Here is my level of sophistication: http://www.hvaccomputer.com/hvac/sizer.asp ;-)

The heating guy I was talking to said he does not do a formal "manual j" heat loss calc. Rather, he used a more general figure for loss as follows: 20Btu/sq ft x 1200sq ft = 24000 loss per hour. DOES THIS SEEM REASONABLE? FWIW, we tend to heat to 67 during the day, turning down to 58 at night, and live in Olympia (like Seattle temps).

The Solo 110 and Excellence 110 units have "input modulaton" of between 30-110 MBH. The Solo 60's is 16-60 MBH. Is that what is telling you that the 110s are/might be oversized (i.e. because 30,000 > 24,000)?

My main questions (in addition to the two above) at this point are:
1. What is the importance of water volume in choosing a system? If the numbers above are right, the Solo 60 with an indirect tank would work. Would a an Excellence 110 _without an external tank_ (it has an internal tank-in-tank) work, or does it need more water mass to operate properly for space and DHW heating?

2. Solar preheat is still in the picture if a Solo boiler is used. I was thinking of a dual-heat exchanger indirect tank, which would save some precious space compared to two separate tanks, but reduce the effectiveness of the solar assist on marginal days. If the solar assist was isolated in its own tank "upstream" of the heating/HW tank, it seems like it would work better. Combining the two exchangers in one tank makes them more "parallel", though we'd set up the solar to use the lower heat exchanger, where the dip tube inserts cold water from the street.

Cost-effectiveness is an interesting term. It seems that too often, rising energy costs are not factored in (I don't think they'll go down). Neither are the non-monetary/non-quantifiable costs, such as "reducing carbon footprint is inherently good".

[Dana]

The slimy stuff mostly sloughs, rather than grows on vertical smooth-bore copper drain, around which these are built. IIRC the anticipated lifespan at which it's efficiency has been reduced to 75% of it's day-1 performance is on the order of 40-50 years, based on studies that started in the 1980s when they were first being analyzed. (I don't know if they're continuing to monitor, or if they just trust the regression analysis on the first 5 or 10 years or whatever it was. Manufacturers' fine print usually gives either a 40 or 50 year lifecycle, but they don't footnote what they mean by that.) It does lose performance over time, but it's a very slow ramp. It'll outlast any solar thermal installaion.

The BTU x square footage estimates are JUST the sort of crap that leads to unbalanced room-to-room heating and oversized boilers, which EXACTLY why you want a manual-J type heat load calculation. I'm saying "...20 btu a foot times 1200 feet..." absolutely ISN'Ta reasonable approach to heating design, even if it might come close in some instances. Every good heating design starts with a real calculation based on real measurements. Heating design & heat loss software packages from boiler manufactures aren't all that difficult to use, and produce good results. Don't waste time and money on a contractor that wants to save an hour or two of measuring and data entry to deliver a potentially less-than-satisfactory result. Even if they charge a few hundred for the service to run a real calc it saves on radiator & boiler costs (not to mention aggravation) to get it right the first time. For $50 you can do your own with cheapie packages like HVAC-Calc . SlantFin used to have a pretty-good downloadable freebie that overshot the whole house number by ~25-30%, but gave the right balance room-to-room- I'm not sure if there's an archive where you can dig it up (IIRC it was called Hydronic Explorer.)

Assuming 24KBTU is the right number (and rule of thumb estimates like that usually overshoot by 1.5-2X, possibly more since you used blown insulation rather than batts and have newer-tighter windows), yes, if the min-modulation of the boiler is over your design-day heat load it's oversized (by quite a bit, actually, but there are very few boilers available in the US with min-mod less than 10KBTU.) A perfectly "right" sized boiler would be running at full tilt at your outside design temperature to keep up. If your real heat load at 6AM on the coldest day of the year is a (more likely) 18-20K, a boiler with a 20K max output would be the right sized, and one with 24K of output at max fire would be modestly oversized. A Solo 60 puts out even more than 2x your heating-guys likely overestimate. But with mod-con boilers, it's the MIN modulation that has the biggest effect on as-used efficeincy- smaller is always better, since it results in longer burns (==less standby loss), fewer cycles (==less flue-purge & ignition cycle losses), and generally greater comfort.

The water volume in the heating system adds thermal mass, and determine the minimum burn length affecting both comfort and efficiency. In mid-April your overnight heat load might be as low as 3KBTU/hr, and your mid-day load half of that. With a lot of thermal mass the boiler can run long enough to be truly efficient, the room temps will be more even, and the time between burns much longer.

On those days your hot water heating load may exceed your space heating requirements by 3-5x, but if it's set up with a tiny internal tank you'll get a lot more cycling losses than if you go with an insulated external tank. The size of the tank + the full-on boiler output determines just how long you get to shower (or how big a tub you can fill in a reasonable amount of time.)

In a net-present-value financial analysis both fuel-inflation and the cost of borrowing are factored in to determine whether it's cost-effective over any specified period. If the present-value of the item is more than the cost of the items over the time period you're considering, it's cost effective. If not, it's not. Some things are never cost effective, even when analyzed over the complete lifecycle of the product. If you're anticipating a huge energy-inflation over the next decade, that will skew the result by a lot. But while energy costs have been volatile since the 1970s, the cost per kwh or therm/ccf hasn't had a clear positive trend- electricity has gone down in inflation-adjusted pricing, natural gas has been both up and down, but with the exploitation of coal seam reserves and recent massive shale gas discoveries beginning to be tapped in the energy hungry northeast, the demand-pressures driving the price higher between 1995-2006 have been largely relieved. A carbon tax would introduce both a step-function on immediate price, and a demand pressure as coal-fired power generation is displaced by gas-burners. Predicting future fuel prices has been, and continues to be, a fools errand (played out daily in the commodities market.) I'm not placing bets in either direction on the price of gas OR electricity, since the cost of readily-implemented efficiency measures are cost-negative even in 1-5 year time frame let alone a couple of decades. (That is, broadly in the market, if not at your house.)

What's not in a NPV calc is the externalities of things like air & water pollution associated with energy production, etc. and what value needs to be placed on those.

Buying the dual-exchanger tank is an upfront cost that gives you an option if/when a signficant thermal array makes clear economic sense. It's a matter of how much you want to pay for that option. Mind you, solar pre-heat unlikely to make more than 100therms or ccf of gas use per year for a 2-person family. If you have a reasonable southern exposure you can probably get that much or more fuel use out of a few thermal air panels at less than half the installed cost.

OTOH, in Olympia's climate and hydro-heavy grid, going with a hydronic air-source heat pump like the Daikin Altherma for both heating and hot water would have a lower carbon footprint and lower upfront cost than a Solo-60 and a solar array, as well as lower operating costs. See: http://www.daikinac.com/DOC/PCAWUSE0...-%20Daikin.pdf

They're not cheap, (but neither is a mod-con + solar DHW) but it might be a better way to go long-term depending on your goals. Your outdoor design temps are high enough that you can probably even disable the backup resistance heaters on the thing and still meet your design-condition load. (I know of an Altherma installation in Maine where the design temperature is about -15F- they definitely need the backup heaters, but you won't- your design temp is 25-30 F warmer than that.)

[Dana]

To get a better handle on the whole-house heat load (not the room-by-room balance), we COULD measure it using your existing heating plant's fuel use, correlated with heating degree-day data, if you have a gas bill during a mid-winter heating period, and the BTU input & output numbers off the rating plate of the old furnace.

Olympia's 99% outdoor design temp is 16F, the 97.5% design temp is a modest 22F. By correlating fuel use to a particular stretch of weather data to determine the heating degree-days thaf fuel was correlated to and convert that into btus-out per heating-degree-HOUR to calculate the BTU rate what you'd need to support the load at +16F or +22F. If the beast is really ancient and has leaky ducts it'll be an upper-bound. If the ducts are tight and the burner has seen regular maintenance it would be about right though.

As a point of reference, my house is more than 50% bigger than yours, 14 years older, glazed with double-hung sashes & exterior storms, blown insulation with some known gaps. My 97.5% design temp is +4F, a temp at which my heat load a bit over 30KBTU, not 35K. Using 68F for an indoor temp my 97.5% design condition delta-T is 63 degrees, to your 46F. That means you're at 75% the delta-T and ~65% the size, so if my house were the gold standard of older houses (which it clearly isn't) your house would have a heat load on the order of (lets bump mine up to 36K instead of 30K just for margin), 0.75 x 0.65 x 36KBTU/hr = 17.6KBTU/hr.

There could be several factors that could add or subtract 3-5K, but that's probably a closer estimate of the truer heat load. The shape of the house counts too, and exterior surface area (from which the heat is lost) doesn't change linearly with square footage of living space. I insulated my basement walls, so if you haven't, maybe you can use a 1.15x multiplier bringing it up to 20K, in which case 24K might be only 20% oversizing factor (which would be about where most commercial heat-load calculators would leave you), but your windows MUST be tighter than mine and I'd already bumped it 20% for margin. (20% here, 20% there, and pretty soon you're WAY oversized.) Most residential heating systems I see in my area are 3x or more oversized for the actual load. In the temperate PNW it wouldn't suprise me if 4x or even 5x oversizing was common, with disaterous effects on as-operated heating efficiency. A ~100K 80% efficiency furnace or boiler would probably be 4x oversized in your house, but is a pretty common size.