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.