Right-sizing a mod-con boiler for 50K heat load

I'm planning to replace my 33-year old natural gas Teledyne-Lars boiler with a high efficiency unit, to take advantage of Massachusetts' early boiler replacement program rebate, and because it doesn't seem worth throwing money at it to fix the noise (flashing?) issues which have worsened over the last couple years. Since the standalone gas hot water heater is also vented to an unlined chimney, it would need to be replaced too.

This forum has been immensely helpful in understanding boiler sizing, and I have a couple questions regarding my heat loss calculation and options that have been proposed.

My design day heat loss, computed from actual gas usage (minus a bit for DHW, cooking and drying) and degree day data is 48,000 BTU/hr. This is based on 77% efficiency measurement of the boiler stack pipe during my last energy audit. Would that be the right efficiently number to use for this calculation or should it be discounted somehow?

The load estimate corresponds pretty well to my energy-modeled design heat loss of 46,000 BTU/hr for this 2-story house with unfinished attic and cellar, 2200 sf of conditioned space, cellulose in walls and attic floor, fiberglass batts sort of held against the cellar ceiling by wire supports, and caulked interior trim joints. Someday I might add a third (short) zone to the attic, which may require 4,000 BTU/hr more. Maybe a mini-split would be a better choice there for year-round conditioning, or maybe the increased heat load can be offset by air sealing improvements (currently about 8 air changes per hour at ACH50). So overall needs might be 48-52K BTU.

Here are three options that were quoted:

* Lochnivar-Knight WHN055 (11-55K BTU input) or WHN085 (17-85K BTU input) with 40-gallon indirect (model depending on heat loss calc)
* Buderus GB142/24 (23-83K BTU input) with 36-gallon indirect
* Navian CH-210 combi unit (20-175K BTU input)

Dana, jadnashua, and BoilerBadger have emphasized the importance of not over-sizing a boiler, and preferring low minimum output, but I'm wondering how close should one cut it? For example, at 51K max output is the smaller Loch-Knight cutting it too close? What about on a design day morning when hot water becomes a priority? My friend's REM/Rate program spit out something like "Required: 50K BTU, Specified: 80K BTU". Why such a plus up? That seems like a lot for piping losses (which I think are already accounted for in the estimate based on actual usage).

The Navien combi unit is a nice package and would save about $2K over a separate tank setup. Is there reason to believe that the combi unit would have less longevity or require more service than the bulkier option? A slightly smaller model is available, although I would be concerned about supporting 2 showers with winter water temps (3.6 and 4.0 GPM @ 77F rise respectively).

Any answers or suggestions would be appreciated.

All three are really good pieces of equipment. I install a lot of Buderus GB's and have had very few issues with them. Combi units are nice and generally will give you plenty of hot water but If you have the money, I prefer a separate indirect tank. Sizing for all three won't be an issue. My choice, the Buderus GB142

On that coldest day of the year's morning, keep in mind all of that showering is releasing heat, too. I've never noticed the place cooling off because I was using a lot of hot water. I have a Buderus BG142 installed with the IWH as a priority zone (it gets heat when it needs it). Except when reheating the IWH, my boiler rarely gets above 140-degrees and during the summer, is often just sitting there at ambient. The house has a lot of mass, so unless it is VERY leaky, it takes awhile to cool off, and at full fire, the boiler reheats the IWH pretty fast.

We install all three here in Minneapolis and don't mind cutting it close. The next priority, and the reason we install all three and a host of others, is the minimum-fire to minimum-load comparison. If I have micro-zones (zones too small and too demanding to properly load the boiler at minimum output) I look to the smallest output at low fire. This is often, but not always the lowest output boiler.

The Navien combi boiler/water heater is a fairly new unit. Thing to know about the Navien is that it is sized to on-demand water heating first and the output, especially at high design water temperature can be quite low when in space heating mode. Even in when operating in an appropriate radiant range we limit our designs to 50mbtuh. It is not an easy boiler to apply without some training.

Most residences in cold climates do best with a condensing boiler sized to the heating load and an indirect sized as one might size any conventional tank-type gas-fired storage water heater. Never mind, "peak loads", over-sizing boilers, the color of the sky at sunset...

jadnashua said:

On that coldest day of the year's morning, keep in mind all of that showering is releasing heat, too. I've never noticed the place cooling off because I was using a lot of hot water. I have a Buderus BG142 installed with the IWH as a priority zone (it gets heat when it needs it). Except when reheating the IWH, my boiler rarely gets above 140-degrees and during the summer, is often just sitting there at ambient. The house has a lot of mass, so unless it is VERY leaky, it takes awhile to cool off, and at full fire, the boiler reheats the IWH pretty fast.

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Unless you're taking literally hour-long showers at outdoor temps where the heat load is greater than the boiler's output there would be no way to feel it.

Even with the boiler off for a a full hour, how much does the house really cool off in that hour? The thermal mass of the house in most homes would limit the temperature hit to less than 2F, even at the 99% outdoor design temp.

Thanks for the replies. You've convinced me not to be concerned about an indirect priority zone on a boiler sized to heat load!

I still have a question about determining the minimum fire rate desirable - for example, whether a 10 MBH vs 17 or 20 MBH minimum fire rate matters in my case.

From another post I gather that the way to do it is to take the zone with the smallest heat transfer at minimum operating temperature. Here, that would be 65' of fin-tube, which radiates 200 BTU/hr/ft at 120F, for 13K BTU/hr. So for my case there would be a benefit in having a boiler that can dial down to 13K BTU/hr out, but no benefit for one that goes down further. Is that right?

If ever ever get around to adding an attic heating zone, short-cycling would become a concern. Applying the formula in that post, even the smallest 11K BTU boiler would raise the temperature of water in 16' of fin-tube @ 140F and 200' of 3/4" pipe by 5 deg F in 2 minutes. Would radiators (like Runtal) be a better choice there, and avoid the need for a buffer tank?

Most of the contractors I've contacted seem to think that as long as the boiler is modulating, oversizing is not an issue, so I appreciate the knowledge conveyed in this forum!

On a mild, but cool day, you won't need the maximum output from a zone even at that low temperature. Continuous (low) heat output is the desire, and longer boiler cycles, so the lower the thing can modulate to accommodate those mild days is the better choice.

Jim has it the principle design criteria we use to size every boiler. First cover the block load, determined by Manual ' Just', then the smallest high load-per-sq.ft. zone and follow up with a boiler with the lowest output.

Of course near piping and proper programming can' t be ignored

MA

Yup- the mod-con with the lowest low-fire output that still has sufficient high-fire output to cover the 99% design condition load is (almost) always the right one.

The only way you can micro-zone a 16' stick of fin tube is with buffer tanks, and it's usually cheaper (and definitely more comfortable) to buy the biggest and highest-output panel radiator that fits with the buffer-tank plumbing money. You'll pay a premium for Runtal, but you don't have to break the bank to get a decent panel radiator, and between the higher output at low temp and the additional thermal mass, it's usually sufficient to manage the short-cycle issue. If you replace a 16' section of 3/4" fin-tube with a radiator of EXACTLY the same 180F output (eg Biasi B-24.48 you'd be adding about 14-15lbs of water to the total, but since it's a single zone that doesn't need to balance water temp with other zones, going by going larger on the radiation the "excess" BTU output of the boiler is smaller, and the thermal mass even higher.

I know it's sometimes hard for the homeowner/bystander to rationalize installing a two by six foot $450 panel radiator in lieu of $200 of fin-tube, but it's both a comfort and wear-and-tear on the boiler upgrade but it's in fact cheaper & easier than adding even a bargain-basement electric hot water heater as a buffer. But that IS a situation where the output of the boiler really matters: 10KBTU/hr out @ min-fire is pretty reasonably matched to the 8255 BTU @ 140F output the the B-24.71, and still less than 2x it's ~5500 BTU/hr @ 120F output, but a boiler with 13K of output at min-fire could be creeping into short-cycle territory again without a bit more radiator.

Dana, Thanks for the radiator tips. So the radiator made in Haverhill, Mass. is the pricey euro-style one and the one made in Italy is the cheaper, hum-drum alternative? Go figure.

I didn't realize that zones could be run at different temperatures. What happens when zones that normally run at different temps call for heat simultaneously?

The lowest fire modcon "is (almost) always" the right choice? Now you've got me curious regarding when it isn't.

Its too bad the combi units don't modulate down more for better efficiency. I suppose doing this would be equivalent to a small modulating boiler for heat and a separate larger one for on-demand hot water.

Thank you for your replies, everyone.

hlv said:

Dana, Thanks for the radiator tips. So the radiator made in Haverhill, Mass. is the pricey euro-style one and the one made in Italy is the cheaper, hum-drum alternative? Go figure.

I didn't realize that zones could be run at different temperatures. What happens when zones that normally run at different temps call for heat simultaneously?

The lowest fire modcon "is (almost) always" the right choice? Now you've got me curious regarding when it isn't.

Its too bad the combi units don't modulate down more for better efficiency. I suppose doing this would be equivalent to a small modulating boiler for heat and a separate larger one for on-demand hot water.

Thank you for your replies, everyone.

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Most system designers will shoot for a single temperature that satsifies all zones rather than going through the complexity of dual-temp systems, though the latter is pretty common in homes where a radiant floor went into the addition that only needs 135F water on design and the rest of the place had baseboard that needs 180F on design day. As long as the radiation temp requirments are remotely well matched, and even the highest-temp zone would take advantage of a condensing boiler most of the time, that's usually "good enough".

The outlier cases on boilers where you might upsize is when you have other intermittent but high loads (usually hot water heating) where you can be forced trade off the size of boiler output against the size of thermal storage. In many ways combi-boilers like the Naviens are a case in point- a monster-burner for the hot water side to avoid the real estate burden of a tank.

Having both a small boiler AND an on-demand isn't usually called for- most of the time you can size an indirect sufficient to handle the hot water loads, which are intermittent, and the output of even the tiniest boilers exceeds that of most standalone 40-50 gallon tanks.

When it looked like my preferred contractor would not be available to install the Lochnivar Knight boiler in time, I solicited a proposal from my own plumbing and heating contractor. He likes Utica, so I directed him to the Utica SSC line and their indirect tank. He is not experienced in mod con boilers, and when I raised this concern he proposed getting a Utica rep on site to instruct us both on the programming aspect of the boiler. From what I've read on these boards I would be comfortable tuning the reset curves once the system is in operation to maximize cycle time and promote condensing.

However, I'm wondering whether proper mod-con training is necessary to properly design the piping of my system. The existing distribution is straightforward - 2 zones, 1 for each floor, each serving about 75' of fin-tube baseboard. Seems like 140F or so average water temp would satisfy design day heat load of 46K BTU/h, with heat loss for each floor being similar. The Utica SSC has a primary-secondary loop built in, so is there much left to decide? Low-loss header? Type of circulators? I wouldn't mind giving him the work if the piping setup (apart from venting) would be like that of a traditional boiler.

Also, re the Loch-Knight option, I met the installer today and asked whether a primary-secondary loop can be omitted (BadgerBoiler and others have suggested that having one can increate the return water temp, inhibiting condensing). He said he always includes p/s now, following the manufacturer's recommendations, apparently after having issues in past systems which omitted it. But he could use zone valves rather than circulators to reduce electric consumption.

Would including p/s necessarily degrade performance? What about valves vs circulators? At this point I'm afraid I'm learning enough to be dangerous, or at least, annoying...

The P/S configuration works well in most applications- the performance hit is primarily pumping power use. As long as the boiler isn't super-sized for the zone loads, the average combustion efficiency will still be about the same. With 140F peak temps you only need a 15F delta-T to get it into the condensing zone even on design day, and the rest of the season it's a gimme- it will be condensing all the time.

Valves are cheaper than pumps. Unless you have unusually high pumping requirements (not likely here!) a single pump can serve both (or all three, if you add one) zones. The difference in fin-tube output between say 2 gpm & 4 gpm is negligible, or even the difference between 1 gpm & 5 gpm. Since the plumbing lengths sound pretty balanced and not ridiculously long, a single ball-valve on the shortest equivalent-length zone (look up equivalent lengths for all of the ells & tees) for balancing flows when both are calling for heat is about the most tweaking hardware you'll need, and even that may not be necessary, depending on how closely the numbers match.

When you add the radiator zone, you may need to add another ball-valve if going with zone-valves and a single radiation pump. (The pumping head of radiators is quite low compared to fin-tube.)