small boiler for small house?

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Old 12-13-10, 11:21 PM
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small boiler for small house?

After house remodel, on coldest day of year expected heat load will be 25000 BTU/h. What boilers supported in the US modulate as low as 15000 BTU/h? Because most days aren't the coldest days of the year.

Seems too easy to overspend. I wonder if there are modulating noncondensing boilers worth considering. I probably prefer stainless but am willing to do maintenance to save total cost, which seems dominated by installation cost.

I've talked to two contractors and one is darned expensive and his method is labor intensive. The other seemed more willing to work out a deal. He is OK with me doing some PEX runs and mounting convectors and such. (Actually, I solder fine but I wouldn't consider it with a boiler.) He is certified by some brands to service wall-hung and he said that compact system boilers with built-in pump, maybe built-in expansion tank, could make sense and save money in a small home if everything fits in a single zone. He seems willing to install about any credible brand that's sized right.

I did notice Camus Dynamax. Plus plenty of makes and models that aren't avail in the US.

If I want to consider Rinnai, I seem to need a Rinnai dealer? Any other brands like that?

(PS, Let's not get confused with hot water heating right now. Not looking for a combi, not looking to store heat, not looking for tankless water heater. If the boiler is really efficient, might be reason to just leave physical space for future DHW upgrade.)
 
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Old 12-14-10, 02:10 AM
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Lochinvar Products - Knight Heating Boiler product line.

300 Maddox Simpson Parkway, Lebanon, T N


This would be my next boiler. I am 1050 sq ft. My current boiler is about 42 I=B=R 65 input

These are the smallest two.

http://www.lochinvar.com/_linefiles/KBN-06.pdf

Model Min. Max. AFUE Capacity I=B=R
Number MBH MBH % MBH MBH
WBN051 10 50 95.3 45 39
WBN081 16 80 95.3 72 63

Hope this helps. All others I looked at were just too big for my home. My current gas bill for my home runs 1,100 a yr. But thats heat,dryer,stove,HW. I wonder what I can save installing one of these with DHW?

Mike NJ
 
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Old 12-14-10, 03:54 PM
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Why not just use a bradford white combi-core or similar tank.
You need DHW, and you can get 25,000 BTU iff the built in heat exchanger.
You will not save any money reseting at 25,000 but load I doubt.

The combo core can be had 50 or 75,000 but... More than the heating load yes, but will give you some nice DHW performance numbers.

It may be worth considering.
 
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Old 12-14-10, 07:54 PM
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thanks for responses so far

lawrosa, thanks for pointer to Lochinvar. The contractor I'm considering is certified by that brand to service their products.

I am wondering if anybody sells a unit for use in USA that's similar to Baxi Megaflo 15, which modulates down to roughly 16000 BTU/h but is 50 Hz as far as I know so not US. Trying to save on installation cost, and Megaflo has an built-in expansion tank and built-in pump. Though I'm not sure it would pump enough for what I need, which I'm guessing will be five convectors. (I haven't been considering underfloor radiant bec. I don't need new floors, and lower crawlspace clearance seems too low to insulate.)

TDHeating, if a water heater type solution saves enough on total installed system cost, I'd consider it. I'm guessing professionally-installed water heater is less than boiler. But I think the water heater would run at lower temp than boiler and so would mean more cost for more/bigger convectors/radiators. Is that about right?
 
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Old 12-14-10, 08:22 PM
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The mod boilers are actually most efficient at low temps such as radiant heat and DHW situations. 140f for radiant and 120f for HW. They run at or close to their rated efficiency such as 95.6 ect... In summer these units are more cost effective for HW heating because they modulate down for lower temp.

In winter the boiler will heat the baseboard 180f but still only heat the HW to what the HW setting is. 110f,120f ect...

Mike NJ

Mike
 
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Old 12-15-10, 11:12 AM
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Maybe for efficiency's sake I should consider keeping the loops at 140 F or below then. If so, would installed cost of water heater be significantly less than boiler?

Not sure yet if 140 is practical. Radiant floor would be difficult in this house, and 140 is pretty low temperature for radiators. Do you think convectors alone at 140 could heat a one-story, wood-frame house with wall and ceiling insulation? On paper, 6 of them would do, but as a practical matter I'm not sure.
 
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Old 12-15-10, 05:10 PM
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Define 'convector'. There are cabinet convectors, which I would avoid in a remodel unless you are reusing. If going new and you only need 6 heat emitters, consider steel panel radiators, or try getting some cast iron rads off craigslist or your local scrap yard. Radiators do fantastic at lower water temps.

Radiant done right is all well and good, but expensive. Steel or cast iron radiators will provide comparable and even better comfort, at less cost.

The little Lochinvar unit would be a good one for this application. Also consider the Triangle Tube Solo 60.

If you want to kill two birds with one stone, you could also consider using a reverse indirect as a buffer tank to mitigate boiler short cycling and also provide your domestic hot water. Turbomax or Ergomax would be possibilities for that, among others.
 
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Old 12-15-10, 08:17 PM
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Originally Posted by xiphias View Post
Define 'convector'. There are cabinet convectors, which I would avoid in a remodel unless you are reusing. If going new and you only need 6 heat emitters, consider steel panel radiators, or try getting some cast iron rads off craigslist or your local scrap yard. Radiators do fantastic at lower water temps

Radiant done right is all well and good, but expensive. Steel or cast iron radiators will provide comparable and even better comfort, at less cost.
By "convector" I meant in-room fan-coil units such as by Smith Environmental or Turbonics. In the Myson catalog they're "fan convectors". At least sometimes, they seem like good choices. I see a lot of concealed horizontal units that work fine in hotel rooms. There's a bulkhead facing my living room, like in a hotel room. My parents' apartment has wall-mounted fan convectors that also work fine. I rarely hear of fan convectors in detached homes except toe-kick in kitchen or bathroom. So in a bedroom, I figure they'd work under a chest of drawers I could build. I definitely welcome opinions about what are the issues with in-room fan convectors.

By "radiant done right" I assume you mean radiant floors, with the nice temperature profile and no air movement. I haven't ruled it out completely but it would be difficult with that low clearance in the crawlspace. I would first have to finish every job I'm ever gonna want to do down there, electrical, seismic...if floor insulation goes in down there there will be no access at all.

As for panel radiators at 140 F, seems to me they are large and heavy and take up more space than a roughly equivalent fan convector (and much more space than a concealed convector). But I guess I shouldn't rule radiators out, esp. if that bulkhead idea doesn't work after all.

Still, considering all that...any opinions about hydronic-radiant wall? There's ugly wainscotting that I plan to replace anyway so and I could maybe DIY the wall. Just not much advice out there about doing radiant wall.

Originally Posted by xiphias View Post
The little Lochinvar unit would be a good one for this application. Also consider the Triangle Tube Solo 60.

If you want to kill two birds with one stone, you could also consider using a reverse indirect as a buffer tank to mitigate boiler short cycling and also provide your domestic hot water. Turbomax or Ergomax would be possibilities for that, among others.
Yes, I'll check out the Triangle Tube unit, thanks.

As for killing any birds with any stones, conventional DHW is cheap. Someday, a spa in the bathroom, I hope. But for now I can't see how to justify spending on anything more than a couple of valves for a future indirect system. If I can modulate down to 15000 BTU or even 9000 BTU, I don't see cycles being short on the boiler (or, at 140 F, should it be a water heater? now I'm really confused...)
 
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Old 12-16-10, 06:25 AM
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Fan convectors require pretty hot water to be effective. Typically >140. You can get low-temp thermostats for them, but really these are for supplemental not primary heat applications.

Radiant in walls can be done, but you have to pay a lot of attention to the wall assembly. In a typical wall, there's not a whole lot of depth to work with for adequate insulation, barrier, etc.

Any boiler that bottoms out at 9-16k BTU/hr is going to short cycle on all but the couple coldest days of the year.
 
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Old 12-16-10, 01:36 PM
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Originally Posted by xiphias View Post
Any boiler that bottoms out at 9-16k BTU/hr is going to short cycle on all but the couple coldest days of the year.
Thanks for your responses...Can you clarify the above? Is that generally true or are you talking about any water that's hotter than 140, or just fan convectors or...?

I thought that as long as emitters are right size, we minimize short cycling on cool but not cold days by picking the boiler having the lowest available modulation, no?

Could I get in the ballpark by using the efficiency curve in a Veissman manual, assuming that other mod-cons have similar efficiency vs. modulation, water temperature, and water temperature drop? If so, looking at curves at Viessmann - Vitodens 200-W, WB2A, click on Technical Manual, see page 2., a mod-con sure looks like it's condensing when the modulation is running anywhere below about 50% rated BTU capacity, regardless of temp. And it's a small house when the smallest available boiler rating is >2x the calculated load.

Originally Posted by xiphias View Post
Fan convectors require pretty hot water to be effective. Typically >140. You can get low-temp thermostats for them, but really these are for supplemental not primary heat applications.
I saw a Modine catalog that lists 160F and drop of 20F as "standard", with multipliers for other temps. Cabinets like Modine are the primary heat/cool for my parent's apartment, but I don't know the actual temp of the water. I'll see if I can find out.

Originally Posted by xiphias View Post
Radiant in walls can be done, but you have to pay a lot of attention to the wall assembly. In a typical wall, there's not a whole lot of depth to work with for adequate insulation, barrier, etc.
I'd only consider inside walls, maybe both surfaces and no insulation. Maybe only four feet high up the walls for temperature profile and in deference to picture-hanging. Another reason I'd consider radiant wall is it seems more DIY and less money, maybe more comfort, compared to fan convectors or radiators. Big downside of wall is, will it actualliy work in the end? It seems like an experiment, with limited advice or experience available from local experts.
 

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Old 12-16-10, 05:13 PM
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Don't confuse short-cycling with condensing.

Short-cycling refers to on-off burner cycles. Generally speaking, a firing time of less than 5-8 minutes is a short-cycle. Particularly if the rep rate is 5-6 times/hour. Any boiler -- modcons included -- is most efficient with long burn times. Taken to an extreme, an ideal modcon would fire in November and not turn off until April, modulating all the time.

Assuming your design load of 25k BTU/hr, you will spend a huge portion of the heating season at half of that or less. If your design load of 25k BTU/hr was done with a Manual J calculation, chances are the actual design load is going to be 65-75% of that, or ~17k BTU/hr. That's because of the inherent conservatism of Manual J.

Point being that with a small modcon you will nearly always be down at the low end of the modulation range. It will short cycle.

Condensing is somewhat related to load, as you note in the Viessmann tech manual. But note that V boilers are a rather different beast than the more standard modcons -- they do a bunch of stuff in their burners and controls that is very custom compared to the current crop of controls in other modcons. So those curves on p. 2 are not directly comparable to other modcons. In fact the 167-140 curve is their design DHW production curve. That V boiler family tops out at a supply temp of 167F per a European specification. Also note that 167-140 is basically a 30F temp drop. Most US space heating systems are not designed for that. You can, but it takes a room by room heat loss and carefully sizing radiation and flow rates to get there.

The modcons generally have only a couple quarts of water to heat. So even at low modulation, you can do a pretty good temp rise through the boiler. Now the radiation sizing comes into play. If you can't meet the heat loss with the radiation at a given water temp, then the water temp has to go up, and with it the boiler may modulate up. Then you are out of condensing range as you've moved up the load curve.

Condensing as it effects boiler efficiency is a function of the return water temperature entering the boiler. For gas-fired boilers, condensing stops above a return temp of about 130F (assuming 100% excess air). So you want to keep the return temp around 130F or lower. The lower the better, to ensure the condensing happens in the heat exchanger where the latent heat of condensation actually gives you the added efficiency. Some modcon installs produce a lot of condensate, but it's in the exhaust, not the heat exchanger.

Now, if you design a heating system for a typical 20F temperature drop, in theory you could send out 150F water and get 130 back. But in practice 20F systems are almost nonexistent. Partly because they are overpumped, partly because design conditions don't last long, and partly because whenever the load is less than design, the deltaT is <20F anyway.

Bottom line is that if you want to ensure the boiler condenses a lot, use a lower-temperature emitter system, like radiant floor, radiators, oversized baseboard, etc.

I would not consider radiant walls for a primary heat source. They are almost exclusively used as supplemental, e.g., in large rooms with tall ceilings, or places that can't have radiant floor (underneath Steinway pianos leaps to mind...).

IMHO, a small modcon paired with steel panel rads and a buffer tank would be an ideal setup. Not the least expensive by any means, but properly set up would be a really comfortable and efficient system.
 
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Old 12-16-10, 05:34 PM
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Originally Posted by watershy View Post
lawrosa, thanks for pointer to Lochinvar. The contractor I'm considering is certified by that brand to service their products.

I am wondering if anybody sells a unit for use in USA that's similar to Baxi Megaflo 15, which modulates down to roughly 16000 BTU/h but is 50 Hz as far as I know so not US. Trying to save on installation cost, and Megaflo has an built-in expansion tank and built-in pump. Though I'm not sure it would pump enough for what I need, which I'm guessing will be five convectors. (I haven't been considering underfloor radiant bec. I don't need new floors, and lower crawlspace clearance seems too low to insulate.)

TDHeating, if a water heater type solution saves enough on total installed system cost, I'd consider it. I'm guessing professionally-installed water heater is less than boiler. But I think the water heater would run at lower temp than boiler and so would mean more cost for more/bigger convectors/radiators. Is that about right?
I believe the combo core will go up to 160 or so
 
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Old 12-16-10, 05:43 PM
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Combicore, or the new HTP combo unit (though about twice the price of a combicore), add a motorized mix valve for ODR.
you can use steel panel rads or the new low water temp fin tubed baseboard (rated down to 110 F).
Installed cost probably 1/3 that of a boiler & buffer tank, and dhw tank.
Comfort the same as a mod con.
marginally more gas used than a mod con, as the energy os stored in the tank and the system will not short cycle.
Plus you get some decent domestic hot water performance.

I am not a real proponent of using DHW tanks, but this application is well suited and should not be overlooked. Just my 2 cents (CAD)
 
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Old 12-16-10, 10:36 PM
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Xiphias, I suspected something like what you said, that design drops of 20 degrees would often be less than 20 in practice.

You also wrote "If you can't meet the heat loss with the radiation at a given water temp, then the water temp has to go up, and with it the boiler may modulate up. Then you are out of condensing range as you've moved up the load curve." Does that mean that during recovery, even with only 25000 BTU worth of emitters, the boiler can modulate up significantly higher than 25000 heat output?

I still don't get your point about short cycling and low modulation. I am under the impresion that low modulation is always better for both comfort and economy. Among all right-size boilers, the ones with the lowest modulation should be the ones that short cycle the least. Any other boilers will short cycle more, no? And I'm pretty sure the same is true of a mod-con forced air furnace. So of course short-cycling is bad, and the way to minimze it is rightsize the boiler and pick one that modulates down as much as possible.

Now as for a buffer tank. There are losses in converting stored heat back to home heating, and there are costs for the tank and for the more complex installation. I don't see a benefit in my case. I have heard of buffer tanks to increase the duty cycle and reduce short-cycling on nonmodulating boilers. And I've also heard of using buffer to actually add to the design load in homes that are so small and super-efficient that any boiler in the world would short-cycle.

Thanks for your detailed response. Yes, I'm starting to look at steel radiators. I wish the US had stone radiators like stoneheating.com for example. I'm guessing the stone's hollow and what's inside is a metal tank.
 
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Old 12-16-10, 11:03 PM
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Originally Posted by TOHeating View Post
Installed cost probably 1/3 that of a boiler & buffer tank, and dhw tank. Comfort the same as a mod con. marginally more gas used than a mod con
OK, if a water-heater closet is 1/3 the cost of a boiler room, the water heater idea definitely has my attention!

Originally Posted by TOHeating View Post
you can use steel panel rads or the new low water temp fin tubed baseboard (rated down to 110 F).
I don't see baseboard in a small house, would need yards and yards of it even at 140F. As for 25000 BTU of radiators I'm not sure what temp you're thinking I"d run at. In your other post you mentioned water heater spec'd to output at 160 F. If so I think the radiators could be kept down to size and not overwhelm the rooms. But tell me, would you install a water heater to heat a home using 160 degree water? Wouldn't you be concerned about service calls?

As for spending anything at all to improve the existing DHW, I don't get the benefit. Maybe on paper, DHW from typical tank isn't very efficient. But DHW costs way less than a phone bill nowadays. I see no payback from any investment in DHW at all.

But if a person can heat their home economically with a hang-it-on-the-wall water heater, that could pay for itself in a reasonable amount of time. It might seem strange to have an older-style DHW tank three feet away but there are stranger things in people's houses, I bet you'll agree.
 
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Old 12-17-10, 04:08 PM
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The boiler doesn't know anything about the radiative capacity of the emitters. It does know, somewhat indirectly, if it knows the outgoing and return water temps. Crank the water up high enough and the dT will drop.

I think you have several good options here

) modcon
) modcon with buffer
) modcon with reverse indirect as buffer
) combicore
) combo unit or something out of the ordinary like an HTP Phoenix
) water heater rated for space heating

pick one (or something else) and design with it. Start with the room by room heat loss, emitter type, and design water temp.
 
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Old 12-17-10, 09:43 PM
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Originally Posted by xiphias View Post
I think you have several good options here

) modcon
) modcon with buffer
) modcon with reverse indirect as buffer
) combicore
) combo unit or something out of the ordinary like an HTP Phoenix
) water heater rated for space heating
Thanks for the hand-holding and laying it out clearly. I should have discouraged topic drift to water heater. Water heaters seem to be for low temp use. Makes no sense to me for small house retrofit. Eg Myson steel panel at 140F is 60% of catalog listed/advertised BTU rating, would take 18 square feet of steel panels in a 10000 BTU living room. They really dominate a small room. Higher temp, I could place one under each window. At only 140 F they're way too big to fit in those positions. I can't be the only small house retrofit that needs more BTU/square foot of radiators. People with crowded conditions like me, worst case they are using air handlers. I'm still investigating fan convectors, I know people who get good service from them as primary heat, a guy I know is in apartment management and is working on an RFP for millions of dollars worth of fan convectors. I've now heard from two residential HVAC contractors who don't seem to like them, can't understand why.

About the "combo unit or something out of the ordinary like an HTP Phoenix". I'll say one good thing about that...it was a somewhat unconventional combo, Embassy Ambassador Onex, which I'm not planning to use but the fact that it promised economy plus performance got me interested in wet heat in the first place. As for the Phoenix, it's too big, mod-ing down to 44kBTU/h, too hot for my toosh. Talk about short cycling. I went some of the numbers in the energy audit and just like you said my house is going to spend a lot of the year looking like a 17000 BTU load or less.
 

Last edited by watershy; 12-17-10 at 09:54 PM. Reason: typos, left out phrases "catalog listed/advertised BTU rating" and "BTU/square foot of radiator"
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Old 12-19-10, 01:34 PM
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follow up

OK, after further investigation I see the problem with my proposed use of 160F water. I see that if the whole shebang is not carefully designed, the mod-con boiler's not going to condense, and could short cycle in spring and fall. Because of limited clearance in crawlspaces plus I'm being picky about emitters, I'm going to need at least some some engineering. Maybe the only DIY step right now is just for me to make drawings of the structure.
 
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Old 12-19-10, 08:19 PM
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The emitter thing is something you have to work through as a personal choice of what you want in the house, what water temp you want to design around, how much of the heating season you want to try to reap the benefit of condensing, etc.

The short-cycling can be addressed by a buffer tank. The way to size a buffer tank is this:

Vbt = t(Qh -qload) / (500 *dT)

Vbt size of buffer (gal)
t desired on-cycle (min)
Qh energy source output (BTU/hr)
qload rate of heat extraction from buffer (BTU/hr)
dT temp rise of tank between on/off cycles (F)

For example, say you want a minimum firing time of two hours at a low modulation and about half design load (e.g., shoulder season), with a 40F rise in the buffer tank over the cycle.

Vbt = 120 * (20000-12500) / (500*40)

Vbt = 45 gal

Lots of stuff can be used as a buffer tank. Most expensive is a purpose-built buffer like a boilerbuddy.com unit. Least expensive is probably a decent electric water heater.

Or rearrange the terms and see what a buffer of given size will give you, e.g.,

30 gal buffer = 120 min runtime at 15k boiler output, load 12.5k, temp rise 20F.
 
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Old 12-19-10, 10:55 PM
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Thanks for your patience, took me a few days to prove to myself it would still short cycle even if can modulate way down.

Now, is the concern about short-cycling mod-con mostly about preserving boiler reliability, or mostly about preserving comfort, or mostly about losses (time needed to reach steady state)? Or a combination?

FYI, to prove to myself it's really true, that even with very low modulation there might be short-cycling on shoulder days in my case, I looked at some seasonal data for nearby airport, chopped up into degree days. Based on the frequency distribution of degree-days, for 50% of the year, the smallest Lochinvar Knight could be running 24 hours a day, modulating between 25000 BTU (load calculation for my house) down to 10000 BTU (minimum modulation for that model). For 40% of the year, I don't need home heating. And for the other 10% of the year the boiler will be running at reduced duty cycle, risk of short cycling.

So again, I accept that it will short cycle, maybe 10% of the year if I don't do something. But what exactly is the downside of short cycling? Is it reduced boiler lifetime?
 
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Old 12-20-10, 04:17 PM
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Short cycling impacts the first (boiler lifetime) and the third (efficiency). Comfort is a different issue and generally not affected.

If you think your numbers are reasonable, then 10% of the heating season for short-cycling isn't too bad.

Keep in mind there are other sources of internal gains -- cooking, people, solar, etc.
 
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