Help with sizing a boiler
Last Post 06 Nov 2014 05:07 PM by Dana1. 49 Replies.
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tom37User is Offline
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17 Oct 2014 08:19 PM
I joined this forum because I saw Rob from NRT is also a member he and his company designed my radiant heat with warmboard and did a great job so hopefully he and all the other talented members can help me size a boiler for me. Although I do understand there are many variables involved with sizing a boiler I'm hopeing you guys could give me your best expert oppinion on what size I should be using. The building is 60x40 one floor building built some where in the 1930s or 40s I believe and the building is used as a club similar to a knights of columbus or elks, the basement is used as a bar with two older type entry doors with 8' high ceilings no insulation in a drop down ceiling with two small bathrooms. The walls are concrete with no insulation on the outside or on the inside, 4' of the basement wall is below grade and 4' of the basement wall is above grade and has two 3'x2' ineffcient basement windows, the floor is concrete with vct tiles. The 1st floor is a large open area with lots of seating and a dance floor. The floor is hardwood the walls are 12' high have no insulation and in this large room there are eight 2'6" x 4' double pane insulated windows with two entry doors not very efficient, the cieling has about 12" of blown in insulation in the attic and the roof is wood framed two gable ends with asphalt shingles, the exterior of this building is brick. The building has 150' of 14" high output base board heaters I believe are 1" copper There is also in the wall one large 6' wide x 3' high output heater in a hallway. Also has 24' of standard residental 3/4" baseboard in the bathrooms. Hopefully thats enough information and I have been at different supply companys in my area which is about 15 minutes north of Boston and thats also where this building is located. The old oil fired boiler we are replacing is 350 BTUs One supply house recomended the Burnham boiler alpine model # ALP285 which has DOE heating capacity 262 and the net AHRI is 228 The other boiler is triangle tube prestige solo model # SOLO 250 with DOE heating capcity 233 and net IBR 194 Both these boilers are mod cons and I'm kind of hoping all the recomendations point to the triangle tube product for two reasons one is its less money and not as big as the burnham and the other is I have a triangle tube solo 110 in my home and it has done very well. Everything I have read and heard is that you don't want to oversize a boiler but on the other hand it would be a nightmare if you under size it. So hopefully thats enough information for all you experts and thank you very much for the help.
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19 Oct 2014 10:41 AM
Anyone !!
Dana1User is Offline
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20 Oct 2014 06:41 PM
You need to run a Manual-J or similar heat load calculation based on the construction and the inside & outside design temperatures.

About 15 minutes north of Boston your outside design temp is probably about +10F, and assuming an interior design temp of 70F that leaves you a delta-T of 70-10= 60F.

An 8"thick concrete wall has a U-factor of about 1 BTU/hr per square foot per degree F, as does a single-pane window. The entry doors are about half that, but we'll ignore that for now. Add all the square footage of the above-grade portion and run the math:

Area x U1.0 x 60F= BTU/hr

With 12' ceilings and 4' of above-grade foundation wall you have 16' tall walls (ignoring the floor) and a perimeter of (60 + 40 + 60 + 40=) 200' for a gross area of about 3200'. So those wall losses are on the order of:

3200' x U1.0 x 60F = 192,000 BTU/hr

With 12" of fluff in the attic you're looking at U-factor of about 0.025 BTU/ft-hr-degree. a 40' x 60' building has an attic area of 2400'. At a delta-T of 60F that's a loss of about:

2400' x U0.025 x 60F= 3,600 BTU/hr

Add it up and you're at about 196,000 BTU/hr.

Assuming you have some ventilation & air leakage, give it a fudge-factor of about 25% to cover those losses and the sub-grade wall/floor losses and you're looking at 250K, give or take a little.

The ALP285 should cover it. The Solo 250 probably will too.

The size of the pipe on those convecting baseboard heaters is less important than the size of the fins on the suckers. The answer dramatically affects the water temperature requirements, and whether you'd actually get some condensing efficiency out of the mod-cons, and how low the temp can go before the thing starts short-cycling.

A the cost of natural gas it doesn't take long years to rationalize R10 continuous insulation on the exterior of the building, with a bit o' cheap siding on it. If you did that your wall losses would drop to about 20-25K (depending on how much window you have, an you could heat the place with an ALP080 or Solo-60.

In MA there are multiple vendors of reclaimed roofing foam that could take the material cost for the exterior insulation down to about 25-50 cents per square foot, or less than $1600 in foam costs to insulate the walls. This is an easier project than you might think, but it does take some planning. But for WELL under the cost of a mod-con installation you can cut the heating load by more than 3/4 and buy a cheap cast iron beast and STILL use a fraction of the fuel you'd be using with a Solo 250 or ALP 285.

If you're going to bite the bullet and do the bigger mod-con anyway know that you can always insulate some or all of the wall area if it doesn't keep up with the load.

The Solo 110 is more boiler than most homes in MA would need- maybe 1 out of 20 homes could make good use of it, but it's oversized for the rest. I'll assume Rob did the math to prove that the -60 wasn't quite enough.
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20 Oct 2014 09:02 PM
Thank you very much Dana for your help I cant tell you how much I apprciate it this club has a small membership and not a lot of money to spend.
I'm trying to remember what one of salesman said about ratings on these boilers, I beleive he said was that you need to look at the net IBR # which I thought he said that was the actual output # on a boiler and on the solo 250 model it was 194 so your calculations of 250k thats not the same as that net IBR # needing to be at 250 also ?
I also forgot to mention this system will have two zones in which one of the zones will be heating 100% of the winter while the other zone with the 12' ceilings will be be opperating 30% of the winter.
Once again Dana thank you for your help and I'm just wondering are you from the Boston area ?
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20 Oct 2014 09:58 PM
IBR is a obsolete except in the rare case where one uses an on/off conventional cast iron boiler without the benefit of outdoor reset. You would also have to have significant distribution losses, as with a converted gravity system without pipe insulation and hung in an unfinished basement. This is what the IBR "discount" was designed for. What is does to a modern system is further inflate the already conservative heat loads.

If you use a condensing boiler (all have ODR standard) you do not have to allow for piping or pickup and the DOE number is sufficient. Furthermore, if you know the design supply water temperature of your system you may get as much as 98% of the rated input of the boiler you choose.

The most important, and always first step in sizing a heat system boiler is and accurate heat load and the second is the control system.

We find that a properly size condensing boiler will save 20% on baseboard fin tube.

Rob is nearly always right, but from your description it looks like you have 100mbtuh fin-tube capacity. Some condensing boilers will not exceed 167°F, makes more sense at 75°C. I would use 90% of the input for an output figure for a high temp convector.

As Dana suggests, insulation in the attic where it does the most good and has the best ROI.
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21 Oct 2014 10:12 AM
The attic already has ~R40 insulation, the walls have NONE.

If you're going to spend ten grand on the place on heating issues, the best investment would be to insulate the walls, reducing the heat load by 3/4 or so. Even if you kept the decrepit oil boiler that would save more money than switching to condensing gas (but it's still cost-effective to switch to gas.)

The net-IBR rating is only relevant if the boiler is in an isolated location, outside of the conditioned space. The DOE number is the right number to use.

With 14" tall convectors of unknown fin size it's pretty clear there is more than 100,000 BTU/hr of heat emitter. (If it were 150' of 8-10" tall x 2"-fins convector it would be about 100K- I'm betting it's 3", maybe even 4" fins.)

Since it's broken into 2 zones, the amount/type of radiation PER ZONE is relevant.

You can buy reclaimed or factory-seconds sheet foam from both Nationwide Foam  in Framingham, or Green Insulation Group  in Worcester at a deep discount from virgin stock distributor pricing. (There are others- search craigslist materials for "rigid insulation".) To hit code-min for new construction on commercial buildings in MA you only need R10, and if you put it on the exterior of a concrete wall you will be BETTER than code due to the mass effects of the wall. Putting 3" of foam (any type) on a concrete wall will do. It can be held in place with 1x4 furring 16-24" o.c. TapConned to the concrete 24" o.c. with 5" TapCons (with washers, to keep the furring from splitting, and you can then hang any type of siding on the furring. It's worth using a liquid-applied weather resistant barrier on the concrete first, and making that the drain plane for the window & door flashing. You'll have to figure out how to deal with the roofing drip-edge & flashing depending on how it's currently set up, but this is not an extremely difficult or expensive proposition, either as a DIY or contracted out (though if contracting out it's better to use a contractor who has a clue WTF they're doing.)

This is a long shot, but if this is just a couple of big open spaces and you insulate the walls, you may be able to heat it with a couple of mini-split heat pump for about the same operating cost as a gas boiler, and it would have the advantage of providing high efficiency air conditioning as well. In it's current uninsulated state there's no way this would be a viable option.
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23 Oct 2014 01:29 PM
Again I want to thank Dana and Badger for there input
Here is a question for Dana
Do you really think the triangle tube solo 250 model with a BTU output of 194 will do it ?
And Im also curious is to why you didn't use the 4' below grade in any of your calculations ?
And there is Two zones and the fins on the baseboard heat are 4"with an 1 1/4 " pipe
As far as spending any more money for some sort of insulation probably cant happen anytime soon
Thanks again for the help
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23 Oct 2014 03:03 PM
Yes, it should do it.

The Solo 250's output is 223K, not 194K, which is it's irrelevant I=B=R spec. Ignore the I=B=R spec unless you are putting the boiler in a separate building. The D.O.E. output is what matters.

The Solo 250 costs about $4900.

The Solo 110 cost about $3500.

For the $1400 delta in just BOILER cost you can buy enough reclaimed foam to bring the heat load well within the capacity of the Solo 110.

The insulation uses no fuel, but you can bet the bigger boiler running a higher duty cycle will.

Then there is the issue of balancing boiler output to heat-emitter output:

A 4" fin-tube convector will deliver something like 2000-2500 BTU/ft @ 210F AWT, about 650-850 BTU/ft @ 130F AWT (start of condensing.) For the thing not to short-cycle, you need enough fin-tube on the SHORTEST zone to deliver the amount of heat that's being pumped in at min-fire.

The min-fire input to the Solo 250 is 65,000 BTU/hr. At condensing temps the fin tube will only be delivering 650 BTU/ft, so you need about (65,000/650 =) 100' of baseboard on each zone. That is unlikely, given you have 150' total.

The min-fire input to the Solo 110 is 30,000 BTU/hr, so at 650 BTU/ft the minimum length that balances is 30,000 / 650 = 46' of convector, a much more likely number.

In-between you have the Solo 175, with a min-fire in of 50,000 BTU/hr, which balances fine if you have 50,000/ 650= 77' of convector. Again, the smallest/shortest zone may not have enough convector to run the thing in condensing mode.

Max-fire output on the Solo-110 is about 100K BTU/hr. If you insulate the walls to R10, that's more than enough boiler to heat the place.

http://s3.supplyhouse.com/manuals/1334949446065/73945_PROD_FILE.pdf

This is the really crude first-order estimate on it, but if you can't run the Solo- 250 in condensing mode it's not worth it. If you have to operate the Solo-250 at high temp to keep it from short-cycling it will run at the same efficiency as a pretty-good ~85% AFUE cast iron beast that costs $1000 less, is simpler to install and lower maintenance to boot. A direct vented 8-9 plate Burnham ESC8 or ESC9 would cover your load in the un-uninsulated condition, and hit the 84-85% efficiency mark. The Solo-250 will run at about 86-87% efficiency at high temp, and is a more expensive installation.

http://s3.supplyhouse.com/product_files/ESC9-Brochure.pdf

About 190K of your 200-250K heat load is from the uninsulated walls. Insulating the walls to R10 will reduce the heat load to about 30-60K, and it's only as high as 60K if the place leaks air like a sieve (probably doesn't), That would be Solo-60 territory.

A sanity check: As it happens my house in Worcester is about 2400' ( not counting the basement) about the same square footage as your building, but probably with a lot more window area, but with less roof/attic insulation than yours. It's a 2x4 framed antique with some cellulose squirted into the cavities, which makes it's U-factor comparable to R10 continuous insulation. Since the house is not even close to being a rectangle, even with the ~10' walls it's probably comparable wall area to your taller but more rectangular building. My heat load at 0F is about 35K, and the way my heating system is set up it can't deliver any more than about 42K, but it cruises through just fine at -5F. It may be tighter than your building.

The Solo 110 is WAY oversized for my actual load but it may be about right for your building, if you just go ahead and insulate it. And it can run at condensing temps with your convectors.  If you reduce the heat load from 200+ K to sub-50K, it will use  something like 1/4 the amount of gas to heat the place.  I your neighborhood if you keep the place 65-70F indoors most of the time, even with a condensing boiler that isn't short-cycling you'd be looking at over7000 therms/year, and even at a buck a therm (it's more expensive than that from N-Star this year), that's SEVEN GRAND.  Insulated you'd be looking at less than 2 grand, a $5K/year savings.  Even if you hired contractors to do the work, insulating a nearly uninsulated building has a crazy-high IRR, a very short payback.
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23 Oct 2014 06:53 PM
Thanks again for responding Dana,
A lot of this stuff is way over my head I am a general contractor who belongs to a club who will be doing this job for no charge with my plumber who is a very good plumber but lacks these calculating skills and I try do the right thing and educate myself on all the newest equitment.
The building is located in winchester I'll try to put a photo of the building on here with help from my children because I have know idea how.
I would even like to know if you would be willing to come to winchester for a fee of course and size this thing up right for us we dont need a design or prints or anything in writing its only about 45 minutes to an hour to worscester.
Last year we spent $ 3000.00 on oil with a 350.000 btu monster thats as old as the bulding so its about 70 years old and even if we did have the funds to do some insulating which we dont the outside of the building is brick and all the walls inside are finished so it would be a somwhat dificult task to insulate.
I'm going to leave my email for you [email protected]
Thanks Dana
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24 Oct 2014 12:07 PM
At $4/gallon that's only about 750 gallons, which implies that either the heat load is a heluva lot smaller than estimated by my WAG U-factors based on your description of the wall stackup, or the place was essentially unheated much of the time, languishing in the 40s F.

You described it originally as "The walls are concrete with no insulation on the outside or on the inside,", and now you're saying it has a brick facing. It's highly likely you have 2" of foam between the brick and concrete given that fuel use, provided the place has been kept at some reasonable temp above 60F. Your heat load is much smaller.

How small is it?

Lessee...

3000 gallons of oil has (138,000 BTU/gallon x 750 gallons= ) 103.5 million BTUs.

Burned at 80% efficiency (best case, for a 70 year old boiler) that means (0.8 x 103.5 m=) 82.8 million BTUs was dumped into the building over a season that ran about 6000 heating degree days.

Thats 82.8 million/6000 HDD= 13,000 BTU per degree day.

Divided by 24 hours in a day that's 575 BTU per degree-hour.

Your 99% outside design temp is something between 5-10 F, but let's call it +5F. The heating degree-day base is +65F (assuming you heat it to something greater than 65F), so that's (65F-5F=) 60F heating degrees.

That implies a heat load of (60F x 575 BTU per degree-hour=) 34,500 BTU/hr.

That's about what I'd expect from an R10-ish walled building that size with few windows and no foundation insulation.

So, WTF?

Would you REALLY install anything as grotesquely oversized as the Solo-250 with that kind of load?

A Solo-60 will almost certainly cover you with some margin, but if you're a nervous Nellie, the -110 won't short-cycle on your radiation, and would only be 3x oversized for your implied load.

Seriously- what temperature was maintained in the building? Got a "K-factor" stamped on an oil bill from sometime between January & April?
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24 Oct 2014 03:32 PM
The second floor which has the 12' high ceilings which is basically one large area except for two small bathrooms this space might have been heated twice last year meaning the heat was turned on for two nights out of the whole entire winter so for the most part thie second floor zone was off.
As for the building it is soild brick I doubt very much there is any foam, as origanlly said that this building has 2 heating zones one of these zones is basically the basement where the members bar is which stays heated all winter the other zone is the 1st floor that is where the 12' ceilings are, down in the basement where the heat is on all the time I said is where the conrete walls are in the basement and it has 8' high ceilings out those 8' high concrete foundation walls is 4' below grade and 4'above grade.
We do not use the upstairs space for anything but we will eventually so this space needs to be sized for the new boiler also
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24 Oct 2014 03:40 PM
If you want to email me Dana I can send you a photo of the building I would put photo on here but I'm not sure how [email protected]
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24 Oct 2014 03:51 PM
We paid around $ 3.20 per/gal. last year so we went through around 937 gallons
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24 Oct 2014 06:56 PM
Even if you went through 1000 gallons and the heating season was closer to 5000 HDD (and we KNOW it was colder than that)...

...that's 0.2 gallons/HDD...

...which works out to 27,600 BTU/HDD source fuel BTUs...

...which at 80% efficiency is 22,080 BTU/HDD net BTUs...

...divided by 24 leaves you at 920 BTU per degree hour...

...times 65 heating degrees implies a heat load of 59,800 BTU/hr.

Even if the crusty old boiler was a shiny-new 86% AFUE oil-guzzler and right sized for the load rather than 5x oversized AND you burned 1000 gallons AND it was only 5000 heating degree days, that would imply a heat load of only 59,800 x 86/80= 64,285 BTU/hr.

Unless the top floor was allowed to drop below freezing (and actually did), there's no way fully heating the second floor is going to do anything even close to doubling the heat load, but it might be 50%. So bump that 64K number by 50% and you're still only at 96K, which is within the output capacity of the Solo-110.

Sounds like there may be an argument for the Solo-110, but not the -175, and CERTAINLY not the -250.

I'm a bit surprised none of the HVAC contractors/suppliers asked for your basic fuel-use numbers, or maybe you were just calling a supply house for some boilers that would deliver roughly the same output the old one might be delivering in it's decrepit state?

With all the uncertainties in the analysis ONE thing is certain:

The annual heat load simply CAN'T be 2-3 times the source-fuel BTUs that actually heated the place!

And that's what you would be saying if you assumed you actually needed the Solo-250 "just to be sure".

You don't need to have enough boiler to be good down to -60F or -125F (temperatures not seen in MA since before the last ice age). Even 1.5x oversizing would be good enough to cover you at the all time lows in Winchester since record keeping began. (So it's only been 150 years or so... ya never know, eh? :-) )

If you have a K-factor stamped on some mid or late winter oil fill-up slips, that number is literally HDD per gallon, measured locally since the prior fill-up. That is the arithmetic inverse of seasonal gallons per seasonal HDD on which I've been reckoning, but has somewhat higher precision since it's an actual measured number, not a WAG. I'm guessing your K-factor is a bigger number than 5.0, but maybe not by a whole lot.

I'm not in the HVAC biz, but I've given up on just putting it out to bid- seen too many proposals that came in clearly 3-4x what the actual heat loads could even remotely be. Based on your actual fuel use you're not likely to be over 100KBTU/hr, even if you're heating the upstairs to 70F when it's +5F outside, and you could be substantially under that.

For a roughly rectangular 2400' building to have a heat load ratio of 50 BTU/ft @ +5F (that would be 120,000 BTU/hr) would be pretty rare. Depending on the thickness of both the the brick & concrete your U-factor for the wall is somewhere between U0.25 and U0.5.

So that 192,000 BTU/hr wall loss when I thought is was a U-1 concrete only wall is no more than half that, and could be as low as 1/4 that number. Assuming it's fully half, and not a quarter, the wall losses then come to 96K, the roof losses come in under 4K, and you're at 100KBTU/hr (which is still a lot, but maybe.)

If the wall losses are more like 1/3 that 192K number the walls lose 64,000 BTU/hr, the roof 3200, and you're at 69,200 BTU, which could be fairly consistent with your fuel use-implied numbers when not fully heating the upper floor.

The old boiler might only be hitting 70-75% efficiency too, remember 80% would be a likely BEST CASE for a boiler that old which skews the numbers upward.

Is there a google street-view shot of the building?

Can you say for sure how thick the brick is, and how thick the concrete is?

Is there an air gap between the brick & concrete?

How about the interior side- is it plaster on lath with an air gap between the lath and concrete? Plaster directly on the concrete? Wood paneling pr sheet rock on furring with an air gap?

The better description of the wall stack up, the better we can estimate the all-critical U-factor of the wall.

How many windows, and how big? (It didn't much matter when I though it was basically a bare concrete wall with minimal coverings in/out, but it does now.)



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25 Oct 2014 09:42 AM
I am not surprised.

First, because fuel use is just a benchmark, and by your own suggestion, inflated by the 10 year low temperatures we experienced last year. Better than measuring radiation, but in the same neighborhood. Last seasons' fuel usage is useful , but the last 10 years is better. Thus Manual "J" . I wish I could say the HVAC industry employed Manual J on every installation or change-out but this is sadly not the case.

The industry generally used a fixed number--everyone makes up their own--for space heating by the sq.ft. adding occasionally cubic feet for cooling loads.

The only downside to under-sizing HVAC systems if the thermostat dropping off a few degrees during the coldest week of the year. The upside is lower fuel bills, more comfort and long systems life.

The down-side to over-sizing your boiler is a list that fills a book...
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25 Oct 2014 11:03 AM
The only downside to under-sizing HVAC systems if the thermostat dropping off a few degrees during the coldest week of the year. The upside is lower fuel bills, more comfort and long systems life.


I mostly agree with a generic "don't oversize". But:

It's degree for degree - so when the temperature drops and stays 10F below what the system can do, then the building is eventually 10F too cold. For example, around here, design temp is 5F and yet -20F is common, for reasonably long periods.

The btu needed is a not very accurate estimate - the "right" size can easily turn out to be noticeably undersized (although Man J usually oversizes).

Oversized air ducts, pipes and radiators increase efficiency.

With variable speed heat pumps, a small amount of oversizing can increase efficiency and yet not suffer from lack of dehumidification capacity.

I'd like to see data that shows that all systems last longer when running more hours/year. Probably not true with many components.

Buildings where setback is used will need significantly more btu to return to comfortable temps. Luckily, under typical (vs design temp) conditions, all units have excess capacity.

Short cycling due to moderate hydronic boiler over-sizing (see above) is easily fixed with a buffer tank.

In Tom's case, I'd consider adding some rigid foam to the walls.
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25 Oct 2014 06:39 PM
if you google this address 18 Raymond place Winchester MA you will see a photo of the building and should explain a lot
Im not sure but if I was to guess whats behind the the brick it is 8" block the wall certainlly measures a lot wider then a studded 2x4 wall behind the brick.
As far as the inside the lower level has panelling on the wall im not sure if it is studded or strapped the upper level walls are plastered with decorative recessed panelling look 3' high agian I'm not sure if its studded or srapped.
We have members there that have been there 40 years and can't recall any updating done so the plastered walls are at least 40 years old.
Another thing I did not mention is in the bar on the lower level area above the bar itself we have three vents with 7" or 8 " regersters attached to 4" ducts that sucks out cigarette smoke from the bar area I have know idea what the CFMs are on this fan but it is probably about 8 "to 10" in diameter if you hold a doller bill up to these regesters with the fan on full blast it barely has the suction to hold on to the doller bill the fan has a dimmer type switch so its not always on full blast because we know its sucking out the heat as well as the smoke so it really depends on how smoky it gets down there but I would say it runs full speed 70% of the time this fan is only opperating durring the times of 1:00 in the afternnon untill 11:00 pm
We keep the lower level heat at 70 and at 1130 pm we lower the heat to 60 and then sometime around 9 or 10 am we put it back to 70.
The supply houses seemed to be concerned with the sq footage of the building and how many floors were being heated which is two and the size of the baseboard and how many feet there was of it which is 150 high output' and 25 ' of standard resisdental baseboard and a 6 ' x 3 ' high output radiator
They came up with the building needing a little over 200,000 btu boiler but wanted go up one size just to be safe
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26 Oct 2014 04:31 PM
I'd install a HRV to deal with your smoke ventilation issue.
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27 Oct 2014 09:40 AM
Where you able to pull up the photo Dana ?
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27 Oct 2014 11:36 AM
The street view I'm getting looks like a very simple shoe-box with a gable type building envelope with a low window/floor ratio. 

So now you're saying it may be an 8" concrete block wall (and not a concrete wall), but it might be a 2x4 studwall (with or without wall insulation with a brick-veneer siding?  Your wall U-factor is probably no more than U0.5 (assuming CMU with cores filled with concrete) and is probably much lower than that.  If it's a 2x4 wall with R11 fiberglass/rock wool you're looking at a U-factor of about 0.12 at most. If the first floor walls are insulated and there is no foundation insulation, the foundation losses could be as much as third of the total heat load.

Find an electrical plug or other location along the paneled wall, take off the cover and peek in.  Or better yet, find an out of the way place where you can drill a 1/2-1"hole and poke around in there with a wire coat hanger or peek in with a boroscope to figure this out.  Bring a can of foam to seal it all up when you're done peeking at it.

The thing can probably be heated with a Solo-60, but if there is enough radiation on each zone that it won't short-cycle with the Solo-110 wouldn't be a disaster.  It remains to be seen whether you're in short-cycling territory, since you haven't given us how many feet of convector there is on the each zone, only the total. 

And just to be sure, you've measured the convectors- 4" fins 14" tall convector housing, right?  And 4" is the shortest side of the fins, if they happen to be rectangular rather than square?

Once you have a reasonably sized mod-con running the system you'll be better off dialing in the outdoor reset curve rather than using overnight setbacks. When you have a boiler that's 5-8x oversized for the load recovery from setback is pretty quick and there is no combustion efficiency penalty.  But with a mod-con controlled by outdoor reset the recovery ramp is slow (especiallty if the boiler's peak output is less than 1.5x the peak load, which is what you want.) If you set the temperature curve high enough to recover temperature more quickly you end up giving up some combustion efficiency. So unless you're leaving the place for a whole day a finely adjusted reset curve will usually beat overnight setback on fuel use. (There are of course exceptions that prove the rule.)
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