Need advice on boiler sizing and Manual J

donr

This will be my first post and it will be a long one. I live in the Montreal area Qc, Canada. I just had an energuide evaluation done on my house to figure where I can save on energy. The house is a prefab unit of 57'x24' built in 1968 sitting on a concrete finished and heated basement. The boiler is an Anthes of unknown power as I can't find any nameplate or serial or model number anywhere. It is located in a small room on the main floor with the water heater and the mains panel. I plan on buying a new boiler probably with an indirect 40 gals hot water tank and have it moved directly underneath where it is sitting now so the boiler and tank is in the basement. I am far from being an expert but I performed a manual J since contractors here don't do this and this is what I came up with:
Design temp for my area is -11F and I like the house at 72F. Construction is 2x4 framing with 3" of bat insulation inside and a black sheeting and aluminum on the outside with no furring between sheeting and aluminum. Windows are original sliders with 4 panes total and aluminum tracks screwed on studs for a total of 168 ft sq. including 4 x 30x18" basement windows. Basement is only 80" from floor to joist and the main floor is 90" to the ceiling. Attic insulation is only 6" thick. Front french doors are 64X80 steel and mostly frosted double pane glass. Back door is 32x80 steel with a slider in it. Total lineal footage of hydronic baseboards is 54' for the main floor and 16' for the basement all tied into a single loop. So with this info, I come up to approximately 57,100 Btu's total of which around 13500 goes for the basement. Do you think this is accurate? My present boiler keeps the house comfy except on the worst of winter days where temps go lower than -30F. Although the basement only has 2 x 8' baseboards, it is very well insulated and keeps temps only a few degrees cooler than the main floor. Now my manual j setting take an ACH of 0.17 while the energuide report states an ACH of 3.83 @ 50PA. I don't really know what this means. The report also states that if I perform all the recommended improvements, my load will go down to 29,413 btu heating and 12,319 for cooling. That seems awfully low!! I tried a manual J assuming all work done and came around to 45,000 Btu's. I will perform the recommended work over the next 2 years but in the meantime, I want to change my boiler. I was going to go with a Bosch Greenstar 100 with a DOE rating of 89000 Btu's and the SIT40 tank but now I am having second thoughts both for the rating and the need of a high efficiency boiler with a 12 to 15 year lifespan. I could save a few thousand by installing an 85% efficiency cast iron direct vent boiler that will last me for the rest of my life unless I live to the ripe old age of 100. My gas consumption for heating and domestic hot water comes to around 3350 cubic meters a year for the last 5 years. So, what are your recommendations? Told you it was gonna be a long one.
Don

Zman

Don,
You have certainly done your home work.
It looks like your calculations may be a bit high.
Consider that most baseboard heaters will only put out about 550 btu/ft. If yours are typical, that would mean that on the radiation side of things, you will max out at 38,500 BTU/Hr. No matter how big a boiler you put in, that is all you can get out. This may account for the house being a little cold on the coldest day.
You sounds like a good candidate for another cast iron boiler. I would think that with the higher water temps you require,even with outdoor reset, you would not get more than 90% efficiency out of a high efficiency boiler. What is 5% if your utility bill?
Carl

donr

So that might account for the 29,000 Btu's after all improvements have been done. I have been looking on line for near 85% efficiency cast iron direct vent boilers in the 40 to 45K Btu range and I don't find that many. Do you think 126 million Btu's is excessive for heating from October to end of April? I do have to take into account that my domestic water is heated all year long.
Don

Zman

That doesn't sound off the charts to me. How many heating degree days do you have in your area?
Weil Mclain CGI-25 looks like about the right size.

donr

Zman said:

How many heating degree days do you have in your area?

Around 8215. I would like to be able to figure out the size of the boiler I have now based on my consumption in cubic meters for the coldest month of the year. From my bills going back 5 years, this is usually jan/Feb. Thanks'
Don

Rich_49

Please keep in mind that after you perform the recommended air sealing and insulation work that your cast iron boiler will be problematic . Your house will be tight as a frogs butt and it will attempt to pull in combustion air from wherever it can , in your case , most likely place will be the exhaust flue . If you use the cast iron boiler you will probably never lower the load to the recommendations made since you will have to allow enough leakage for the boiler to have make up air . A damn near completely isolated room from the living space would be one option but when this si done and the house is heated sufficiently there will be no call , the boiler will be idle in a very cold space .
I urge you to think this through and investigate the many options that exist that are not super expensive but will allow you to have the tight efficient house without killing yourself . The mentioned Bosch would not be my first choice for a home with the loads you will achieve after the work , although it would allow you to remain breathing .

Look for a mod con that has a firetube HX , no primary / secondary piping requirement and a high turndown ratio . This typer of boiler will heat the house now and as you tighten it up will get low enough to not be oversized then either .

You may have enough baseboard to keep the home warm now using whatever temp you must and give up a bit of efficiency until the watter temp becomes lower required based on work done . In either case the new mod con will be more efficient than the cast iron boiler .

Zman

It would be easier to just clock the meter. Observe how much gas it uses in a set period of time (running constantly with everthing else off), then convert to BTU/Hrs.

donr

Now I really am confused. I had 2 contractors submit quotes to change and relocate the boiler. One proposed the Lochinvar Knight WHN 085 with an indirect SIT40 while the other proposed the above mentioned Bosch setup. Both quotes were close in price, the Bosch unit being a little cheaper. I favored Bosch because of the 5 year parts and labor warranty. I know this forum does not discuss price but, labor for a licensed hvac contractor here is out the roof and, no one is cheaper than the other. Same for equipment compared to the US even accounting for 35% exchange rate. Now, I returned to the Bosch site and saw that they have a smaller unit, the Bosch Greenstar Gas-Fired Wall-Hung 57 #ZBR16-3. As for Lochinvar, the WHN055 might fit the bill. Knowing nothing about boilers, I read many good reviews about Bosch and that is why I favored it. What do you think of this choice? Any suggestions?

Zman said:

It would be easier to just clock the meter.

I did and for a 2 deg.C average on a 24 hour period, I used 7 cubic meters of gas. Thank you for the help.
Don

Zman

The Bosch unit has an aluminum heat exchanger. In some areas these are prone to water issues.
The Lochinvar has a stainless steel firetube exchanger.This design has a good track record and is used by many manufactures.
Personally, I would not install the Bosch if it was given to me for free.
As for the size, both proposed boilers are oversized both to the heat loss and the available radiation.
If you are trying to determine the size of your existing boiler, it would be easier to clock it while running. A boiler from 1968 would normally have a nameplate somewhere. Did you look inside the cabinet?

donr

Ok now, I am really stumped. I can't for the life of me find a tag on this boiler anywhere. I removed everything that was removable and the only tag is the boiler name "ANTHES". Now as I have mentionned before, this is a 1300 sq.feet house with a 1300 Sq ft basement for 2600 sq. ft. I have a 40 gals DHW heater of 38000 btu's and 59% efficiency. I have compiled the gas consumption for the coldest month of the year, usually December/January from 2010/2015. I came up with 3223 cubic meters for 170 days. This is total gas consumtion for a little over 5 cold months. From my summer bills, I figured I was using around 1.43 cubic meters a day DHW for a family of 3. So if I substract this from my gas consumption, I am left with 2980 cubic meters for those 170 cold days, which equates to 17.53 cu. meters a day or 0.73 an hour which is roughly 27716 Btu/hr. What am I doing wrong? From everything I have read for the last month trying to figure out the best replacement boiler size for my house, all pre 1970 boilers were over sized and around 65% efficient. I am trying to cut down on my $2000 a year fuel bill but, can't seem to nail either the boiler sizing or my manual j calcs. The energy report that I got states that my BTU ratings will go down to ~29K once the house is tight so, it has to be more than this now.
Don

bmwpowere36m3

donr said:

I did and for a 2 deg.C average on a 24 hour period, I used 7 cubic meters of gas. Thank you for the help.
Don

That can't be right... 7 cubic meters in 24 hrs? So 0.3 cubic meters per hour?

That's about 11k BTUs/hr.

As far as the ACH (air exchanges per hour) of 3.83, are you sure its the ACH50?

If so, your house is TIGHT. An ACH50 of 3.83 is ~ 0.21 ACH (natural). Most recommend at least 0.35 ACH (natural) for air quality reasons and anything less should have a forced air/ventilation system.

bmwpowere36m3

Considering you might only have ~30k (BTUs) of baseboard... then that's all your boiler can put out. Regardless of whether its 30K or 150k DOE. The bigger boiler will just heat everything up quicker.

So your gas bill is in-line with that.

bmwpowere36m3

If you want to reduce costs... besides envelope upgrades, then you need to consider more emitters (lower boiler supply temps), modulating and condensing* boiler to get that last bit of efficiency out. *Condensing if you can install enough emitters to keep return water temps below 130* most of the heating season. Or keep supply temps below 150* (assuming a 20* delta system).

dtrani

You probably don't have enough radiation to run a modcon at a low enough reset to condense much, but, assuming you don't oversize it too much, it will be able to turn down and run one, long, continuous burn for every heat demand. That in itself is a much more efficient mode of operation than an iron boiler cycling on/off. Condensing only accounts for the last few % of a mod/con's efficiency.

donr

bmwpowere36m3 said:

As far as the ACH (air exchanges per hour) of 3.83, are you sure its the ACH50?

I think so. It's in french but check attachement. The report states that if I do the mods it will reach 3.45 ACH@50PA.

bmwpowere36m3 said:

I did and for a 2 deg.C average on a 24 hour period, I used 7 cubic meters of gas.

Actually, it was 9 but that was with DHW. Temp. was averaged for that day from figures taken on an hourly basis by the weather station located a few miles from my home.

I can't really install more baseboards but I think I am getting a handle on this now. I will check other solutions for now then. Energuide report recommends a heat pump so I might look into this. Thanks'
Don

bmwpowere36m3

So ~70' of baseboard on a single loop… what's the boiler set-point or supply water temp? If its around 180*, then for example SlantFin baseboard (regular) will put out 510-540 BTUs per foot with a 180* supply, 20* delta and 160* return water temp. Thus you have ~37k BTUs/hr of baseboard. Remember baseboard or any emitter output is based mostly on water supply temps (higher temps = more output, lower=less). So again, regardless of whether you have a 40k or 150k boiler… you can only put in 37k into the house.

Now try and find any nameplate or tag on the boiler. When its REALLY cold out, does the boiler run flat out? Or does it still cycle periodically?

My experience… manual J calculations gave me the lowest heat loss values and IBR calculations the highest. THe spread was ~12% in heatloss on design day. The calculations are very sensitive to what you enter and they make some assumptions, good or bad.

So on design day (-11*F), your manual J calcs are saying you have a heat loss of 57k BTUs/hr? In reality, when the temps have dipped to that level was the system capable of keeping the indoor temps were you set them? If so, then your heat loss is likely closer to 37k (limited by baseboard) or lower. Now when the temps dip WAY below design day… that's another story. How long was it -30*F outside… a few hours, days, how many days, etc…?

bmwpowere36m3

What exactly are you trying to accomplish?

If the tested ACH50 is correct, then your house is VERY air-tight… and air infiltration is usually the largest source of heat-loss. I'm no expert, but I find the infiltration value to be really low considering the age and construction of the house. Is there work you did to the structure?

Next, with "good" heat loss calculations you can determine where the highest heat loss is…. windows, doors, ceiling/attic (insulation), walls (insulation), foundation (insulation), etc…

If the current "system" can maintain indoor temps down to design day and your gas usage is around 27k and baseboard limited to 37k BTUs/hr… then something around 50k with modulation should be sufficient. I installed a WHN055 in my house with a design day heat loss of 37k and 50 gal indirect tank. Not sure what boiler options exsist at such low BTU outputs.

donr

Thanks' bmwpowere 36m3. It is comfy in the house up to design temp. but starting to get chilly below that. I never noticed if the boiler is continually on but I don't think so. At the temperatures we have now, close to 0C (32F) the boiler kicks in for 10 minutes every 11/2 to 2 hours. Boiler set point is 180* with a return closer to 150*. As for ACH50, why do they recommend reducing air infiltration to bring it closer to 3.45? I was under the impression that higher numbers meant more infiltration!! As for work done, I finished the basement and insulated the walls with foam panels on the concrete, air space and 4" bat insulation between metal studs and 8" in the ceiling to sound proof the main floor. Still have to insulate exterior walls before re siding and insulate the attic. As far as what I want to accomplish, I was planning renovations and no one wants to move my old boiler downstairs. They will install a new one with a hot water heater and I hear how the new mod/con boilers are efficient so figured to save on my $2000 a year gas bill. Both contractors told me I needed around 80K btu's as explained by the models they want to sell me. Then I have a pro come in to estimate all heat loss in the house. He even put a fan in the door. When the report came back, I was told that if I tighten up the house, I will only need 29K btu's for heating and 12K for cooling.
Don

jonny88

Just a quick note that Rich pointed to.Combustion air is very important,I saw first hand last year.Super tight home with atmospheric boiler.Homeowner actually deactivated Carbon monoxide alarms as they would bother him when they went off.When I went there the negative draft was scary.It sure was a learning curve and the installer sure heard about it.Don't want to harp on about it but take his advice seriously.

Zman

It is great that you have done a heat loss and blower door test. The information has given you great direction on improving the envelope.
With the blower door/heat loss info and the fact that your radiation cannot (and never has) been able to utilize a boiler with an output of more than 38,000 BTU/hrs, you should probably stop listening to contractors that want to install an 80,000 BTU model.
As far as the mod/con vs atmospheric debate goes, The mod/con will be a little more efficient. As far as the concerns about combustion air and CO, You should never install an atmospheric boiler (or any other) in a home without providing the required combustion air piped from the exterior. This is especially true with tight construction.

donr

I did not think the house was tight. In very cold days, I can feel the draft coming from the electrical boxes on the outside wall. And the boiler was in that small 40 sq.ft room along with the water heater when I bought the house almost 20 years ago. I sometimes open the door in winter to let the heat escape from that room in the house.

Zman said:

you should probably stop listening to contractors that want to install an 80,000 BTU model.

That is why I joined this forum. Great info from people that know what they are taking about. Now I know that it does not matter the boiler size I have, as long as it supply the necessary heat to my baseboards, I don't need more. Not one contractor out of many that I talked to ever explained that to me and the ones that made quotes never checked. I had my water heater replaced in 2012. The contractor insisted that the flue pipe be changed for a bigger one or else he would not install a new heater. He was suppose to come back and drill the outside wall and install a vent. Never did even after I called the gas company that supplies everyone here and sent us their certified partner. From what I understand, we only have one gas supplier up here and it is a big corporation so no competition. They make the rules and that is one of the reasons I can not buy my own equipment. No one will install it. I have to go through a certified partner whom of which will sell me what ever he makes more profit on. At + 11K $$ for the equipment and install, you now understand why I am so hungry for all this info.
Don

Rich_49

Plomberie Chauffage Normand inc . Don't know Canadian geography but maybe they are a possibility . Morgan Tallman is the best you'll find

donr

Thanks' Rich. Just sent him an email.
Don

bmwpowere36m3

donr said:

Thanks' bmwpowere 36m3. It is comfy in the house up to design temp. but starting to get chilly below that. I never noticed if the boiler is continually on but I don't think so. At the temperatures we have now, close to 0C (32F) the boiler kicks in for 10 minutes every 11/2 to 2 hours. Boiler set point is 180* with a return closer to 150*. As for ACH50, why do they recommend reducing air infiltration to bring it closer to 3.45? I was under the impression that higher numbers meant more infiltration!! As for work done, I finished the basement and insulated the walls with foam panels on the concrete, air space and 4" bat insulation between metal studs and 8" in the ceiling to sound proof the main floor. Still have to insulate exterior walls before re siding and insulate the attic. As far as what I want to accomplish, I was planning renovations and no one wants to move my old boiler downstairs. They will install a new one with a hot water heater and I hear how the new mod/con boilers are efficient so figured to save on my $2000 a year gas bill. Both contractors told me I needed around 80K btu's as explained by the models they want to sell me. Then I have a pro come in to estimate all heat loss in the house. He even put a fan in the door. When the report came back, I was told that if I tighten up the house, I will only need 29K btu's for heating and 12K for cooling.
Don

Lets call it 5 min every hour or so... sure sounds like an oversized boiler to me. However that's only at 32*F and your design day is -11*F.

Just to throw out some random #'s... according to MY heat loss calcs (my house 1600 sqft, un-insulated basement, only first floor heated, 2x4 construction, R15 walls, R50 attic and some air sealing, 0.4 ACH) when you go from 32*F to -11*F the heat loss doubles and when you drop to -30*F its 2.5x greater (from 32*F).

Boiler supply 180* and return 150*, so your average water temp is 165*F. You use the average water temp, when looking at BB output.

Yes, the higher ACH50 or ACH the more infiltration... however at some point there's NO "fresh" air getting inside and that is bad for air quality. Search the web... but here in the US, ASHRAE and the DOE recommend a MINIMUM of 0.35 ACH (natural, NOT ACH50). Below that, they recommend installing mechanical ventilation to force air inside the building.

That's why I'm a little dubious of your result... my understanding is to get very low ACH requires a great deal of sealing that isn't easy or done with existing construction. Also the ACH50 value is calculated based on sqft of the house. Do you happen to have the actual CFM reading from the blower when they did the tests? Were any doors between the basement and first floor open (they should have been)?

Going back to boiler size... your limited by baseboard. So if you don't plan on adding more (and can live with the lower indoor temps at EXTREMLY cold outdoor temps), then you should have your answer. If you want a "cheap" way to increase the output of the baseboard when it gets really cold, consider changing the circulator pump. Right now you have a 30* delta... if you brought that down to 20, 15 ,10* you end up with a higher average water temp and thus more BTU output. At the cost of running a more powerful pump.

donr

Below that, they recommend installing mechanical ventilation to force air inside the building.
They recommend installing Heat Recovery Ventilator and also state that I need ventilation because I do not have enough ACH.
On the other hand, I don't have blower readings but door to basement was open.

bmwpowere36m3

Yup and all that adds expense and complexity. If it were me, I wouldn't build the house that tight to "need" mechanical ventilation... beyond the occasional usage in bathroom and kitchen.

Gordy

Controlling your make up air comes at a cost for an HRV when needed. With that being said you also are controlling the amount of air exchange, and the recovery of energy in the exchanged air .

Uncontrolled infiltration is what kills heat loss numbers, and can make them unpredictable.

donr

Thanks' guys. One last question. It is 0 C (32F) here today. I clocked my boiler and it consumes 1 cubic foot every 39 seconds. I figure roughly 92.3K Btu/hr. Is this a correct assumption and if so, that would also be the capacity of my present boiler?
Don

bmwpowere36m3

Sounds right.... not taking into account efficiency I assume.

donr

bmwpowere36m3 said:

Sounds right.... not taking into account efficiency I assume.

Forgot about that. So would that mean if it is, say 65% efficient, then it would be 125K Btu/hr which, is what I remember seeing on the boiler before the last service call. I guess the tag was lost at that time.
Don

Brewbeer

If your boiler is consuming 92,000 BTU/hr, and your boiler is 65% efficient, it is providing roughly 60,000 BTU/hr useable heat.

If your boiler doesn't modulate, the outdoor temp doesn't factor into how much heat your boiler produces.

donr

Got it. thanks'
Don

Gordy

Just because the meter is saying your gross input is 92 k is not an indication that is what your boilers gross input rating actually is.

SWEI

As I recently learned, you can not trust the nameplate rating either. Truly an eye opener for me.