Boiler estimates, decisions

northlakes

So after some effort and expense trying to get two 30-yr old gas boilers running and rotating, one decided to take a leak. So I’m down to one, which will probably have to last me the winter.

The current plan is to get estimates for a new boiler ASAP.

I can do a SlantFin heatloss calc, installers I’ve talked to so far say they don’t see the point of a “full calc” but can do a rough one they say gets close enough. Current boilers were sized at DOE 135,000 btu each. Official government design temp here is 0 but the heating guys say design is -20. About 150 ft of baseboard emitters, 3 levels including boiler level (heated), about 4K square feet total. 1900 house, but has been insulated a bit and windows aren’t too bad. Does get some good wind exposure on top two floors in winter from the north/west. 

So the usual mod con vs CI questions, I always assume baseboard is a bad match for the desired low return temp of mod con, is this actually the case? I can maybe see the advantage in shoulder seasons but does it make sense with -20 design temp (rarely happens) to consider mod con over CI? Installer says in addition to mod cons (of a three-letter brand I haven’t heard of) they also put in some WM GV90+ units. 

Currently piped as primary secondary, all copper runs. Tankless HW, I’d rather have an indirect but I don’t need the HW concern to influence boiler decision. 

Currently has two gas units but it seems from initial estimates that 135,000 btu is probably close to correct size and that the second unit it may have been for very cold days or redundancy as it was formerly a building with healthcare services. 

Any help thinking this through is appreciated. 

Hot_water_fan

So the usual mod con vs CI questions, I always assume baseboard is a bad match for the desired low return temp of mod con, is this actually the case?

No - modcons can do just fine with baseboard and this isn't an obstacle. Yes, they need higher design temperatures, but if your design temp is 0 (or -20? what's the zipcode?), you'll be condensing most of the winter if you need 180 at design temp, which might not even be the case to start.

If you have 150 feet of baseboard (please double check ), your ceiling is 90kbtu at 180 degrees. If you go with a modcon, it will likely have a 10:1 turndown, which is about as good as you can do. If you go with a CI, then I'd focus more on nailing down the heat loss - How much gas did you use last year?

mattmia2

I suspect the 2 boilers were to make enough dhw for the health care facility.

Look at how much baseboard you have and what your heat loss is on an average day, you may be able to run a reset curve that gets you in the condensing range most of the time if you have enough baseboard.

The design temp isn't the coldest temp you ever see, it is based on an assumption that the mass of the structure can carry it through a couple hours of very cold weather.

hot_rod

DHW is by tankless heaters, or coil in the boiler?
What is the building use now?

northlakes

@hot_rod the DHW is a cheap gas independent tankless that last owner put in but based on what I know I think I’d prefer to change to indirect with a boiler replace. Building use now will be residential.

northlakes

@Hot_water_fan

Hot_water_fan said:

So the usual mod con vs CI questions, I always assume baseboard is a bad match for the desired low return temp of mod con, is this actually the case?

No - modcons can do just fine with baseboard and this isn't an obstacle. Yes, they need higher design temperatures, but if your design temp is 0 (or -20? what's the zipcode?), you'll be condensing most of the winter if you need 180 at design temp, which might not even be the case to start.

If you have 150 feet of baseboard (please double check ), your ceiling is 90kbtu at 180 degrees. If you go with a modcon, it will likely have a 10:1 turndown, which is about as good as you can do. If you go with a CI, then I'd focus more on nailing down the heat loss - How much gas did you use last year?

Thanks for catching that... Baseboard feet number is wrong, I forgot to include the lower level (which is about 2/3 below grade.) total feet of emitters is ~315', with some bare pipe in between.

-20 is what the installer said they design for here, but -1 is what EnergyStar.gov lists for the county. My assumptions are partially based on previous owner running this boiler in constant on/constant circ, but it might have just been his way of solving for it. The gas usage for last year isn't available to us, as we just acquired it over the summer, the gas co. will only give us high and low months, which is 376 and 8 therms. I'm guesssing closer to 200-250 therms average.

My heatloss calc on the Slant Fin app comes to around 105,000 btu at -1. But as I said I'm hearing design is -20. If it's supposed to be based on our [EDIT] coldest week, that feels close.

There is a fair chance there will be about 800 sq. ft added to this floorplan within 2 years. Proper way to plan for scaling up in this case? Would that just mean adding additional boiler/branches, or is there something we should be doing to allow for future scaling.

Hot_water_fan

-20 is what the installer said they design for here, but -1 is what EnergyStar.gov lists for the county

This installer seems scared and not confident in their calculations. It's understandable, they get the call backs, but there's no good reason to use the wrong design temp. With that amount of baseboard, you'll be able to condense nearly always if that's the way you want to go. That said, 376 therms/month is pretty low for a house with a 105kbtu heat loss, maybe they were keeping the thermostat low?

hot_rod

Really no need to guess on temperature conditions. -1 to -20F is quite a spread

Weather data is available for most anywhere going back decades.
Put it into a graph like this to get a realistic visual of what has been the temperature history.

JakeCK

In addition why oversizing is bad, that second graph really drives home the utility of a heat pump doesn't it? Even if it couldn't heat effectively below 20f, and back up had to be utilized, that's like maybe 500hours a year if I'm reading that graph right? At least for Syracuse NY.

jhewings

What is the design temp for Syracuse? I see 99% is -16F and 99.6% is -19F in an ASHRE document (2005). These are both in the most left "bin" in hot_rod's graph and appear to happen for only a few hours per year

Hot_water_fan

What is the design temp for Syracuse? I see 99% is -16F and 99.6% is -19F in an ASHRE document (2005). These are both in the most left column in hot-rod's graph and appear to happen for only a few hours per year

Indeed, that’s the definition of a design temp. 24x365x(1-.996) is only 35 hours per year, usually spread across a few days 

JakeCK

Hot_water_fan said:

What is the design temp for Syracuse? I see 99% is -16F and 99.6% is -19F in an ASHRE document (2005). These are both in the most left column in hot-rod's graph and appear to happen for only a few hours per year

Indeed, that’s the definition of a design temp. 24x365x(1-.996) is only 35 hours per year, usually spread across a few days 

The sanity of sizing a system to even that is questionable.

northlakes

Hot_water_fan said:

-20 is what the installer said they design for here, but -1 is what EnergyStar.gov lists for the county

This installer seems scared and not confident in their calculations. It's understandable, they get the call backs, but there's no good reason to use the wrong design temp. With that amount of baseboard, you'll be able to condense nearly always if that's the way you want to go. That said, 376 therms/month is pretty low for a house with a 105kbtu heat loss, maybe they were keeping the thermostat low?

They probably were keeping it low. Most people I know do ;-) My btu calcs might be off, I'm trying to understand the inputs a little better. Is there an additional "overshoot" calc for boiler capacity? or do you try to size it directly at your heatloss calculation.

I'd say -20 is rare here, I would say there are two things at play, one is the fear of callbacks and the other is we are higher up in a kind of microclimate where we can get 100" more snow and sometimes colder temps than the rest of the county below. One issue here is that snow moving tends to be everyone's second job, so maybe a callback is a bigger deal.

I can never find historical data for our specific area, the station is at a location lower, somewhat warmer (and with less snow.) Every time I try to access noaa's data/site I feel like I'm navigating a game I don't know the rules to. If there's a straightforward way to get temp history I'd love to know it.

Jamie Hall

One thing the figure juggling up there misses a bit is that in the real world the highest heating demand is commonly not on the coldest temperatures. The coldest temperatures tend to be associated with very low wind speeds, and the highest heating demand tends to be associated with higher windspeeds, even with a very tight modern house. Something to keep in mind...

northlakes

Jamie Hall said:

One thing the figure juggling up there misses a bit is that in the real world the highest heating demand is commonly not on the coldest temperatures. The coldest temperatures tend to be associated with very low wind speeds, and the highest heating demand tends to be associated with higher windspeeds, even with a very tight modern house. Something to keep in mind...

A good point I hadn’t considered, and we get a lot of wind exposure here.

northlakes

Most recent installer quote says he’s been doing it so long that a full heatloss calc is a waste of time. He does it by square feet, looking at the house, where it is, estimating insulation level by age/condition. Normally this kind of attitude would make a lot of sense to me. He claims to end up at the same numbers either way. 

Hot_water_fan

Most recent installer quote says he’s been doing it so long that a full heatloss calc is a waste of time. He does it by square feet, looking at the house, where it is, estimating insulation level by age/condition. Normally this kind of attitude would make a lot of sense to me. He claims to end up at the same numbers either way.

If you get a modulating boiler, this approach is fine. The contractor's numbers will be garbage, but the modulation will likely cover up the errors . If you have 315 ft of baseboard and no other emitters, than 200kbtu boiler than can modulate all the way down to 20kbtu will be perfect.

Robert_25

northlakes said:

@Hot_water_fan

Hot_water_fan said:

So the usual mod con vs CI questions, I always assume baseboard is a bad match for the desired low return temp of mod con, is this actually the case?

No - modcons can do just fine with baseboard and this isn't an obstacle. Yes, they need higher design temperatures, but if your design temp is 0 (or -20? what's the zipcode?), you'll be condensing most of the winter if you need 180 at design temp, which might not even be the case to start.

If you have 150 feet of baseboard (please double check ), your ceiling is 90kbtu at 180 degrees. If you go with a modcon, it will likely have a 10:1 turndown, which is about as good as you can do. If you go with a CI, then I'd focus more on nailing down the heat loss - How much gas did you use last year?

Thanks for catching that... Baseboard feet number is wrong, I forgot to include the lower level (which is about 2/3 below grade.) total feet of emitters is ~315', with some bare pipe in between.

-20 is what the installer said they design for here, but -1 is what EnergyStar.gov lists for the county. My assumptions are partially based on previous owner running this boiler in constant on/constant circ, but it might have just been his way of solving for it. The gas usage for last year isn't available to us, as we just acquired it over the summer, the gas co. will only give us high and low months, which is 376 and 8 therms. I'm guesssing closer to 200-250 therms average.

My heatloss calc on the Slant Fin app comes to around 105,000 btu at -1. But as I said I'm hearing design is -20. If it's supposed to be based on our [EDIT] coldest week, that feels close.

There is a fair chance there will be about 800 sq. ft added to this floorplan within 2 years. Proper way to plan for scaling up in this case? Would that just mean adding additional boiler/branches, or is there something we should be doing to allow for future scaling.

The next 100 days will tell you a lot about what the house needs. See how evenly it heats, and what kind of cycle time you observe on the one boiler.

In the meantime, I would use the Slant Fin software to do a heat loss calculation based on what you know. Since you do not know how well the house heated in the past, your heat loss calculation will not only help you size a new boiler, it will also give a good idea of how your radiation is sized. If you discover that your radiation requires 200F water to cover the calculated heat load, that is important to know. The calculations may also alert you to rooms that have had radiators removed, etc. and you will know what to watch for when the mercury drops.

Considering the age of the house, I would not make any capacity allowance for an 800 sq. ft addition, but I would pipe the new boiler to allow for the easy addition of another zone or two. The new addition should be much better insulated & sealed than the existing house, and likely won't add anything significant to the heat load. I have a home of similar age and size, and put a well insulated 750 sq. ft addition on it two years ago...my fuel consumption has not increased.

random12345

The Slantfin app appears to be gone on the Apple app store and their website. Maybe this is good enough, but my guess is you get what you pay for.

https://slantfin.com/slantfin-heat-loss-calculator/

My understanding as a non-pro is that the right way to do a heatloss calculation is with ACCA-approved Manual J software:

https://acca.org/standards/approved-software

There are apparently many companies that can provide this service for a couple hundred dollars. Whether or not it's worth it in your case, I defer to the pros on that question. I have fooled around with Coolcalc which is one of the approved software packages and is relatively cheap. I wouldn't recommend it. I'm pretty computer savvy, but I was not confident in the results. What do the pros on here think about hiring a third party to do this kind of work? I also found this article helpful:

https://greenbuildingadvisor.com/article/sizing-a-modulating-condensing-boiler

hot_rod

The concept of heatloss is just temperature difference across a wall or window. I'd guess all load calc software, excel, etc is based on Manual J calculations.

If you don't mind old school the Pikes peak Building Dept, among others has them manual, fill in the blanks, sheets for free. It doesn't offer them option to create unique wall builds up, but much better then guessing.

The Steam Whisperer

northlakes said:

@Hot_water_fan

Hot_water_fan said:

So the usual mod con vs CI questions, I always assume baseboard is a bad match for the desired low return temp of mod con, is this actually the case?

No - modcons can do just fine with baseboard and this isn't an obstacle. Yes, they need higher design temperatures, but if your design temp is 0 (or -20? what's the zipcode?), you'll be condensing most of the winter if you need 180 at design temp, which might not even be the case to start.

If you have 150 feet of baseboard (please double check ), your ceiling is 90kbtu at 180 degrees. If you go with a modcon, it will likely have a 10:1 turndown, which is about as good as you can do. If you go with a CI, then I'd focus more on nailing down the heat loss - How much gas did you use last year?

Thanks for catching that... Baseboard feet number is wrong, I forgot to include the lower level (which is about 2/3 below grade.) total feet of emitters is ~315', with some bare pipe in between.

-20 is what the installer said they design for here, but -1 is what EnergyStar.gov lists for the county. My assumptions are partially based on previous owner running this boiler in constant on/constant circ, but it might have just been his way of solving for it. The gas usage for last year isn't available to us, as we just acquired it over the summer, the gas co. will only give us high and low months, which is 376 and 8 therms. I'm guesssing closer to 200-250 therms average.

My heatloss calc on the Slant Fin app comes to around 105,000 btu at -1. But as I said I'm hearing design is -20. If it's supposed to be based on our [EDIT] coldest week, that feels close.

There is a fair chance there will be about 800 sq. ft added to this floorplan within 2 years. Proper way to plan for scaling up in this case? Would that just mean adding additional boiler/branches, or is there something we should be doing to allow for future scaling.

I'm in Chicago, southside, which has a design temp of 4F ( north side is about -1F) We have historically been about 6500DD65 winters, though over the past 25 years I strongly suspect that number has dropped considerably. Our coldest winter months are usually about 1200 to 1300 DD65 and our peak winter use is about 200 Therms, which is about 28,000 btu/hr equipment input on average. Due to being relatively well insulated and having an all face brick exterior, we don't need heating until the average starts getting below 60F outside. Our heat load @ 0F is about 45,000 btu/hr. If you have similiar typical northern city weather patterns and your home used about twice as much fuel as ours, I would say your heat load is probably around 90,000 btu/hr. If a heat load calculation gave you about 105,000, that is what I would expect. It is well known that typical ASHRAE based calculations overestimate peak heat loads quite a bit for typical hot water and steam heated homes due to the impact of a large number hard to calculate factors... such as thermal mass (thermal flywheel effect), ground contact tempering, solar gains, etc.

Since you have a good boiler going into the winter, and it sounds like it is plenty big, you can probably get an accurate heat load number if you track fuel usage during extreme weather events. If you track equipment on time, make sure that the equipment is actually running at its rated input. I've seen way too many boilers where people have tinkered with the design gas pressures, etc.

northlakes

@The Steam Whisperer thank you, I’m not sure I know what you mean “tracking equipment on time.” I will check my gas consumption for sure to see if I can measure how it’s going. 

northlakes

hot_rod said:

The concept of heatloss is just temperature difference across a wall or window. I'd guess all load calc software, excel, etc is based on Manual J calculations. If you don't mind old school the Pikes peak Building Dept, among others has them manual, fill in the blanks, sheets for free. It doesn't offer them option to create unique wall builds up, but much better then guessing.

Thank you for this. 

northlakes

Robert_25 said:

@Hot_water_fan

So the usual mod con vs CI questions, I always assume baseboard is a bad match for the desired low return temp of mod con, is this actually the case?

No - modcons can do just fine with baseboard and this isn't an obstacle. Yes, they need higher design temperatures, but if your design temp is 0 (or -20? what's the zipcode?), you'll be condensing most of the winter if you need 180 at design temp, which might not even be the case to start. If you have 150 feet of baseboard (please double check ), your ceiling is 90kbtu at 180 degrees. If you go with a modcon, it will likely have a 10:1 turndown, which is about as good as you can do. If you go with a CI, then I'd focus more on nailing down the heat loss - How much gas did you use last year?

Thanks for catching that... Baseboard feet number is wrong, I forgot to include the lower level (which is about 2/3 below grade.) total feet of emitters is ~315', with some bare pipe in between. -20 is what the installer said they design for here, but -1 is what EnergyStar.gov lists for the county. My assumptions are partially based on previous owner running this boiler in constant on/constant circ, but it might have just been his way of solving for it. The gas usage for last year isn't available to us, as we just acquired it over the summer, the gas co. will only give us high and low months, which is 376 and 8 therms. I'm guesssing closer to 200-250 therms average. My heatloss calc on the Slant Fin app comes to around 105,000 btu at -1. But as I said I'm hearing design is -20. If it's supposed to be based on our [EDIT] coldest week, that feels close. There is a fair chance there will be about 800 sq. ft added to this floorplan within 2 years. Proper way to plan for scaling up in this case? Would that just mean adding additional boiler/branches, or is there something we should be doing to allow for future scaling.

The next 100 days will tell you a lot about what the house needs. See how evenly it heats, and what kind of cycle time you observe on the one boiler. In the meantime, I would use the Slant Fin software to do a heat loss calculation based on what you know. Since you do not know how well the house heated in the past, your heat loss calculation will not only help you size a new boiler, it will also give a good idea of how your radiation is sized. If you discover that your radiation requires 200F water to cover the calculated heat load, that is important to know. The calculations may also alert you to rooms that have had radiators removed, etc. and you will know what to watch for when the mercury drops. Considering the age of the house, I would not make any capacity allowance for an 800 sq. ft addition, but I would pipe the new boiler to allow for the easy addition of another zone or two. The new addition should be much better insulated & sealed than the existing house, and likely won't add anything significant to the heat load. I have a home of similar age and size, and put a well insulated 750 sq. ft addition on it two years ago...my fuel consumption has not increased.

I got one quote for a straight replace, with no significant re-pipe or redesign of the system, tying into existing vents. The boiler they recommended is the WM CGa-7. As I understand it this is conventional CI high efficiency but no condensing like the GV90+. They did not quote a modcon, as they explained they didn’t think return temps below 140 would be good with the system, and so they didn’t think it would be worth it to go that route. 

I’m not against old school CI, especially as remote as we are, I do like the idea of a simpler system, even if it’s a bit less efficient. But I do admit some sticker shock as it appeared to me to be about a day (maybe1.5) of work, maybe I’m missing something

WMno57

@northlakes Please remove the pricing from your post. We can talk about just anything else related to heating, but not price. Site rules.
That said, we are here to help you and want to see you make good decisions regarding heating upgrades.
My math says the Weil Mclain CGA-7 is about 30 percent larger than your current boiler. Do I have that right? The proposal is from a contractor that is not willing to do a heat loss calc. You currently have two boilers, one working, and one leaking. Is the working one in good shape? Age means nothing. Perhaps you should start a new thread with some pictures of your current boilers.

northlakes

WMno57 said:

@northlakes Please remove the pricing from your post. We can talk about just anything else related to heating, but not price. Site rules. That said, we are here to help you and want to see you make good decisions regarding heating upgrades. My math says the Weil Mclain CGA-7 is about 30 percent larger than your current boiler. Do I have that right? The proposal is from a contractor that is not willing to do a heat loss calc. You currently have two boilers, one working, and one leaking. Is the working one in good shape? Age means nothing. Perhaps you should start a new thread with some pictures of your current boilers.

Done, sorry about that. Ok I’ll start a new thread. The boiler I currently have that isn’t shot is “DOE” rated at 137,000 btu, input 167,000. It’s running, heating the place fine so far, but is 30 yrs old, sorta like an old couple where one dies and you don’t expect the other to last too much longer.

Winter is a bit extreme here and waiting until it leaks to replace makes me nervous. Not even sure what my options would be at that point other than totally shutting down and winterizing the place.

The Steam Whisperer

northlakes said:

@The Steam Whisperer thank you, I’m not sure I know what you mean “tracking equipment on time.” I will check my gas consumption for sure to see if I can measure how it’s going. 

This is simply measuring how long the firing on time is and the off time between firing. Measuring this early morning on a very cold day for a couple cycles will allow you to calculate the percent on time. Percentage on time multiplied by the boiler btu/hr input should give you the input needed to maintain comfort at the current temperature outside. I would add some to this number if you have a high mass exterior wall ( brick or stone), since this mass reduces the impact of nighttime lows on the heat load for a building.