Replacing old NG boiler / water heater - check my sizing please?

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MarkP1

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Hi,
Wow... so much great information, hard to know where to start thanking people! So after reading more threads than I can count, I think I have done my homework on sizing for a new boiler / water heater.
House is a 1971 built home, in Topsfield MA. 2500sq ft, 2x4 walls (r-11 insulation), double pane windows, R-35 attic (16" blown in plus spray sealed all holes / gaps). There are 4 zones, all standard baseboard heaters (not cast iron). In the winter we keep the house at 69 during the day (little kids) and 62 at night.
I took my natural gas usage from National Grid, and did a daily linear regression analysis outlined on degreedays.net. This resulted in a base temperature finding of 66F, with an R2 of
0.996, so I am fairly confident in the correlation. The coldest period I have data for is 12/31/13 to 1/29/14. 30 days, the home used 285 therms, with 1228 heating day degrees (HDD) for that same period. We use NG for our water heater and stove, which I expect to be the same year-round. During the summer months (when we turn the boiler off completely), we average 20 therms used for 30 days. This leaves me with 265 therms used to heat the house to 66F on 1228 HDD's.
Dividing 265 therms into 1228 HDD's gives me 0.21065 therms per degree day, or 21065 BTU's per degree day. Per hour, this is 878 BTU's per degree hour.

The current boiler is a standard Burnham cast iron boiler, made in 1983. When it came out, the AFUE was 84.6% (164K BTU input, 138K BTU output, 120K BTU net). So assuming I've actually *gained* a bit of efficiency (calling it 85%), that means the boiler put into my home (878 * 0.85) 746 BTU's per degree hour. The 99% design temp is 5F. So 66F - 5F = 61, times 746BTU's per degree hour, gives me 45,510 BTU's. (if I go with 75% which is more likely, it drops to 40k BTUs).

If I understand this all correctly (and did the math above right), that means I need a boiler that will deliver an IBR net output rating of at least 45,510 BTU's.

I am less interested in a super high efficiency boiler than a longer lived / cast iron model, so I am looking at the Weil McLain GV90+ series. The GV90+3 (91.9% efficient) has a BTU input of 70k, DOE of 65k, and IBR net of 56K. I will add to this an 80 gallon indirect water heater (3 showers, large bathtub, and laundry runs all days with my kids...), on a priority zone. The next model bigger is the GV90+4, which is (91.2% efficient) has a BTU input of 105k, DOE of 97k, and IBR net of 84K.

It would seem that the GV90+3 is more appropriately sized (1.25X coldest thermal demand) than the GV90+4 (1.87X coldest thermal demand).


Am I on the right track? Is there something I am missing? Any advice is greatly appreciated.

All the best,
Mark
 

Dana

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You had it right up until you used the I=B=R output of the boiler, which would only make sense if the boiler was located on the exterior of the wall insulation in an attached shed or garage with insulation between the boiler room and the house. If the boiler is in the basement (even if you don't actively heat the basement to 69F), the distribution & standby losses of the boiler still offset the heat load of the house. Use the D.O.E. output for sizing.

But since the GV90+3 is the smallest in the line, that would be the right choice, if going with that series.

40,000 BTU/hrr @ +5F for a 2500' house or 16 BTU/hr per square foot of conditioned space is about right for a 2x4/R11 house with clear-glass double panes (no low-E). With the Burnham being ~3.5-4x oversized for the whole house load and with the house cut up into 4 zones, INSANELY oversized for any individual zone the thing was probably short cycling on zone calls much of the time with average burn times probably under 3 minutes, which would make your estimate of 75% for as-used efficiency credible. With a heat-purging controller it might have been able to beat 80%, but without it, not so much.

Gas use for hot water in January is typically ~50% higher than summertime average, since A: the incoming water temps are 15-20F colder, and B: Laundry use goes up due to more/heavier layers of clothing use in winter than in summer, but difference that doesn't introduce a huge error on calculating space heating loads from the fuel-use numbers.
 

MarkP1

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Thank you Dana!! I really appreciate it

So seems like part of step 1 in designing a system is done... gross BTU usage, now onto zone loads...

Doing more research, depending on the length of the zone's in the home right now (4 zones... 3 on first floor and 1 on second?) might take me down the mod-con route, so The GV90+ series might not be the way to go for me.
 
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MarkP1

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Ok, did a load calculation for each room of the house, and combined them into the 4 zones for the house:

Zone 1: 11k BTUH, 48 linear feet of baseboard = 235 BTUH per linear foot output needed
Zone 2: 9k BTUH, 32 linear feet of baseboard = 270 BTUH per linear foot output needed
Zone 3: 15k BTUH, 52 linear feet of baseboard = 280 BTUH per linear foot output needed
Zone 4: 12k BTUH, 102 linear feet of baseboard = 120 BTUH per linear foot output needed

Zones 1 through 3 are all on the first floor.
Zone 4 is the 2nd floor with all the bedrooms... hence the amount of linear baseboard being so high.

The total is 47k, which is a bit higher than I got through the daily degree day calculation (not sure which is more accurate though).

Based on the above, I believe I should be looking for a boiler that can modulate down to as close to 9k BTUH as possible, and be able to get to the total of 47k BTUH if needed, right?

So the Lochnivar WHN055 looks like it will fit the bill:

55k BTUH input
52K BTUH high burn output
10.5K BTUH low burn output

Am I on the right track? Much thanks :)
 

Dana

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Heat load calculators have inherent bias to the high side of reality. It's pretty common for the calc to come in 15-25% higher than fuel-use measured reality. Part of it is due to unrealistically high air infiltration defaults, or the fact that fuel-use against degree days doesn't necessarily tell you what the absolute peak load would be in a high-wind, and shoots a bit to the low side. Either way you know what ball-park you're in (it's Fenway, not Wrigley :) ).

The lower a boiler can modulate, the better it is from a condensing point of view, since it's the min-fire output that determines how low the water temp can go before the radiation can't emit as much heat as the boiler is delivering. Fin-tube may deliver 580-600BTU/ft at 180F average water temp, but it can only spit out ~200-220BTU/ft @ 120F AWT. If the boiler is pumping more heat in than the fin-tube is spitting out, the temp rises a bit causing the burner to trip on/off multiple times during a call for heat, which is hard on the boiler and bad for efficiency to boot. But with some tweaks to the radiation that Lochinvar is probably a decent choice.

With a peak of 280 BTU/ft peak loads the thing should be able to run in condensing mode at least 95% the time, since that implies a peak average water temp of 135F, which would be returning the water back to the boiler at just above condensing temp. It has to be 125F or lower coming back to the boiler to hit even 90% efficiency. This will vary a bit from manufacturer to manufacturer, model to model.

With 10,500 BTU/hr of output at min-fire on your shortest zone of 32', means it won't balance if it's not emitting 328 BTU/hr per foot. To hit 328 BTUH/ft you need an average water temp of about 140F, which means the return water would just be barely in the condensing zone, limiting you to ~90% efficiency. If you dropped the output water temp to under 125 F to get some condensing efficiency it will be prone to short cycling on that zone, eating into efficiency and adding wear & tear on the boiler. You either need lower modulation (very few boilers out there run even that low), or more radiation on that zone.

The next smallest zone is 48'. With 10,500/48= 219 BTUH/ft, that would work out to about 120F average water temp, (say 125F out, 115F back, depending on flow) which is fully in the condensing zone. With fin-tube baseboard you want to keep the output temp to no lower than 120F anyway, since the output of fin tube becomes very non-linear below that, and you'd never be able to find a satisfactory outdoor-reset curve. So it's really only the shorty zone that's an issue.

If there's room to add another 12-15' of baseboard in a balanced way to that zone you'd be able to rock that Lochnivar WHN055 in the sweet condensing zone all fall, winter & spring without short-cycling it into an early grave, and that's probably going to be the easiest/cheapest way to get there. If you can't fit more baseboard into the that zone you can do it swapping in panel radiators of sufficient size to emit the full 10,500 BTU/hr @ 120F AWT, which will also give the boiler a bit of thermal mass to work with due to the water volume of the radiators.
 

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Thank you again Dana.

Turns out there is 12 feet of radiator pipe suspended in the open part of my basement. It is not accounted for in the above figures (I forgot it was there). It is currently part of zone 3, but I could swap it to zone 2, giving zone 2 44 feet of baseboard. Would this be enough to balance it out?
 

Tom Sawyer

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Baseboard or fin tubing is rated at X amount of Btu per lineal foot at a specific water temperature. Usually, 180 degrees and somewhere between 570 and 600 BTU/Ft is pretty common for most standard emitters. So 44' @ 580 BTU/Hr @ 180 degree inlet will give you 25,520 Btu/hr. Lowering the water temperature will lower the BTU/Hr output and depending on the delta T across the loop may very well lower the return temperatures far below the proper operating parameters. Unfortunately oil boilers don't fire that low. Most small oil boilers will have an input of around 80,000 BTU/hr with an output in the lower 70,000 range. You can down fire the boiler with a smaller nozzle but that may cause return temperatures that are too low. Unfortunately there is no cheap solution to your problem and in fact doing nothing more than having your current unit properly cleaned and adjusted and adding an outdoor temperature re-set device is probably about the best you can do on a budget.
 

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Tom, he's looking at installing a mod-con gas burner, not an oil burner.

MarkP1: I've given you the napkin math for answering your question, and encourage you to do it. Divide the minimum-fire output number by the number of feet of fin-tube on the zone to come up with the lowest BTU/ft number that has do balance. Then look up what the average water temp is required to balance at that BTU/ft level on some fin-tube spec or another:

Slantfin%20baseboard.JPG


If the AWT that balances with the min-fire output is above 130F you won't beat 90% efficiency, but if it's under 120F you 'll hit the mid-90s:

Condensing%20Chart.jpg


The return water dew point temperature at which condensing begins shown in this chart is a bit optimistic for natural gas, but it illustrates the point. If you have to run at a higher temp to keep the thing from cycling on/off in rapid succession during a call for heat you won't be able to reap the condensing efficiency that the boiler is capable of.

So, min-fire output of 10,500 BTU/hr divided by 44 feet is how many BTU/ft, and what does that correspond to for water temperature?
 

MarkP1

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Hi Tom, I am on natural gas. I have already been approved for a $3500 rebate through mass save on a new boiler as well.

If I understand correctly, the min-fire of the Lochnivar is 10,500 BTUH, which at only 32 feet of baseboard meant it would need to expel 328 BTUH per foot of baseboard, which would require a higher output temperature (150ish) than my other zones (130 output temp). If I add 12 feet of baseboard, bringing it to 44 feet of baseboard, then only 238 BTUH per foot of baseboard would be expelled, which would mean an output temperature of around 125. I believe this would mean all 4 of my zones would be balanced in the sense of needing the same temperatures. Is that right?
 

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You are getting the gist of it. Getting the zones to run at about the same temperate is important, sure.

But it's not as important as getting the smallest zone to balance with the min-fire output of the boiler at a temperature well into the condensing zone. (See the graph in my previous post- those are return-water temps, not average water temps.) If you don't, you'll never be able reap that condensing efficiency without short-cycling the boiler into an early grave.

So, if you need 238 BTU/ft that's an AWT of something like 125-128 for it to balance. Assuming the flow is set for a delta-T of something like 10F that would put the return water in the 120-123F range, and the boiler output in the 130-133F range, yielding an efficiency of maybe 90%. It doesn't take a lot more fin tube to move a bit to the left on that condensing curve, edging into the mid-90s. It's probably going to be a pretty good return on investment gain the extra 3-5% efficiency.
 
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MarkP1

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What about combining zones 1 and 2? Basically, zone 1 is a family room, dining room, entryway, and kitchen, all over basement (somewhat heated by boiler being in the basement) with a second floor above. Zone 2 is a bonus room over an unheated garage (3 walls exposed to outside, attic above). This would give 80 feet of baseboard, which 10500 / 80 = 131 BTUH per foot. The extra 12 feet of baseboard in the basement I would put on zone 3, giving that 64 feet of baseboard, at 164 BTUH per foot. Zone 4 (the second floor), would still be 102 feet, which would be 103 BTUH per foot. This would allow for AWT of 120, so 125 out, 115 return assuming delta-T of 10. This would put it at about 92% efficiency on the chart. If I could get a delta-T of 20 then I'd be up to the 95% efficiency range.
 

Dana

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Combining zones is a standard solution to this problem. When combining zones it's important that the room-to-room temperatures will still work, which is easier to do if the heat loads of the rooms per foot of baseboard are similar, and the general characteristics of the rooms are similar. It rarely works to combine a basement zone with a first-floor zone, for instance, and even balancing first floor & second floor temps when operated as a single zone can be squishy. It's not clear how similar the bonus room's heat loss/gain characteristics will be relative to the zone 1 rooms. If you replaced the fin-tube in that room with a tweakable Buderus panel radiator of similar output you might be able to make the room temps balance even if it needs to be tweaked a bit seasonally. It's probably cheaper just to add baseboard to the shorter zones though, keeping the zoning as-is.
 

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So after doing a room by room manual J load calculation, and slightly reconfiguring my zones, I have:

Zone 1: 13.5k BTUH, 41 linear feet of baseboard
Zone 2: 15k BTUH, 42.5 linear feet of baseboard
Zone 3: 8k BTUH, 53 linear feet of baseboard
Zone 4: 22k BTUH, 102 linear feet of baseboard (1" start splits upstairs into 2 separate 3/4" loops)

So my new total is 58.5K BTUH for all 4 zones... a good bit higher than the degree day calculation of 45.5K BTUH (I'm not sure what to make of that though).

Zone 1 and Zone 2 are the main portions of the first floor of my house. The BTUH and linear feet of baseboard are nearly the same... and both thermostat's are always programmed the same. What about combining zone's 1 and 2 into a single zone (I'd have them on separate zone valves, but would open both valves off a single thermostat)? This would give:

Zone 1-2: 28.5k BTUH, 83.5 linear feet of baseboard
Zone 3: 8k BTUH, 53 linear feet of baseboard
Zone 4: 22k BTUH, 102 linear feet of baseboard

With zone 3 being the smallest at 53 linear feet of baseboard, and a planned delta-T of 20, an AWT of 120 at 4GPM would give me 10600 BTU's for my smallest zone.

So, I am now puzzled as to which direction to go for a boiler... any suggestions on a make / model based on the above?

Much thanks.
Mark
 

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Manual-J always shoots to the high-side of measured reality on a fuel use basis. The absolute worst case (highest 99th percentile wind speeds winds at the 99th percentile outdoor temperature) is something like a 99.99% probability, but under those conditions the true heat loss might approach the Manual-J number. What was your air-infiltration/ventilation load in the Manual-J?

The Lochinvar WHN055 isn't a bad choice, provided you can find local contractors/suppliers to fully support it.

What is your foundation & band joist insulation? If the foundation and foundation sill/band joist have little or no insulation, it's worth fixing, which would peel off a double-digit fraction of the heat load, if you're concerned that the WHN055 would not keep up at the 99.99th percentile condition.
 

MarkP1

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Foundation is concrete (8"), 90% is under ground. The band joist doesn't have much insulation at all.

My total infiltration from manual-j was 28.5K BTUH.
Walls were another 8.5K BTUH.

So I've got until the end of October to have this installed, or my $3500 rebate goes away. I have a plumber I have worked with for years (and really like), so I am hoping to just purchase and provide him a layout of how I want it all plumbed. I haven't found anyone who knows more than my own homework on modcon's, and am running out of time.

What I am thinking is:

Lochnivar WHN055, primary / secondary plumbing, 4 zones off the secondary on zone relays with a Grundfos
Alpha or Taco Bumblebee pump for the secondary. Grundfos or Taco 3 speed pump for the primary, and another grundfos or taco 3 speed for the priority DHW off the primary loop. DHW would be a 60 gallon indirect.

I can play around with the combining of zones by tying both zone 1 and zone 2 to the same thermostat this way.

Thoughts?
 

Dana

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The air infiltration numbers are gonzo-nuts high unless you sleep with the windows open on the coldest night of the year. If your house truly leaks that much it's fix-able for not a lot of money. A symptom of high infiltration rates would be extremely dry indoor air in winter. If it stays above 25-30% RH without running humidifiers there is no way that your true infiltration rate will add up to literally HALF the total heat load(!).

An 8" concrete wall has an R-value of about R1, so even if you only have 12" of above grade concrete that top foot represents a huge heat loss (and the next 2 feet are pretty big too). Even if the basement is allowed to drop to 55F, that's a delta-T of 50F @ +5 outdoor temps, losing 50 BTU/hr per square foot. The band joist is losing about 25 BTU/hr per square foot a that temp. Together that is about as much heat as all of your 2x4 x 9-10' wall area is losing.

The band joist and foundation sill are also a bigger air leak than all of the window & door crackage combined.

Code-min basement & crawlspace foundation insulation for new construction in MA is now R15 continuous insulation (recommended) or R19/2x6 (not recommended.) As a DIYer it's possible to find reclaimed roofing foam for from multiple vendors) for less than the cost of batt insulation from the box stores:
http://www.greeninsulationgroup.com/
http://www.greeninsulationgroup.com/
http://nationwidefoam.com/
http://nationwidefoam.com/

It takes about 3" of XPS or polyiso (either is fine for wall-foam, but don't use polyiso for insulating slabs) to hit the nominal R15 code min. It can be held in place with 1x furring through-screwed to the foundation with TapCons (4.5" minimum if you're going with 3" foam- you may have to buy them online if your local box store doesn't have 'em), and mount the wallboard on the furring. It's better to use 2- layers with staggered seams. Seal the seams of the foam adjascent to the foundation with houswrap tape, and seal the seams of the layer closes to the interior space with fiber-reinforced duct tape. If polyiso, keep the bottom edge of the foam off the slab to prevent wicking of moisture.

Another route is to use 1-2" of foam trapped to the foundation with a 2x4 studwall insulated with unfaced batts.

Either way, the foundation sill & band joist need an inch or more of foam, sealed to the wall foam with can-foam at the seams & edges. Cutting in rigid foam at the band joist, make sure to leave enough space to insert the tip of of the can-foam applicator- air-tightness is critical. You could also just insulate & seal the bandjoist & foundation sill to your wall foam with closed cell polyurethane, either a kit (TigerFoam, Fomofoam, etc) or hire a pro. You might qualify for MassSave-subsidised band joist/foundation sill foam, which would make it cheaper than a kit-foam solution. But even out-of pocket it's going to be worth it.

If your plumber has experience installing mod-cons, you can probably just sketch it out for him with suggestions, and make him do the math on the pumps, etc (or punt- take a WAG and go for it- most would probably be right.) The warranty on the Lochinvar would still be good if installed by a pro, but not if installed by you. IIRC Lochinvar (and a few others) will void the warranty on any system that isn't configured primary/secondary, even though you can sometimes skip that nicety if you do all the math and size the pump correctly. There will be some flow and outdoor reset tweaking to do no matter what, but the approach is sound.
 

MarkP1

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So I will definitely look into having the joist / walls spray foamed, thank you!

I found a Viessmann WB2 6-24, brand new, for $2400. For that price, it seems like a better choice than the Lochnivar WHN055 (which is about $2600). Thoughts on the Viessmann?
 

Dana

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This is a discontinued model- not sure how easy it would be to find parts if there were problems with it down the road. Figure out where the nearest distributor is.

Viessmann is a well known & respected European vendor, not some no-name. The stainless steel heat exchanger in the Vitodens series seems pretty robust. I have no direct experience with them though.

The min-fire output of the Viessmann WB2 6-24 is ~ 23,750 BTU/hr, more than twice that of the Lochnivar you were looking at. It may be a bit oversized for operating in condensing mode with your radiation if you keep the same zoning scheme. If you had big high mass radiators it would probably work, but with fin-tube maybe not. Run the BTU/ft numbers at 23,000 BTU/hr and look up the temperature at which it balances- you'll see what I mean.
 

MarkP1

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I am *slowly* starting to understand this (I think)...

smallest zone if I combined is 53 feet. 23750 BTUH into 53 feet would mean 450 BTU per foot would need to be dissipated to keep delta-T of 20 running constantly. 450BTU per foot is a AWT of about 165 degrees, putting my return temp at 145 degrees, which means I would be out of the condensing zone. So what would happen is anytime the 53 foot zone was on by itself, it will short cycle. I suppose the question is how often will that zone be on by itself... no way to know that.

So the Lochnivar Knight WHN055 is a better choice for having a zone that small (10500 BTUH into 53 feet is 200 BTU per foot, which puts me at an AWT of just over 120, certainly in the condensing zone).
 
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