Advise on Boiler Replacement annd Sizing

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J.NAughton

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I have been lurking on this board for the last week and could use some advise. I am looking to replace a 25 year old Weil McClain GV4 series one 105000 BTU boiler that is connected to a Amtrol Indirect H/W heater. I live in upstate NY (Syracuse)which I believe is in heating zone 6. My house is a 2050 split level built in the mid 1970 that has been update with new Windows and Insulation recently. The system is split into 3 heating zones and the indirect HW heater. 85 feet of Slantfin aluminum baseboard split 44 on main level, 34 on second floor and 7 feet in a basement media room. We used 172 Therms according to our last utility bill which cover 2/10 thru 3/10.Last month was unseasonable cold never getting over 30

We have no heat or hot water supply issues with the current system. I have noticed since gaining some knowledge on this forum that the system short cycles a lot, cycling multiple times an hour.

My plan is to eliminate the small basement zone and tie it into the 1st floor zone. I have gotten 2 quotes so far from local HVAC firms with solid reputations The first company quoted Lochinvar Knight KBN106 using the exist indirect H/W tank. The second company proposed a Navien Combi unit NCB-210. I like the idea of the Combi but have read some negative thing mostly on the older C series.

My questions are 1. Are the units the proper size 2. Is the Navien a good reliable option hot water supply should not be an issue (3 people basic needs) 3. What other brands should I be looking at

Thanks in advance you guys are a wealth of knowledge.
 

Tom Sawyer

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Go with what is locally available and you can get replacement parts for. The unit you have now is probably twice as big as it needs to be.
 

BadgerBoilerMN

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Tom is right.

Your radiation will only handle about 50mbuth. Installing a condensing boiler with higher output would put you in the same short-cycle situation you are in now. When sizing a ModCon to and old, zoned, fin-tube system you have to the look to the low-fire.

Naturally, the KBN 105 is silly, and it's advocate is misinformed. Pass him by or show him a proper Manual 'J' heat load and insist he follow it.

We install many combi boilers and combi water heaters. Which we use is determined by climate, space and DHW loads and the clients life style. In cold climates we lean to combi boilers where they fit.

Since you already have an indirect any locally supported ModCon in the 60-80mbtuh range should suffice noting the minimum fire rate and the annual maintenance. All should start with a Fernox boiler commissioning kit, F5 before the old system is removed and F1 when all is done.
Tying the basement zone to the coldest upstairs zone makes sense. We often allow the valve to operate but do not let it call the boiler independently giving us control of a possible over shoot from people and appliances for example.
 

Dana

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The outside design temp for Syracuse is +2F, and is in US climate zone 5. So, using a base 65F balance point that's 63 heating degrees.

According to degreedays.net data, from 2/10 to 3/9 the Syracuse Hancock Airport weather station logged 1472 HDD, during which time you burned 172 therms at about 87% efficiency (when it was new), so the heating system was really only delivering at most 0.87 x 172= 150 therms of heat into the house (the rest went up the flue.) Thats 150/1472= 0.102 therms/HDD which is 10,200 BTU/HDD.

Converting to BTU/degree-hours you get 10,200/24=425 BTU per degree hour.

So at +2 F (63 heating degrees) the implied heat load is about 63F x 425= 26,775 BTU/hr.

For a 2050' house that's a realistic 13 BTU/ft^2- in the "good" range for a 2 x 4 framed 1970s house, which means the insulation contractors on the recent upgrades did their job pretty well.

That also means that your existing boiler is nearly 4x oversized for the existing load, and the recommended Lochinvar would be just as ridiculous.

The NCB-210's minimum input is 18,000 BTU/hr, which puts it's min-output at about 17,000 BTU/hr. Done right it would modulate some, during mid winter, but most of the time it's min-fire output would be above the instantaneous heat load and cycling on/off, and when serving the ~40-44' zones it would not be able to run at condensing temps without cycling. (17,000/44'= 386BTU/hr per foot, which balances at an average water temp of about 150F, which is well above condensing temperatures), and the 34' (or 34' + 7') zone would be slightly worse.

Ideally with ~40' fin-tube zones you'd want a mod-con with a min-fire output under 10,000 BTU/hr, which would balance at an AWT of about 125F or lower, which means it could at least run at 92% efficiency. Alternatively, you could us a self-buffered high-mass combi system like the HTP Vertex, which can definitely fill the bill, and run in the mid-90s without undue cycling. You wouldn't have to combine the basement zone to another zone either.

In total you have 170' of baseboard for a ~27,000 BTU/hr load, or 160 BTU/hr per foot of baseboard. That is a sufficiently low output requirement that theoretically it could keep up at an AWT of 110F, which means just about any condensing tank type hot water heater could be set up as a combi unit using heat exchangers, at a storage temp of ~125-135F.
 

J.NAughton

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Would a Navien NHB-80 be a better solution it has a TDR of 10:1 bringing the minimum BTU output to 8000?
 

Dana

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Yes, the NHB-80 could be set up under outdoor reset control and it would modulate in condensing mode most of the season since it's min-fire output is ~7500BTU/hr.

You would need to continue to use the Amtrol for hot water.

The zone with only 7' of baseboard would still be a potential short-cycling issue, but the 34' zone can deliver 7500BTU/hr of output at an AWT of ~120F which would deliver return water well into the condensing temperature range once you've tweaked it all in.

The heat loss characteristics are very different from those of above-grade floors. Rather than combining zones it would be better do add another 20-30' of baseboard to that zone, or find an old fashioned high-mass radiator on craigslist or something to replace the 7' stick of fin-tube. (A 45-55" Arco Sunrad or Burnham Radiant kneewall-cut-in type 20" tall x 5" deep radiator would do it.)
 
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