Replacing old NG boiler / water heater - check my sizing please?
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Yes, you're starting to get it.
When you have low-mass radiation like fin-tube cut up into a bunch of zones short-cycling issues have to be watched like a hawk.
If you had 30-50 gallons of water in radiators in each zone it matters a lot less, since the thermal mass of the water and the (usually programmable) differential temperature around the outdoor reset setpoint allows you to adjust the minimum burn times up, which will reduce (but not eliminate) the rate of cycling during zone calls, hopefully to something "reasonable". Ten or twenty burns an hour when just the one zone is calling for heat it becomes both an efficiency and a wear & tear disaster. At 3-5 burns/hr, not so much.
When the min-fire output is much more than half the 99% design-condition load for the whole house, that's something of a problem too, since it won't modulate much except during mid-winter. With the bigger Viessmann you would be getting pretty close to that mark too (though with high-mass radiation it wouldn't be a problem).
The ideal is to have something that can be tweaked to run 3-5 burns per DAY, where the outdoor reset is dialed in so close that you barely need a thermostat. With fin-tube that would be tough, since the output becomes very non-linear below 120F, but you should be able to get it to modulate most of the time in winter, if not the shoulder seasons.
I don't have sufficient experience to pick Lochinvar over Viessmann( or conversely.) Both have good reputations, not sure how well supported they are locally.
When you have low-mass radiation like fin-tube cut up into a bunch of zones short-cycling issues have to be watched like a hawk.
If you had 30-50 gallons of water in radiators in each zone it matters a lot less, since the thermal mass of the water and the (usually programmable) differential temperature around the outdoor reset setpoint allows you to adjust the minimum burn times up, which will reduce (but not eliminate) the rate of cycling during zone calls, hopefully to something "reasonable". Ten or twenty burns an hour when just the one zone is calling for heat it becomes both an efficiency and a wear & tear disaster. At 3-5 burns/hr, not so much.
When the min-fire output is much more than half the 99% design-condition load for the whole house, that's something of a problem too, since it won't modulate much except during mid-winter. With the bigger Viessmann you would be getting pretty close to that mark too (though with high-mass radiation it wouldn't be a problem).
The ideal is to have something that can be tweaked to run 3-5 burns per DAY, where the outdoor reset is dialed in so close that you barely need a thermostat. With fin-tube that would be tough, since the output becomes very non-linear below 120F, but you should be able to get it to modulate most of the time in winter, if not the shoulder seasons.
I don't have sufficient experience to pick Lochinvar over Viessmann( or conversely.) Both have good reputations, not sure how well supported they are locally.
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MarkP1
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More reading... more thinking... ahh!
So having my first floor currently split into 3 zones seems very silly, and will be quite prone to short-cycling. Zones 1, 2 and 3 are all the first floor, and the house has rather large openings between areas (at least 8 foot wide openings or more between the zones):
Zone 1: 11k BTUH, 48 linear feet of baseboard, ~7.2 feet of head pressure @ 1.1 GPM
Zone 2: 9k BTUH, 32 linear feet of baseboard, ~6 feet of head pressure @ 0.9 GPM
Zone 3: 15k BTUH, 52 linear feet of baseboard, ~ 8.4 feet of head pressure @ 1.5 GPM
Each zones is piped with 3/4 copper lines. What about running all 3 zones in parallel as a single zone to the boiler? I'd then have 2 zones, first floor and second floor:
Floor 1: 35k BTUH, 132 linear feet of baseboard = 265 BTUH per linear foot = 135F AWT @ 3.5GPM
Floor 2: 22k BTUH, 102 linear feet of baseboard = 215 BTUH per linear foot = 125F AWT @ 2.2 GPM
That puts the GPM if both zones are open at 5.7GPM, which is less than the max of the Viessmann (6.2GPM), and with only one zone open I'd be above the minimum flow rate as well.
This would mean I could plumb it without primary / secondary plumbing, so I would only need 2 pumps and 2 zone valves... a Taco Bumblebee set to delta-T of 20 degrees with 2 zone valves for the 2 zones, plus a 3 speed pump for an indirect water heater.
Going with a 40-50 gallon indirect water heater, with a drain water heat exchanger and I'd have plenty of hot water all the time.
Any thoughts?
So having my first floor currently split into 3 zones seems very silly, and will be quite prone to short-cycling. Zones 1, 2 and 3 are all the first floor, and the house has rather large openings between areas (at least 8 foot wide openings or more between the zones):
Zone 1: 11k BTUH, 48 linear feet of baseboard, ~7.2 feet of head pressure @ 1.1 GPM
Zone 2: 9k BTUH, 32 linear feet of baseboard, ~6 feet of head pressure @ 0.9 GPM
Zone 3: 15k BTUH, 52 linear feet of baseboard, ~ 8.4 feet of head pressure @ 1.5 GPM
Each zones is piped with 3/4 copper lines. What about running all 3 zones in parallel as a single zone to the boiler? I'd then have 2 zones, first floor and second floor:
Floor 1: 35k BTUH, 132 linear feet of baseboard = 265 BTUH per linear foot = 135F AWT @ 3.5GPM
Floor 2: 22k BTUH, 102 linear feet of baseboard = 215 BTUH per linear foot = 125F AWT @ 2.2 GPM
That puts the GPM if both zones are open at 5.7GPM, which is less than the max of the Viessmann (6.2GPM), and with only one zone open I'd be above the minimum flow rate as well.
This would mean I could plumb it without primary / secondary plumbing, so I would only need 2 pumps and 2 zone valves... a Taco Bumblebee set to delta-T of 20 degrees with 2 zone valves for the 2 zones, plus a 3 speed pump for an indirect water heater.
Going with a 40-50 gallon indirect water heater, with a drain water heat exchanger and I'd have plenty of hot water all the time.
Any thoughts?
With a 40 gallon tank and a ~60K+ boiler behind you probably won't need a drainwater heat exchanger for showering performance, unless you take unusually long showers (an hour or so, maybe?) It will save a bit on gas, but at current natural gas pricing the payout is pretty long on fuel savings alone, unless you & your family really do run long showers on a regular basis. If gas prices go up, the economics get better- do you have a crystal ball on future energy prices? 
If you DO go the drainwater heat exchanger route, note that only Renewability's PowerPipe series is on the pre-approved plumbing products list in MA. Others would require a variance- not hard to get, but it's another step.
The cheapest source I've found for them is EFI in Westborough, but you have to open an account to buy off the wholesale/contactor supplier list- they don't web-retail them. EFI makes it pretty easy to open an account though- a credit card over the phone is enough. IIRC the drainwater units get shipped from their Wisconsin warehouse (a state that subsidized them) , not Westborough, but they don't tack on handling charges to the freight- you pay what they paid for the shipping, not more.
You need to run the numbers on how much pumping head you have before relying on the boiler pump as your heating pump. It's safer & saner for those without the experience/software/math to plumb it primary/secondary. You don't want any expensive mistakes, even if it costs you a bit more in pumping power use to have separate primary & secondary pumps in the design.
If you DO go the drainwater heat exchanger route, note that only Renewability's PowerPipe series is on the pre-approved plumbing products list in MA. Others would require a variance- not hard to get, but it's another step.
The cheapest source I've found for them is EFI in Westborough, but you have to open an account to buy off the wholesale/contactor supplier list- they don't web-retail them. EFI makes it pretty easy to open an account though- a credit card over the phone is enough. IIRC the drainwater units get shipped from their Wisconsin warehouse (a state that subsidized them) , not Westborough, but they don't tack on handling charges to the freight- you pay what they paid for the shipping, not more.
You need to run the numbers on how much pumping head you have before relying on the boiler pump as your heating pump. It's safer & saner for those without the experience/software/math to plumb it primary/secondary. You don't want any expensive mistakes, even if it costs you a bit more in pumping power use to have separate primary & secondary pumps in the design.
MarkP1
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Then the power-pipe can wait
I spoke to a Viessmann technician, and he suggested although I am over the min and under the max, it will run better with their low-loss header (which will provide the primary / secondary piping). Part of this is due to their being a temperature sensor inside the LLH which the boiler uses to modulate more effectively.
So the questions I am facing now are:
1) Do I combine all my zones on the first floor into a single zone?
1A) If yes to combining, then running them in parallel, do I need any kind of balancing valves between the parallel runs (if so, what)?
2) What pumps to use for boiler circuit / DHW circuit? I'd prefer an ECM for power efficiency, so either Taco Bumblebee or Grundfos Alpha's set to fixed speed?
3) For heat zones off the secondary, if there are only 2 zones, then the cost of 2 Bumblee or Alpha's (one for each zone) is the same price as 1 of the pumps and 2 zone valves. I would think a pump on each zone would be better, using Taco Bumblebee's on each zone, so each pump can alter the flow rate to get a delta-T of 20 from each zone. So the question is, 1 pump with 2 zone valves, or 2 pumps and no zone valves?
I spoke to a Viessmann technician, and he suggested although I am over the min and under the max, it will run better with their low-loss header (which will provide the primary / secondary piping). Part of this is due to their being a temperature sensor inside the LLH which the boiler uses to modulate more effectively.
So the questions I am facing now are:
1) Do I combine all my zones on the first floor into a single zone?
1A) If yes to combining, then running them in parallel, do I need any kind of balancing valves between the parallel runs (if so, what)?
2) What pumps to use for boiler circuit / DHW circuit? I'd prefer an ECM for power efficiency, so either Taco Bumblebee or Grundfos Alpha's set to fixed speed?
3) For heat zones off the secondary, if there are only 2 zones, then the cost of 2 Bumblee or Alpha's (one for each zone) is the same price as 1 of the pumps and 2 zone valves. I would think a pump on each zone would be better, using Taco Bumblebee's on each zone, so each pump can alter the flow rate to get a delta-T of 20 from each zone. So the question is, 1 pump with 2 zone valves, or 2 pumps and no zone valves?
