Combo boiler and water heater
Thank you for your advice and opinions. Here is some further information on my existing system. Oil furnace is 98,000 BTU for main house and oil boiler is 105,000 BTU for back room with 56 feet of baseboard and hot water coil for domestic hot water (no longer used). I calculated my design heat load using wintertime oil fill up history and came up with 48,000 using the 1.4 ASHRAE factor (59,000 using 1.7 factor). I am attaching the proposed heating equipment specs for the Buderus gas boiler, 22″ square water-to-air heat exchanger and a future 55 gallon indirect water heater (which says a 140,000 BTU boiler is required). My present 40 gallon gas fired hwh loses hot water on a 3rd consecutive shower. My gut feeling says to use the mid-sized boiler (87,000 BTU output) but I want to make sure I get enough heat transfer across the exchanger since my ductwork is not tight (I lose about 30 degrees from the furnace to the farthest bedroom). I tend to be very conservative and would want to stick with a conventional boiler unless you feel this new system will be very inefficient and/or problematic. Thanks again.I live on Long Island,NY and have a 1600 sf house built in 1950. It currently has an oil furnace for heating the main house and an oil boiler for heating a large back room addition. I have a separate 3 year old 40 gallon gas hwh. Both heating systems are due for a replacement. I am thinking of replacing them with a gas boiler that would have 3 zones…one for the hw baseboards in the back room, one for a water to air heat exchanger with blower unit, and one for a future 50 gallon indirect hot water tank. I would like a more efficient 85% traditional boiler, not a condensing boiler due to simpler and less costly maintenance. I have received estimates from a few contractors who are recommending a 130-140,000 btu/hr unit. This seems to be oversized, perhaps due to my future desire of the indirect hwh. How should the boiler be sized? Does this combined system seem reasonable for my situation?
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>" I have received estimates from a few contractors who are recommending a 130-140,000 btu/hr unit. This seems to be oversized, perhaps due to my future desire of the indirect hwh. How should the boiler be sized? "
It's (almost) never the right thing to up-size a cast iron boiler to serve up domestic hot water with an indirect. (Even the smallest cast iron boilers put out more heat than the typical 50 gallon standalone.) You would only need that much burner to get sufficient domestic hot water out of an embedded tankless coil, not an indirect. Simply making the indirect the "priority zone" would be sufficient even with the smallest cast iron gas burners.
A reasonably tight 1950s house with storm windows over single panes and some R11s in the walls is likely to come in under 30,000BTU/hr @ +15F outside (the 99% outside design temp for most of L.I.) maybe even under 25,000 BTU/hr. If you have wintertime oil fill up history with exact dates and amounts it's possible to infer the design heat load using this methodology:
https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new
A 130K boiler would CRAZY oversized for your heat load, but there may be other issues. Do you already have a hydro-air handler currently being served by your oil burner? If yes, what's the model number? Or are you simply planning to use the existing ducts of the way oversized oil fired hot air furnace?
Even a right-sized cast iron boiler might still short-cycle on zone calls from the baseboard zone. How many feet of baseboard do you have? Multiply the length in feet by 500 BTU/hr, what does it come to? (I'm betting it's a LOT less than 130,000 BTU/hr) While you're at it, multiply by 200 BTU/hr, which is what it would deliver at domestic hot water (and condensing) temperatures. It may be worth spending some money on flat panel rads with more output & thermal mass to increase comfort and reduce the short cycling potential.
It's not always cheaper to install a right-sized cast iron boiler than a modulating condensing boiler, and the mod-con approach is more flexible, and can be set up to work quite well with right-sized hydro-air systems.
Cast iron boiler and indirect water heater is a very expensive and inefficient way to heat water especially in the off season. You are wasting a lot of energy bringing the boiler up to temperature in the summer, even with heat purge a lot of heat is lost. At best it will be about the same efficiency as a budget vented water heater. Plus an indirect is not a cheap part.
Depending on the cost of your oil and electricity, it would probably be about the same cost to go with a resistance heat and much cheaper to go with a heat pump water heater. If your oil is around $3/gall that is with an 80% efficiency boiler that works out to an electricity equivalent cost of $0.10/kWh.
Thank you for your advice and opinions. Here is some further information on my existing system. Oil furnace is 98,000 BTU for main house and oil boiler is 105,000 BTU for back room with 56 feet of baseboard and hot water coil for domestic hot water (no longer used). I calculated my design heat load using wintertime oil fill up history and came up with 48,000 using the 1.4 ASHRAE factor (59,000 using 1.7 factor). I am attaching the proposed heating equipment specs for the Buderus gas boiler, 22" square water-to-air heat exchanger and a future 55 gallon indirect water heater (which says a 140,000 BTU boiler is required). My present 40 gallon gas fired hwh loses hot water on a 3rd consecutive shower. My gut feeling says to use the mid-sized boiler (87,000 BTU output) but I want to make sure I get enough heat transfer across the exchanger since my ductwork is not tight (I lose about 30 degrees from the furnace to the farthest bedroom). I tend to be very conservative and would want to stick with a conventional boiler unless you feel this new system will be very inefficient and/or problematic. Thanks again.
>"...a future 55 gallon indirect water heater (which says a 140,000 BTU boiler is required)...
An indirect water heater doesn't "require" a particular sized boiler. A boiler size might be included in the specification simply to be ABLE to specify some performance factors such as first-hour gallons, etc. But a heat exchanger in the indirect big enough to deliver 140,000 BTU/hr is also going to work just fine with a 40,000 BTU/hr boiler- roughly the same recovery rate of a standalone 50-60 gallon residential water water.
With the TT-55 indicated in your attachment 140K of boiler only "required" if you need the full continuous flow of 187 gph (more than 3.2 gpm) at a 90F rise, or 277 gph (4.6 gpm) at a 70F rise (35F in, 105F out at the shower head) in a truly INFINITE gusher-shower that would deplete the 55 gallon tank and rely on burner alone. It's highly unlikely you need anywhere NEAR that much hot water delivery in a normal sized house with normal bathing equipment.
A 2 gpm shower @ 70F rise is 70,000 BTU/hr. The burner doesn't need to be that big when you have 55 gallons of indirect buffering the heat, especially if you set the temp higher. A typical 40 gallon gas-burner has a burner output of only 30-32,000 BTU/hr- you won't need to even double that to make it work, and you certainly don't need to more than QUADRUPLE that with a 140,000 BTU/hr boiler.
For about a grand you could probably turn the 40 gallon gas burner into an "endless shower" experience if you have 5' of vertical drain downstream of the shower where a EcoDrain VT-1000-4-54 or PowerPipe R4-60 drainwater heat recovery heat exchanger could be installed, delivering more than 50% of the heat going down the drain back into the incoming water stream:
https://ecodrain.ca/en/products/VT1000/
http://renewability.com/wp-content/uploads/2017/07/20170913-Power-Pipe-Spec-Sheet.pdf
Adding one of those is thermally equivalent to adding more burner onto the water heater for long continuous simultaneous drain/potable flows, but it's a "burner" doesn't use more gas.
Finally, oversizing the boiler for the space heating load to deliver normal levels of hot water usage ends up reducing both comfort and efficiency. Simply giving the indirect priority from the zone controller works for at least 19 out of 20 homes. Only homes with truly large domestic hot water needs or miniscule heat loads would need to do something different.
>"Oil furnace is 98,000 BTU for main house and oil boiler is 105,000 BTU for back room with 56 feet of baseboard and hot water coil for domestic hot water (no longer used)"
Given the extreme oversizing of the boiler for the baseboard zone the fuel use based heat load estimate of 34,000 BTU/hr is probably at least 10% higher than reality. The boiler's nameplate efficiency may be 83-85%, but it's as-used efficiency is probably closer to 65-70% due to the low duty cycle, and the fact that it will cycle on/off quite a bit during continuous calls for heat from the zone, since at 180F output the 56' of baseboard is only about 28,000 BTU/hr. It probably even short-cycles, with sub 3-minute burns during long calls for heat. Some of those cycling & distribution losses are heating the boiler room, but probably excessively so.
>"...I want to make sure I get enough heat transfer across the exchanger since my ductwork is not tight (I lose about 30 degrees from the furnace to the farthest bedroom)."
The "farthest bedroom" heating problem is usually more about low duty cycle than duct leakage. By oversizing the hydro-air handler you make the problem worse (or at least don't fix it), since even on design-day the 22 x 22 air handler indicated in your attachment runs only about a 25% duty cycle, with fairly short bursty on-cycles. To hit decent comfort levels and to get full temp air to the farthest register takes a duty cycle well north of 50%. ASHRAE's 1.4x multiplier would deliver a 71% duty cycle under design conditions, and would increase from there as outdoor temps drop below the 99% temperature bin.
Even the smallest air handler on that sheet would be suboptimally oversized for your your loads even at low gpm and the lowest blower speed, but FirstCo has quite a few air handlers that can deliver the 40-50K you would be looking for. But since you're not heating the whole house with the air handler, you'll have to figure out the load of that zone. You may be able to do that measuring & tracking the duty cycle of the existing oil burning furnace when temperatures are near the 99% outside design temperature. I suspect you're looking at a 1.5 or 2 ton hydro air handler. Check out the heating specs on these few (among many other) series from FirstCo:
https://www.firstco.com/documents/ProductDocuments/hbqb1117.pdf
https://www.firstco.com/documents/ProductDocuments/hbxb-hw1117.pdf
https://www.firstco.com/documents/ProductDocuments/uc116.pdf
It would be good to review Nate Adams' short videos and free download chapter on how oversizing the air handler is going to DESTROY your chances of delivering comfort with the existing ducts, and how it can actually, finally be made to work with a right-sized hydro-air handler:
http://www.natethehousewhisperer.com/home-comfort-101.html
http://www.natethehousewhisperer.com/hvac-101.html
http://www.natethehousewhisperer.com/hvac-102.html
With leaky ducts it's even more important to keep the cfm on the air handler down, for a lower static pressure (=less leakage) and longer duty cycle (= higher temp air at the far registers). Of course sealing up the ducts to the extent possible will only help.