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Dana

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I haven't been able to calculate my home's heat loss yet, but in the meantime, I had emailed Slant/Fin about downfiring the TR-30 because the instructions that came with it show four nozzles for the Beckett with the same burner and same burner head: a .85, 1.00, 1.10, and 1.25. It came with the 1.10 installed and they also shipped a 1.25 in the accessory box.

Slant only said it could be indeed be run with the .85, but was hoping they would give more info as to real world fuel usage or efficiency. Also, I read that the boiler should be matched to the air handlers, correct?

Is downfiring the boiler good practice as long as it's setup for that nozzle size?

John

As long as the EWT at the boiler doesn't drop below 140F when both air handlers are running it's fine to dial it back. I'm estimating with a boiler output temp of 180F the air handlers would be pulling only 110,000 BTU/hr, whereas with the 0.85 gph nozzle the burner is delivering about 0.85gph x 85% efficiency x 138,000 BTU/gallon = 99,705 BTU/hr. That's about 10% shy of what the air handlers deliver at 180F EWT, but that only means the boiler would drop back to ~170F-175F out (roughly the EWT at which the pair of air handlers would be drawing the full 99,705 BTU/hjr) with a return water temp in the 150F range- there is still plenty of margin above 140F.

With the 0.85 gph nozzle you could conceivably run into hot water capacity issues when the air handlers are running if you're family is fond of taking extra-long showers. If that happens, setting up the air handler controls to inhibit the blower when the EWT drops below 155F - 160F would be safe for both the boiler and the person in the shower. Unless you have a real gusher of a shower you're not likely to be drawing the full 97.7K with just the tankless coil.
 

JohnCT

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You mentioned several times of keeping the return water above 140F. Since I have an air handler in the attic above the insulation line, I can imagine there are times (particularly with a set-back thermostat) when the circ for the upstairs starts, I could be getting maybe 50F water returning on startup for a minute. Is this a thermal shock issue to the boiler that we're concerned about? The Slant is a cast iron boiler and the Thermodynamics is a steel boiler. I wonder if the steel boiler might be more durable in such a situation since there are no sections to deal with. The Thermo is close to 30 years and never leaked.

I'm estimating with a boiler output temp of 180F the air handlers would be pulling only 110,000 BTU/hr, whereas with the 0.85 gph nozzle the burner is delivering about 0.85gph x 85% efficiency x 138,000 BTU/gallon = 99,705 BTU/hr. .

If this were your call, would you run the .85 or the 1.00 nozzle?

Would I be better off not using the tankless coil and running an electric HWH instead? I know the new control "learns" usage and has an option for an outdoor probe.

Thanks.

John
 

JohnCT

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Existing boiler did you ever have the fin tube warm up in summer using the tankless coil? If so a flo control valve would be needed to stop the gravity flow.

The boiler runs all summer just for DHW and maintains its full set temperature as established by the typical Honeywell aquastat/control. If there's any thermosiphoning going on, I'm not aware of it because the air handlers won't turn on unless the thermostats are calling and the temp in the air handler coil is above the min set temp. In any case, each of the two trunk lines has it's own one way flow check valve (they're brass with a finger adjustment on top and directional arrows cast into the body).

Thanks.

John
 

Dana

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You mentioned several times of keeping the return water above 140F. Since I have an air handler in the attic above the insulation line, I can imagine there are times (particularly with a set-back thermostat) when the circ for the upstairs starts, I could be getting maybe 50F water returning on startup for a minute. Is this a thermal shock issue to the boiler that we're concerned about? The Slant is a cast iron boiler and the Thermodynamics is a steel boiler. I wonder if the steel boiler might be more durable in such a situation since there are no sections to deal with. The Thermo is close to 30 years and never leaked.

There isn't enough thermal mass in that distribution plumbing + hydro-coil's initial slug of cool water to induce any thermal shock on the cast iron. (What do you think the temperature differential is or the temperature slew rate on the fire side of the heat exchanger plates when it's intially firing?) The issue is only about condensing the fairly acidic exhaust onto the heat exchanger plates &/or flue liner.


If this were your call, would you run the .85 or the 1.00 nozzle?

Would I be better off not using the tankless coil and running an electric HWH instead? I know the new control "learns" usage and has an option for an outdoor probe.

Thanks.

John

If it were my call I'd opt for the smallest nozzle, cap off the tankless coil, and install an electric heat pump water heater in the boiler room to "harvest" the standby & distribution losses from the still-oversized boiler. During the summers the heat pump water heater dehumidifies the boiler room, lowering the latent cooling load in the whole house. During the winter the boiler room is often the warmest (and lossiest) room in the house- the ~1F reduction in room temp actually lowers the heat loss out of that room. Dumping that heat inside an insulated tank is more efficient than letting that heat conduct to the outdoors, or to the slab.

Using an outdoor reset control doesn't work well with air handlers, especially when they are already oversized for the boiler at 180F output. At 150F output the air handlers becomes even smaller, making the boiler LUDICROUSLY oversized for the heat being delivered, a recipe for short-cycling.

Read the manual on the controls to see if you can figure out how it works, and what parameters are tweakable. Many use simple algorithms to purge heat from the boiler down to a programmed low temp (140F is fine) before firing the burner, and auto adjust the high- limit based on the behavior of recent calls for heat. Some of them will anticipate the end of a call for heat and kill the boiler, letting the air handler draw down the boiler temp to finish the call. That parks the boiler at a lower idling temperature between calls, resulting in lower standby losses.
 

JohnCT

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Interesting update: my son is looking at houses, and last night we saw a house in our town with a brand new Slant/Fin TR-30-1.10 (prepackaged with the Beckett AFG and 1.10 nozzle) - the same boiler package as I will be installing.

The two things that caught my eye is that this house is about 500 sq *smaller* (2300sq) than my house, and it had two circulators (like I do) and they were both on the return side. This one had a standalone HWH and no tankless coil, otherwise the same boiler as I have. The other difference is that this house used hot water baseboard radiators not hydro-air air handlers like I have.

I realize the consensus is that the TR-30 1.10 is oversized for my home at 2800 sq and I agree with that after reading the explanations here, but the same exact boiler setup is used in a home smaller than mine. Since the new plumbing around the boiler is Propress, I'm assuming the installation was professionally done (plus the home's seller is elderly). So is oversizing boilers a common or even normal thing?

John
 
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Jadnashua

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Oversizing is very common. SOme of the old rule of thumb calculations were for 2x4 walls, and very little (by today's standards, anyways) insulation and quite a bit of air leakage. TOday's standards call for a much tighter home with better windows and insulation. So, as a result, when using old rule of thumb, many people are way oversizing the heating systems. While properly sizing it should be part of the codes, it is not...they just cover safety of the install, not appropriateness. Long gone are the x BTU per sq foot rough estimates. When used, those can produce maybe a 2-3 or more oversized unit. Same thing with a/c units. Because of the newest standards requiring more insulation and air tightness to meet them, that needs to be looked at, too.
 

Dana

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I realize the consensus is that the TR-30 1.10 is oversized for my home at 2800 sq and I agree with that after reading the explanations here, but the same exact boiler setup is used in a home smaller than mine. Since the new plumbing around the boiler is Propress, I'm assuming the installation was professionally done (plus the home's seller is elderly). So is oversizing boilers a common or even normal thing?

John

It's an extremely common thing, almost impossible to avoid with oil fired boiler in new code-min construction with less than half the design load of a comparable sized 1940s house with single pane windows and no insulation, the paradigm where most of those idiot rules-of-thumb sizing came from.

Often boilers get ridiculously oversized if a tankless coil is serving up the domestic hot water, but there's no excuse for that level of oversizing any 2300' house that has a separate water heater, even an older house with minimal insulation or window updates, which might have a 99% design load in the 50-60,000 BTU/hr range.

Even if it was a retrofit to a high-mass radiator system with excessive radiation size the "right" solution is to plumb it primary/secondary, running the radiation at a lower temp and include a system bypass branch to keep the entering water temp above the specified minimum (usually 140F for oil burners.)

The minimum nozzle sizes for even the smallest commonly available oil burners would be more than 2x oversized for a new IRC 2018 compliant 2300' code min house in CT. Using heat purging smart controls it's possible to keep efficiency from falling off a cliff due to the low duty cycle (= high standby loss), but even with heat purging controls it's better and easier on the equipment to keep the oversize factor as low as possible (down to 1.4x or so), resulting in fewer but longer burns.
 

JohnCT

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It's an extremely common thing, almost impossible to avoid with oil fired boiler in new code-min construction with less than half the design load of a comparable sized 1940s house with single pane windows and no insulation, the paradigm where most of those idiot rules-of-thumb sizing came from.

Often boilers get ridiculously oversized if a tankless coil is serving up the domestic hot water, but there's no excuse for that level of oversizing any 2300' house that has a separate water heater, even an older house with minimal insulation or window updates, which might have a 99% design load in the 50-60,000 BTU/hr range.

Even if it was a retrofit to a high-mass radiator system with excessive radiation size the "right" solution is to plumb it primary/secondary, running the radiation at a lower temp and include a system bypass branch to keep the entering water temp above the specified minimum (usually 140F for oil burners.)

The minimum nozzle sizes for even the smallest commonly available oil burners would be more than 2x oversized for a new IRC 2018 compliant 2300' code min house in CT. Using heat purging smart controls it's possible to keep efficiency from falling off a cliff due to the low duty cycle (= high standby loss), but even with heat purging controls it's better and easier on the equipment to keep the oversize factor as low as possible (down to 1.4x or so), resulting in fewer but longer burns.

Thanks again Dana. Slant says the boiler and the installed Becket burner head combo will work at 0.85, so I'll install that and have the HVAC tech set it up for that nozzle. If I didn't already have the boiler sitting in it's crate in my basement for 3 years this winter, I'd have gone for the TH-20 instead.

I'm still up in the air about hot water. I think the "thermal targeting" feature of the Hydrolevel Hydrostat might be more efficient if I run an external water heater.

Does it make any sense to put the tankless coil in series (intake side) of a standard HWH, or would I be better off with a storage tank that runs off the coil? I haven't abandoned the idea of the heatpump HWH either.. :)

John
 
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