New Boiler: Size, Brand, Fuel

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Dana

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How much total radiation/baseboard do you have?

How many zones?

If it's pretty limited and it's low-mass stuff like fin-tube baseboard (not cast-iron rads/baseboard) you can probably get away with a simple boiler bypass branch, no need for a thermostatic mixing valve as in the diagram. It may already have a boiler bypass branch, with or without a ball valve for tweaking it. Some good pictures of the near-boiler plumbing would tell.

If it's short-cycling like crazy and never hitting the low-limit there's something wrong with the controls. A bad thermal connection between the sensor and the boiler's sensor well is one of several possibilites.
 

Alex Frost

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Dana, we already went thru total radiation and total number of zones in earlier threads.

There is something wrong with triple aquastat I have. We changed the boiler temperature gauge as I thought that might have been part of the problem, but now I still have the same issue. The temperature is set for low 140 and high 170 and the temperature never drops below 170 because after burner finishes firing the temperature reads 180 or 190 on the temp gauge.
 

Dana

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Shockingly, I don't have perfect recall on the radiation particulars ...;)

Sounds like the aquastat is screwed up, and that's the primary cause of the short cycling independent of the radiation & zoning particulars.
 

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I have installed Hydrostat 3250 Plus, which I really like based on what it suppose to be doing and how easy was to install. However, I noticed the the "new" temperature gauge on my boiler reads different temperature than the LED display on Hydrostat. It's off by about 20F..give or take. also noticed, while I was installing new Hydrostat (it took me 45 min) which during this time the temperature (according to the boiler gauge) dropped almost 50F (from 180 to 130) and all of this kind of happen after I took the old Aquastat probe out of well and since then gauge and LED display on Hydrostat have been out of sync. Any idea what could be causing this?
 

Dana

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The idling losses of a barely insulated boiler are pretty high when you park it at 180F. I can easily believe it dropped 50F down to 130F over a major fraction of an hour, but it would take hours to drop the next 50F.

Did you put any thermally conductive compound in the well when you installed the new aquastat? If it was a dry fit (no compound) it's probably reading a bit low.

The point on the boiler where the temp is being sensed matters, and there can be real differences of several degrees, especially when burner is firing and the circulators are running. I can also believe a 25 year gauge can be more than 5F off calibration, but 20F, or even 10F of drift would be rare (but not unheard of.)

If there's a tap on or near the boiler for draining the system, a meat thermometer in a styrofoam cup of boiler water would be able to give a third opinion as to what the boiler temp really is.
 

Alex Frost

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Wow, I can't believe that temperature would drop 50F just as standby loss in that short period of time.

I didn't put thermally conductive compound as manufacturer recommended not to put it in. Also, that would only help get the reading and temperature higher. Note, that Hydrostat reading is higher by 20F than temperature gauge. Also, I installed brand new temperature gauge. Today I brought the IR thermometer from work and measure temperature at the outlet point and it matches the reading on temperature gauge, which tells me there could be something wrong with the Hydrostat sensor.
 

Dana

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IR thermometers aren't very precise for measuring absolute temperatures, are more useful for measuring differences in temperature of similar materials. They are sensitive to the emissivity of the surface. It'll read 15-20F or more low if reading a copper or bronze fitting compared to a black-iron fitting. I've sometime put splash of latex paint on copper or bronze fittings, even PEX to make quick-checking of the temp with an IR thermometer more consistent, if not accurate in absolute terms. (Some people prefer hockey tape.) If you can get a reading from the iron on the boiler near the sensor well with the IR thermometer it'll be within 3-5F of reality.

A 180F boiler in a 70F room has a 90F delta-T. Most boilers don't have more than R3 of jacket insulation, many are in the R2-ish range. A square foot of R3 at a 90F delta-T will be transmitting 90F/R3= 30 BTU/hr. Without looking up the size, volume, and weight of YOUR boiler, I'll hazard it has no more than about 40lbs of water-equivalent thermal mass, and about 25 square feet of jacket area (not all of which is insulated). At 40lbs water-equivalent thermal mass, a 50F swing means it lost 50F x 40lb= 2000 BTU. With 25 square feet of R3 jacket it's initial loss is 25' x 30BTU/ft= 750 BTU/hr. The difference is really the uninsulated or less-insulated parts such as the burner port, flue, plumbing connections, which lose heat at a much faster rate than the insulated jacket.
 

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Would the red valve in the attached picture be the place to drain some water to measure the temperature of the water as you suggested? Is it safe to drain water from here?
 

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Dana

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If you turn off the water on the potable side of the auto-fill valve in that picture it may be a good enough spot. If you don't turn off the auto-fill, as soon as you've relieved some of the system pressure the auto fill will be letting in some cold water from the potable supply.

How much pipe is there between the valve and where the plumbing enters/leaves the boiler?
 

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It's about 6 inches give or take of piping. Why does it matter if cold water gets in once I fill my cup with hot water?
 

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The auto-fill valve won't wait until your cup is full to open up- it'll start diluting right away, and it's closer to your cup than the boiler.

With only 6" of pipe it's a good spot to test if that's the output side. If that's on the return side from radiation it'll be measuring the return water temp, which will be lower than the boiler temp.
 

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Dana,

I think the Hydrostat unit might be bad. I am getting replacement tomorrow, so I shall see the results.

Regarding the 12RLFCD (concealed duct) unit, I have contacted local distributor (they have duct/sheet metal shop) and spoke with their HVAC/Duct Designer and he told me these units are not designed for multiple ducts (bedroom) purposes. He is saying the static pressure is so low that bedrooms would not get enough air and I would struggle with comfort level. He said these units are designed for large open area (same as wall units) but for those that do not like the look of wall units. So, basically you hide the concealed duct unit in the ceiling or closet with potentially short duct that would basically blow air in the one open area.

What do you think about his feedback? Does it make sense?

The other option I have in mind is to use ceiling cassette unit and put it right outside of bedroom doors and point the “louvers” down toward the bedroom doors.
 

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The air handlers on the RLFCDs are much better than it's Mitsubishi cousins, and can be used with longer duct runs, and they are NOT just for open areas.

You don't wan't to go hog-wild on length, and both the supply ducts and the return paths have to be very low impedance (=high cross section) to move enough air. There are many existence proofs that it's possible to heat a whole floor with them in small foot-print houses. The lower the actual load, the lower the cfm it needs, but I recently reviewed a proposal for heating a code-min ~800 square foot upper floor zone on Martha's Vineyard with an 18RLFCD that looked like it would work. (But they went with a different option.)

A ceiling cassette "aimed" at the doors to the bedrooms is more likely to be a comfort problem than a ducted unit.
 

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That’s what I told them. What difference does it make whether you are pushing the air thru 3 smaller vs one large duct/opening? I mean I understand there is little bit of the air loss, but not to the extent that room temperature would suffer. I will also add manual in-line dampers to control the air/do adjustments if needed. Also, I do not see each supply duct being longer than 5ft. However, return air ducts will have to be longer since I would have to place them at the opposite side of supply air registers.


How do I figure out heat loss for the basement. I would like to use 12RLFCD with option to heat the basement if needed (not on constant basis) since I will not have any residual heat from the boiler once I start using 100% mini splits. I do not want to get bigger unit to heat the basement as I would not use it all the time or should I?
 

Dana

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You calculate the heat loss of a basement just like any other room, while noting that the dirt on the outside of the below-grade section and below the slab don't track the daily lows, but are more like a constant, with some big fudge factor.

So, it depends on the U-factors of the windows and of the construction type. An uninsulated 8-10" poured concrete foundation runs about U0.65 BTU per hour per square foot. If you insulated it to current IRC code min (= R15 continuous insulation, say 2.5-3" of rigid polyiso) that U-factor drops to about U0.050 (more than an order of magnitude lower.)

Calculate the above-grade area of exposed foundation, less windows & doors, etc. Assume a 70F temperature difference (+68F indoors, -2F outdoors) which is approximately your 99% condition. Then assume you measured the above grade concrete and it came to 250 square feet, and the below grade wall area comes to 1750 square feet:

U0.65 x 250' x 70F = 11,375 BTU/hr for above-grade wall.

At code min it would be:

U0.050 x 250' x 70F= 875 BTU/hr

If you assume the average temp of the below grade dirt is about 48F by the end of winter (could be cooler or warmer- it depends on the conductivity and thermal mass of your soil), that's a difference of 20F:

U0.65 x 750' x 20F= 9750 BTU/hr

At code min it would be

U0.050 x 750' x 70F= 750 BTU/hr

So on just wall losses you could be at 21K of loss (completely uninsulated) or as low as 1625 BTU/hr of wall loss if insulated to code min. This is why it's important to insulate the basement walls even in parts that you are not going to be finishing, since even at lower delta-Ts the losses are pretty large.

Typical clear glass double-pane basement windows run about U0.5. An uninsulated band joist will run about U0.35-U0.5 depending on the siding type. An exterior door runs about U0.5 unless it's insulated. Insulated doors will have a published U-factor (typically U0.25- U0.3).

A 4" uninsulated slab on top of 6" of dry gravel runs about U0.3, but in your location us 50-52F as the deep subsoil temp, and use 65F as the air temp at the floor, for a 15F delta-T.

Run all those numbers, add them to the wall losses, and you have the approximate heat load.

To fine tune the U-factors I'd need to know really what the stack-up is on the foundation walls, including any finished studwalls, exterior foam, whether it's CMU (block) vs. poured concrete, how thick, etc.

With the RLFCD series there's no efficiency down-side to oversizing, since the 1.5 tonner can modulate down just as low as the 3/4 tonner, and IIRC the air handler for the bigger one can move a bit more air.
 

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So if the price is not a driver I can get 18K one and still not worry about cycling...that's good. If this was your house and you were putting this unit what duct size you would use? I was thinking to start with 10" round take off and branch of 7" round duct to each bedroom. It would be short runs 3-5 ft or should I go with 8" diameter for each bedroom?
 

Dana

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A 7" duct would be fine, a 6" duct would also be just fine.

With oversized ducts the air velocity is slow, and you can get better throw and mixing if you avoid similarly oversizing the registers. (How important that is depends on the particulars of the room.
 

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Dana, are you saying I should select register size that meets 6" duct air flow even if would chose 7" duct or are you saying that my registers output would need to match output of the duct?
 

Dana

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Make the register on the smaller size than which would be usual for the duct size. That makes the register the determinant of air flow, not the duct resistance, but it also makes the exit-air velocity a bit higher than the duct air velocity for higher mixing with the room air.
 
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