Condensing Gas Boiler Questions (Forced Air)

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Old 09-01-17, 03:22 PM
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Condensing Gas Boiler Questions (Forced Air)

Hello, so I am in the process of converting from oil to propane and switching most of my house from fin tube baseboard to forced hot air with an aqua coil (adding a/c later).

A small loop in my finished basement will still be baseboard as there is no feasible way to run duct work down to the basement from the attic. Although I'm also considering just putting some electric baseboard down there as it is such a small space that it may not even be worth hooking the boiler up to it but i'm not sure yet.

Forced hot air sq ft - ~1000
fin tube basement sq ft ~200


My question is, I would like to install a high efficiency condensing lp gas boiler. How well does that work with condensing boilers? What steps must I take to get and maintain that efficiency with an aquacoil?

Also when it comes to boiler sizing... I have a hot water storage tank that I plan to continue using. The house has only 1.5 bathrooms and only 2 people. Not much of a constant hot water load.

I did a heat loss calculation and came out to around 57,000 btu for the entire house based on my region and house. ~1200 sq ft, built in 1958, shoreline Connecticut.

I am also in the process of a complete re-insulation overhaul, ripping out all the insulation in the walls. Dense pack R-15 with 1" polyiso on the outside of the house (R~5.5 in the winter) and re-insulating the attic. I am doing this room-by-room so the whole process won't be complete until sometime next year (if i'm lucky) that being said, I assume that would considerably lower my heat loss calculation.

I know a lot of these new boilers can modulate their btu/h so it really all just comes down to upfront cost, I don't plan on buying a 200k btu/h boiler that can run at any range when I can buy a 60,000 btu/h, right?

tl;dr

How hard is it to operate a condensing boiler with an aquacoil?
What boiler size would be a safe bet on my house?

Any information would be greatly appreciated, thank you!.
 
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Old 09-01-17, 04:32 PM
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I'm just out of the hospital so my endurance level is pretty low right now. I'll add a couple of comments and then let others (Mike Lawrosa, specifically) give their opinions.

What you propose IS do-able provided you allow extra size for the ducts and air handler/heating coil. Design the duct work to the cooling load and you should be fine. You will want a larger (slower) blower unit than commonly specified for such a system and the heating coil itself needs to have a higher BTUs/hr rating to overcome the lower temperature drop over the coil. The whole secret to getting maximum efficiency from a condensing boiler is to run the output water as low as possible. My gut reaction is to shoot for design heating with no more than about 130 water temperature and 125 is better. This will also give you the ability for fast warm-up by having the boiler temporarily going to 140+ degrees.
 
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Old 09-01-17, 06:18 PM
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Hey thanks for the response and the information.

If I was looking for 120-130 degree water I don't think the aqua coil I got will work... I asked what the btu/h ratings were and with 180 degree water it was 44k btu/h @ 600 cfm 51.5k @ 800 cfm and 57k @1000 cfm.

Maybe that will work? Again I'm a complete beginner at this and would love to learn more... Either way if I need to send it back I will, still unopened.

edit: I will look up the info on the air handler, I know it is a generic 2 ton unit with lo/med/hi speed
 

Last edited by obg88; 09-01-17 at 06:21 PM. Reason: forgot
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Old 09-01-17, 06:33 PM
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The amount of heat (BTUs/hr.) a coil will output is depended upon its physical size along with the entering and leaving water temperatures and air temperatures. A bigger coil will allow for a greater temperature drop thereby releasing more heat to the air stream.

If you can't get the circulating water temperature below about 150 degrees, the lower the better, there is nothing to gain from a condensing boiler.


The problem is that as the temperature differential between the water and the air come closer the heat transfer RATE between the two becomes less and less. Do not make the mistake of assuming you need high air temperatures from the coil. 100 degrees output is going to still be some 30 degrees warmer than room temperature, more or less and depending on what terminal temperature you desire. Large air flow (larger fan and ducts) will heat just as well as smaller ducts with higher temperature air. Bonus when using larger ducts with slower fans is greatly reduced noise.
 
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Old 09-02-17, 05:20 AM
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Here is a link to the info for the aqua coil I have.

http://www.aquecoil.com/specs/hhuspe..._TV-Series.pdf

Mine is the HHU-TV-1. Again, I can return this and order a larger coil if necessary.

Also is pumping head a concern? I see it listed in the specifications for the coil, mine is from the basement to the attic which is probably 18-22 ft vertically.
 
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Old 09-02-17, 03:56 PM
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The pumping head given in the specs is the dynamic friction pressure loss across the device at rated flow. The 18-22ft you listed not relevant - that is the static pressure drop seen when you fill the system when it's empty.
 
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Old 09-02-17, 04:25 PM
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Thanks for the reply, will that size coil work well with low operating temp of 125-140?
 
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Old 09-02-17, 04:34 PM
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Sorry to be so late in responding. I am unable to look at any specifications or do engineering calculations at this time. Please see my first response.
 
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Old 09-02-17, 04:51 PM
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no worries man, hopefully someone can spare some time to help
 
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