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Condensing Gas Boiler Questions (Forced Air)

obg8 Member Posts: 22
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?


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!.

I also posted this on the DIY Forums but my thread seemed to run out of gas :neutral:

The aqua coil I purchased is a HHU-TV-1 along with a 2 ton air handler.


They were mentioning increasing the size of the aqua coil to constantly run the unit at lower water temperatures. The air handler I bought runs at a maximum 1046 cfm and a minimum of 646. This means at 140 degree entering temperature the most btu/h I could get out of my coil is 39,000. This is not enough to meet my heat loss calculation but also not taking into account me re-insulating the house. Do you think it would be a safer bet to just increase the coil size? The HHU-TV-2 at 1000cfm produces 47,000 btu/h. I imagine that might be ok, it's about 10,000 less than my initial heat loss calculation and it doesn't take into account the added insulation. My initial heat loss also included that small 200 sqft basement which is poorly insulated now (If i remember correctly that was a pretty large source of heat loss).

Anyway, sorry for the wall of text, any info would be appreciated. I'm looking to get this all installed before winter sets in here.


  • NY_Rob
    NY_Rob Member Posts: 1,370
    edited September 2017
    obg8 said:

    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.

    Did anyone on the "DIY" forum question that heatloss figure?

    A good ballpark heatloss number to use on modern/post war 2X4 construction homes in the USA is between 15-30BTU's/sq-ft, with the many coming in under 20BTU's/sq-ft. There are longtime contractors here who have stated that they have never seen a 30BTU/sq-ft heatloss home during their entire career.

    You calculation puts you at 47.5BTU/sq-ft heatloss!

    Give the Slant/Fin app a try:

  • obg8
    obg8 Member Posts: 22
    I will attempt to do another calc using that site later today, if that's the case and my calc is way overboard than my coil might be fine? it can produce 28k-39k btu at 600-1000cfm. @ 140 degree water temperature. But wouldn't I want slightly lower water temp than that to guarantee I'm maximizing my efficiency? Someone said around 130 degree water is what to aim for. Also how do you calculate the load of an indirect water heater, again.. 35 gallon capacity and only 1 shower with 2 people. I assume it's not much of a load and we will run that at 180 degree water temp. What size boiler should I be looking at? 60-80k range?
    HVACNUT Member Posts: 4,870
    First, your calculation is waaay off.
    47.5 btus per sq ft would be ok in Antarctica.

    Why do you want to get rid of your nice comfy baseboard heat and install a hydro air system?

    Installing a hydro coil in the attic will almost certainly necessitate adding anti freeze to the system.

    Plan for a high quality steam humidifier to combat the morning bloody nose syndrome.

    This is a complete new install? High supply ducts in the ceiling? Central high return in the ceiling? Not the best way to comfortably heat a house.
  • obg8
    obg8 Member Posts: 22
    When this house was built in the late 1950's it was originally an all electric house, the first homeowner converted the house to oil baseboard. Because of that the amount of baseboard in the house is already a bit wacky. They did what they could when they installed it but it looks like ****. They got pipes running everywhere lol. That being said I can squeeze ductwork into the attic above all the rooms and get them into the living room and kitchen. It won't be easy but it will be doable.

    And the house is already small....4 bedrooms 1.5 bath 1200 sqft, removing the baseboard will give me a few extra feet of room. Forced air looks better and the added heat/cool adds value to the home.

    Also the rooms will have supply and returns except the bathroom/kitchen.
  • Ironman
    Ironman Member Posts: 7,085
    What you're planning would be comparable to trading in a Cadillac for a Yugo, plus paying a hefty sum, then expecting the Yugo to be better than the Caddie.

    Replacing the oil with a gas mod/con is probably a good move depending upon fuel costs in your area. Getting rid of hydronic heat with forced error (no typo) is not a good move. Especially a forced air system that supplies and returns from the ceiling. Hot air will be blown across the ceiling and returned from there. It would not surprise me to see you having 85* at the ceiling and 65* at the floor. Then there's the issue of duct heat loss in the attic; probably 15 - 20% of your heat wasted.

    Forced air requires the thermostat to be set higher than a house with hydronic heat. The reason for this is because forced air is trying to warm your body with air moving across your skin that is lower than your body temp. This actually removes heat from your body making you feel cooler. The lower the relative humidity, the cooler you will feel. This will cause you to raise the thermostat setting to feel warmer. This whole process is is exactly the opposite of how hydronic heat works: radiators emitt heat rays which travel equally in every direction heating every object they strike (including your skin). They do not heat the air; the heated objects do that. Forced air is forced error because it tries to heat in a manner that is 180* opposed to the natural method. Think about how the sun heats the earth and its objects which in turn heats the air and you get the picture.

    Your house needs only the smallest mod/con available: usually 55k btus. That's still more than sufficient for your domestic since most gas 40 gal. water heaters only have a 32k burner and are only 70% efficient.

    If your looking for a/c, then I'd suggest ductless mini splits like a Mitsubishi. Most all of them come as heat pumps which work well for heating in the shoulder seasons.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    HVACNUT Member Posts: 4,870
    Ok. You're sold on hydro air.

    You mentioned you have a hot water storage tank. Indirect or direct fired.

    If your set on doing the attic job, have you given thought to installing a 95+% gas furnace and A/C evaporator coil?
    Still recommend a steam humidifier and a MERV 10 or better air filter.
  • obg8
    obg8 Member Posts: 22
    @Ironman If you saw the baseboard setup currently installed in this house you would not be surprised why I wanted to switch. It looks horrible, they stuffed radiators where ever they could get them and I completely understand, they did the best they could. There is only a foot stub in the bathroom, there are three different sections in the kitchen, the piping in the kitchen runs exposed through the kitchen cabinets lol. It really is awful.If I told the wife I wanted to keep them after I told her I was pulling them out she might kill me. Do you feel like any forced air system from the ceiling does a crappy job? I mean I grew up with an oil furnace with ceiling registers and I never thought the heating was inadequate.

    @HVACNUT Storage tank is indirect. I haven't really given it a thought because the 'basement' area still has baseboard heat and there is no way to get duct work down there, I was considering putting some electric baseboard down there but I'm not sure if it would be worth it... My indirect is 8 years old, I did some searching and I was under the impression gas furnaces were a lot more expensive than what they're listed at... I would maybe consider doing it.It would probably be cheaper than the air handler + aqua coil + boiler combo to just do furnace + water heater + electric baseboard.
    HVACNUT Member Posts: 4,870
    edited September 2017
    @obg8. Is heat in the basement a priority right now?
    Just throwing it out there.
    Gas furnace with evap. in the attic.
    Gas power vent water heater.
    Sell indirect on Ebay.
    Ductless mini split for the basement will provide heat and A/C when budget allows.

    I love spending other people's money!
  • Ironman
    Ironman Member Posts: 7,085
    Have you looked at panel rads?

    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.