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First Step: Sizing the boiler

Good morning. We are in the process of installing staple up 1/2 " Pex-Al-Pex to heat our home. We live in Maine. We'll be heating <1100 sf of living space in 3 zones. (Zone 1=3 loops, Zone 2=2 loops, Zone 3=2 loops initially, with option to add a third if 2 aren't adequate - this zone will be in the walls of a 200 sf basement room). I don't have exact figures yet for each loop size, but all will be between 200 and 300' max.

We are looking at Bosch Greenstar LP boilers. (Heat only, we have a hybrid electric/heat pump water heater).
We've been given a wide range of BTU recommendations from several installers. Anything from 40k to 100k.
I'd done some research online and thought we should end up in the 50-60k range, but after poking around this forum some, I'm afraid even that will be too big and we'll end up with short cycling.

A little more info: On the main floor, we're stapling to sub floor, and it has a combination of porcelain tile over plywood and cement board (small bathroom), vinyl tile directly over subfloor (kitchen and dining), and 3/4" hardwood directly over subfloor (living, hall, 3 small bedrooms).

In the basement we are stapling to 1/2" plywood, which is used essentially as paneling, so no other barrier between the PEX and the room other than primer and paint.

We are using 4' omega channel aluminum plates over the PEX and using rFoil reflective bubble insulation fit between the joists/studs, leaving an air pocket between the rFoil and PEX.
Joist ends have been insulated with 2" EPS sealed with spray foam. The house was built in 1989 and has Rockwool type insulation in the attic crawlspace. Its a 1 story home of around 900 sf after deducting interior walls (988 total) and as mentioned we'll also be heating the 200 sf basement room.

For the main floor, the radiant floor will be used to supplement 2 heat pumps in Spring and Fall, but used as primary heat in the colder months. It will be the only heat source for the basement room, and that room will be heated continuously throughout the heating season.

So, I'm hoping you fine people might be able to give me some insight into how to size a boiler using only 1/2" PEX for heating ~ 1100 sf in Maine. I appreciate any help you can give.

Thank you!


  • Jellis
    Jellis Member Posts: 228
    A Heat loss calculation should give you your answer.
    The below link is a simple heat loss calculator, do a room by room calculation and see what you come up with.

    Where in Maine are you located? I may be able to recommend a local contractor.
  • pecmsg
    pecmsg Member Posts: 4,837
    1St step is doing an accurate Room by Room Load / Loss calculation. Now you know what size boiler is needed and if a buffer tank will be required.
    2nd is to calculate BTU output of the tubing at what temperature and find out if there's enough floor space for the required BTU's.
    Supplemental heating may be required!
  • gaabbee
    gaabbee Member Posts: 43
    So we just converted my 900 sf house to radiant last year and have some pointers. My heat loss calculations came in around 30-40k. I was struggling to make the numbers work with the amount of floor space available and target temps I wanted.

    Don't worry about the boiler first. Focus on getting a heat loss calculation either professionally done or use slantfin's app to get a general idea. Then see if you can get enough btu's out of your floor space. We ran the pex 6 inches on center and barely hit my targets. Now you can always run a bigger temperature differential but we wanted that to be a buffer in case of really cold weather.

    We ran part of my house with basement access as staple up with aluminum plates and the other rooms with the pex above the sub floor. We used strips of plywood to create a pattern for the pex to run through and then put a floating floor on top. I have to say I wish we ran the pex above all the sub floors because the performance is way better.

    We put in a reverse indirect tank to take care of the hot water and buffer tank needs. So far so good. If you really come back with needing 60k btu or more I think you may have some trouble with the amount of available square footage. I run a 10 degree temp differential and you may get away with 20 degrees but comfort is sacrificed. Definitely run an outdoor reset and beefing up the insulation to lower that heat loss as much as possible.
  • Ironman
    Ironman Member Posts: 7,373
    FIRST STEP: Do a scientific heat loss calculation. That's the foundation for everything.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • John Ruhnke
    John Ruhnke Member Posts: 882
    Forget the reflective foil stuff. A gap between the insulation and the plates is a bad idea. Any gap will negate the effectiveness of the insulation during the entire length of the joyce bay. The air will flow through the air space in the gap and down any holes or gaps in your insulation.

    Push the insulation right up against the plates. No air flow to be allowed above your insulation. Again it is critical to eliminate air flow between the plates and the insulation. The reason is at least where a hole may be you only lose effectiveness at the hole. A very miner loss. A small hole in your insulation will devastate the performance if there is a air gap above. Because you will create a air current that causes equalization of temperature above and below the insulation.

    I am the walking Deadman
    Hydronics Designer
    Hydronics is the most comfortable and energy efficient HVAC system.
  • John Ruhnke
    John Ruhnke Member Posts: 882
    Experiment to try.

    On a cold day go outside with a winter jacket on. lower your zipper 2'' but keep the jacket tight around you. You wont notice the difference that much. Just a tiny bit colder at the neck line.

    Now with the same jacket create the same 2" opening at the top. Then pull the whole jacket about 2' away from your body. But just leave the 2'' opening at the top. You will quickly start to be freezing cold. What happens? The cold air enters the 2' gap and circulates downward until your entire stomach is as freezing cold as it is outside rendering the entire jacket useless. It wont take long before you pull the jacket close and tight again.
    I am the walking Deadman
    Hydronics Designer
    Hydronics is the most comfortable and energy efficient HVAC system.