mod/com location - venting length vs proximity to manifold

hot_rod

I'd position the boiler close to the venting termination. They use a special vent pipe material, concentric, and it is a bit expensive.

Insulate the S&R piping between the two locations of course. I like PAP or FostaPex for remote manifold piping. Clean, quiet, no couplings needed, and easy to slip insulation over.

hot rod

Dan_51

mod/con placement - flue vent length vs proximity to manifold

I will be installing a mod/con boiler (either Baxi HT330 or Laars Mascot)and I'm struggling with where to best position the boiler. The manifold for the 5 loop single zone is in the middle of the basement - 1/2" PEX buried in slab on grade, so it's not moving The nearest wall to vent is 25-30 ft from the manifold. So the question is, should I position the boiler so that it is close to the manifold (requiring a 30 ft vent length) or position the boiler so that it vents directly out the wall but would require a 30 ft supply run to the manifold?

Brad White_184

I would favor shorter venting...

Piping is much more forgiving. Venting is ultimately a safety issue so start there...if the venting is compromised, the solution is a re-installation which will affect both piping and venting. Even with the cost of pipe material these days, just do it, install it once.

Dan_51

thanks

will put the boiler so the venting is direct.

Dan_51

Laars Mascot design constraints

Thanks hot rod for the advice. Given now that I will have the boiler remoted from the manifold, I have to determine if I can run the system without any need for a secondary pump. Here are the details of the install:

1) 1250 ft of 1/2" PEX (1 zone, 5 loops of 250' each)

2) 25 ft from boiler to manifold

3) 1800 square ft of concrete with 1.5" blueboard
insulation under slab

4) walkout basement in lower MI, heat loss calc of ~30kBTUs

So the questions are:

1) Can the Mascot be used without a secondary pump for this configuration?

2) What size pipe (3/4" or 1")should I run from the boiler to the manifold?

3) Given that the lowest mod. rate of the Mascot is 32.8k, will I have problems with short cycling?

hot_rod

I would P/S it

their installation directions are not very clear, but why risk it for a few fittings and a 80 dollar circ.

hot rod

Dan_51

short cycling

what about the short cycling?

Bob Forand

No need for P/S

In your application, if you are only heating that slab, you will not need to P/S pipe it. You can go directly to the manifold. The beauty of the Mascot-HT330 (same boiler) is that you can set the max water temp on the boiler, and it will modulate from there. I would also strongly recommend that you use the outdoor sensor. Also, keep in mind the HT380 is now available. It offers a larger DHW plate for more GPM. I believe it is 4 gpm now. The Siemens controller that come with the Baxi versions, also can be used as your room thermostat. They now come in Fahrenheit, you just have to change parameter 47 from celcius. You shouldn't see any short cycling, it is a fully modulating boiler. Given that your BTU load is approx 30K you could probably run 3/4" pipe. HOWEVER, your heat loss calculations should tell you what you head pressures and necessary GPM's needed are. That is how you would size that piping. Good Luck..

Dan_51

min. modulation and short cycling

Thanks Bob. The issue I brought up concerning short cycling is because the HT330 only can modulate down to 32.8k, which is about the same as my heat loss calc for the room. SO the concern is short cycling even when the boiler is modulated to it's lowest level.

Didn't know you can use the Siemans controller as the thermostat - good to know.