boiler size question radiant and indirect DHW

scpt17

I have been reading this forum for awhile now.  Learned a lot, but need

to ask a couple of questions.

1st here is a little history.

I am in the process of installing a hybrid Radiant system. I currently

heat with forced air, which does not get the job done.

I do plan on having a professional do the final set-up of the boiler to

make sure it is set-up correctly.

I plan on running all the PEX and soldering of the copper for the loops

myself.

 

The house is appox. 2600 sqft(not including basement) and the heatloss

is 68000BTU's(Slant/Fin), this includes the basement.

The 1st floor will use staple-up 1/2" PEX with formed heat transfer

plates, most of the 2nd floor will use radiant panel radiators(Buderus

PK21).

The rest of the second floor(480sqft) which is above the garage will be

staple-up. Basement will use baseboard or radiant panel radiators.

I plan on using a water temp of 150*max, I will size up the panel

radiators to accommodate the lower water temp. I will also be using a 50

gal.  Indirect water tank. I currently plan on a Peerless PureFire for the

boiler and a Bock for the indirect.

 

 

I have a couple of questions.

 

1) What size boiler do I need? I was told to get the 140, but that seems

to be twice the size I need. He said it was because of the

indirect.   Seems like a waste, since I should only need to heat the

indirect for maybe 3 hours a day. I was thinking the  PF-80 or the

PF-110.

 

2)  Will the 150* water temp be OK for the indirect, I know it will slow

the recovery time down, but again why heat the water to

180* when it will only be used maybe 3 hours out of the day.

 

3) Does the outdoor reset really save that much fuel compared to the

cost of the additional parts needed.

If the temp. drops below the D.T.D, you can manual increase the

mixing valves and boiler if needed, correct?

 

4) I have attached a pdf of the primary and secondary loop setup I plan

on using. I have had two guys check it out and they said it was OK.

Would just like to double check.  The boiler is about 25' from were I

have secondary loop.

 

5) mod/con or not.  It seems to me the cost over the life of a mod/con

is more than you would save in fuel cost over a reg. cast iron boiler.

 

Any help you can give me would be greatly appreciated



Thanks,

Scott

<strong>Added the pdf.</strong>

CMadatMe

Re-Think This

Just for starters that heat loss is probably not accurrate. What compensation did you make in the program for the radiant? A radiant loss is different then a conventional heat loss. Secondly, why are you running 1/2" pex? Run 3/8" it's easier to pull and you get the same btu output. The only advantage to 1/2" is a 50' longer loop length. You can also run 3/8" to the panel rads.



I'm concerned with the fact you make no mention of any type of mixing for the radiant. How do you plan on controlling it. You mentioned forced hot air. Is this radiant more for floor warming? If so, you also did not mention the use of any floor sensors/set point control. How about the panel rads. Are you using thermostatics?



Yes this is the proper application for a mod/con. Personally I would use a Viessmann Vitodens 200 for this application. Boiler can run 2 water temps. Boiler reset for the rads and  a modulating 3-way valve for the radiant. It's all plug n play.  You can also control domestic on a seperate curve and have the ability to program it on time if you choose. The domestic function works similar to a programmable thermostat. You can tell the boiler when and for how long you want domestic.