Please review my combi boiler hydronic system design numbers...

OG_Simon

Hello all,

I'm a home owner taking on a combi boiler install. This isn't my daily so I don't have anything to compare the math to. I'm in Vancouver BC, it's mild here but we get maybe 3 cold months. My house is small, 1500 sq ft over two floors. I used a couple online heat loss calculators and they both returned the same heat loss.

I'll need a combi boiler that needs to provide the following heating, I was thinking an IBC SFC 125 because it can modulate down but maybe that's a bit much. I'm open to suggestions:

heat loss calculation = 46140 btu over 3 zones.
- zone 1 = 20140 btu, 5 runtal 4 bar radiators, various lengths.
- Zone 2 = 10000 btu, 3 runtal 4 bar radiators, various lengths.
- Zone 3 = 15,500 btu, 5 runtal 3 and 4 bar radiators, various lengths.

Required GPM = 4.6
Total Head loss = 9' (rounded up, this seems low??)
Zone 1 = 3.5'
Zone 2 = 1.6'
Zone 3 = 3.5'

One single variable speed Circulator - Grundfoss 15-58 or a Taco 007e

All radiators are fed by 1/2" PEX home runs. The longest is a 70 loop. Radiators to have TRV's and each zone will be controlled by a zone valve.

Picture attached for proposed design.

Thanks,
S

hot_rod

Move the purge valve up to the return line, just left of where all the returns tie in.

So flow goes from the fill, thru the pump, around the zones, stops and purges at the purge ball valve.
Still include an iso valve at the pump discharge for service.

Add a dirt/ mag sep if you use any of the electronic circulators.

Another option is a hydraulic separator that provides all 4 functions. Instead of the P/S header under the boiler.

Is that size combi adequate for your DHW needs?

OG_Simon

Hi Rod,

yes the 125k btu is sufficient for my DHW. Not sure about the 88 mbtu output for the heating… The SFC 125 can modulate down to 14.8k mbtu and still maintain condensing. Will it short cycle? 

Thanks for commenting on the other design details. 

S