Gravity System: Appreciate Advice

Jason Quinn

I looked at a gravity conversion the other day.  I am new to this type of hot water heating,  I am responsible for the boiler installation and to tie-into the existing supply and return branch mains in the boiler room only- the distribution piping and radiators are not in my scope of work.  The owner would like 2 zones.  There are 3 sets of branch mains: Group 1-(1) 5 inch supply with (2) 4 inch returns ( 1 return is for the radiators above and 1 is for fan coil units mounted below the height of these branch mains in the cellar- this return runs in the cellar floor), Group 2- (1) 3 inch supply and return main and Group 3 is (1) 2 inch supply and return main.  The owner would like Group 1 on one zone and connect Group 2 and 3 together forming another zone. The energy consultant on board  came up with a heat loss of approximately 550,000 btu's.  From Dan's book "How Come?" I have an idea of how to size the near boiler piping, but I am a little confused how to do so when 2 branch mains are connected together and or when there are 2 return lines for 1 supply?  Any ideas on this and other things to look out for greatly appreciated. 

Alan (California Radiant) Forbes

Gravity Conversion

The easy part is that your main header will be 2½" for a 20° delta T.  As for sizing the separate zones, you will need to have some idea of the BTU's of the connected load.



You have 3 branch mains.  One zone will supply one branch and the other zone will supply the other two branches.  All the returns will combine.  Is it that easy?

Jason Quinn

Yes. Thank you.

Does anyone know if NJ allows Schedule 10 pipe and gruvlok couplings and fittings on hydronic installations?

Unknown

Gravity boiler conversion

Here in Minneapolis if the system is gravity, it is probably split down the middle of the building...hard to zone.

Radiation can be measured against the heat load to come to a design temperature for the radiators. The fan coil may take a much higher design water temperature.

A 20 degree delta T is optional as the design temp drop was considerably higher.

But for the fan coil, gravity conversions should be done with condensing boilers featuring outdoor reset.

Jason Quinn

How about the expansion tank?

Thanks for the insight.  Any tips on sizing the expansion tank based on the info I have provided?

clammy

gravity systems zoning thoughs

Jason as others have stated it would make sense to do a heat lose for each zone as for combining 2 returns and sizing i would reduce them to the same sizing as the supply  is being reduced to and install globe valves on each return to balance them if needed on another note i have talk to some who say they prefer to find the last radiator connection on the return and run a new properly sized return back to the boiler room and then cap the old return in the boiler room which will give that zone a reverse return piping system and more even heating which can be a issue with  some gravity conversions .On another note if you do a heat lose and compare it to the outputs of the existing radiators you may find that you do not need 180 degree water to heat the home on design day temps in your area.If the load is what you have stated about 500mbtu then i would think 2 boilers and some primary secondary piping coupled with boiler staging and out door reset would be a nice set up and would save the HO money on there fuel use.I think i would also be thinking about using  low head energy efficient pumps  Peace and good luck clammy

Jason Quinn

Thanks, Clammy

Great ideas.  The energy consultant onboard has done the heat loss calcs. We are proposing (2) Triangle Tube Prestige Solo's with the "mini-mod' multiple boiler staging control. 

Unknown

sizing expansion tank

Expansion tanks are sized by volume and design temperature (never as high as 180F) when using ODR you will find the tank remarkably small if sized correctly.