Gravity Hot Water Conversion

BrianK

This is my first post to this site but I have been using it for years whenever I need help from the best. Thank you everyone for for making this such a great site for homeowners and contractors.

I am looking at converting a gravity hot water system to a 2-zone forced hot water system. The customer would like to reuse the existing radiators, but wants all the piping in the basement removed due to space issues. I am thinking about using a Buderus G124X/32II Natural Gas boiler with outdoor reset, possibly a Taco PC700. I was going to run my supply and return piping on to boiler in to 1” x 3/4" manifolds and then run separate PEX runs to each radiator. I will have to connect my 3/4" pex to the 1-1/4 supply and returns to each radiator in the basement (at least the ones going to the second floor). I will be using primary secondary piping on the boiler. I will also be putting balancing valves on the return manifolds. Does anyone see any potential problems with this system?

Thanks

Brian

Brad White_108

Working on something similar now

My Susan's 1922 gravity HW system with 2" head-banging mains. Vitodens 6-24 going in with a low loss header so, like yours, primary-secondary.

Heat loss is 47,000 BTUH and there are 11 radiators, a couple sharing a riser. TRV's on all radiators where I can and regular control heads on a manifold where I cannot readily use TRV's.

Main is 1" copper to start (5.5 GPM rounded up) and this, after continuing 1" for a few branches, will split into a 3/4" loop around the basement.

One cluster of radiators will come off of a manifold with 5/8" Kitec PAP, the rest will be valve-connected from copper to iron with Kitec PAP and K1 fittings. Each direct-to-copper main connection will have 1/2" metered balancing valves much as you are doing.

So... see enough similarities, Brian? Aside from the fact that the radiators will operate as one large zone with TRV's granted, it is essentially the same as yours. I am not familiar with the Buderus setup you describe but it seems the goal is to do the same thing, OD reset to the maximum possible.

As far as the distribution, I do not see a problem!

Have fun-

Brad

kevin coppinger_4

I have just done...

two GHW conversions....In both I used the Buderus 2107 the RTU room sensor (much better than the pc700).......they will run off of constant circ....in one I did add trv to the second floor(not real easy on a couple) ...very comfotable system when it runs...next best thing to radiant floor....kpc

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Constantin

Hmmm...

There are more efficient boilers out there. Given where gas prices are and what kind of a system the boiler will be hooked to, I would consider a boiler that can run at a lower internal temperature and which can accept colder return water temperatures.

IMO, the insulation on the G124X is pretty thin and given those boilers minimum standby temperature requirements, that loss really adds up over time. Buderus has a number of non-atmospheric boilers like the G215 that can handle lower water temps, never mind their condensing range which are likely to be ideal for this sort of customer.

The conversion you describe will probably work, I would do the math to see how many and what kind of circs you will need to keep all the radiators happy.

Paul Pollets

G124X

If I used the G124X it would be with the R2107 control and mixing card, and a 4 way motorized mix valve. TRV's on the radiators with one RTU or "BFU".

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Phil_15

Gravity fed and condensing

This question has been bugging me for years: Weren't all of the great old gravity fed systems with the large diameter piping and the fantastic big CI rads actually running in condensing mode for the majority of the time? My understanding is that if the return water temp. is less than 140 deg. F, (for gas) you are dripping H2SO4 (acid) over the CI boiler segments.

Is this true? If so why did they last so long. If not, what am I missing?

Thanks

Ron Schroeder

Phil

Not sure where your from but ususally around here boilers of the old beast era were kept at 160* for the tankless coil so issue resoilved.

Paul Pollets

answer

the large boilers had a huge water content...often 50-100 gallons (or more) The cold water return was mixed due to the large water content, and the boiler did not condense. With modern boilers, where the water content of the boiler may be 5-20 gallons, thermal shock shows up FAST!! Either a mixing valve or thermal bypass needs to be installed.

The older systems were designed for 180 degree supply water, not 140. The radiators were rarely in condensing mode for too long. Again, when the return temps came back to the boiler and mixed, the dilution was rather insignificant to harm the boiler. Of course there are always exceptions. One of the biggest problems facing boiler replacement is thermal shock. It must be dealt with or the new boiler will be toast, unless it's designed for a condensing load.

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Cosmo_3

word of caution

Watch your flow rates. If you end up not doing the TRV's then be careful how you re-pipe the system, the dead men that put the old system together made sure that every radiator would get warm fairly evenly with the proper flow rate.

Many times w/ econo jobs, I just re-use the existing mains, and loop them into a secondary circuit, using a properly sized circulator. Then via closely spaced T's inject the heat into the loop. Have a sensor on the boiler return to keep it from condensing. I would just keep the radiator loop circ running all winter, it will help balance the home temps in the different rooms. I have had VERY good responses from customers, finally the whole house is comfy instead of only some of the rooms. And if money is the issue, you can do the job for a good price (just adding one circ and a few t's) and still keep em' happy.

If you have to remove the beautiful mains, then just make sure you design the new distribution to flow evenly through each radiator using properly sized two pipe reverse return. You would be surprised how many guys messed this up.....

As I said before, if you can talk them into TRV's, then that is the ultimate in comfort, Balancing the piping, though still important is less critical with TRV's.


Cosmo