CH System Overhaul with Cast Iron to PEX Conversion

troutmaster444

Hello everyone.  I am planning to upgrade my CH system and would really appreciate some input. 

Current System:

1. Two pipe, direct return. 140MBtu Dunkirk boiler serving 8 CI radiators of different shapes and sizes.  82% efficient boiler per nameplate and combustion analysis.
2. Separate, radiant floor system. Takagi tankless "boiler" (I know...) serving two zones of heated floors. I built this as a prototype when renovating/eliminating some CI rads and it has worked well for around 4 years.
   

Home Details: Three story 1925 construction (brick and block exterior) with moderate insulation added over the years. 2400 sq ft.  Pittsburgh area.  Detailed heat loss calc results in 114MBtu (68F/0F design criteria, including piping loss).

Objective: Ultimately, replace both boilers with one high efficiency boiler to serve the high and low temp zones.  Run PEX home runs from manifolds to existing radiant floor loops and CI radiators (not modifying any).  Add three panel radiators to the low temp zone.  Eliminate the existing two-pipe system piping in the basement (which runs the perimeter of the basement and is a real head banger in most spots).  This will result in a significant amount of space savings in the basement, correct several design flaws with the existing CI system, reduce maintenance, and hopefully gain some efficiency.

I attached a schematic for the planned boiler piping.  I wanted to point out Zone 4, which is a very small load.  However, in >4 years of operating, it never calls for heat on its own and is always in tandem with Zone 3 (which is the kitchen/pantry/hallway with open spaces and lots of windows).  Hopefully this is not an issue with the new design.  

Do you see any major concerns or flaws with what I am planning? Also do you have any recommendations for boilers to suit this application? 

Thanks in advance! 
Al

Schematic R1.pdf

troutmaster444

Forgot to mention on the schematic, thicker lines (supply/return for CI) is 1-1/4", thinner lines (low temp zones) is 1". 3/4" PEX to all CI rads, 1/2" PEX to all low temp emitters/floor loops.

mattmia2

A mod con is probably a better boiler for that system, you can run odr on it set for the ci radiators then mix it down for the other loads but the mass of the system is likely something you'd need some protection from low return water temps on a ci boiler. Might want to use a control to do the low temp zones that can do odr too.

Do you really need 170 to the CI ever? It is possible you might but is likely you don't.

Kaos

First step is to get better handle on your actual heat loss. I know you said "Detailed heat loss calc results in 114MBtu" but that is unlikely for a place that big.

If you have last years winter fuel bills, a rough heat size is to convert the fuel used to therms and multiply by 42. (ie to have a 114m heat loss you would have used about 114000/42=~2700 therms).

Once you have a better handle on heat load, you can work on sizing boiler and required system temps.

With a mix of high temp and low temp, I would do a PS loop instead with closed spaced T for each temp zone. Put the high temp zones pulling off the loop first followed by the low temp zones. This way the low temp zones will be partially fed by the return water from the rads. Increases loop delta T which is better for efficiency.

hot_rod

Some of the mod cons have multiple temperature outputs. But only one runs at a time. Maybe the high mass warms and can be off while the boiler goes to the low temperature load?

Not many series primary loops being used these days. If the high temperature load is not running, the low temperature will not see that lower return. Plus you need a loop pump if the boiler needs hydraulic separation

mattmia2

If you pick a boiler with multiple DH calls with different setpoints or reset curves you can make the boiler only heat to the current temp needed but you will still need a control to mix down the hotter water when there is a simultaneous call for the higher temp zone.

hot_rod

https://forum.heatinghelp.com/discussion/comment/1837852#Comment_1837852

I thought they can only do one temperature at a time. So it could not provide the lower temperature load while the high temperature is calling?

mattmia2

https://forum.heatinghelp.com/discussion/comment/1837853#Comment_1837853

It can only do one temp at a time but when only the lower temp zone is calling it can use that as the setpoint instead of using the higher setpoint then having to mix that water down to the lower temp.

It should be a little more efficient when only the low temp zone is calling but it still needs a thermostatic mixing valve or injection mixing for when there is a call for the high temp zone.

Kaos

https://forum.heatinghelp.com/discussion/comment/1837849#Comment_1837849

I was not talking about a series loop. Standard primary secondary piping with the high temp zone pulling from the first set of closed spaced Ts, the low temp from the 2nd set of closed spaced Ts. You would still need a mixing valve for low temp and a pump for primary(if not inside boiler) and for each secondary temp zone.

EdTheHeaterMan

I believe Taco makes a ODR mixing valve! You set the ODR from the boiler to operate the high temperature zone and you set the Taco mixing valve ODR for the low temperature zone. Basically you’re resetting the low temperature zone off of the already reset high temperature zone.

https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Taco%20-%20I075U3R-1%20-%20Install%20Instructions.pdf

Ironman

https://forum.heatinghelp.com/discussion/comment/1837853#Comment_1837853

I can’t speak for every brand, but the ones that I’m familiar with will run both zones if they’re calling, but default to the higher heating curve. Obviously therefore, a mixing device is needed to limit the SWT to the lower zone when both are calling.

leonz

UM, how about giving us a schematic of the heating system as is??