Gravity Flow Hot Water Retrofit

Old 10-16-09, 08:11 AM
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Gravity Flow Hot Water Retrofit

Hi Folks,

Let me introduce myself: the names Chris, I am a Mechanical Engineer by day and an avid Do-It-Yourself'R by night. My wife and I life in a 1909 farmhouse in Portland Oregon.

After years of avoiding the inevitable it is time to replace the 100 year old boiler sitting in the basement. Why replace a systems that has worked flawlessly for 100 years? Glad you asked, our family is expanding and we would like to finish the basement into a family room and guest quarters. The lowish hanging pipes of the gravity flow system and the massive footprint of the current boiler mandate a change.

This is where you guys come in...

After exaustive research and a few hours spent doing manual J calculations(67K BTU @ 24 degrees DD) I have decoded that a Triangle Tube Solo 110 will fit our needs.

Now for the hard part... How to lay the system out. My first thought is Primamry Secoundary with 3+1 zones. The Zones would be as follows:

Upstairs: 19.3K BTU 5 stamped sheetmetal radiators.

Main Floor:18.9K BTU 4 stamped sheetmetal radiators

The plus zone an 8K BTU undefloor pex loop in the kitchen and bath

Basement: 21K BTU Combination of underfloor hydronic and wall hung radiant panels.

The control stragety would be 3 simple (plus a floor sensing) thermostats with a relay block and end switches. Let the TT computer handle the outdoor reset and such

This seemed like a good starting point until I read about the horrors of short cycling and micro zoning a Mod/Con.

It also requires 4 pumps (plus the system circulator) running the T-stat wires, a pretty impressive PS block, lots of copper, lots of valves, etc, etc... I don't mind the time and expense of this system but a gut check is in order.

Doing a complete reversal and going to the simplest system possible would look like this:

Re-Plumb all the current radiation in a two pipe reverse return loop (all accesable from the unfinished basement, do NOT want to get into the walls so some radiator supply pipes will remain relatively huge). Use the 110's 3 speed circulator to handle the roughly 9 GPM at design load and use the radiator valves for fine tuning the existing rads, a mixing valve for the floor loop, and thermostatic control valves in the basement for some local temperature regulation... This layout probably wouldn't even need a pressure differential bypass valve as the radiator valves are not full shutoff...

I know those approaches are extremely different from each other but I am torn between the Engineer technology geek who wants to add all the whistles and bells and the pratical Engineer homeowner who knows that components left out of the system cost me nothing and don't create any service issues...

Ideally I would like to have a T-stat upsatirs but may sacrifice that for simplicity.

Has anybody ever done a hybrid system where the Boiler pump feeds (in my case) the main floor and basment and the 2nd floor is on its own circulator(T'd in a parallel loop with a spring seat check, would get some ghost flow with the primary call regardless) . This would mandate that the upstairs call for heat would by default trigger the main floor but we always want the bedrooms a bit cooler than the living areas anyway. The more I think about this the more I think could work... Reality Check Please.

If anybody has other options I would be glad to hear them.

Thank You

Old 10-16-09, 02:16 PM
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Join Date: Jul 2007
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Why have almost double the boiler size. I would use the 60 or if you are not comfortable with that there are other choices on the market that will get you closer. You want to size the boiler for the DOE or Gross output to match the heat loss so you can take advantage of full modulation. You are modulating down with that selection to 30 when 60%+ of the time you will be operating below the minimum modulation of that product.
Watch the micro-zoning!
Old 10-16-09, 02:26 PM
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You are right on the boiler size... The reason I have focused on the 110 is I will also add a 40 gallon indirect and want ALOT of hot water capability. A 60 would be marginal in that respect.

Also note that my heat loss calculations represent a future state in the building envelope (upgrading windows attic insulation and finished basement) which will not be complete for the first couple of heating seasons.

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