I've searched and read posts here about using pumps in series to obtain additional head to overcome loops that are too long in in-floor hydronic systems. I think I know what I’m going to do, but wanted to ask for comments first. As an incentive to help and for your collective amusement, I will first provide a list of the comedy of errors associated with this system.
Background – this is 1,615 square foot home consisting of two, uninsulated buildings about 15 feet apart. Construction is stuccoed concrete block with high (14 foot) ceilings that comprise a concrete roof topped with Spanish tile, There are many large wood-framed single pane windows and doors throughout both buildings. It was built in 2009, the owner died about six months later, and we purchased the home in 2011.
The home is 7,200 feet above sea level, in Pátzcuaro, México, where in-house heating systems consist of a gas or wood fireplace or portable ventless gas heater at best, or more typically, nothing at all. Insulation is never used in construction, and gaps around doors and windows are standard. Low temperatures in December and January are in the low 40s with daytime highs in the low 60s. Summer rainy season can be cold and damp after a few days without sunshine. People wear a lot of wool sweaters and bundle up in quilts at night during both seasons, not your typical image of México. We cheat at night with electric mattress pads, and run unvented portable gas heaters and a gas-log fireplace when it’s cold.
Unvented gas appliances pump a huge amount of water vapor into the air, which then condenses and runs down on the cold window glass, staining the woodwork. (Weather station data and webcam that we installed last November can be seen at http://tinyurl.com/pfbm7b7 by entering a custom date range beginning on Nov. 5, 2013.)
The home has an improperly installed in-floor radiant heat system. It was the architect’s first and only system, and he relied on an installer who claimed to know what he was doing. He lied. Here’s what I’ve discovered:
The Comedy of Errors:
1. The installer imported a boiler, mixing valve assembly, and zone controls from Spain and Italy that are 230 volt 50 cycle. Mexico is 127 volt, 60 cycle. They fixed the voltage problem with a step-up transformer, but the 50 Hz pumps are running 20% faster on 60 Hz A.C. In our case, this is not a bad thing.
2. They insulated under the slab but not around its edges. The concrete floors extend under the outside walls and doors to various outdoor patios.
3. The construction photos and architects’ drawings were lost when his hard disk crashed. The paper plans were lost when the original owner died and friends emptied the house. There are no engineering designs for the system – no heat loss or sizing
calculations. No one knows the tubing layout or lengths.
4. The 15-foot underground PEX runs from the manifolds to the main building were uninsulated. I’ve since dug them up and insulated them.
5. The zone thermostats controlled the wrong zones. That is, the zone 1 thermostat operated the zone 2 valve, zone 2 controlled zone 3, etc. I’ve fixed that too.
6. Thermostats were placed on outside walls instead of centrally located in their rooms. Also fixed by me.
7. Mixing assembly bypass valves were closed, making what should have been a primary/secondary loop system into a single loop.
8. The differential valve was at its lowest setting, so it was always bleeding.
9. The balancers were misadjusted.
10. The 1” copper pipes in the boiler room were all uninsulated.
11. The big one – they ran 1/2" PEX in the slab with only three zones, all longer than 300
I’ve added pressure gauges, a flow meter, a SpiraVent, and made as many corrective adjustments as possible. I’m running a 176F boiler with the mixing valve set to its maximum of 131F. Supply and return manifold delta-T is 40F, and delta-P is 11.75 psi with one zone on, 11.0 psi with all three zones on. Flow with all three zones on is 2.45 gpm with all balancers wide open. Flow for the longest zone by itself is 0.67 gpm.
Using tables of PEX pressure drop, I estimate my loop lengths at 829, 432 and 576 feet. Zone areas are approximately 715, 412, and 488 square feet, excluding the connected areas outside the perimeter walls. I characterize these zones with Cv’s of 0.202, 0.289 and 0.247 respectively.
Zone 1 is the living room, dining room and kitchen in an open floor plan. Only the dining room floor and part of a hallway gets warm. There isn’t enough head to push warm water fast enough to the rest of the loop.
Zone 2 is the master bedroom, bath and walk in closet. It gets warm enough to be comfortable, though the floor temperature is far from uniform.
Zone 3 is the guest bedroom, bath and studio in the second building. The guest bedroom comes up to temperature, but the studio receives no heat and requires a portable gas heater to be comfortable on cold evenings.
The secondary loop pump is a Wilo RS25/7, developing 7 meters of head at 50 cycles, or 8.4 meters at 60 cycles. That’s 27.56 feet or 12 psi, which corresponds to my measurements. I’ve posted a PDF with system drawings and photos at
https://dl.dropboxusercontent.com/u/12631920/System%20Notes.pdf, also attached here.
Page 1 shows the overall floor plan and zones.
Page 2 is a diagram of the system plumbing and valves.
Page 3 is a graph of my estimated system curves as well as pump curves of the present pump and pumps I’m considering to put in series.
Page 4 is a photo of the 18 KW boiler (less derating for altitude) and mixing value assembly.
Page 5 is a photo of the supply and return manifolds.
Page 6 is a photo showing a typical floor section in the main house.
I also have graphs of the boiler supply and return temperatures as well as the manifold supply and return temperatures, recorded with a 4-probe Hobo data recorder that I had left over from an old engineering project. But I think that’s overkill for this discussion – I’ve adjusted things so the boiler no-longer short cycles when supplying two zones.
My thought at this point is to add a 230 volt, 60 cycle Grundfos UP26-116F or UPS26-150FC downstream of the secondary Wilo circulator. The graph in the PDF shows how I expect the system curve to look with additive heads. The three-speed UPS25-150FC can be dialed back to a lower speed if there’s a problem with such a large head mismatch with the Wilo. However, my reading on this forum indicates that such problems are most likely at the right-hand, low head, flow-cutoff end of the pump curve, not at the high-head, low-flow end of the curve where this system will operate.
I would like to have some confirmation that the heads will be additive and that there won’t be problems of cavitation or wind milling.
For those that have made it this far in this overly long post, I would appreciate any thoughts and comments. I should also mention that finding these pumps in Mexico is difficult, and they typically cost 2.5 times what they do in the US, so trying and swapping if it doesn’t work isn’t much of an option. I’d like to go with a best guess, and have someone driving here from the US bring the pump and flanges with them. Swapping out the existing Wilo is not an option because it’s European and uses 1.5” threaded unions on 180 mm centers, whereas all the US pumps seem to be flange based. That’s why I’d like to use two pumps in series and develop a whopping 32 psi (73.7 feet) of head to see if I can finally push the water through fast enough to spread the heat around the floor. If flow increases as the square root of pressure, going from 11 to 29 psi (all zones on) should increase my flow rate by 62 percent (square root of 29/11).
A secondary consideration is the high cost of propane and electricity here. The present system will eventually make the main house and the guest bedroom comfortable, but it runs very hard and consumes a lot of gas. I’d rather spend more on electricity and have the slab be more uniform in temperature.
Thanks in advance for anyone’s thoughts, including those who might say I should just abandon the system and buy more sweaters.
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