Estimate of head and flow feels wrong

Discussion in 'HVAC Heating & Cooling' started by alternety, Jan 29, 2013.

  1. alternety

    alternety Like an engineer

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    I am in the midst of draining and refilling my hydronic system. That discussion is in another thread here.

    Initially, my system design was for a variable speed circulator pump that held constant pressure in the distribution system. At the time, the only pump I could find was a Grundfos that had external speed control. I got a pressure sensor to control it, but the pump was way too noisy. I believe the noise problem was a result of using a Triac on a normal AC motor. Not what I wanted, but all I could find.

    Today there are alternatives. PM motors with variable speed controls.

    While looking at alternatives, the Taco Viridian series is the most impressive. Taco does not recommend this series of pumps for my application. Looking at the pump curves it is clear that they are correct. My system has such a small requirement that it would be operating way below any reasonable speed/flow on the curves for any of the VS pumps. That is my problem.

    The calculated numbers for the system are 13' head at 12 gpm. This is so far off the curves for pumps designed for residential hydronic systems that I have to question the load calculations. This is a 6,000sq ft house. Plus heated greenhouse and garages. I believe that it is all piped with 1/2" PEX. I have 6 manifolds and a total of 32 solenoid controlled loops. Theoretically the least load could be a single bathroom, and the maximum all of the circuits. The max is unlikely; the minimum, not so much. Does the 13' and 12 gpm make sense to you guys for 6,000 sq ft, some outdoors (sort of)? It just feels wrong to me.

    The house is very heavily insulated with good glass. When I had no boiler, a 1.5 KW heater in the great room (plus 2 people, fridge/freezer, cooking, TV, computer) kept the central part of the main floor of the house above 60 degrees with outside temps going into the upper 20s.

    I need to decide if there is a fundamental error in the sizing and, if so, redoing it. If the numbers are way low, I have a much better set of pump options.
    Last edited: Jan 29, 2013
  2. Dana

    Dana In the trades

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    If it's 32 equal sized loops of 200' running ~0.4gpm each, 13' @ 12gpm sounds like the right ballpark. The distribution lines to/from manifolds are likely to be bigger than half inch, and may add a bit to the total if they're skinny and long, or if there are several tight elbows, etc. . Order of magnitude feels right, for all valves open.

    Micro-zoning it into 32 separately controlled zones is a disaster for the efficiency of the boiler though, since it's min-mod output is WAY over the load of boiler, and it will short cycle on zone calls for the smaller zones, even if the radiation is all concrete slab.

    "Heavily insulated with good glass", is a meaningless for estimating the design condition heat load, as is the square footage of conditioned floor area. The actual construction type & R-values, U-factors of the windows/doors, (with total exterior surface area of wall/attic/winows, etc) and air-tightness (got a cfm/50 number?), and the 99% outside design temperature are what it takes. But the design heat load could be met with anything from 1gpm to 100gpm, depending on the radiation design and the designed water temp.

    At 12gpm with a 10F delta on the radiation you're looking at about 60,000BTU/hr, which could easily exceed the heat load of a well insulated 6000' house in a temperate climate. (Or even a code-min house at IRC 2012 prescribed R values & U-factors.)
  3. alternety

    alternety Like an engineer

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    Thanks. I understand the process, but all the contractor calculations have been thrown away and I lost the application and the dataset I used when I did my own calculations when my computer crashed a few years ago. I got the number for the house yesterday: 58K Btu. So what you are saying seems to be in the ballpark. That is what I was after. Just a quick order of magnitude sort of idea. Once the system is cleaned out and running again, I plan to keep an eye on floor temperatures and try to balance the contribution across the house. I think a factor in heating unevenness is from me making a mistake about the way some upstairs zones and thermostats are layed out. In retrospect, 2 should be in different locations. It is kind of a dumb configuration. That I can't do a lot about.

    Regarding short cycling - I have an insulated buffer tank in the system (about 85 gal I think). Overall the efficiency of a 98% modulating boiler + the tank should be pretty decent compared to a big old boiler with a massive water jacket. It is a small boiler (85K).

    I am taking a look at the Grundfos pumps again for variable speed. Initially I had one in with an external pressure sensing control loop. But the pump was just way to noisy. Same pump without variable speed was much quieter. The issue was, I believe, that Grundfos had chosen the cheap and inefficient way to put in a variable speed motor. They just added a Triac to the existing motor. The new ones use the much superior PM motor and digital control. They ought to be quiet. If it works out, I can just close off the pressure regulator/bypass between the main supply and return pipes and let the pump control pressure. Which is what I wanted initially.
  4. Dana

    Dana In the trades

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    Micro-zoning to that level almost universally calls for a buffering thermal mass, so it sounds like the original designer did their homework on that. Without looking it up I'd assume the min-mod on that is probably on the order of 20-30KBTU/hr, so with 85gallons/700lbs of water in the buffer your minimum burn times will still be decent with as little as 5F of hysteresis on the ODR setpoint.
  5. alternety

    alternety Like an engineer

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    Actually, I was the original designer.

    The boiler modulates up to about 80KBTU/hr. I don't remember the low limit at the moment. It is this large for several reasons. One factor was that is the smallest boiler in this product line, but I could have gone to another boiler. I preferred the characteristics of this line. But the major reason it is this large is for indirect DHW capacity and some potentially large but initially and currently unquantifiable loads. Things like a greenhouse (if built), makeup air for a variable speed commercial range hood (turns out we do not need to heat that air), and a garage which would be heated intermittently. It might get involved with a pool, but that will probably be straight electric. When I was looking at things, electric heat here was cheaper than propane. And yes, the actual burn times are reasonable currently.
  6. alternety

    alternety Like an engineer

    Messages:
    671
    Location:
    Washington
    I am not having any success at finding a constant pressure capable pump at these low head and flow requirements. Grundfos Alpha does not appear to match, and they have odd control parameters. Not designed for constant pressure. The one I like the best by far is the Taco Viridian. Unfortunately they are wayyy to big. Seriously nifty control.

    Looking at what some manufacturers are calling "residential" pumps, they have not all been paying very much attention to what a modern well insulated house requirements are likely to be. Even a whole bunch of light commercial. The engineers need to go upstairs and explain to the marketing/planning people where things are. If that is not the problem, the engineers need to get out more. Take some time out from the Disco bar and smell the water vapor from 98% condensing boilers, poke some foam, look through "good" glass.The world has changed. It does not appear the pumps have.
    Last edited: Feb 6, 2013
  7. Dana

    Dana In the trades

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    It's not just the pump manufacturers- even the smallest standard condensing gas furnaces & boilers are oversized for most code-min 1500-2500 houses that meet IRC 2009 code for U-factor & R-values and air-leakage, which have been tightened up a yet further for IRC2012 (which hasn't been widely adopted by states yet.) Code-min in IRC 2009 states is no longer the same crappy efficiency standard that it was 20 years ago, but the HVAC equipment vendors seem to think it's still 1983 or something.

    To be fair the world for them hasn't changed THAT much yet- the US market for residential heating equipment is still dominated by replacement/retrofit into pre-1983 houses, not new construction. But even there right-sizing it brings most houses down to the smallest-in-product line range. eg: My ~2400' 1920s antique is no marvel of high-performance glass & insulation, but any 50K boiler, even a code-min 82%AFUE cast iron beast has substantial margin on the real heat load at my +5F outside design temp, and could still heat the place without losing ground at the all time record-low temps for my area.

    There is a broader selection of tiny-output equipment in Europe & Asia, and people in much better than code houses have turned to heating & cooling them with (mostly Asian) mini-splits, or cobbling combi systems onto hot water heaters.
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