I have a self-installed Spacepak high-velocity chilled water system. It works well in the shoulder seasons, but cannot keep up on the hottest days of summer. I am short on air handler capacity.
I really like this system, and don't regret buying it. The system provides excellent humidity control, aspiration distributes even temperatures. ERV provides fresh air year round, silently. The system is quiet and no drafts. Problem is the inability to reach design temperature during hot weather, and electrical usage is much higher than I expected.
1,500 square foot ranch, full sun exposure. Original R11 walls, unventilated, closed-cell R28 foam attic. 300 sq' addition has R20 walls and a R40 cathedral ceilings. We installed the AC system. I had a blower door test done as part of an energy audit, prior to the spray foam and addition, it was too tight. They recommended an HRV or ERV.
I used load-calc.net to perform a series of manual-J calculations - 70 F @ 88 ambient called for 21,600 BTU, while 70F @ 100 degree ambient was 26,000 BTU. These were with 70 CFM of fresh air. Average of all was two tons of cooling.
I installed a 3-ton Spacepak SCM-036 chiller, with two Aircell air handlers. Both Aircells use 7" rigid galvanized trunks (R8 insulated), feeding the 2" Spacepak supply ducts. An ERV exhausts main bath air, with the fresh air piped into the fresh air intake directly on the Aircell air handlers. The attic over the original house is conditioned space. It used to be an oven - with the spray foam, it's comfortable up there now.
1.) In hot weather, I can't get the house temp below 74 degrees. I purposely designed it for 70 degrees. If we return from vacation, and we had the air off, it can take two days to reach 74, from 85. My wife likes it at 70 degrees.
2.) Excessive electrical usage. August peak is 1,800 kWh / month greater than my January usage. Chiller runs anytime the Aircells call for cooling - constant in hot weather. To improve performance., the manufacturer's rep had me turn the water down to 33 to 42 degree operating range - this is using excessive energy and increased load on the compressors. We shut if off if gone for more than a week for this reason. A major contributor is the excessive pumping involved - described below. I've read a lot - Caleffi, some of John S's articles, and training on the manufacturer's web site - some of which is also from John. I know it needs improvement.
A year or so after purchase, the Aircells were de-rated from 18,000 BTU to 9,000 BTU
, under ideal conditions. Due to this change, I am at least 6,000 BTU short of what the Manual-J calls for (24,000), and probably more. I have not been able to get any relief from the manufacturer. I suspect this is a significant contributor to the problem.
When I first installed it, I had questionable advice on the hydronics side in terms of water lines and main pump. I followed the reverse-return design called out in the installation manuals. After the first season, I consulted with a hydronics engineer in FL, and he had me change to a load-match approach with closely spaced Ts. On the air side, I also switched from home-runs off the Aircells to trunk lines, except for one group of four supply lines.
I have good airflow at all outlets, measured with an anemometer. Don't have #s handy. Tech support does not think there is an airflow problem. One Aircells is zoned (bedrooms/addition). The other Aircell is dedicated to the front of the house, living space, faces west. This side gets a lot of solar gain. All of the trunks and supply/return lines are properly air-sealed with foil tape.
40% Fernox glycol, single loop system. 1" copper off the chiller, converts to 1" Oxygen barrier PEX. Armacell insulated lines. The first Aircell in line has two closely-spaced T's (directly adjacent) - a Taco Bumblebee HEC-2 feeds the Aircell via 1/2" copper, 1/2" copper return is converted to 1" PEX and back into the other Tee. Next Aircell is the same. On the return path, I have an air-scoop with exampsion tank, next in line is a 20-gallon HWH acting as buffer tank. I'm using 1" copper to two 3/4" inlets - and two 3/4" outlet converging back to 1" copper. Flow calcs I've done show this is not a bottleneck - but I'm not a hydronics engineer. On the other side of the tank is a Taco 2400-50 (this is the noise generator) which "pumps away" and back to the Chiller, via 1" PEX again. All of this is in the attic - Chiller is outside. I'll attach diagrams to illustrate. Per advice, I installed a "balancing valve" (full-port ball valve) in the middle of the two Aircells - the theory I was told was to slow the water to allow for greater heat absorption - didn't help so it's wide open.
1.) Should I add another Aircell to cover the BTU shortage resulting from the manufacturer de-rating, or replace the two existing with a single 2-ton hydronic unit? The Aircells are discontinued - but I can still source one from a couple of supply houses. These seem to have high-resistance on the coil - 11.3' WC @ 5.5 GPM. I'm leaning to adding a third, unless this is going to drive further inefficiency.
2.) What changes are needed to make this an efficient hydronic design?
3.) Should I switch to a primary/secondary loop arrangement? Would this allow the Aircells to realistically run off the buffer tank for a short time, versus always running the chiller? Attic is 4' vertical on center - no room for a tall buffer tank, BTW. 4:12 pitch roof.
My original calculations, counting Chiller pressure drop, was about 37' of WC on the main loop. I was estimating 10 to 12 GPM on the main loop with the 2400-50 pump, and about 4.5 on the Aircell loops with the Bumblebee HES-2 pumps. I've tried Pumpsim to model this - but this is really beyond my capabilities.
I suspect a primary/secondary design would allow for a downsizing of the main pump - thoughts?
4.) Should or could I get away from needing individual pumps for each Aircell? They don't draw a lot - 90 watts, if memory serves, at setting 4
I've measured the AirCell coil temps as low as 35°, of course the temperature swings with the normal cadence of the chiller range. 4 to 6 degree differential - which I assume means flow ... so I am confused about this. The chiller has a flow switch set at 7 GPM - max flow resistance is 21' WC @12
GPM. I believe flow is sufficient -but have no reliable way to measure it, other than temperature using a laser gun. I've seen designs with a single pump running multiple Aircells - in parallel. If there were zone valves with a single pump/manifold - would that resistance to the pump triple, for three Aircells? With electricity, resistance is additive in series, and reduces in parallel (been a long time since tech school).
I want to re-work the hydronics so I can raise my water temperature back to "normal" ranges - which are above 40 degrees. I'm located in the Hartford, CT area. I would really like to consult with a professional that understands high-velocity systems, but more importantly, hydronics. I checked out the "find a contractor" link, nobody is close. I would appreciate any advice, or for someone to recommend the right person or themselves. Feel free to contact me directly - if the site supports that.