After a couple of months of fiddling, I now have a very efficient hydro-air system that I thought I would share with you folks, seeing as I've learned a lot from reading here and having my questions answered.
First, the house. It's an Integraspec ICF house, south facing with low-E double glazed windows. Energystar rating of 92. 2250 sq. ft. on the main and 2250 sq. ft. in the "basement". Uses 14,384 BTU per hour on a windless 30 degree F night, cacluated from both boiler run time and buffer tank energy drain (see below).
Second, the equipment. A Buderus GA124/17 gas boiler generates all the hot water. It has a 85% AFUE. It is piped to a manifold that has 3 checkvalve protected takeoffs. One goes to an AMTROL 40 gal Boilermate indirect DHW tank, one goes to the Air Handler via a 40 gallon AMTROL buffer tank and one is a spare. The return manifold is also checkvalve protected, with returns from each of the AH, DHW and spare. The AH is a Lifebreath unit with a huge 4 layer coil and a Grundfos 15-42 B7 pump for circulation. The AH runs with an ECM motor that draws 60 W and pushes 350 cfm on it's low setting.
Third, the control. A Tekmar 262 runs the show, with some twists. It has indoor temp sensing via a Tekmar 062 RTU and outdoor reset, via a Tekmar 071 sensor that is strapped on the south facing side of the house. Why south ? Because we get extensive solar loads, and the control needs to take that into account. Yesterday, for example, it was 23F and sunny. The WWSD kicked in at 10 am (45F in the direct sun apparently) and the boiler shut down and the house still rose to 74F with no heat and our normal daytime setpoint of 69F. Heat came back on at 9 pm.
Fourth, the operation. The DHW side works with DHW priority. Nothing new there. The AH side is more interesting. The boiler output goes to the top of the Amtrol buffer, and the bottom of the buffer tank goes to the AH, where it drops about 30 F and comes back to the boiler. The AH circ pump runs under control from the Tekmar 262. It is essentially running in continuous circ mode. A temp sensor in the Amtrol tells the Tekmar what the buffer tank temp is and the differential is set at 24 degrees. So it works this way. When the buffer tank is cooled down to the setpoint minus half the differential, the boiler fires. Since this temperature is usualy around 85 F, a Johnson controls A419 in the boiler's well interrupts the AH circ pump off untill the boiler hits Buderus's recommendation of 104 F. This takes about 1.5 minutes. Then the circ pump starts again, the tank heats up from the top, pushing the colder water out the bottom to the AH, and when it hits the setpoint plus half the differential it turns off the boiler. This takes about 20 minutes. It then takes about 40 minutes for the buffer tank to give up its heat through the air handler. Note that the circulation loop includes the boiler, but it's so well insulated and a cold start boiler, that I don't worry about any real loss or condensate problems. All pipes have 2 inch insulation on them too. The mechanical room is in conditioned space. When there is a DHW call, the DHW pump starts, the AH circ stops and a relay is thrown that now uses a sensor in the Buderus boiler well to measure the target temp of the boiler. So the buffer tank is untouched by the DHW circuit.
Fifth, the savings. The design temp of this system is 115F for a room temp of 70F on a design day of 14F outdoor temp. This is way, way, way below what is traditionally required for air handlers that cycle on and off. Essentially I have something more like a continuous flow radiant zone but using the AH on low (so no drafts). I have full outdoor reset even without a modulating boiler. The relatively cool air coming out of the registers does not shoot to the ceilings of our house (10-14 ft) and stagnate as it does with 180F air. The AH circ pump and fan together use 145W. The HRV gets to work with the always on AH and keep the humidity at 35%. And I get 20 minute run times with a 70 minute cycle time instead of 6 cycles per hour of 3.5 minute run time that I got before I put the buffer tank in. The boiler runs about 20F warmer than the current buffer tank temp when it's running at steady state. That means about 135F on design day.
If I had to change anything, I'd replace the GA124 with a GB142 or a Viessmann or NTI or Munchkin that could modulate down to near my shoulder season loads. The house is so tight and insulated that the smallest modulating boilers out there are still too big. But the low water return temp from the AH is perfect for a condensing boiler. But the GA124 is what came with the house, and so it shall stay.
[Edited by odcman on 02-13-2006 at 05:47 PM]