Hello All, DIYer here. I've been working on the design of my heating system for the house I'm building and am getting ready to start purchasing and digging. I would appreciate it if anybody cared to give me some feedback on my plans. Broadly, it will be a water to water heat pump with horizontal trenches as source and radiant floors as load. It is a 2800 sq-ft log home with expansive windows on NE and NW sides. The house sits on top a ridge exposed to occasionally nasty winter wind. I followed the Manual J and came up with 38,500 Btuh heat loss at design temps. The trenches will be in "sandy-gravel" soil, well drained, but the location (Olympic peninsula, WA) gets a fair amount of rain. The load is heating dominant. Many people around here don't even have air conditioning, although I plan on putting in connections for chilled water to air handlers so if I want to install them later. I'm looking at getting a 5 ton HP. Either a Bard GW060-1S1A1RCCX or a ClimateMaster TBW060-AGD01B0BS or its equivalent Comfort-Air. I used the 09 IGSHPA Ground Source Heat Pump Design Guide to come up with my trench design. 4 trenches, 2 pipes/trench (1 loop/trench), 6' deep, 24" wide, 10' between trench centers. Trenches would be 265' long but if I add 10% for safety then aprx 290' long. Trench pipes (4 parallel branch loops) would be 3/4", with a 190' home run (x 2 pipes) of 1-1/2" header pipes between the trench field and the house. I expect I will go with a non-pressurized circulation system. Haven't selected the pump(s) yet. I calculate that I will have about 35' of head loss at 11.25 gpm. I would very much appreciate comments or suggestions of any kind. I have a few specific concerns: The IGSHPA books (I also have their soil book) present a very broad range of diffusivity and conductivity values for 'sandy-gravel' soil, depending on the moisture content. I really don't have a clue on this. We usually get plenty of winter precipitation, but the soil drains easily, and what if we have a dry winter? I used the values at the dry end of the range. If I use the values at the wet end, the trenches can be at least a 100' shorter. How to account for moisture variability when moisture affects the calculation? The 5 ton heat pumps I'm looking at are somewhat oversized. A 4 ton heat pump would be marginal and I don't see a simple way to provide supplemental heat to a radiant floor system. I don't mind paying extra for a bigger heat pump but the whole point is to have an efficient system. I'm hoping the 2 stage Bard will do this for me. Any thoughts? Are there any other 2 stage W-W HP's that sell direct to consumers? The Bard HP Performance Data "http://www.bardhvac.com/digcat/S3364_TechDoc_CD/TechDoc-PDF/S3465.pdf" has a column called 'Heat of Absorption' in their heating performance table, under 'Source'. Does anybody know what this is and how it relates arithmetically to the Heat Capacity, Power Input or COP? Note, this is not Heat of Extraction, as defined by the IGSHPA text, the values don't match what that would be. I contacted Bard and the technician that I talked to at first couldn't provide an explanation but then emailed me that Absorption and Extraction are the same thing. I may be an amateur but I can see that the math doesn't work for these values as Heat of Extraction. Can anybody add clarity to this? Anyway, sorry for the long post. I appreciate any comments at all.