Dear people, I’m trying to come up with a practical efficient HVAC plan that has the potential to be made more efficient down the road in a project that may seek a net zero certification. Envelope design and existing conditions: In this high efficiency gut reno of our 1890s house in Chester CT ‘green’ features include 2 layers of continuous 2” Rmax with taped staggered seams at all above areas and as much XPS exterior basement insulation clad to protect from insects. Cellulose in stud and rafter bays for a total of R52 at attic and R49 at walls. Currently the envelope is 95% complete and the main services are connected but there is no cellulose nor systems installed, the frame is entirely exposed at the interior. With mostly triple paned windows, load calcs show 12 btu/hr/sf and a total design load with DHW of 24,000 btu/hr. I’m also testing a new automated single hung window design (for night flush cooling) with 80sf of vertical semi tracking solar thermal flat plate collectors as operable automated exterior shutters at an oversize (7’x14’) south facing window. The insulation on the ST panels will insulate the big window on a winter night. With solar thermal infrastructure given, I’m inclined towards mostly radiant with a couple of fan coils for the minimal cooling and dehumidifying load. As I've (clearly) invested too much already, further improvements will come after a c of o as time and money allow, including 16 PV panels, which I’m interested in making hybrid PV - Solar thermal (PV/T) panels as I’m already committed to some solar thermal infrastructure with the shutters. The core conditioned living space is 1970sf though with the studio and basement the total conditionable sf is 2930. There is a wood stove so it's not necessary to plan for over the calculated load for the main living space here in southern ct at 21,500 btu/hr. The property has three abandoned wells one of which is an accessible dug well some 30’ from the house. (Image shows the site and the locations of the wells) The new deeper potable well is 6” in diameter and 150’ deep with a Grundfos 10S05-9 pump at 115’; water line 30’ below surface. If i understand the pump docs properly this pump should offer 13 gpm sufficient for 5gpm (or even 7) to the geo unit. The well produces 40 gal per min. There is a culvert 100’ down the block that goes right to the stream and digging a ditch to bury the dump pipe would have other drainage benefits for us and our neighbor. The city is fine with pump and dump,, so everything suggests an open loop except the water quality: “2.47mg/l iron, bacterial iron present, 179 mg/l chloride, ph 6.12”. HVAC Options: Install a hydronic distribution system (mostly radiant with some fan coils) and a water to water heat cool unit (thinking Nordic W25 or Waterfurnace 500W11) with a sacrificial external heat exchanger and run a pump-and-dump setup through that. Down the road maybe replace the external heat exchanger with a ground loop that connects two to four of our existing wells. Perhaps a cheap narrow 48” trench will connect the wells with a single pipe and U tube down each in series. The trench would be about 290ft, and the well loops would total possibly more than 410 lnft for a total loop circuit length of 700’. Perhaps even less total length would suffice if it is supported by 10,000 KW/year in hot water from the PV/T system which would prime the incoming water in winter and charge the ground loop while cooling the PV in summer. In either of these pump and dump setups, would it be cost effective to replace the pump with a variable speed pump? Would it be better to use the dug well as a buffer, taking the source from the dug well and running the existing deep pump at full capacity for an hour or so at a time to limit cycling of the deep submersible? Could the dug well effectively serve as a heat exchanger if I were to drop a coil in it and flush it with water from the deep well? Install a direct pump and dump 1.5 ton water to water heating only unit to the primary core living areas (1500sf) then also install a Mitsubishi Electric 1800 btu low temp air to air heat pump. This air to air unit would handle the full load efficiently with temperatures down to say 32f, then the water to water would take over down to say 5f, then either the direct vent wood stove and water, or water and air, would handle extreme low temps. Perhaps with such a small load on the geo, (18k btu half of the winter only at 4gpm) we could enjoy the efficiency of the direct pump and dump without worrying about fouling or corroding the unit over 20 years or more. ??? Or should this hybrid option 2 still employ an external heat exchanger? A classic closed loop system feels out of reach because it leaves me without a heating system until I can afford the 7k for the well and 3-4 k for the landscaping. I’m also not convinced it’s the most practical or efficient solution, further If i'm going to invest I would rather it be more innovative. Just do two (one 18k and one 12k) air to air units and keep things simple. Any feedback, thoughts, or useful notes would be appreciated!
since no one has replied, I'll give it a shot. First, you have clearly spent a ton of time and money getting the envelope as tight as possible lowering your peak heating load to 2 tons. So, regardless of the system type you choose, it will be comparatively small relative to most homes that size. With such a tight envelope I would highly recommend a way to have purposeful air changes most commonly done via a heat-recovery ventilator (HRV). What most people neglect when they tighten up the home envelope is how stale and humid the indoor air can get (because it can't leak through the envelope both in and out). Yes, it wastes energy/money exhausting air that you heated and bringing in cold air from outside but the HRV should temper it recovering some of that energy. Otherwise you will be living in a terrarium. This also means your wood stove better be using exterior air for combustion. Most tight house envelopes don't incorporate fireplaces/stoves for precisely this reason because there tends not to be a practical way to implement how to provide combustion air only from the exterior and have a draft to induce exhaust up the chimney. Being in CT, I don't think you want to install a system that doesn't have air conditioning. It can't be that different there than here in NJ and summers can be brutal with both heat and humidity. I can't imagine not having a/c for June-Sept. And like most, forced air is a much better way to get both heating and air conditioning with the same system. So that would suggest a radiant system isn't the best fit for the location. Do you already have some infrastructure related to a radiant system that you are trying to re-use? No space for duct work? If you are stuck on having radiant heat, then a geo system for radiant heat and a separate air source system for a/c might be the best scenario. But this doubles your systems (and likely the cost). Geo units with our ground temps essentially provide free a/c (ground is colder than air and energy wants to run downhill so compressor/refrigeration cycle barely has to do any work). Its why you see water to air systems being the predominant system installation. I personally have a closed loop vertical bore system and didn't research any type of open loop system as I don't have the water availability that you have. yes, water temps are more moderate in open loop systems so there is a gain in efficiency there. But, open loops main deterrent is water quality running through the unit. With your iron, manganese, low pH, and the iron bacteria, you may find that an open loop system simply won't last and get fouled. So upfront and ongoing costs might be cheaper (pump and dump with existing wells), but you might be replacing units every 10 years or dealing with intermittent fouling/maintenance due to the water quality creating high ongoing costs. Lowest total cost that includes initial install+ongoing costs should be the metric to shoot for assuming funds are available for initial install. I should also point out that pulling water from a well for both domestic uses and geo use tends to be very inefficient. Domestic operates at much high pressures and is intermittent. Geo operates at lower pressures and is more continuous. So whatever pump you get will be a bastardized setup doing neither domestic or geo well. If you are set on pump and dump, I would separate your pull for domestic water and geo water from different wells. Search the forum and you will see difficulty using a combined setup. I question the ability of an air source heat pump to provide both heating and cooling in your (and my) climate. This winter - sure would have done fine. Most winters they will get quite inefficient attempting to provide heat when it is 10 degrees. But in my travels through the northeast, I have seen plenty of them in NH and ME so someone thinks they work okay. My gut says they were installed purely due to cost and the owners aren't particularly thrilled with winter heating but it was "cheap".
Thank you! I have installed a heat exchange ventilator. I am concerned about humidity. Less so cooling but yeah it would be nice to have cooling now and again we do have an automated night flush cooling setup. Yes there is some hydronic infrastructure already in place so that's an issue. Maybe if the upper floors are run predominantly with hydronic fan coils they can be used for heating and cooling, radiant floor heating on the lower levels. I'll have a look at the thread on shared well pump issues. Thank you for taking the time!
It depends on the heat pump. Some cheapo single-stage ducted heat pump is going to be lousy in the winter in New England. But a modern mini-split like Mitsubishi, Fujitsu, or LG, or a the Carrier Greenspeed ducted unit will have no problem meeting design load in Connecticut. The design temperature for Middlesex County is 6F, which would allow a mini-split to offer full heat output at design. With super-insulated homes, using a mini-split is going to be a lot cheaper than geo, as the heating and cooling loads just aren't that high.
