"We don't have a furnace, we have a boiler." You saved me the trouble sue of explaining that! OK, what you have is an aqua stat that controls the fan in your air handler. When the t stat calls for heat either a pump comes on or a zone valve opens downstairs and allows 180 degree water to flow through the coil in your air handler. On the hot water pipe that leaves the coil should be a thermostat attached to this pipe. When it senses the water is hot enough going back to the boiler it brings on the blower. That thermostat, known as an aqua stat should be set at 140 degrees. Thats the temperature the fan comes on. The fan off setting is usually 100 degrees. Yours is probably set to low. Check the aquastat settings, there may be 2 settings (on and off) or there may be just 1 and a differential. If its that one set it at 140 with a 40 degree differential. The nice thing about a system like yours is you can fine tune them. If 100 degrees feels to cool you can tweek it up a few degrees to find something more to your liking. You don't have to worry about overheating the equipment. Just don't adjust the two settings to close together or else the blower my cycle on and off to ofter before the t room stat is satisfied

Datagrrl, I'm not sure it's clear that you intend to keep the return "open" by fronting the banquette with perforated metal. As a second approach, any chance of leaving the left and right ends open or partially open to improve air flow? My gut reaction is that the perforated metal will allow adequate air flow - certainly no 'hard science' I know of to back it up. Let us know what you decide!

Don't do that! If you hook a dryer into the HVAC return you'll blow lint-fines all over your ducts and quickly clog the heat-exchange radiator in addition to quite obnoxiously heating and humidifying your entire space every time the you run the dryer. Bad news! Sounds to me like you found the actual duct. Explore that. Assuming its the right one, verify that its clear to the outside and use it.

Re: Symie “Can any one of you answer the question of how to install a HP without the need for backup or fossil fuels? That was the original question.” I see you’ve already made your choice, that’s great. As you have already chosen a gas system, I’ll try to give you the short answer. An Air-Source Air-to-Liquid HP utilizing Variable Speed Compressor technology (VRF) & Enhanced Vapour Injection (EVI), that you already indicated that you know something about, is the choice of HP able to operate at very low Outdoor Temperatures averaging a ‘Seasonal’ COP fairly close to that of geothermal - without the need of a ground loop (geothermal is still the Gold Standard - but at a price). The indoor distribution is via hot or chilled water feeding in-floor radiant, heat emitters (radiators) fan coils units to change indoor temperatures quickly (heating or cooling), hydronically fed central fan coil units (to condition an entire floor), chilled beams or any combination there of. The way the Defrost Mode can work without backup is that in heating mode, the only mode requiring Defrost, is that a hydronic system employs the use of a buffer tank that stores hot water. The buffer tank can have many functions besides the prevention of short cycling (as when one small zone such as a bathroom in-floor radiant zone or even just a hydronically heated towel warming rack calls for heat), depending on design of the system. It can be configured that during Defrost Mode, the thermal energy stored in the Hot Water Buffer Tank is diverted and used to defrost the HP condenser coil while simultaneously continuing to heat the home, as there is enough stored thermal energy to do both. Depending on the design of the buffer tank system there might be as much as a 5˚F -10˚F drop in temperature of the stored hot water - but that temporary drop in water temperature would hardly be felt in the home because unlike a ‘Conventional’ Air-Source HP that completely switches over to Air-Conditioning, the Hydronic system might see that drop in buffer tank stored water from possibly 120˚F to say, 110˚F - not enough to feel any real difference in the home for perhaps the 20-minutes it takes to defrost and even less so with a system designed with all emitters operating at a design water temperature of 110˚F. Afterwards, the buffer tank quickly recovers when the HP fully switches back to heating. Keep in mind that when doing things the ‘Conventional’ way you still have at least 2-separate systems, such as a gas furnace and an air-conditioner. The system described above can be 1-fully integrated system for space heating, air-conditioning, Domestic Hot Water production - all with 1-source of energy, 1-utility entrance and 1-bill. It can also be designed to perform all functions all at the same time. “Current HVAC has 88K BTU and can't keep up as we get down toward single digits and 3 ton cooling which struggles to keep up over 90 degrees.“ Given your situation, I personally would be looking to do something quite different. While you should be able to sort out your HVAC challenges with conventional forced air technology, I would be concerned that some issues while ameliorated were not quite resolved leaving me unhappy. I might therefore choose water knowing that if I could deliver the right temperature water to all distribution points within the envelope, an easy task with proper design, that I could with the right emitters and fan coils, control the temperature in all parts of the home - heating and air-conditioning. Keeping to a specific temperature, water is easier to control than airflow. A modern water ECM circulator (pump) might drawing 50-watts (small incandescent light bulb) for the whole house instead of a ½-HP Blower Motor. Hydronics are much easier to zone and control than forced air. A 1-inch insulated pipe might be easier to install than duct work. "A given volume of water can absorb almost 3500 times a much heat as the same volume of air, when both undergo the same temperature change.” 3/4"tube = 20"x12" duct 3/4"tube = 18" ø duct Having said all this, I would still insist on an electric boiler backup in case of a real Emergency, such as a Compressor Lockout Condition or if for any reason the HP could not keep up with the envelope or DHW heat loss, if that were part of the design - but not for reasons of Defrost! While I’m not on-site to first hand evaluate your situation, if this were my home, I would look to do something cutting edge and radially different, rather than ducted forced air that has given you no real satisfaction. Please keep in mind - I am NOT trying to sell you anything. A HP Hydronic conversion would be a much more expensive Premium solution! ‘Think Water’ (But don't drown in the details) IMPO SR