Combined radiant and forced air Geothermal Retrofit - need advice and opinions

[frumper15]

I'm in the home stretch of designing this project (yes, I'm a little slow on it). I have 4 distinct designs that I've settled on (that will find use beyond just this project).

My "best" design is the waterfurnace Synergy3D (actually branded as american standard) unit to provide both the forced air and hydronic side of the equation. The place I'm getting hung up is the domestic hot water production (still). When I'm supplying the hydronic side of things I'm also including the garage floor heat (currently supplied by a separate water heater). That bring my heat pump size up to 4 ton from 3 tons. Air conditioning is still OK, my first stage cooling doesn't exceed the calculated load, but I can't go any higher than 4 tons.

That being said, I would still really like to offer Geo-heated domestic as an option, but it's looking like I would either be on electric backup heat (if setting the hydronic side to priority) or won't be able to keep up with the domestic load (if leaving the air side as priority).

The plan is to use a Triangle Tube SME Multi Energy tank that has 100 gallons of primary boiler/hydronic water surrounding a 60 gallon inner tank of domestic hot water. The Geo unit would be tasked with keeping the water in the inner tank at 120 degrees so the outside water should be also at 120 degrees when everything is said and done. Radiant floors would draw from the 100 gallons of primary water and the desuperheater would also temper a preheat buffer tank (mostly for summer benefit as the Geo will be supplying the heat in cold months anyways.).

Am I making this too difficult? Am I chasing that last dollar of savings but adding a lot of cost and potential disappointment? Should I just KISS and put in an instantaneous Hot water heater behind a buffer tank and call it a day? I'm looking for a sanity check here and hoping you guys can set me straight.

[frumper15]

This has me looking at things again and I thought I would add this wrinkle: if I use the existing water heater as the buffer tank and leave it powered (propane) it could effectively serve as an auxiliary heat source for both the domestic hot water and the hydronic floor application. I will also have the electric strip heat for the air side backup as well, so then it would be a decision of putting the unit in hydronic or air-to-air priority mode.

I would effectively have 160 gallons of 120 degree water when the tank has satisfied and having the Geo in air priority would mean the house would never be without heat and that would leave all the gaps between cycles to work on heating the water back up to temperature (and a large portion of the house heating would be covered by radiant floors as well). I haven't found an effective way to measure how much of my demand would be taken up by domestic hot water production and it really varies depending on usage. I think it's more important for me to use a reasonable assumption and make sure I have a viable backup source of both air and hydronic heat.

[frumper15]

I think I may have stumbled my way into a viable idea on this topic: Rather than use the Geo as the final domestic water heater, I should use it on the preheat tank. Sort of.

Tell me what you think about this: I leave the Geo in Air priority so that I'm certain the house is always kept warm (priority #1). Keep the existing Hot water heater in place and set to a comfortable temperature (120 degrees). Set the SME tank at a temperature slightly higher than the powered water heater (125 degrees) for the Geo to shoot for. No need for a desuperheater as all the heat would be coming from Geo anyways.

The way I picture things going is this: Geo will not have trouble 90% of the time keeping the SME tank at 125 degrees so the powered water heater will do nothing but serve as another holding tank, and hopefully standby losses are low enough that it won't even need to fire as the entering water will be hotter than its setpoint. If it does get to point the Geo can't keep the SME tank at 125, the hot water heater will kick in (only after it has used up its heated water) and even then it will only need to heat water that should still be above 100 degrees. The lower water temperature water for the radiant shouldn't be a problem as it is for comfort not heating (with the garage being the exception, but that is kept at 50 degrees anyways.)

[arc8]

My "best" design is the waterfurnace Synergy3D (actually branded as american standard) unit to provide both the forced air and hydronic side of the equation. The place I'm getting hung up is the domestic hot water production (still). When I'm supplying the hydronic side of things I'm also including the garage floor heat (currently supplied by a separate water heater). That bring my heat pump size up to 4 ton from 3 tons. Air conditioning is still OK, my first stage cooling doesn't exceed the calculated load, but I can't go any higher than 4 tons.

With a 4 ton Synergy3d unit, is there enough capacity from the hydronic side to take care of the load? Can this Radiant heat meet the need for the whole project? If not, the "AIR" side is the priority!

You can use any means of supplementing the Radiant heat as you wish.

The plan is to use a Triangle Tube SME Multi Energy tank that has 100 gallons of primary boiler/hydronic water surrounding a 60 gallon inner tank of domestic hot water. The Geo unit would be tasked with keeping the water in the inner tank at 120 degrees so the outside water should be also at 120 degrees when everything is said and done. Radiant floors would draw from the 100 gallons of primary water and the desuperheater would also temper a preheat buffer tank (mostly for summer benefit as the Geo will be supplying the heat in cold months anyways.).

I would use the desuperheater (water assist) to heat a pre-tank before any DHW device/equipment.

Am I making this too difficult? Am I chasing that last dollar of savings but adding a lot of cost and potential disappointment? Should I just KISS and put in an instantaneous Hot water heater behind a buffer tank and call it a day? I'm looking for a sanity check here and hoping you guys can set me straight.

You can. You do want it simple and less costly and maintenance free, don't you. What would your warranty be after everything is installed??

[CraziFuzzy]

I don't think multi-stage chillers are out there in small loads because it really doesn't matter that much. The fact that you are using a chilled water system already gains you a lot of the advantages of multistage (being able to scale the system to the load). With a properly sized buffer tank, it doesn't matter that the compressor itself cycles - the cooling can continue just as much as needed.

[SolarMike]

If you are chasing that last dollar of savings, I would suggest to remember that your biggest annual load is heating and your best efficiency is when you can drop that down as much as possible in temp.

I don't know what percentage of the house is floor heated but if possible Treat your floor as the primary heating system. Put all your heat into a 120 gal tank and heat it to 40C (104F). Use a 3 way mixing valve with an outdoor reset (Taco has a stand alone unit) on the floor heat which i assume is in a gypsum cement (please). (you can increase the tank temp to 50C (120F) setpoint when the outdoor temp is below freezing. Depending on the tubing layout, your return water temp to the tank can determine when the fan coil comes on.

As propane is the current energy source and, in most places, there is not much difference between propane and straight electricity, for DHW just use electric elements to boost the incoming to 50C (120F). This, of course, assumes the DHW load is not huge, as mentioned earlier.

Use a second tank (50gal) for a chilled water buffer and take it down to (4C) 38F. The system would use 2 pumps, one for the heating tank and one for the cooling tank. of course the HP has to have a reversing valve. A normal fancoil will distribute the cooling.

This is probably the simplest way to really do it.

[Oldsolarguy]

I use to do retro fits with my old company. With radiant and geo, the w2w dedicated is the easiest way control wise to go. Install a buffer tank(size according to load) and then distribute with either a central pumping method and zone valves or use a primary loop with injection pumping to the zones. As far as keeping the gas as backup and installing a water coil, I think that would be a good idea to promote for the customer having a 5k propane bill. As for another idea, water furnace makes a dedicated w2w with a tank for domestic hot water use. You can do multi units on one loop, just size the loop for the total tonnage load, and then pipe to each unit as normal with flow centers. Overlapping controls could be done with a dedicated radiant stat, either wireless or hard wired as allowed.