Maryland Replacing gas HVAC with geo - what to do about DHW?

Discussion in 'General Discussions' started by Emily, Mar 19, 2018.

  1. Emily

    Emily New Member

    I'm fairly new to geothermal. I've been reading a ton of information and have just received my first proposal.

    Background: My 1950s brick rancher (1900 sq ft incl 900 sq ft finished basement) has two HVAC systems: 1) baseboard radiators on a natural gas boiler, and 2) natural gas furnace with ducts. Most of the upper floor has only ducts, and a two-story addition on the back of the house has only baseboards; most of the basement has both ducts and baseboards.

    The gas boiler is more than 40 years old (!!), so my primary goal is to replace it with a GSHP before it dies on me and before that 30% federal credit starts to phase out. With today's gas prices I'm not embarking on this project expecting huge savings. Cost is a factor as it is for anyone, but I'm also motivated to reduce my carbon footprint to the extent I can afford.

    So, here's the broad strokes of the plan:

    1) Boiler replacement: I've settled on the WaterFurnace 504W11 because it can get hot enough to work with the existing baseboard radiators.

    2) Furnace replacement: The furnace still has years of life, so I will might leave it as is for a few years. Eventually the HP will heat the whole house (and is being sized in advance to do so), but if I decide to delay adding an air handler I may wait til I can afford to run radiators to the duct-only part of the house. (I kind of hate forced air heat from a comfort standpoint.) At least one installer has recommended keeping my gas furnace as a source of aux heat. Would anyone here recommend differently?

    3) Cooling: I'm currently using a decidedly un-environmentally-friendly 7 window A/C units for cooling. The WF will take over cooling whenever the NAH is added.

    4) Domestic hot water: Current DHW is a fairly new (2009) 60-gallon gas water heater. This is where I'm feeling the most confused about my options. Here are the options the first proposal came with:

    Desuperheater - $1475 w/o storage tank; $2000 w/ 50-gal tank
    As I understand, the DSH (with or without storage tank) is just for preheating and cannot replace the gas HWH. I've also read on these forums that DSH perform poorly with gas HWH - would there be any point to this if I keep the gas HWH?

    Electric water heater - $2892 for a 75-gallon Marathon water heater
    Is there any advantage to replacing my gas heater with an electric one given it's still pretty new? How would Marathon with DSH performance compare to gas HWH alone?

    On-demand geo water heater - $8848 for NSW025 water-to-water with geo storage tank
    Aside from operating costs and carbon footprint, are there performance differences between electric and geothermal water heater?

    Solar - $9626 for 20-tube flush roof mount with 80-gallon tank
    I'm honestly not sure I get enough sun exposure for this to be viable due to many tall trees (cutting them down would be illegal). I once plugged my address into a website to see if rooftop solar was a viable electric source for me and was told it wasn't, but maybe hot water doesn't need nearly as much sun as solar panels? How do operating costs compare to geo water heater?

    Change nothing - $0
    Any special considerations or pitfalls to be aware of if I just leave the gas HWH running like it is alongside my geo system?

    Edit: After doing more research it's looking like it's going to come down to I don't enough space for the geo heater to make sense. Space is already pretty tight with the furnace, boiler, and gas heater. It also looks like the solar solution would require yet another extra tank to store the solar heated water, although maybe not as huge as the existing 60 gal water heater? At this rate my basement will be full of tanks! Hot and cold buffer tanks for the heat pump, a solar storage tank, and the existing water heater.

    So now I'm leaning towards just sticking with the gas heater until it's getting ready to die and re-evaluate then. Maybe someday when I build my upstairs laundry room I'll be able to take the washer and dryer out of the utility room to make space for this tank farm and add the solar at that time.
    Last edited: Mar 19, 2018
  2. ChrisJ

    ChrisJ Active Member Forum Leader

    New air systems can be very comfortable, slow and steady air flow. I have to look at a plants leaves moving near a register to know the heat/cool is on. Some of the comfort will come from envelope upgrades air sealing and insulation.

    Having a water to water high temp heat pump, using that with an indirect tank for domestic hot water (like having another zone from the heat pump).

    I would think this can be added after the gas HWH dies, switch tanks out, add a zone valve or separate circulator pump.

    Without perfect sun exposure, solar hot water is not worth it.
  3. Emily

    Emily New Member

    Thanks Chris! I actually had a sealing & insulation package done last year so I'm pretty tight now (other than the old windows on the main level of the original house, which I use plastic wrap sealing and thermal curtains to mitigate).

    It's good to hear that the new forced air systems are more comfortable. My main gripe with forced air is probably particular to my house's old system now that I think about it. For starters, there's just a single vent near the floor in each room, so the heating is pretty uneven and...

    ...I don't have and can't really install a whole-house humidifier without return ducts, so the rooms heated by the furnace have relative humidity in the 16-19% range, AKA "I wake up in the morning and down two glasses of water to recover lost fluids" range, AKA "I wish my cats understood that static shocks aren't me trying to hurt them" range! :D
    But I suppose I would just add return ducts and humidifier to the new system to resolve those issues.
  4. ChrisJ

    ChrisJ Active Member Forum Leader

    Low humidity has nothing to do with the heating ducts, it has to do with air infiltration, cold dry air getting in. So there may need to be more air sealing done.

    Have a blower door test done and use it to find more air leaks, that will help with the low humidity in the winter.

    My house we built 8 years ago has only supply ducts in most rooms, but because we used spray foam and an interior vapor barrier it's very tight. I actually have to dehumidify in the fall and early winter.
    Emily likes this.
  5. Emily

    Emily New Member

    Well, I learned something new today! I had always assumed the furnace heat was drying out the air.

    I suspect the dry air infiltration is probably because I elected not to seal (or repair) the ducts themselves when I had the efficiency package done. At the time I was uncertain whether I was even going to keep the furnace, so I didn't want to spend any skrilla on duct sealing if I was going to end up not using them. As I recall the company who did the audit said there was a duct in the attic that was actually largely crushed in one section and duct leakage was estimated at about 32% :eek:

    Now that I know I'll for sure be using the ducts, it sounds like I should go ahead and do the repair and sealing now, and it'll start benefiting me in terms of humidity even before the geo goes in. Thanks for the insight - very helpful!

    So then assuming tight envelope and duct sealing, it looks like I shouldn't need to bother with a whole-house dehumidifier if I use the geo for cooling. Research suggested that it would only really be helpful in heating-dominant climates where the system would end up grossly oversized for cooling if sized properly for heating and not run often enough to sufficiently dehumidify, but in CZ4 the loads should be more equal, yeah?
  6. moey

    moey Member

    You could go around in circles debating this but your carbon footprint might be more with a geo system rather then using natural gas. Theres a good chance the majority of electricity comes from natural gas factor in the line loses by the time it gets to your house and your using more natural gas to make electricity then you would be if you heated your home with it.

    I think I would just stick with the natural gas water heater unless you dont like having gas in your house. The options for making hot water with a geothermal system are all by products of heating your home. This leaves 4-6 months of the year where your making hot water in a less efficient manner.
  7. Emily

    Emily New Member

    I pay extra to source all my electricity from wind, but I'll probably always have gas in the house because I prefer to cook on a gas stove. Maybe someday when the gas company starts piping in biofuels instead of LNG I'll truly be off fossil fuels.
  8. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    Hi and welcome!
    I replaced a oil boiler many years ago for geo. Looking at all the available options to replace dhw, I settled on a on demand propane unit and never looked back. House already had propane, small footprint, no heat for storage, unit big enough to flow constant hot water to Mrs. Pirate when in her soaking tub in master bath. Problem solved.
  9. Liam

    Liam New Member

    I advise you to take a closer look at these models if you still doubt what kind of device to choose :) When I chose for myself personally at home, this site helped me a lot.

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