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Thread: Heat Pump Domestic Heating
03-29-2012, 10:47 PM #1
Heat Pump Domestic Heating
Not sure what part of the site to post this in, but I figured I'd start here (please move as appropriate).
I'm looking for opinions/hang-ups on heat pump domestic water heating. It seems to make sense to me on the surface, just want to make sure I'm covering all the concerns.
Here's the situation. A Medical Office building (6 stories), with relatively low hot water usage (mostly just hand-washing), but quite a large recirc flow/losses. (Rough estimates based on the current system is somewhere around 50,000BTU/Hr losses to the recirc, with peak demand near 75,000 BTU/Hr. Currently, the demand is served by some AERCO steam heat exchangers, but the steam piping is running from a really inefficient boilers system across the campus, with quite a bit of transmission loss, and these heat exchangers are the only things on this steam line. It makes sense that separating it out and going with something else would have gains, but it's kind of a toss-up which way to go. The building has no gas in it, and would have to be run about 1200 feet across ground, and up 4 stories. The current system is in a penthouse at the 4th floor level.
Here are my thoughts:
- Condensing gas tank heaters - ideally something like 3 50-60MBTU, for redundancy, and future capability. Low operating costs, but high installation costs - due to the gas line requirement.
- Electric Tank - ridiculously high operating costs, far exceeding even the poor current situation.
- Electric Heat Pump - Middle of the road operating costs, middle-to-low installation costs (the heat pump is likely expensive).
- Solar Collectors - High installation, but very low operating costs. Still would require some backup source of heat.
What my first desire points to is to go with the Solar collectors, combined with a heat pump for backup-supplemental. Being in a large mechanical room, it is likely we could duct the cold-air discharge to the air handler intakes, and utilize that cooling when it is in use. Being in SoCal, we have pretty decent sun-hours, so I'm hoping we'd be able to get the tanks hot enough during the day to keep the demand through the night. Am I missing any problems with this setup?
03-30-2012, 02:14 AM #2Professional Member*
- Join Date
- Feb 2009
- Portland OR
So peak load is 75k BTUs and your putting in (3) condensing 60k units?
Why not do something like a 200k tankless preheating the water for a single 80 or 100 gallon storage tank. Less cost in tank, gas line is already being ran, fairly efficient with high fast recovery and a lot of buffer room water.
Or just do a single condensing water heater and set output to 140+ degrees and add a mixing valve so you have more heat energy stored in the water.
Solar would still take some sort of an electric backup and I don't know how well or what percentage solar would provide for you.
03-30-2012, 10:01 AM #3
The control scheme I was envisioning would be a single large tank, with the building recirc pumps. A separate pump for the solar loop, run when panel temp > tank temp. Then the heat pumps would only run if tank temp drops < 130°F (off at 135°F). If ducted correctly, and with a switching damper, the coldest inlet air to the heat pump would be about 75°F when drawing from return air, or up to 110°F when switched to outside air (it would switch to whichever dry bulb was higher). The heat pump could share the main recirc pump flow (3-way valve perhaps to bypass when the heat pump is not online). That way it is drawing from the coldest point in the system (recirc + make-up mix), and discharging into the tank.
We would of course still use a mixing valve for the building supply.
In the end, gas would of course be ideal, but I feel that may not be feasible. Solar may be easier to sell to admin. They are somewhat 'green' conscious, and the parking structure right next to this particular building already has 1MW of PV's on the roof.
03-30-2012, 10:50 AM #4Professional Member*
- Join Date
- Feb 2009
- Portland OR
Hmm, good to know about the N+1, I have never worked in a medical situation nor do I plan on it but I see now why you designed that way. Based on everything else you just said, it really looks like the solar option may be the best way to go and sticking with electric with a 1MW solar system in place, how big is this building again lol.
03-30-2012, 11:12 AM #5
hehe... that 1MW doesn't feed directly to that particular building, it goes into our power grid, but lets just say it, at it's peak, would be contributing about 10% of our campus wide electric demand. The PVs just went online 2 months ago, so we haven't hit 'peak' season yet, but I've already seen it at 750kW in late afternoon in March...
The n+1 isn't necessarily a medical requirement, but it is company policy to ensure non-interrupted service at all times.