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Community and Q&A

Electrifying a Multi-Unit Apartment Community With a Heat Pump

user-6603341 | Posted in Mechanicals on

Hi There,

Our company is in the process of designing a new heating, cooling, and domestic hot water system to serve a 260-unit garden-style apartment community is Sacramento, CA. The property is comprised of ~ 20 single-story buildings and was built in the 1960s. We are looking for ways to electrify space heating, cooling, and domestic hot water heating. Currently, the apartments are served by underground distribution piping, connected to a central plant that houses heating boilers, a chiller, and domestic hot water boilers. The existing equipment and infrastructure is old, has a lot of deferred maintenance, and is at end-of-life.

The intent is to decommission the central plant and the underground piping and replace with unitized equipment. For space heating and cooling, we are planning to specify a heat pump, situated outdoors, and connected to a ducted fan coil in the attic. Are there any manufacturers making systems that can also provide domestic hot water? You would think that a heat pump that can provide heating, cooling, and hot water would be the holy grail, especially for multifamily housing. I was hoping to leverage the collective knowledge of this community to see if anyone had any thoughts or suggestions. Thanks!

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  1. Expert Member
    NICK KEENAN | | #1

    Have you penciled out cost of the unitized equipment? Unless the existing piping is shot, I would think it would be lower total cost of ownership to keep the hydronic infrastructure you have and just replace the boiler and the chiller. Do you have separate piping for the chiller and the boiler, or do you just supply cold water in the cooling season and hot water in the heating season? More flexibility would be a reason to unitize I guess.

    If you've got a chiller, you already have an air-to-water heat pump. There are heat pumps where one side provides cold water for cooling and the other side provides heat for domestic hot water and space heating. When there is an imbalance you need a place to dump the heat. This might be a good application for geothermal.

    A lot depends on your specific circumstances so it's hard to generalize.

  2. user-6603341 | | #2

    Thanks for the thoughtful response.

    Unfortunately, the existing underground piping is in very bad condition, with a lot of leaks, no isolation valves, and is also "insulated" in asbestos, making repair/replacement very expensive. Furthermore, we are securing a sizable amount of incentive dollars to electrify. Lastly, ownership would like to transfer the utility costs onto the resident meter, and is willing to make a sizable investment to do so. This also encourages conservation, when you have to pay for what you use.

    For domestic hot water, we are entertaining the idea of designing a system that serves each building rather than each apartment unit, since the annual cost of heating domestic hot water with a heat pump is minimal, especially if you can program the heat pump to run when the cost electricity is less expensive.

  3. Expert Member
    NICK KEENAN | | #3

    If each building has an average of 13 units, it seems crazy to put 13 pieces of equipment on a building instead of one. Especially if you already have hydronic units in every apartment. If the underground piping is shot, fine, cap it where it enters the building. Running 13 line sets through the building would tear it apart. Air to water heat pumps are real common in larger installations but they're just in their infancy in single-family residences.

    If you want to bill people individually just use smart thermostats and log their usage. Figure out a formula for assigning costs and put it in the lease.

  4. Expert Member
    BILL WICHERS | | #4

    I ran into a datacenter project like this once. It was a repurposed semiconductor manufacturing facility around 700,000 square feet or so. The new owners wanted to turn it into lots of smaller datacenter spaces to lease out to tenants. They also wanted to keep the efficiencies associated with the large-scale (42" chilled water lines! Yeah!) chilled water system. But they wanted to bill tenenats individually. What killed their plans was that they didn't want to MAINTAIN the large, central plant -- they just wanted to be "regular" landlords, but they wanted the much higher per-squarefoot rates that technical customers would pay.

    In your case, there exists thermal metering equipment, so you can bill units for BTUs used from a central plant. This is common in cities with steam service as a utility -- and remember that you can use absorption chillers to air condition using steam too. You may find that a central plant is actually more efficient than a unitized system (which is often the case), because you can operate the central plant with a higher utilization factor compared to a lot of smaller units. There is some efficiency gained with large, central equipment in many/most cases.

    If you have an old piping system, it might actually be worth replacing with something like Aquatherm pipe, but that depends on many factors. Something like DC suggested might be an option too, using smaller per-building common plants. You'll get lower system efficiency with per-building systems compared to a campus system though.

    I would do a serious cost study of upgrading the central plant system and possibly adding BTU metering before I'd look at putting per-unit heat pumps everywhere. Keep in mind that a large heat pump deployment may required upgrade costs too -- to the building's electric distribution system.


  5. Jon_R | | #5

    +1 on centralizing to some extent and then metering.

  6. Expert Member
    Akos | | #6

    I have seen a number of ways this has been done at smaller buildings.

    The most common one around me is the packaged water sourced heat pump. These buildings have a central water loop that the heat pump in each unit is connected to. Tentants pay for the hydro but there is some operating and maintaince cost to the building loop. These always have a resistance electric tank for hot water. You could make hot water from the hydro loop but the equipment to do so is way too expansive to have any ROI.

    For smaller places, the most common one is a higher BTU gas fired water tank with a small hydro coil in an open loop configuration. The equipment is pretty simple and small (hvac+tank fits into a 3x3 closet). This still needs an outdoor unit plus a gas connection, so I think this would be an expensive retrofit.

    Overseas it is common to have split systems with wall mounts or a single ducted cassette in the ceiling. You do have to find space for an outdoor unit, most places this tends to go onto a balcony. Hardest part is not making the building look like this:

    Hot water was from a small natural gas tankless unit.

    Trying to find a per unit system to supply both heat/AC/hotwater is still not possible in North America. There are some magic box type system available in Europe, but they have not made it here.

    I think the BTU metered semi centralized setup suggest is probably the easier retrofit. I've lived in a place that was supplied by a ground source heat pump where my heat and cooling was BTU metered, pretty straight forward setup.

    1. charlie_sullivan | | #7

      What are your per-unit heating and cooling loads? Chiltrix is one company that makes a small air-to-water heat pump that can be configured to do heating, cooling, and DHW. Howver, it might be oversized for a single unit, and it doesn't offer a mode where waste heat from cooling goes into DHW.

      1. Expert Member
        NICK KEENAN | | #9

        I generally like Chiltrix but I don't like the way they do DHW. The HW heat exchanger is just plumbed as a separate zone, when it turns on all of the other zones turn off and if the heat pump is in cooling mode it switches over to heating mode. Nothing elegant at all about it, and no practical benefit compared to just having a HPWH.

        1. charlie_sullivan | | #11

          Agreed. In a heating dominated climate it can still be good, but in balanced or cooling dominated climate, you are losing out on the free cooling offered by either a more sophisticated system or a simple HPWH.

  7. Expert Member
    NICK KEENAN | | #8

    I'll just point out that even mini-split systems need to be engineered. This isn't like buying a microwave and washer-dryer for each unit.

  8. jonny_h | | #10

    From an "I just live here" perspective, I would vote more for the central plant or central building approach. I've spent a couple years in a similar complex -- 20 buildings with 14 units in most, in zone 5 -- except everything was per unit. Each unit (that I saw) had a ducted air handler in the wall (with a central return and terrible flex duct distribution above the ceiling), with a heat pump coil and a hydronic heating coil. The hydronic heat was just DHW circulating; DHW was provided by a gas water heater in a closet. The system execution was mediocre at best, but it also didn't age well:

    - With 14 units per building, there were also clusters of 3-4 outdoor condenser units around various corners of each building. Probably 80% of them were original from the 80s or early 90s? and the remainder were a hodgepodge of mixed low-end replacement units. In the winter there'd always be a couple ice cubes if you walked around looking at them. Basically they got run and minimally maintained until they failed to the point that a resident complained, then replaced with whatever was cheapest.

    -The gas water heaters, I assume, were a similar hodgepodge.

    -The controls were old and poorly understood by the current management. One fall, maintenance came around and cut / patched together a couple wires in the units to "make the heat work" (they just disabled the heat pump and made the hydronic pump run any time the fan turned on). I fixed it myself when spring, and then summer came and the promised "fix" involving "some relays they ordered" never materialized.

    The point I'm trying to make is that a per-unit system makes billing easy, but it also makes neglect easy. Whatever you design may function flawlessly and efficiently now, but what happens in a few years or a few tens of years when the management company may have changed, the institutional memory of how and why things were done is lost, and there's little motivation to fix or improve things until each resident gets frustrated enough to complain about their unit. A centralized system can also be neglected, but if it loses performance or fails, an entire building or complex complains so there's a bit more incentive to maintain it. And, I have to think there's some economy of scale both in efficiency and cost when considering larger vs. individual-scale systems.

    Now, you may not have any leverage in the decision to be centralized or unitized. Given that, here's what I would have liked to see in a per-unit system:

    - Per unit heat pump. It looks like Sacramento is zone 3, so with the combination of warmer weather and more modern equipment, the hydronic backup should be unnecessary. Depending on the unit layout, could be ducted or ductless.

    -Per unit HRV. Especially post-COVID, I wish these units had them. Fresh air is important, especially in buildings that would otherwise stack-effect air between units on different floors. I'm not a mechanical engineer, but I wonder if in, say, a one-bedroom unit, you could get away with a ductless heat pump on the wall in the main room, with HRV exhaust from the bedroom, bathroom, and kitchen and supply to the main room near the mini-split -- allowing the HRV to pull conditioned air into the bedroom thru a transfer duct or undercut door.

    - In your area, I'd be real tempted by making DHW per-building using one of the packaged Sanden multi-unit systems. As a retrofit like you're doing, I don't know where you'd put a traditional water heater.

  9. walta100 | | #12

    I agree that a central plant costs less to operate but the people that pay the bills want what they want you have no choice but to give them what they want or they will find someone else who will.

    At this point I bet management is fed up with down time big bill they are paying to keep the old central system running combined with the fact they want out of the heat, AC and hot water included pricing model.

    My wild guess is the electric panels in the unit are currently are way too small to supply power for HVAC and DHW.

    My guess is if you change the panels they will have to upgrade everything to today’s code. Most likely have to open every wall to add outlets on every 6 feet.

    For DHW you have 2 choices find room for tanks or go tankless and more than double the size of the electric service panels.

    This line is pure baloney “since the annual cost of heating domestic hot water with a heat pump is minimal”

    Unfortunately the equipment you have imagined is yet to be invented.

    Is your budget in the 75k per unit neighborhood?


  10. user-6603341 | | #13

    Thank you all for providing this wealth of information and feedback. I am going to take some of these ideas back to the design team and might revive this thread if any follow questions emerge!

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