HPWH: Unheated Basement...is it worth the risk?

[Dave Hudgins]

Backdrop: 32 yr old (duplex-attached) home, with separate boiler heating/hot-water systems...meaning, main side has separate boiler from apartment side and each boiler supplies hot water base board heat along with potable hot water. The semi-burried basement under main house is finished/heated. Adjacent basement under apartment is unfinished with very limited heat and has a garage door for car storage. Adjoined basements are burried on front side, while back sides are exposed...we live on a hill, with walk-out basements on backside. Boiler room is in the unheated (or very limited heat, see below for details) section of basement and is walled off (un-insulated) from rest of unfinished basement.

Recent upgrades & conversions: I've been meaning to convert over from the inefficient/obsolete boiler system (& $4K annual oil consumption) to heat pump system for years, but procrastination...
I finally pulled the trigger this fall and converted our heat over to 16 SEER American Standard Heat pumps...4 ton unit for the main house, and a 3 ton for the apartment. Our boilers are still installed, supplying hot water. So now, I'm looking to get rid of the boilers and replacing with tanked water heaters. I'm also going to be installing a looped (w/recirc pump) hot water line on the main house side, since it can take 5 min for hot water to reach the far end of the house, in current config. I did install a manual supply register off the apartment 3-ton unit main trunk, to keep the garage above freezing temps in the winter time.

So HPWH or resistance WH?

My problem, I don't know how cold my basement will actually get without the boilers, and with the HVAC supply installed...not enough data. I'm fairly confident I can keep the apartment side basement above freezing during really cold days, but really cold days are fairly infrequent in the mid-Atlantic region. My boiler room (soon to be water heater room) is fairly large (approx 150 sq.ft. w/10 ft ceiling) and could handle 1 HPWH under normal conditions, but how about 2 HPWHs?
Maybe split it with HPWH supplying main and resistance HWH supplying apartment?
Is the backup resistance heat on an HPWH more or less efficient than a standard resistance HWH?
I can also keep the WH room door open to the garage to help, if there's any useful temp differential between the spaces.

I know there's lots to consider, and I've rambled on, so...

Appreciate any feedback?
-Dave

 

[Phog]

Is the question, how to know whether or not you'll get frozen pipes?

 

[Sponsor]

 

[Dave Hudgins]

...frozen pipes?

There were several questions in there. None of them talked about frozen pipes. Guess I should have been less wordy, and better framed the question...apologies.

The main question is HPWH or resistance WH?

 

[Phog]

It depends on the electricity rates in your area, the hot water usage patterns of the household & the cost of installation. You live in a humid climate most of the year which is good for HPWH-- more latent heat to be captured. You need a decently sized room or ducting (yours should be ok) and a condensate drain nearby (don't know if you have that available). After installation the HPWH will be much less expensive to run in "heat pump" mode, but if you have frequent large drawdowns it will need to run its backup electric heat element to recover enough hot water to meet demand, negating savings. So not ideal for households > 4 persons. Areas with low electricity rates have a longer payback time on the additional initial upfront costs. Every situation is unique. The majority of the time, but not always, it comes out in favor of HPWH.

 

[WorthFlorida]

To cover for those few super cold days in the mid Atlantic, you can install heat tape around the pipe if you fear they could freeze. MOst now have a built in thermostat that will automatically turn on at the set temperature.

If you look into the Rheem heat pump water heaters they really got it together. Though the price of a HPWH is more than double than a resistive heating unit, you'll be ahead on the electric bill with the heat pump. A HPWH has the same resistive heating elements as a standard water heater to meet the high demand times. The Rheem HPWH has a recovery rate of 29 GPH at a 90 degree rise. A standard unit is less 21 GPH.

Going all electric is easier to budget since the price of kilowatts varies little when fuel oil cost spikes. There is far less maintenance but HPWH do have a filter that needs to be cleaned regularly.

https://s3.amazonaws.com/WebPartners/ProductGroupDocuments/C6BC483A-3721-483F-BCED-726D4C96C821.pdf

 

[Jadnashua]

The colder the area, the less efficient a heat pump will become, and cause it to rely on the resistance heating elements more and more. THey work best where it is warmer. Since they are taking heat out of the area around them, that will act like an air conditioner in the summer, but will do the same in the winter, making the room colder. Maryland tends to have more hotter days than colder ones. You would want to check to see what the coldest temperatures are recommended for installing the heat pump unit. Also, the larger the temperature of the tank is versus the room it's in, the more standby losses you'll have. With your boiler, those standby losses weren't really all losses, as they'd help heat the area, at least useful in the wintertime. A heat pump unit will be cooling the area all year long.

 

[Dana]

A 150' room with 10' ceilings doesn't have the volume to support a pair of HPWHs even if it were fully conditioned heated space. Using a ducted version it could make use of the main basement's volume of air, which is much more reasonable. (The Rheem Premier Platinum series available at box stores is already set up for ducting, but there are others.) With 1000 square feet of un-insulated basement ceiling below conditioned space a heat pump water heater serving a family of 4 won't lower the basement temperature by more than 2 degrees F.

I'll quibble with my friend Jim here a bit- the lion's share of the standby losses of the boilers would be true losses if the foundation walls aren't insulated. I'll bet the boiler room was the warmest spot in the house when it's cold out, and likely the least insulated spot too. No matter what it's going to be worth air sealing and insulating the foundation walls to the current IRC's code min continuous R10 or better, even on the unfinished portions of the house. That will raise the temperature of the space, improving the performance of the heat pump water heaters, and lower the amount of mechanical dehumidification needed to keep "must basement" smell in summer at bay.

A 4 ton heat pump would be sub-optimally oversized for most normal-sized houses in your location unless there is a lot of single-pane glass and barely or un-insulated exterior walls. Many or most homes built in 1987 had clear-glass U0.5-ish double panes, and at least R13 in the walls. Unless there was a serious attempt at measuring or calculating the loads (say a Manual-J, performed by an engineer, not an HVAC installer) they're probably more than 1.5x oversized and not quite hitting their HSPF/SEER numbers (but still cheaper heat than typical 2-3x+ oversized oil boilers.)