Adding heat pump water heater alongside ancient boiler coil

[ajs317]

I currently have a large beast of an American Standard oil-fired boiler that dates back to the 50s. In addition to heating the house, this thing does a decent job of providing domestic hot water for my 2400 sq ft, 3-bathroom house. The reason that I say decent is because we experience a fair bit of temperature fluctuation during showers and sometimes water will go cool for a number of minutes, which I assume is due to the boiler being used for heating. As you can imagine, the boiler costs a ton to run (and my basement utility room sits at about 82 degrees even in winter).

I'm looking to add a heat pump water heater to even out temperature fluctuations and decrease reliance on using oil to heat water. Until we replace the boiler, which isn't in the immediate budget, I assume the HPWH will benefit from using the heat put off in the utility room. I'm wondering if it's possible to plumb things so that hot water will be supplied in priority from the HPWH tank and then by the boiler coil if the tank runs out. The reason I think this would be helpful is because with a family of 4, I think we could mostly get by with a 50 gallon HPWH but we may need a little extra help when we have guests and as our kids get older.

I know that some have placed water heaters in series after the coil, but because I understand that a heat pump is generally going to be more efficient and cheaper than heating water in the boiler (especially my ancient one), I wouldn't think you'd want to do that with a HPWH. What I'm imagining would be like a valve setup that switches supply over to the coil if the tank needs to recover. Is a setup like this possible? If possible, would it be cost effective and sensible to do?

 

[WorthFlorida]

You can search this site and reads reviews about the heat pump water heaters. Bottom line is they do not have a quick recovery rate as an all electric water heater and they can be noisy. It is an air conditioner siting on top the of the tank. However, it will help recovery the excess heat you have in the boiler room back into the heating system. As far a added it to your system for additional hot water wouldn't work. If it is down stream and the boiler feeds very hot water to the tank for brief periods, the temperature pressure relief valve may open quite often. Temperature to hot water heating systems as old as yours is usually about 160 degrees for it to keep up with the very cold days for home heating needs. New furnaces, however, with an outdoor temperature sensor will vary the circulating water temp, therefore saving a lot of money on fuel cost.

The best way to get an even, safe temperature domestic hot water is to install a SuperStor tank or any other named system. It is a heat exchanger for your set up. High temp hot water from your old boiler is circulated through this tank. The tank is connected to the cold water of your home and the "OUT" is you domestic hot water. It is controlled by an added thermostat and a circular pump. The temperature for domestic hot water should not be more than 120 degrees. A 50 gallon tank will be suffice for most homes.

This can get pricey but it is far safer and more comfortable way to go. You'll need a plumber for this since you would need to disconnect your domestic hot water from the boiler. The Superstor tank will needs it own pressure relief, check valves, or anti back flow devices as to the local codes require. Heat Transfer Products, Inc., now owns SuperStor.

http://csiworldwide.com/HTP/SuperStor Storage Tank.htm

You are in a 60-70 year old home and it be well worth it to finally upgrade your system. Check with your oil or gas company, state department of energy, etc to see if there is any rebates or tax incentives to upgrade your furnace.

Did a quick search and this popped up for NYS.

https://www.nyserda.ny.gov/About/Ne...Announces-Proposal-for-Rebate-Program-for-RHC

 

[Sponsor]

 

[Dana]

A quick recovery rate isn't an issue if the boiler's coil is feeding hot water into the heat pump water heater (HPWH) but the thermal mass of the tank will even out any fluctuation in temperature, even more so with a code-mandated temperering valve on the output of the HPWH.

With the huge standby loss of that pig of an antique boiler the boiler room is probably the warmest room in the house most of the time, even if it's not insulated. In the higher temperature of a boiler room a heat pump water heater will operate significantly more efficiently than in a 55F basement.

If you're not using the tankless coil directly, you can lower the operating temperature of the boiler (no lower than 140F for a low-limit) and save quite a bit on standby & distribution losses too. With most tankless coils you have to keep the boiler well north of 150F 24/365 to get barely-adequate hot water performance out of it.

An indirect tank would offer more capacity and faster recovery than a HPWH, but in most local electricity markets heating hot water with an oversized and poorly insulated low-efficiency oil boiler would have a 2-4x higher operating cost of an HPWH. An oil boiler that went into service during the Eisenhower administration won't be doing much better than 70% steady state efficiency. If it is 3x oversized it'll be doing much lousier. They were ALL oversized by 3x or more for the space heating load back in the 1950s, partly to get reasonable hot water performance out of the tankless coils. In hot-water heating mode even with an indirect and turning the low-limit down to 140F with the age & oversizing factor it would struggle to hit 50% for just heating hot water, and annual average heating + hot water annual efficiency (as-used AFUE) no better than the mid-60s efficiency.

Read this Brookhaven Nat'l Labs study as a bedtime story. In particular, see Table 2 and Table 3. Your current setup would be comparable to a burnt-out well past it's prime lower efficiency version of system #1. Adding an indirect tank and lowing it's operating temperature would deliver you something akin to system #2. What you WANT is system #3 0r #4, sized correctly for the space heating load, either of which would still be more expensive to heat water with using an indirect tank than a HPWH, but not 4x as expensive.

A boiler like yours isn't worth keeping going, even though they're as simple as a box o' rock and about as rugged. Replacing it with something more appropriately sized for the space heating load is worth it if you're going to be living there for awhile, and you can make the indirect vs. HPWH decision at that point.

Since you have a heating history on the place you can use the name-plate efficiency of the boiler to establish a firm upper bound on the actual heat load using fuel-use against heating degree day data.

 

[WorthFlorida]

From another post, member name STUFF replied: Expensive but here is an 80 gallon hybrid water heater. Supposedly uses 70% less electricity than conventional. http://www.homedepot.com/p/Rheem-Pe...-and-10-Year-Warranty-XE80T10HD50U0/300620269

I looked up the reviews on the HD site an it seems the heat recovery and noise have been addressed and there are a lot of good reviews. However, this tank is over $1500 plus install. Now your over $2K in cost, what would a new replacement furnace cost installed? The best information is from Dana as he wrote above.