More DHW Capacity...Add Second Water Heater or Indirect Tank?

AdmiralYoda

Hello all! I normally lurk around the Strictly Steam forum but I've got a DHW question. A couple years ago we had a 50 gallon heat pump water heater installed and it works great. The problem is my kids are becoming teenagers and with the added showers, laundry, etc we are getting low on hot water.

I'm thinking of adding an additional 50 gallon heat pump water heater in parallel or another alternative would to add an indirect hot water tank to my steam boiler.

I'd probably configure the indirect water heater to work off of an Aquastat. Some logic like:
- IF the boiler water is greater than 140F and the indirect water heater calls for heat...circulate the hot water.
- IF the boiler water is less than 140F, don't circulate.

I wouldn't want the steam boiler to fire up just to heat the water, but I would take advantage of it while it is in use...essentially the winter months.

Overall installation cost is pretty similar for both scenarios. Thoughts?

Hot_water_fan

Can you fit a drain water heat recovery pipe? 

AdmiralYoda

@Hot_water_fan Interesting, I didn't know those existed...I had to look them up! I don't think I could get that to work short of a major renovation. It is an old house and the waste water goes into cast iron main pipe at various places.

WMno57

Some of the newer HPWH store water at 140 and have a built in mixing valve to deliver at 120. This gives more capacity than storing at 120.
You also may be able to add a basic 40g electric tank downstream from the HPWH. Do you have space in your electric panel?

AdmiralYoda

@WMno57 I have my HPWH set to 135F. The kids are old enough not to scald themselves and can throttle the output temperature appropriately. Plenty of room on the panel.

Even with low flow faucets and appliances we find that if we all take showers back-to-back the last person gets cool water. Of course if dishes are being done, laundry etc it compounds the problem.

Jamie Hall

Always a conundrum. Adding another heat pump water heater will help a little -- you would have the extra storage capacity, which might be enough. Might. It won't help much with the fundamental problem, which is that their recovery rate is painfully slow. The biggest single problem with two (or more) water heaters in parallel (which is what you would have) is trying to make sure that the draw is close to the same from each. This is an intriguing plumbing problem, but if they are fed from, and feed into, a very large low loss header and the plumbing between the tanks and the headers is identical, one can do pretty well.

You would have a somewhat similar problem with balancing the flow from an indirect, however.

The problem with the indirect is that while you could get a large enough one to handle the needs of the household, and its recovery is good, you don't intend to use it all the time -- so I really don't see that as a viable solution. Unless, that is, you piped the heat pump water heater output into the indirect tank, and used the latter as essentially a large storage tank when you weren't using the boiler. That would give you a lot more storage -- though you would still have the lame recovery rate of the HPHW to contend with. The other problem with that solution would be that unless you used the hot water quite regularly, the water in the indirect would cool...

hmm.

What fuel do you use for your boiler? One of the houses we care for did install a heat pump water heater -- a big one -- but had much the same problem you do, and we ditched it and put in a small oil fired tank type. No complaints since.

AdmiralYoda

@Jamie Hall We use NG to fire the steam boiler.

I hadn't considered a traditional electric water heater in series after the HPWH. The HPWH will do most of the work heating the water and the traditional electric heater would act as storage. Recovery may not be fast if they are both emptied but at that point no hot water would be needed for awhile.

Jamie Hall

AdmiralYoda said:

@Jamie Hall We use NG to fire the steam boiler.

I hadn't considered a traditional electric water heater in series after the HPWH. The HPWH will do most of the work heating the water and the traditional electric heater would act as storage. Recovery may not be fast if they are both emptied but at that point no hot water would be needed for awhile.

There you go.

AdmiralYoda

@Jamie Hall To keep this ball rolling....I suppose I could install a NG water heater instead of an electric one since it will be doing minimal work and NG is cheaper than electric where I live.

The HPWH replaced an older NG water heater so the gas line and flue connections are still there (capped).

Hot_water_fan

A typical electric tank uses less than a kWh/day to combat standby losses, so very cheap energy wise to use it in series. 

Larry Weingarten

Hi, If there is a way to tie tanks together, both top and bottom, you'll get convection between them, allowing the HP to heat both tanks. Then use the electric elements as backup when needed. About the shower drain heat exchanger idea. Is there any vertical drop from the shower to the main drain?
Yours, Larry

hot_rod

If you have a flue to connect into, it seem gas would be the fastest recovery and least expensive to operate

EBEBRATT-Ed

Put a timer on the hot water to the tub/ After 10 min no HW.

Wash the clothes in cold water. They last longer.

JMWHVAC

I installed an indirect that is heated by my oil boiler to help my HPWH. I have the domestic water going thru the indirect first and then into the hpwh. The indirect is heated ONLY by the return flow from the hot water baseboard, which means the hpwh does ALL the water heating in summer but less as it gets colder. Works swell because the hpwh has plenty of heat to use all summer but much less in winter. We also tend to use a tad more hot water in winter (I love cool showers in summer but scalding in winter).