Can I use my indirect water tank with my heat pump water heater?
user-7940936
| Posted in Mechanicals on
Hello, I currently have a boiler-fed, indirect water tank that supplies our domestice hot-water. To avoid $5/gal oil cost and short-cycling the large boiler during the 6 months of non-heating months I decided to invest in a 50 gal heat pump water heater. My question: can I plumb the new water heater so I can use the indirect storage tank as additional hot water storage space? The hope would be to allow additional hot water capacity for my family. Thoughts?
Thanks
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The simplest would be to plumb the indirect in line with the HPWH downstream of it (coldwater ->HPWH->indirect->house). From the drain of the indirect connect a small pump (recirc ones are fine) to the drain of the HPWH. The recirc pump can be driven by the existing controls.
The idea is that in most cases the indirect is just a holding tank, as long as the HPWH has enough hot water in it, all the water going into the indirect will be hot. The only time the recirc should run is to make up for standby losses when there is no water flow.
The better option is to sell your indirect and buy a plug in 120V tank water heater. Set this up the same way after the HPWH and set the thermostat on it to a bit bellow your HPWH output temperature. This way the extra tank would only use power to make up for standby losses or if ever there is a large enough water draw to empty out both tanks.
Akos - is there such a thing as a plug in hot water heater - in the 40-50 gallon range ? Also, you said to set up the plug-in the same way. Do you mean to set it up with the recirc pump.. and if so, what is benefit of using the 120v versus it recirculating to the primary tank ?
I’m putting in a 80 gallon HWHP and need extra capacity on occasion for large soaking tub. I’m guessing that I could do the same ? Cold water —> HWHP —-> 2nd tank —-> house
EDIT: sorry i misread thinking OP wanted to use propane. In any case, my question still stands. Is this the best way for extra capacity using a HPWH ? I don’t think increasing the temp/ mixing valve will be enough.
There are no big plug in units. There are larger 120V units which can be hard wired to a near by receptacle provided that it is a dedicated circuit.
In your setup, you won't need a recirc. You also have to be careful with HPWH and recirc as some setup can cause the heat pump to run too much.
Simplest is to plumb in series as you suggest and set the resistance tank temperature a bit bellow the setpoint of the HPWH. This way the resistance tank will only run to make up for standby loses or where there are very large draws, rest of the time the hotwater used will be refilled from the HPWH tank. Since it is mostly a buffer tank, the fact that it has only a small element and very slow recovery doesn't matter. If you do empty out both main and buffer tank, it will take a while though to re-heat everything.
Akos - thanks for the clarification. i suppose I could purchase a standard 5o ga HW tank and wire 120v to the 240v element. That will be equivalent to about 1,500w, I think. Would that be enough to take care of standby loss ?
So my setup would pretty simple. A 80 ga hybrid HPWH in series with a 50 ga HW heater acting as a storage tank. Set the storage tank below the HPHW set point. Maybe wrap the storage tank.
A 240 unit will use 1/4 the power at 120V, I can't see why it won't work but not sure about code requirements. You won't quite get 1500W, that would mean a 6000W 240 tank, but close enough not to matter.
Standby loss on a resistance tank is a small fraction of the 1500W, it won't have any issues at all.
Akos - again, thanks for your input on your response below. And thanks for the correction . Yes, 1/4.
You may find this interesting. I modeled a few scenarios and then chose to install with a 40gal HPWH in front of the indirect. Works very well—indirect hardly ever runs and recovery times are very good when we do wind up with a few simultaneous baths and dishwasher running at the same time.
Here’s the setup:
https://www.greenbuildingadvisor.com/article/hot-water-system-retrofit
But outside of heating season you have to either keep the boiler warm on standby, or have it fire up and warm the whole boiler for the rare occasion that it's needed for hot water.
If it's cooling season all of that waste heat then needs to be removed by the cooling system.
Agree that indirects are inefficient in summer. But heating pumps are more efficient in summer, so indirect will run less. If the indirect is needed, you just pulled a whole lot of hot water out (I have a 40 gallon HPWH and a 50 gallon indirect, so 90 gallons total), at which point you really want the catch-up capacity of the indirect. Regardless of the source (resistance elements, natural gas heater, indirect), any catch-up heat is going to be less efficient than a heat pump. And the bigger HPWH single tank solutions don't have any bigger of a heat pump, so upgrading that doesn't help you.
Ari - thanks for the info and link. Very interesting and great write up. I don’t have NG, so my choices are limited. I looked into a thermal storage tank with recirc, but that is more expensive and a little bit more complicated. A standard 40 or 50 gallon water heater seems to fit the bill.
Bummer. I guess you could do a two-stage electric the same way, or put two 40 gallon heat pumps in series or parallel, but I didn't model those scenarios.
The key insight is that, at least for the model I used, the 80 gallon HPWH tanks don't have any bigger of a heat pump on them than the 40 gallon, yet they're a lot more expensive. So might be worth installing two and then you get redundancy plus twice the heat pump capacity so more of your overall generation comes from the highly efficient source.
I’m not sure if I would get the output with two 40 gallon heat pumps. My soaking water tub takes about 100 gallons. Any hot water usage after a bath might not be too good.
I thought an 80 gallon heat pump is more efficient.