Heat Pump DeSuperHeater (DSH) and hot water production

[Jerry_NJ]

I have discussed here a few months back my hopes and experience with the DSH generation of hot water.

My experience started almost 22 years ago with a WaterFurnace two speed geothermal HP, it produced little to no hot water due in part to a poor design on how the water was circulated between the hot water heater and the HP... through a combined in/out fitting in the drain of the hot water heater. I replumbed the loop to the modern arrangement of out from the cold water inlet to in via the drain on the water heater. Didn't see much improvement. Perhaps Waterfurnace has improved the DSH design.

When I replaced the old WF with a 5 Series two speed, again 4 ton, I spent some time here and elsewhere on the value of the DSH to generate hot water in a home that uses resistive heating elements to heat water.

I decided to go again and when the new HP was installed in November the weather was already cold enough in NJ to require heating, days in the 40 or less and nights sometime below 30 degrees, not real cold, but HP operation was significant (don't have a number, but will estimate perhaps 40% run time) I found I was able to turn the hot water heater off, a new 80 gallon AO Smith, and obtain enough 125 degree hot water to supply my household of two seniors, light on showers, cloths and dish washer operation.

Then in spring I had to turn the hot water tank back on as there was little or no heating or cooling needed. Then as summer progressed I expected to get "free hot water (in heating mode I am taking heat that could be used for my house heating) but so far not so much... nothing like when heating.

i don't have a run time clock other than my electric usage, which I'll look at carefully for the month of June. My WaterFurnace computer run predicted no need for resistive hot water when in the cooling mode when temperatures are in the 80s, they have been. I just turned the hot water heater off yesterday and I already notice a lowering of the temperature of the hot water. Temps may reach 90 today.

I post this here for two reasons:
1) Share my experience, with the knowledge a given time run in cooling produces less hot water than the same run time in cooling. This seems nuts to me but it is what I got on a geothermal heat pump forum.
2) To learn from others if they have the same experience.

No, I do not have the two tank setup, but I am comparing the same setup when in heating and in cooling modes. So far I am disappointed at the low amount of hot water produced when my HP is trying to get rid of heat, not use it. But, the very effective hot water production when heating may be sufficient to justify the extra expense of having the DSH included in the installation.

 

[sesmith]

1) Share my experience, with the knowledge a given time run in cooling produces less hot water than the same run time in cooling. This seems nuts to me but it is what I got on a geothermal heat pump forum.
2) To learn from others if they have the same experience.

No, I do not have the two tank setup, but I am comparing the same setup when in heating and in cooling modes. So far I am disappointed at the low amount of hot water produced when my HP is trying to get rid of heat, not use it. But, the very effective hot water production when heating may be sufficient to justify the extra expense of having the DSH included in the installation.

My system works similarly. In winter, you make use of some waste heat plus you heat water with the heat pump at the COP it is operating at (and cutting into a little of its heating capacity for room heat). In AC mode, you are only making use of waste heat from the compressor.

In the winter, my DSH supplies almost all my hot water (at a very efficient COP). In summer, it's a tempering tank, at best. We do have a 2 tank system, which is the recommended way to install a DSH for maximum efficiency (I have read that you can kill its efficiency using only a single tank). My personal opinion is that overall it is worth it, on a yearly basis, to pay for the DSH option. The payback seemed to be there on paper, at least. If I remember correctly, we expected about a 60% contribution from the DSH for water heating on a yearly basis.

 

[woodgeek]

Interesting. My HPWH in my semi-conditioned tuck under garage prob does COP 2.5 in the summer months, but more like 1.5 in the winter.

 

[Jerry_NJ]

Thanks, I feel better and I have now looked at the electric power consumption as recorded by my electric service provider. Still, in the background I got from a geothermal forum that has several contractors and suppliers responding to posts: I don't have the technical reason given in memory, but I do recall being told the cooling cycle 1) runs less, and 2) produces fewer transfer BTUs via the DSH. This latter point may not be true, but looking at the power record I extract the following.

June 2014 500 KWH total which includes all hot water from resistive heating. June 2015 (est) 700 KWH again with hot water from resistive heat, no heat pump run time.

From this I take that when the heat pump is not running my home uses about 600 KWH The only other months this low are October and March
August came in at about 800 KWH.

November last year, first month running with hot water heater turned off, and April this year consumption was1000 KWH. with the high use month being February at 2,000 KWH All my hot water November through April came from the HP.

Hum, clearly I am seeing so far a much lower HP run time in June (really mid May to mid June) perhaps I can associate at least 500 KWH to additional run time HP (remember the hot water tank is off) November-Aprio, and my HP runs most of the time in Stage I, about 2KW. This easily translates to 250 hours at 2KW for HP, or about 8 hours per day generating hot water via the DSH over the run time for June air conditioning. From history my high summer cooling run times are August and September and they are far lower than even November, my lower heating season usage. Thus, no surprise the cooling isn't producing the amount of hot water I have gotten from heating.

My 5 Series is the current model WaterFurnace, the 7 Series is different only in the variable speed compressor. The 5 Series is a soft switch 2 speed. The COP is over 4 and the EER over 20, don't have the numbers handy.

No doubt the hot water produced saves KWH not used by the water tank and can be used at a high efficiency to feed the HP.