I am finishing up my Geothermal system and was told by my Geo contractor that an electric water heater is much more efficient to use with the Desuperheater.  I understand the concept of a preheat tank but I was told it wouldn't work as well with a Gas water heater, which I had planned on using.  So here are my two questions: Without a preheat tank, which water heater works better (gas or electric) with a Desuperheater and why?  If I am going to use a preheat tank, which primary tank is better to use gas or electric?

There's considerable material on this subject in the archives.

Summary: without a preheat tank, a DSH will reduce a GSHP's performance if the HWH is gas - an electric HWH is required.  With a preheat tank, it doesn't matter what the HWH is - gas or electric.

Best regards,

Bill

Thanks Bill.

I am new to the site and I am finding lots of answers to questions I never even thought to ask. There was alot of disscussion between my plumber and I on that, and I had not found a suitable answer yet.

Among many contributors contributing nicely on this subject, lLook for comments from 'engineer' - his comments are worded in an exceptionally clear manner, and he covered this subject within the last month.

Best regards,

Bill

One thing we didn't clarify is what kind of gas. In my area natural gas is still less expensive than an electric water heater. Also imporant is demand. An empty nest couple with natural gas available recieve very little benefit from a HWG.
Regardless of other conditions, without a preheat tank you are chasing your tail.
J

I second what Bill and Joe have written.

Are standby losses greater with a gas WH vs electric due to the vent? Perhaps the reasoning was that the water heater will run infrequently with the dsh and pretank so the standby loss difference between the gas and electric wh becomes more significant than the cost of gas or electricity.

Just to add my own documented experience with using a gas hot water tank without a preheat tank together with a desuperheater. It worked so well during our first winter that the gas company thought I was fiddling the meter and called to make an inspection. Our gas consumption declined as the weather got colder (due to the heatpump running more) to the point that, in the coldest months of winter, we use around $1.70 (that's one dollar seventy cents Canadian) of gas per month for hot water and our gas stove. We do have a gas fireplace insert but don't use it at present. In the spring and fall gas consumption rises to around Can$20 per month and declines somewhat in the cooling season. All these figures don't include the meter charge. Actual lowest bills in Jan/Feb are around Can$13 per month and the highest in May are about $35. So to those who say it can't work without a preheat tank, my experience is to the contrary. Of course, it might work better with a preheat tank, but it's hard to see how much more I could save if the gas bill has already declined to less than two dollars. At a couple of hundred bucks for the preheat tank + install, it would take decades to pay back. Your own mileage will of course vary as we're only a family of two and therefore don't use a vast amount of hot water. What I do notice, though, is that in winter the temperature of the hot water is noticeably hotter since the thermostat on the gas tank is at 125F and I think the DSH puts out water at 135F.

Paul.

I don't see where electric vs. gas should make any difference in terms of needing a preheat tank. Shouldn't it come down to the temperature the tank thermostat is set at? I could see where electric tanks can be controlled more tightly since the element can cycle on and off as often as needed, but otherwise, why would a natural gas tank heater be any different than an electric one in terms of what the DSH sees? I see a lot of DSHs plumbed with a special tee in the bottom of the tank - is there some reason that natural gas heaters tend to have hotter water on the bottom of the tank? I can't think of one. Maybe if Bill turned the thermostat on his gas tank down it would work a lot better!

The biggest difference I know of between electric and gas water heaters is that electric water heaters are foam insulated, completely around the tank. Gas water heaters tend to be fiberglass insulated, and cannot be insulated on the bottom of the tank.

With a foam insulated tank, you're far better off using the electric tank.

Why would the tanks need insulation on the bottom? The water is cold there and, if anything, will pick up heat from the room it's installed in. One thing I did find is that the longer the warranty, the better the insulation tends to be - though if your tank is in conditioned space the insulation is more useful in the summer than in the winter, where any heatloss from the tank just adds to the incidental gains of the building.

Paul.

I agree.  In fact, last Winter I did this very thing.  Problem was that in order to get reasonable value from the DSH I had to turn down the gas HWH's tstat so far that it prevented still being able to enjoy a pretty hot shower.  In heat mode, the DSH just doesn't contribute enough additional heat unless there's a pretty good difference between the the HWH's water temp and the water circulating through the DSH.

The situation is particularly challenging with R410A GSHP systems, like mine.  My compressor's gas discharge temp rarely gets past 125°F, and in fact takes a while to get to this temp.  At least with R22 systems the discharge temp get higher on order of 150°. 

What's worse is in cooling mode, where the direct-connected gas HWH's hot water can be hotter than the compressor's gas discharge temp, thereby increasing, not decreasing the refrigerant gas's temp before it makes it's way to the TXV and evaporator.  And thus making the GSHP less, not more efficient (slightly warmer LAT).

Again, the situation is particularly challenging with R410A systems.

So in either mode (heating/cooling), it's a marginal at best situation.

I agree that:
if you're in an extreme heating climate where the compressor runs long and hard each day in heat mode and where little cooling ever is needed,
if you're using an R22 system,
and if you don't have a lot of HW demand,
then you can get a direct-connected DSH to a gas-fired HWH to provide benefit.

But, the benefit would be significantly greater with a pre-heat tank.  And, the benefit significantly goes away with R410A systems and/or with a more balanced heat/cooling mode.  And, the benefit turns into a cost with the combination of R410A and running the compressor a lot in cooling mode.

There's a lot of detail on this subject in the archives here.

Best regards,

Bill

Yes, I remember now - I've got too much going on! Interesting comment about R410A vs R22 - I know that 410A has a much higher saturation pressure for a given temp, but I think that would tend to increase discharge temp rather than reduce it. In general R410A compressors are rated for a lower condensing temperature than similar R22 compressors, but maybe that is strictly due to the pressure difference. I know it's apples and oranges, but my R410A w-w heat pump had discharge temps over 220F last winter.

Where are you measuring the discharge temp? Is the sensor insulated?

I am also confused about the R410 discharge temps. Maybe it’s because we have a water to water system I don't see this. We have an electric (not connected) 40 gallon pre heat tank for the DSH and a 50 gallon natural gas tank second for the house. Our DSH tank will max out my thermometer at 160F in really long runs with little hot water usage. In my case this is ok because the main 50 gallon tank buffers the incoming 160F water with its 115F water. As you take a shower you slowly have to turn it colder :)

Is this because I have a water to water setup that I get higher DSH temps?

I think that everyone here gets the difference between heating and cooling dominated climates. I just heard today from an IGSHPA instructor that desuperheaters in MI are not worthwhile because of the quality of our water..... Usage matters, water quality matters,maintenance matters, fuel costs matter..........I think we have covered why preheat tanks often have an edge, but every rule does have exceptions. That's why we're all here. I for one appreciate the experience of P-Paul as I have a few pre-tankless installs out there and worried that I had done a dis-service to my clients.
J

I'm in the process of turning back on my DSH that's connected directly to a gas-fired HWH, along with creating some meaningful charts to illustrate the challenges of not having a pre-heat tank.  Now's a good time to do this because, for my climate, we're still running in cooling mode, but it won't be long where a little bit of heating will kick in.

We'll learn from data and fact.

I'll report back here when I can show obvious conclusion.  Look at http://welserver.com/WEL0043/ for the work in progress (already in process of changing my charts that show measurement of R410A gas discharge temp).

Best regards,

Bill

I wonder if the observed difference in discharge temps between 22 and 410 originates from the fact that 410a is used in newer systems. Newer systems may offer less recoverable superheat for 3 reasons:

1) Newer systems often employ a TXV to help achieve higher efficiency. The TXV controls evaporator superheat. That controlled superheat may be a lower value than the superheat associated with older fixed orifice systems.

2) Newer, more efficient equipment also uses large and / or more efficient condensers, reducing refrigerant saturation pressure and temperature, thus the degrees of superheat are added to a lower base.

3) Newer systems use more efficient (scroll vs reciprocating) compressors, further reducing system superheat and thus recoverable heat.

As for pp99's good results with reducing gas consumption for hot water without a preheat tank, I attribute that to low hot water demand (couple vs family) and lengthy heating season (more recoverable superheat in heating mode vs cooling mode). The economics of cheaper gas, short cooling season, and lower household demand for hot water may make a preheat tank unjustifiable.

Houses without relatively cheap natural gas, but with longer cooling seasons and more occupants still likely benefit from a preheat tank.

A difference between gas fired and electric water heaters is that with gas (or propane) heat is applied to the very bottom of the tank instead of part way (lower electric element) or most of the way (upper element) up. A gas storage water heater will exhibit less or no thermal stratification within the tank. Thermal stratification within a tank lets cooler water from the bottom be fed into a desuperheater and warmer water from the desuper be fed back.

The situation we need to avoid is feeding hot water from a non-stratified gas fired water heater into a desuperheater while a system is in cooling mode. It is possible, indeed probable in some circumstances, to flow heat the wrong way, that is from the water heater to the hvac system. Both appliances suffer and operating costs rise.

This is the case for me - in cooling mode heat to/from the R410A refrigerant in the DSH will flow in the wrong direction unless my gas-fired HWH's tstat is set exceptionally low.  We'll see it in the numbers.

Best regards,

Bill

Here's some preliminary data:

Water-to-air R410A GSHP, direct connected to gas-fired HWH.

Lines:

• Red = hot water measured immediately after exiting HWH
• Violet = compressor R410A refrigerant measured immediately after exiting compressor
• Green = hot water from HWH into DSH, measured at GSHP entrance
• Orage = hot water from DSH to HWH, measured at GSHP exit
• Blue = DSH (GSHP) on/off

More data is at http://welserver.com/WEL0043/ .

I'll need to see some more cycles before I'm ready to offer observations / conclusions.

Let me know of any mods/improvements to the chart you'd like to see.

Best regards,

Bill

That warm green line (and somewhat cooler orange line) suggests to me continuous loss of heat from HWH into DSH (and thus GSHP) whenever GSHP not running.

Why is green always relatively warm even when GSHP off? Is DSH circ pump running continuously? That would not be good. Cause could also be natural convection.