GeoExchange and DHW Priority

Chap

Wondering if anyone has done DHW priority on a water to water geo-exchange system. I have an application where I have all in-floor radiant, a Tekmar TN4 system, and am wanting to create DHW off the Geo unit as well. There is no desuperheater on this unit, and this unit doesnt come with DHW as an option. I was going to control it just like a seperate cooling buffer tank with 3-way valves. Has anyone done this, and if so what should I be aware of? Is there a particular type of sidearm I should be looking for? I appreciate the input.
Chap

Unknown

geo dhw

I've seen several schemes to achieve this: Two are done much like a boiler using a second priority pump to a dual coil indirect with the coils piped in series to get an adequate amount of heat exchange surface area. This is ok for smaller heat pumps, but for larger applications you would need a heat exchanger sized to handle the full output from the heat pump, a pump from the heat pump to a heat exchanger, and a pump from the heat exchanger to a commercial storage tank. I haven't had the opportunity to try either yet, but these seem to be common practices.

There's some good info here: Viessmann Heat Pumps (see page 26).
and here: Next Energy Design Guide (page 12).

Mark Eatherton

Buffer/DHW preheat

Chap, you are going to have to have an auxillary DHW heater because the heat pump can only feasibly raise the water to around 110 degrees F, not hot enough for most people. We use the Thermomax 2000 for our combination buffer/DHW preheat tank, and pipe it in series on the potable side to an auxillary water heater. I just read an article that Dave Yates wrote on the cost of storing 140 degree F hot water versus 120 degree F hot water, and for the potential health impacts of exposing users to legionella, it is just not worth the $6.00 per year savings using natural gas.

Pre heat for sure, but final boost to 140 degrees is really necessary to kill off the bugs. Down forget to include antiscald valves at the point of generation and the point of use.

ME

Rich_46

that a great guide. do you know of any guides like that for solar. i have found that all the solar installs lately are producing more energy then the next one. what i would like to find is a system design that i would not end up dumping any energy.

BC_5

I'm doing a similar setup with my w-w heat pump. The output from the heat pump goes to a 3-way valve (high flow) that can switch between the buffer tank for heating/cooling or a plate heat exchanger that feeds a DHW preheat tank. I went with a seperate heat exchanger and circ on the DHW side because I was concerned that a traditional indirect wouldn't be able to handle the full output of the heat pump at low temp differentials. Most indirects are spec'd at 180F or higher boiler water and I wasn't confident in estimating the heat transfer capability with 120F "boiler" water. With my oversized plate heat exchanger I expect to get closer approach temps so I can maximise the percentage of heating done with the geo.

I don't have the DHW running yet, so I'll report back when it is and let you know how it works.

I agree with ME about output temps - I expect to preheat with the geo and then top it off to 140F with an electric heater. I don't have the numbers in front of me, but the geo ends up doing 80% of the work or so.

There are a very few hydronic heat pumps out there that can heat water to 140F, but most seem to be limited to 115-120. With mine, even at 120F water out my compressor discharge temperatures are in the 230F range which isn't great for long term durability according to Copeland.

Chap

Good Info

Thanks for the good info. Good to hear from you Mark, hope things are well.

Chap

Chap

Boost Temp

How about boosting the additional heat in the buffer tank with an electric tankless hot water heater? Taking the temp from 110 to 140 or so. Are there any disadvantages to this approach as opposed to another 40-50 gallon electric water heater in series?

schiller

climate master makes a add on desuperheater, I used it a couple of time when the disitributer was out of units w/ it factory installed. It looks like you could add it to a standard heat pump or condenser but i havent worked out how to deal w/ the potable water being exposed to the out door temp.

jp_2

sounds good to me

if you only need 20F- 30F temp rise, a small on demand electric should work dandy.

need to look at the rate of hot water you need.