Ground source heat pump design questions

landmobile

Four years ago I purchased a house and inherited a forced air heat pump with resistance backup. I don't have to tell you guys how hard it is to extract heat from winter outdoor air in Philadelphia, so I have been designing a replacement HVAC system since we moved in. I have decided on a hydronic system using as much radiant floor as possible. I have subfloor access to the entire house and plan to install heat transfer plates with PEX tubing. I also built a two story garage/workshop which is attached to the house by a mudroom, all of which have poured concrete floors that are insulated under the slab and have loops of imbedded PEX.



As much as I hate the air-air heat pump, I am fascinated with the idea of using the earth as a "thermal flywheel", and want to install a ground source water-water heat pump to run the hydronic system. It seems like an ideal match since the lower water temperature of the heat pump would be nicely suited to the lower temperature requirements of the radiant floors. I also plan on using the heat pump in the summer to provide chilled water to separate air handlers that will air condition the house in three zones. The air handlers will also be used in the winter as second stage heating to help the radiant floor on cold days. I will run all of the hydronic zones using primary-secondary pumping and design a control system to supply water to the zones/stages that call as required.



A heat loss/gain analysis program based on ACCA Manual J puts the heating load at 64000 BTUH and the cooling load at 45000. If I size the individual air handlers to the cooling load of each zone will that ensure that they run long enough to provide adequate dehumidification? I'm a little concerned because it seems like the heat pump will be oversized for cooling, especially if only one zone is in use. I have read about the problems with oversized heaters and air conditioners short cycling and want to avoid that before it starts. I plan to have a storage tank of heated or chilled water that the heat pump will maintain as a reservoir for the system as needed. Do you think that will be an acceptable way to load the heat pump, or should I consider a two stage system or perhaps separate heat pumps for heating and cooling? Also, are there any unusual piping requirements involved in a system that is used for heating and cooling?



I realize that this is a rather complicated residential design but I think I can make it work. I'm an electrical engineer so the control system shouldn't be too hard for me, but my practical experience in mechanical systems is somewhat limited. 



Thanks in advance for your thoughts,



Chris

Brad White

Cost it out.

I have yet to see geothermal be cost-effective in a residence especially with a deep bore well and without significant incentives/rebates. Even then, the system proposals I have seen still had payback periods within range of the useful life of the major components. Get a few quotes and cost it out against a condensing boiler for example, even just for heating.



For AC you have other options.



For commercial work the payback is better and yes, you still need rebates. The reason commercial projects work better is higher hours of use, more consistent demand, higher loads and balanced heating and cooling needs year-round with total energy use ranges to justify the capital expense. Most houses, especially insulated and row houses, have fairly modest loads. You have to spend to save.

landmobile

I'm a better engineer than a businessman . . .

Thanks for your reply about costing the design Brad.  I pretty much knew that I'd be dead before the system broke even, but I can't seem to get it out of my head.  I've been up at night comparing horizontal slinky and deep well heat exchangers, reading about direct exchange systems, and basically doing everything other than earning a living in my own business (which is radio communications).  Needless to say, my wife is thrilled

Brad White

I'm a better engineer than businessman too, FWIW...

But my practice requires that every facet have an ROI or at least a number the client can accept or reject. That is our currency and you know this very well too.



Our office recently designed a deep quarry cooling system for an office complex near Boston, which achieved LEED Platinum. Not the same thing as compressor-based refrigerant cooling, but rather, using a "constant within a degree" replenished source of chilled water 250 feet down in a 400 foot deep quarry, available for the cost of pumping it. Now it is February and we are in ice-making mode. Indeed.



My point here is, some thermal storage is easier to grab hold of than others.

I totally agree with you, the thinking never stops.



Put the capital you saved into a nice trip and memories for you and your beloved wife. The ROI is immediate

NRT_Rob

there are several

cold climate air source heat pumps, and there are a few air to water heat pumps out there as well. We work with the Daikin Altherma.



You still need low temp hydronics to make it shine, but it's a fair bit more cost effective than geo. Whether it's actually cost effective or not though depends a bit on whether you are heating and cooling with it or not. If it saves you a condensor/chiller, that helps quite a bit on the payback calcs.

TonyS

Similar system

I have a similar system in my home. High mass radiant downstairs and low mass on second floor. I have a 3 ton climatemaster on a open loop with a 40 gallon buffer tank.

You must use about 1 gallon per thousand btu buffer tank. Very inexpensive to operate!

There really isn't much to them. Just a heat pump with a water cooled condenser. I think they are a little overpriced for what they are but the payback is quick. My system is simple on the source side. I pump 4-1/2 gallons a minute through it and dump it in a stream. My water is 57 degrees all year round. On the load side I use a outdoor reset in the buffer tank and use 100 degree water most of the year. One word of advice would be to try to really isolate the sound. Compressors can be noisy inside. I don't use mine to cool, chilled water is a pain and that would require duct. I see enough duct in the course of a week to know I don't want any in my home. I use inverter mini splits for that.

landmobile

Thanks for the replies ...

 

A gallon per 1000 BTUH would work out to something like a 70 gallon tank for my residence, which seems right around where it should be.  Thanks for the tip on noise, TonyS, I will keep it in mind when locating the compressor.



NRT_Rob, I read your other post on monitoring the Daikin Altherma air-water COP, and I'm amazed.  How does that heat pump manage to work so well in such cold air? 

tim smith

Re daiking Altherma Geo etc

 I have been looking at the Daikin also, it intrigues be but also I am a bit skeptical on the longevity of the equipment. No founded reasons just a complicated package that has many parts. I have found water source heat pumps to last for many yrs, 20+ in commercial apps and I have still many  that are approaching 30 and still purring along. Water source hps are the same as geo hps except for the heat sink method. I do agree with Brad that when I have tried to make $$ sense of Geo units it has always been hard due to we are in City and vertical is the only way to get wells in.

landmobile

My property measures 100' X 325' Tim,

so I probably have room for a horizontal ground loop.  I could install that myself, which would be a big cost saving over deep wells, but my DIY trenching depth is limited to about 6 feet.  The earth temperature at that depth deviates about 11 degrees from the mean seasonally, so I would lose some of the benefits of constant temperature deep wells.  In any case, it still seems easier to extract heat from 45 degree water than 10 degree air.  



I have also read a little about direct exchange systems.  From what I understand they save the energy required to circulate ground loop fluid and the energy lost in the heat exchanger by putting the refrigerant tubing directly in the ground.  They also use buried copper, which has much better thermal conductivity than polyethelyne or polybutylene, and requires less acerage. Any thoughts or experiences?