Im working on the design for a house Im building that will use a 2 ton Waterfurnace Heat pump for (the Envision NSW) does not have a hot water generator. I'm having some trouble keeping the system simple while being able to use 3 different sources of heat; Solar, Geo, Electric (as Back for both DHW and Space Heating) Right now I will be installing 4x SPP30 evac tube collectors. total of 1.4Therms a day. Also an electric 10kw boiler The house has a very low heat loss. 20,000 btu/hr at 0*F outdoor.

How are you heating the house? Radiant, Hot Air, or baseboard? I would think you would need to keep water temp below 125 to get decent efficiency from the geo. The solar will need to get very hot at times of lots of sunshine to get the most storage of the heat.

Do you have space for a two storage tanks? If so, maybe go with solar dedicated to tank 1, then Geo with electric back up on tank 2. The larger tank 1 is, the more heat you can store from the solar, but heat loss and cost go up. Use a mixing valve to temper water going from tank 1 to tank 2.

Sometimes integrating all thermal systems into a single mechanical system is more trouble than it's worth. A self-contained heat pump water heater would draw heat from the room at reasonable efficiency, heat that would have to be made up by the other systems in winter to maintain room temp, but that's a high efficiency heating system.

If your design-condition heat load is only 20KBTU/hr at a design temp of 0F it usually means your cooling season will be nearly as long as your heating season (even if your neighbors with the 40K heat loads @ 0F have an 8-9 month heating season), and a heat pump water heater would lower your space cooling load slightly.

If 0F is your design temp and 20KBTU/hr is the design load, it may be worth looking into a Daikin Altherma air-to-hydronic solution rather than GSHP, since it may be half the cost even after the geo subsidy and nearly the same efficiency (not that the absolute efficiency matters very much when the loads are that low). Altherma systems have ready-made DHW options too.

If it's a relatively open floor plan, mini-split or two might be an even cheaper option, as the backstop to your solar-thermal space heating, further simplifying the system integration problem.

Integrating hydronic heating systems with DHW directly only make sense when the water temps for the space heating are at or above DHW temperature. To get the geo to beat a mini-split or Altherma requires much lower temps. Solar space heating has similar efficiency issues- lower temp=higher efficiency, always. Depending on the subsidy picture, in a house with a equal heating/cooling seasons you may get more bang/buck out of grid-tied net-metered PV than solar thermal systems, if all thermal systems (integrated or separate) are heat pumps.

Posted By Dana1 on 15 Feb 2013 04:05 PM
Sometimes integrating all thermal systems into a single mechanical system is more trouble than it's worth. A self-contained heat pump water heater would draw heat from the room at reasonable efficiency, heat that would have to be made up by the other systems in winter to maintain room temp, but that's a high efficiency heating system.

If your design-condition heat load is only 20KBTU/hr at a design temp of 0F it usually means your cooling season will be nearly as long as your heating season (even if your neighbors with the 40K heat loads @ 0F have an 8-9 month heating season), and a heat pump water heater would lower your space cooling load slightly.

If 0F is your design temp and 20KBTU/hr is the design load, it may be worth looking into a Daikin Altherma air-to-hydronic solution rather than GSHP, since it may be half the cost even after the geo subsidy and nearly the same efficiency (not that the absolute efficiency matters very much when the loads are that low). Altherma systems have ready-made DHW options too.

If it's a relatively open floor plan, mini-split or two might be an even cheaper option, as the backstop to your solar-thermal space heating, further simplifying the system integration problem.

Integrating hydronic heating systems with DHW directly only make sense when the water temps for the space heating are at or above DHW temperature. To get the geo to beat a mini-split or Altherma requires much lower temps. Solar space heating has similar efficiency issues- lower temp=higher efficiency, always. Depending on the subsidy picture, in a house with a equal heating/cooling seasons you may get more bang/buck out of grid-tied net-metered PV than solar thermal systems, if all thermal systems (integrated or separate) are heat pumps.

I'm not quite sure what you mean by my cooling season is as long as my heating, Here in Colorado we only have about 4 months of cooling while we have 8 months of heating

I hear you on the fact that sometimes its more work than its woth. the cool part of this equation is that I get most every thing at cost, Heat pump, controls (PLC based), plumbing parts (pipe, pumps...) so it afford me the opportunity to build an advanced multi-load/multi-source system. I do understand that geo-hp's lose a good amount of efficiency above 130. My outdoor design temp of Zero, results in a target floor temp of 110*F. Ill be using Warmboard radiant for the heating system as well a microduct cooling system made up of atleast two Aircell modules from Spacepak. I drew up my ideas, it also allows me to push heat through the microduct system as well as the radiant during the cycles that the HRV are running to keep drafts down. I will be using PV to off set any extra power usage. I think I've come up with a way to inigrate all of them togeather. Take a look and tell me what you thing.

I have a small problem with the size of the solar system. If you are doing only DHW and you have a typical load for a family of 4, you want to size the system for about 90% summertime load and take what you can get in the winter. If you use glycol in your 4 panels, the chances are that you will be changing it every 2 years with an oversized system. The exception to this is if you use a drainback system but in my experience, you are way better off with flat panels than tubes in a drainback. They will last a lot longer and are not subject to the thermal stresses of the tubes when in stagnant conditions.

What kills tubes is not taking heat out of them on a regular basis. They can be very efficient if you can keep the max temp down to under 100C but if they are allowed to stay above 150C on a regular basis, the glycol will degrade much more quickly and the likelyhood of tube breakage will be quite high, possibly 5-10% of tubes could fail annually in these conditions. Most people don't notice it because it is not easy to see on a roof.

BTW, nice Solidworks or Inventor drawings...

Good eye! The thing I didn't put in the drawings is a heat dump loop, either an external fan coil mounted under the array (will be a ground mount) or a loop of buried pipe. I prefer ET vs flat plate when doing space heating as the ET perform much better in the supper cold of Colorado; weeks below freezing. Heck we even had a week here this year where it was below zero for a full week. I've done at least 50 of each and the data shows that flats just don't perform when needed. They are great for DHW but for space heating they don't. Also depending on the type of glycol used I've had it in systems that maintain a good PH level for 8 years. As long as care is taken to not let it over heat It lasts a long long time. Drain back are goods but to maximise the efficiency of the collectors while also maintaining long life the et's like you said need to be kept from overheating. in fact the warranty says no more than 30 days of dry run time.

Be sure the up front cost AND cost of operation of the solar hot water system is reasonable compared with reasonably projected savings.

I modified it a little. I realized I had a three loop system... kind of over kill

I added the heat dump.

Posted By engineer on 16 Feb 2013 12:16 PM
Be sure the up front cost AND cost of operation of the solar hot water system is reasonable compared with reasonably projected savings.

I get everything at cost. so its super cheap for me to put in large amount of PV and ST :)

Trying to see if there is a way to eliminate some of those pumps. They can really reduce the system COP.

I could remove some of them but I don't think they will kill the COP leaving them in. Also the redundancy part of having multiples is important to me.

The one I could remove would be the bypass on the solar loop. I could use the primary solar pump but there again if the main pump fails there is no way to keep the system from over heating. The balance of that is that the system will ONLY run the bypass if the temps get to high. with the size of the tank 400 gallons and an upper limit of 180*F it wont be running often.

I thought about putting the E-boiler in line with the HP but realized that during cooling mode condensation could become a problem on the boiler housing

I could use valves in some sports but the thing i dont like about valves is when they fale they fail in what ever spot they where left in brining down the entire system. Im not sure that the use of pumps really kills the COP being that not all pumps are running during all operations of the system. for example the DHW pump doesn't run while the HP is loading the buffer tank. The only thing it does is cost. but a good motorized valve cost more than a 15-58 so not really sure I would be saving anything.