[Mobile Master Tech]

Daox, that sounds about right to me. You will have more to worry about with bowing of the supports than the burst strength of the plywood. Your tank isn't too big so you won't have that much bowing. Have you thought about going bigger vertically instead of horizontally to gain stratification?

Strider,
This will be for DHW and all space heating combined. I installed an open loop radiant hydronic floor heating system (details on the radiant heat thread) for comfort, quietness, efficiency, resale value and the ability to use a multitude of heat sources.

I have now had the system in for over a year so I have details on energy usage. My family of 5 used 805 therms of natural gas in 12 months, with 150 therms peak usage in Jan. Hot water (and cooking!) used only 7-15 therms each month during the summer-not enough demand to be worth the bother and expense to size solar for, and only a drop in the bucket compared to the needs during the winter months when you have the least solar available. This is why I say having a large ability to store useful amounts of energy when you can get it so it is available when you need it is important.

Our hot water is from an Eternal hybrid gas heater. It's 98% efficient most times but more like 90% when return water temps are hotter because the floors are on a long time. Taking 10% off of 150 therms for efficiency losses, allowing a 20% reduction in demand from further improvements in the building envelope and adding a 10% margin because last year was milder than usual, I feel that 120 therms(12 million BTU), is the January usage I should size for. That works out to about 400,000 btu/day. However, mother nature sends us several days in a row where weather is harsher causing demand to be greatest while availability from ASHP and solar is at its' lowest. That 2300 gallon tank seems huge until you realize that the 1.6 million useable BTU it can store between 110F and 195F is 4 days of average January demand at 100% fraction.

This brings in the next benefit. Having that big 4 day reserve evens out demand so each heat source can be sized for average demand instead of peak demand, reducing cost due to downsizing & improving efficiency due to infrequent cycling. I plan on having 4 heat sources: a 1 ton (12,000 BTU) hacked groundsource heat pump (thanks to AC's inspiration on the GSHP thread) 2 solar collectors with 30 evacuated tubes each, a desuperheater on my 22 SEER inverter air conditioner, and a small electric water heater to make up the last few degrees in case the hot water output drops below 130F.


Those solar collectors together will probably output 30,000 btu/day for a January total of 900,000 BTU. This is assuming 70% of the time they will operate under the SRCC's D-mildy cloudy rating and 30% of the time under C-Clear. Decades worth of government observations show that the average cloud cover in NE Georgia is 63-65% from Dec to Mar and the percent of totally clear days is only 14-15%.

The GSHP will be switched on by the solar controller whenever the top of tank temp gets below 135F. If it needs to run 24/7 it will give around 288,000 btu/day, for 8,640,000 btu/mo. Combined, solar and GSHP can give 318,000 BTU/day. This might not even be half of the demand on peak cold and cloudy days. But with a total of 9,540,000 BTU/mo to average over the peaks and valleys, that's an 80% alternative heat fraction under the harshest conditions all by themselves, thanks to the peak-shaving storage capacity of the large tank.

An AC desuperheater produces around 2-3000 BTU/hr per ton of air conditioning. Since we have the downstairs A/C dismantled for basement renovations, the "1.75 ton" upstairs air conditioner runs about 20 hrs a day when it is about 90F. It has gone over 90 several times already this year, so our A/C load is at least 35 “ton-hours” per day for at least 4 months out of the year. I therefore expect to get over 100,000 BTU/day from the desuperheater. The solar collectors will also be putting out around 90,000 btu/day during the summer. In both cases, they will be blasting out the heat during the day and almost nothing at night.

I am going to retrofit ground loops under my basement so I can inject that heat, timed by depth to return about 6 months after it was put in (I will start a thread with details on that one). Hopefully this will raise the average ground temp under the entire house from 62F to 72-73F, hot enough to add heat in winter while still being lower than the desired temp in summer. There will be a hotter core centered around 15’ below the slab that warmed during the summer. That heat will make its way to the surface during the winter. Because you have to stay at least 15' from the edges of the foundation to make sure your heat doesn't leak out the sides and you can only go so deep without groundwater carrying it away, there is a limit to how fast you can put it in the ground. Again, the big tank lets the underslab exchangers hum away, 24/7 if needed, while smoothing out the temp swings.

Since our Summer demand is approx 1.2 million btu/mo, and we’re getting 5.7 million btu/mo at least 4 months out of the year, that leaves a surplus of at least 4.5 million btu/mo. If we get even half that energy back either directly through the basement slab adding heat to the house while reducing demand (the basement will now be a net-positive heat source instead of a net-negative load sitting in a 62F heat sink) or by increasing the efficiency of the GSHP field surrounding the house, we now are capable of greater than 100% heat fraction for the whole year except Jan, and it will probably be darn near 100% then-all because of a large tank allowing multiple smaller systems to work together!

There’s not a huge difference in the component count/cost and effort required for a system that can provide 100% of your needs vs one that is noticeably smaller, especially weighed against the actual savings. I could build a basic solar DHW system that can cover about $120/year of my energy expenses before being oversized for peak supply periods. Instead, I could go to a little more trouble so I can drop my gas company and their monthly minimum charges while covering almost $2000/yr of my current expenses and become more self reliant. Big tank + little system-bad idea! But if you build a system big enough to be useful, greater storage capacity is important. I will be doing some more research on phase change materials to get more storage density in a given tank also.

Craig
The MMT