DIY Geothermal Kits
Last Post 19 Oct 2015 01:05 AM by docjenser. 43 Replies.
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chrsUser is Offline
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15 Oct 2015 08:37 AM
It's certainly true that if you want a heating system to respond quickly to a change in setpoint, you need extra capacity if the delivery is through heating a high-mass slab. The idea that a high-thermal-mass wall leads to a requirement for extra capacity is exactly the opposite of conventional thinking. The conventional thinking is that the thermal mass of the wall helps carry you through a cold spell or a cloudy day without as much help from the heating system, which can simply coast along with the slab kept at the same temperature, a little above the setpoint. The only scenarios where I can see the thermal mass of the wall requiring higher capacity in the heating system would be if you were trying to warm up the house from a 50 F unoccupied setpoint, or adjusting the setpoint for some other reason and wanting fast response.

If you want to oversize for fast response on arrival to a house with a set-back thermostat, or if you want to oversize because of anticipated air leaks, that could make sense, but if you want to claim that the thermal mass makes things worse for daily variations in weather conditions, that's an extraordinary, claim, contrary to the benefits normally claimed for thermal mass, and requires some evidence.

And although the response time from a setback is a real issue, for an extra $9k, you can buy a thermostat that connects to your smart phone so you can start the house warming up a day or two before you arrive, and have at least $8k left over.

Aside from the question of whether the $9k is well spent, putting in loop capacity sized for 5 tons and running a 10 ton heat pump can mean that the short-term temperature drop in the loop is more it would be with the 5 ton unit, thus lowering the COP, even though the long-term drop is not an issue, as you explain. And it also requires more care to ensure that the pumping power is not excessive, although it can certainly be done, and that would not be a big concern if you have a skilled, educated designer. At the other end, you might need a big buffer tank to prevent short cycling. A slab can help, but if only one zone is attached to a slab, that doesn't really solve the problem. Perhaps the $9k includes a big buffer tank?

By the way, I've been wondering why buffer tanks are so expensive, and I think a big part of it is that the one I know about are rated for about 250 F and 150 psi, neither of which is at all necessary in a GSHP system. Are there any sources for insulated tanks rated for 25 psi and 150 F? It seems like if you want something rated for less than a boiler tank, you might need to go all the way to an open, non-pressurized thank.
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15 Oct 2015 04:37 PM
chrs: You need to differentiate between thermal mass as passive living space storage and thermal mass in an active radiator. The latter is always a bad thing (although how bad depends, it can work OK). The former can go either way, but is typically beneficial in a modern residence (setback being a counter example).

Another point is that as houses get better insulated, the cost and discomfort of anticipating a cool-down and pre-heating a high mass radiant floor goes down. For example, if you pre-heat a floor to 75F before any heat is needed, it will never overheat the space to more than 75F. Yet 75F might supply plenty of heat later with the building never falling below 70F. Sub-optimal overshoot, but not terrible.

Hydronic systems work well with water storage - another way to address high short-term btu requirements.

Summary: don't use a high mass radiant floor with simplistic controls.
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15 Oct 2015 04:39 PM
Posted By docjenser on 15 Oct 2015 02:29 AM
Posted By Dana1 on 14 Oct 2015 10:47 AM
Yes, it's a log home, as decribed: " ...these logs will be 20" plus 20" logs. " That's not a thermal bridge.

With 20" logs there both R-value well in excess of code-min wall performance just on raw insulation value, and a very significant thermal-mass effect. It should beat minimal ICF wall performance if air tightness can be maintained. Air tightness is the Achilles heel of log construction more so than raw R-value at 12" or fatter thickness. But 3ACH/50 air tightness is achievalbe.

Doubling the size of a heating system for DHW loads is a pretty silly approach compared to buffering domestic hot water in tanks. A heat pump that can deliver 30KBTU/hr at DHW temps would have faster recovery times than a typical electric HW heater. A heat pump water heater drawing it's heat from the room air would be able to utilize the thermal mass of the house, and it wouldn't increase the necessary size of the GSHP by more than a tiny fraction.

Do you think anybody can design & implement a GSHP system that delivers his presumed 60-80KBTU/hr for less than $40-50K and still make a living? (I'm not even talking the more expensive southern New England pricing here, since it's not in southern New England.)

If the real heat load is something like half his presumptive loads, the cost won't be half of $40-50K, but it'll still be well worth paying for the engineering analysis to come up with the real number, and sized the GSHP system accordingly.





The main issue is the drying and shrinking wood, causing air infiltration over time, and the high thermal mass on days without solar gain. Lesser of an issue with a forced air system, bigger issue with a slow responding radiant floor.The doubling is not for DHW, it is the extra capacity needed to overcome the thermal mass of the floors and the house, in order to put some heat into the house. The DHW need is small compared to that load. It is not silly, it is real. It will take hours to respond to a heat call, unless you have extra capacity.
Once you have the full hot water generating capacity of a hydronic heat pump, why putting in an air sourced heap pump within the thermal envelope?
Again, a 10 ton system does not have to be significantly more expensive, since only the heatpump size is different, taking the same time to bold in.
What is the alternative? Most w-w energy star rated HPs are a max of 5 tons, making about 50 kbtus/h, the next energy star units are 7 or 10 tons I am aware of, the 7 ton being a high temp, costing as much as the 10 ton. We install them for $33K without DHW, and $37 with DHW (no A/C, not including the floors). It is about $9K more than a 5 ton.
Again, the system needs only higher peak capacity, the rest of the system, including the loop field, can be much smaller.

Want a project on Martha's Vineyard, MA?

A friend of mine there was recently quoted $38K for a ducted propane fired heating with a cooling coil in the air handler and a 3 ton AC.

The alternate proposal was $40K for a 4 ton GreenSpeed, which is both oversized and overpriced.

That's for a ~ 36-38K heat load and 17K cooling load (calculated per Manual-J by an independent contractor after I told her those proposals were crazy both on capacity and price.)

I'm in the rong biz, I guess...
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19 Oct 2015 01:05 AM
You certainly need to differentiate between high mass radiant floors, which generally is bad, and high mass buildings, which I would argue is very good.



Lets say the temps drop outside considerably after a sunny day, the floors have not turned on due to the solar gain with a glass wall and southern exposure, even on a very cold day. Now the solar gain is gone, and the house cools quicker due to the entire wall being close to an R-value of 3. Wind is usually coming from the SW. Now the temps drop down to the 0.5F below setpoint usually after a few hours past sunset, and turn on the large zone. For the next few hours the heatpump must first heat up the floors, satisfy the heat loss of a cold evening/night, will pause heating the floors since the DHW is calling for it for 30-60 min, because someone turned on the dishwasher after dinner.

So if you don't have those extra 20-30kbtus to heat up the floors quickly to transfer some heat in those 14ft high spaces, your house will get quite uncomfortable just about every night between the end of December and the beginning of March when the sun is shining during the day.
How do I know? I own one of those houses which broadly fit this description. When the sun is not shining during the day, conversely you do not need the extra capacity since the floors are already somewhat warm.



The oversized heatpump will not lower the COP, since the loop has heated up during the time of no heat call, so it starts warmer. It will not drop very much below 30F either since now you are making ice which means the moisture in the surrounding ground will go through a phase change, and temporarily release significant amount of energy, which will be the source for some peak capacity in the loop field.
www.buffalogeothermalheating.com
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