If I do away with the fireplace, I can drop 8250 btu/h

What is it - a big masonry one extending through the roof? There have to be some better designs.

What is it - a big masonry one extending through the roof? There have to be some better designs

Yep. You guessed it. It was one of the items on the dream house list. I guess I never considered the fact that it makes for a giant heat sink with most of the surface area being in the house. The only consideration I made was that the damper would be closed when not in use, and using a rumford design, we would put more heat in the house than up the flue when it's burning. Possible impending design change here.

The masonry can't stop at the roof and just have a metal (preferably stainless steel) chimney continue? That would be minimal thermal bridging and still look nice inside. I also wouldn't be surprised if someone had a design where the masonry could continue after a thermal break.

Possible impending design change here.

Sealed and certified fireplace with exterior combustion air feed.

The masonry can't stop at the roof and just have a metal (stainless steel?) chimney continue?

I stopped the masonry at supporting the second floor and went on from there with a wood framed chase and vent pipe.

Posted By danreed76 on 01 May 2013 08:44 PM
On another note... is going by the heating size in selecting the equipment tonnage going to grossly oversize the system for cooling?

No

Posted By danreed76 on 01 May 2013 08:44 PM
On another note... is going by the heating size in selecting the equipment tonnage going to grossly oversize the system for cooling?

Second "No". the air handlers should be sized for cooling for the zone, for their runtime, and they reject as much BTUs out of the buffer tank as they need.

As Doc suggests, a properly sized buffer tank prevents short cycling and picking the right fan coil design, air flow, reheat options and outdoor reset algorithm prevents issues with lack of dehumidification. End result - no oversizing problem (other than upfront cost) no matter what the size.

Posted By joe.ami on 02 May 2013 08:45 AM

Posted By danreed76 on 01 May 2013 08:44 PM
On another note... is going by the heating size in selecting the equipment tonnage going to grossly oversize the system for cooling?

No

In Michigan, no, but in Hotlanta, maybe.

Per your calc latent load exceeds 20% of total cooling load. Two concerns arise from being oversized for cooling:

 1) Adequate dehumidification - In a nutshell, tonnage cools a house, but runtime dries it. A five ton system, two stage, develops nearly 3.5 nominal cooling tons in low stage. Proper dehumidification requires that the system run long enough for the indoor coil to become fully chilled so as to condense water out of air. An oversized system often does not run long enough for that to occur, especially at night, during mild, cloudy or rainy weather. If you addressed elements of your load so as to pull latent load down to 15 % of total load, I'd be less concerned

2) Efficiency - All the number ratings we have come to know, trust and love...SEER, HSPF, EER, COP, etc are only achieved in near-lab conditions and only after the system has been running long enough to get to steady state conditions. An oversized system will rarely run long enough to attain anything near rated efficiency.

I highly recommend seeking ways to reduce load such that a 4 ton system becomes a good fit.

Sounds like you are applying ducted system ideas to a hydronic system. The size, SHR and runtime of the fan coil units are completely independent of the size of the HP.

Even perfectly sized ducted systems cycle on all days less than design days (ie, almost always). But with hydronic, a sufficiently large buffer tank can make the HP run time as long as desired - independent of the weather.

Makes sense. The buffer tank essentially becomes a heat sink, and is only going to take what you deliver to it. In that case, could the EWT of the hydronic load be adjusted (increased or decreased within reason) in relation to the relative humidity to allow for shorter / longer run times depending on the actual load (sensible vs. latent)? Is there a controller that will allow for that?

A buffer tank will also provide extra capacity for the hottest (summer) or coolest (winter) hours of the day. For example, 500 gallons can provide an extra ton for 4 hours. This allows a reduction in HP size. Very large tanks allow the use of off-peak power.

Posted By danreed76 on 03 May 2013 07:45 AM
Makes sense. The buffer tank essentially becomes a heat sink, and is only going to take what you deliver to it. In that case, could the EWT of the hydronic load be adjusted (increased or decreased within reason) in relation to the relative humidity to allow for shorter / longer run times depending on the actual load (sensible vs. latent)? Is there a controller that will allow for that?

Now that is neat idea.  A kind of an indoor dew point reset controller.  I don't know if anyone is making such a device for hydronic homes. 

I guess the same effect is achieved on non hydronic ducted air units by slowing the fan down on high humidity.

For example, 500 gallons can provide an extra ton for 4 hours

That's a lot of buffer tank. Maybe an insulated underground tank?

8'x2'x4' which could fit in most utility rooms and cost maybe ~1/10 of what a ton of geo costs. For off-peak, an underground cistern makes sense.

Now that is neat idea.  A kind of an indoor dew point reset controller.  I don't know if anyone is making such a device for hydronic homes. 

I don't know of one either, but if you want something that uses indoor and outdoor temperatures and humidities to control buffer tank temperature, let me know and I'll make one for you.

Humidity-reset control based on outdoor dew points would be irrelevant, since indoor humidity has many other sources than mere ventilation air.

Surprisingly little, but it doesn't matter. A good control system also senses interior conditions and can adjust buffer tank temp for internal latent and sensible sources.

Posted By jonr on 03 May 2013 09:12 AM
8'x2'x4' which could fit in most utility rooms and cost ~1/10 of what a ton of geo costs. For off-peak, an underground cistern makes sense.

I did plan a rather large mechanical room (i figured it was better to plan a clean space where equipment would be serviceable than to shove it in a broom closet or pantry, or maybe under the stairwell like so many) Should be plenty of room for something of that nature.

Posted By jonr on 03 May 2013 08:26 AM
A buffer tank will also provide extra capacity for the hottest (summer) or coolest (winter) hours of the day. For example, 500 gallons can provide an extra ton for 4 hours. This allows a reduction in HP size. Very large tanks allow the use of off-peak power.

Not a good idea. A buffer tank is different from a thermal storage tank. There is a reason why manufacturers recommend 10 gallons per ton. 500 gallons will significantly reduce your response time.

Lets say temperatures start to drop close to the design temps. Now the system does not get the supply temperature up since you have 450 gallons more in thermal mass in the radiant system. Now your house temps will drop below thermostat settings. Now you will never get up to the 44 kbtuh you need to heat your house to design temps since you made the HP smaller.....

Outdoor reset controllers allow you to put in whatever margin (extra temperature) you need for rapidly changing conditions. So you have extra capacity immediately available and can use it for heating up the houses's thermal mass. A smaller tank doesn't provide as many btus to do this (ie, is MORE likely to fall behind that a properly configured system with a larger tank).

For example, rapidly changing outdoor conditions mean that a concrete floor slab (huge thermal mass) needs to rapidly change from 80F to 85F. What is better able to do that - 50 gallons of 95F water or 500 gallons of 95F water?