Back Up Heat Source and Boosting Loop Temps

[jeff5may]

Originally Posted by wewantutopia

Thanks for the reply.

If we get a 12,000 btu unit does that mean in the coldest months it will run 45 minutes of every hour?

That seems like a lot. Is it?

If it were me, I would probably go with 1 1/2 tons (18kbtu) of capacity. The reason being is your heat load will vary above and below average. During sub-zero temperatures with wind, your heat load may rise above the 1 1/2 tons. The thermal mass indoors should help the house from cooling off too fast, and if not you could always run a backup heat source that 1% (or less) of the year.

A heat pump saves the most energy when it runs constantly without shutting off. With a constant capacity unit, it makes sense to size the unit a little smaller than your maximum heat load. The new inverter units take advantage of this fact with "fuzzy logic" that calculates heat load like your car's cruise control senses hills. When they can slow down a bit to keep demand happy, they avoid cycling on and off to save energy.

[NiHaoMike]

It might make more sense to use a good 12000BTU/hr unit most of the time and a quickly hacked 5000BTU/hr window unit (look up how to build a PC chiller) for extra capacity. You can also build a geothermal heat pump water heater with an extra coil between the outlet of the heat pump and the tank. When extra capacity is needed, run the water heater and a fan on the coil.

[AC_Hacker]

Originally Posted by NiHaoMike

Geothermal heat pumps aren't affected much by outdoor temperature.

This is not the whole story...

The work that a heat pump does is ultimately a function of delta-T. So it is the source or input temp (Ti) minus the 'sink' temperature or output temp, which would be your home (To).

When it gets cold, an ASHP needs to scavenge heat from a declining heat source (cold air)... it also needs to supply heat to an increasing load because your house is leaking heat faster than than it would in warmer periods, so the scenario is the worst of both worlds. Just when you need heat the most, your heat pump's source is in decline.

With a GSHP, the source temp is declining very slowly, so we pretend that it stays the same. However, the heating requirements are increasing. So it is not as unfavorable as a ASHP.

I just did a search on his post, but stevehull posted a very useful graph of his system which showed the ambient temp and also power consumed.

I wish I could find it, because it was clear that when ambient temp dropped, energy use increased, but with a ASHP, the energy use would be much higher (maybe quadratic) when ambient temp drops.

-AC

[AC_Hacker]

Originally Posted by jeff5may

I found a diagram that illustrates the associated heat transfer values in a typical system:

Can you explain how this diagram is used to understand heat pumps.

-AC

[stevehull]

Wewantutopia,

Your average figure of 9,000 BTU/hr. for the winter month is just an average. If you looked at both winter and summer use, your average of heating and AC might be close to zero BTU/hr. Again, an average.

Average consumption does not apply to your situation. Rather, you need to figure out the heat loss (for heating) at about 90% of your maximum heating load. Your location in Illinois is a cold one. I bet your typical low in January is in the single numbers with typical yearly lows at about -10F.

Lets assume you want the house at 70F at a low of 0F. That is your delta T. Now figure, from manual J, the resistance value of the walls, windows, doors, ceiling and floor. Calculate the BTU need for that.

Now add in the extra BTUs needed for an air change every 3-4 hours. Add that in. The total will be your need for heat.

My estimate? About 24-36 KBTU (2.0-2.5 ton).



Steve

[Mikesolar]

I wonder what program created the diagram?

[NiHaoMike]

Originally Posted by stevehull

Average consumption does not apply to your situation. Rather, you need to figure out the heat loss (for heating) at about 90% of your maximum heating load. Your location in Illinois is a cold one. I bet your typical low in January is in the single numbers with typical yearly lows at about -10F.

If the outdoor temperature stays near worst case in that period (and no other exceptional events like going on vacation), the average is valid for sizing equipment.

Granted, oversizing a heat pump for heating is nowhere as bad as oversizing it for cooling (the latter can cause poor dehumidification), but a single heat pump sized just right for heating in a heating dominated climate is going to be oversized for cooling needs if A/C is necessary during the summer. That's where multiple units or an inverter unit can make more sense. And a unit oversized for heating would be even more oversized for cooling.

Do, however, feel free to oversize the geothermal loops. It will likely improve efficiency, especially if it is multiple parallel loops instead of a single large loop.

[jeff5may]

I grabbed the illustration from the website of:

TechniSolve Software

They have a whole suite of programs for whatever (air or water) heating, cooling or refrigeration solution you'd like to simulate. Not cheap.

The illustration breaks down a freeze-frame in time of a running phase change system. It appears this one is something like a walk-in refrigerator. All relevant temperatures, pressures, heat flows, and mass flow are recorded for respective devices.

The long and short of this system is this: the evaporator absorbs 163.1 kw from its environment and the compressor uses 39.1 kw to pump refrigerant, yielding 202.2 kw of heat rejected by the condenser. Since the unit is in cooling mode, COP is the ratio of heat absorbed by the evaporator divided by compressor power, or 163.1/39.1 or 4.17. If the unit was used as a heater, the COP would be higher at 202.2/39.1 or 5.17.

[AC_Hacker]

S-Hull,

Originally Posted by stevehull

...My estimate? About 24-36 KBTU (2.0-2.5 ton)...

I'm curious how you are utilizing your quarter century of geothermal heating expertise (* "consulting on geothermal heating/cooling & rational energy use since 1990" *) to come up with this estimate?

I often hear from furnace installers with no university degrees in any kind of science that their method is to 'guess' what the heat load might be and double it.... seems to me to be a pretty blunt ax they are swinging, but it's almost exactly what you came up with.

Originally Posted by stevehull

Your average figure of 9,000 BTU/hr. for the winter month is just an average.

I agree that using the average heat input for a month is not going to give the answer we are seeking.

But wewantutopia did give us some kind of a clue...

Originally Posted by wewantutopia

...I live in a ~1000 sq. ft. 1910 bungalow that I have insulated to just under passive house standards (the builders grade double pane windows the previous owners installed are the week spot) including R80 attic, 1.5" exterior poly iso, 3" poly iso foamed in between the studs, 1.5" XPS that I hand dug around the basement to below the footings. The walls come to about R30.

Also...

Originally Posted by wewantutopia

[the gas consumption indicates]That comes to 9125 btu/hour average.

I know that the Passive House allowable heat loss is 4.75 kBtu/sf/yr, or 4750 Btu total for his 1000 sq ft (* This is the YEARLY cumulative heat loss, not hourly rate. *). So it's safe to say that we're not quite at Passive House standards.

But still, from the impressive list of improvement given above, it seems like the method of just doubling the estimate might be a too-easy approach. I mean his sample month is a month from a very harsh winter. So in that average there were HDD heat demands above and HDD heat demands below that average. Surely there would be a way to go from an average to a likely distribution of heat demands.

So his average of 9125 btu/hour would be right at the center of the bell curve.

There must be a way to mathematically to approximate the values that would suggest this curve.

I suppose that wewantutopia could seek out actual recorded daily HDD data from his area and then find out what the 95% highest heat requirement might be for his house.

It's times like this that it would be so useful to have a data logger reading his current heating system and also reading ambient temperature data.


-AC

[stevehull]

AC,

I lived up north in that area. It is cold. That area of IL is also very cloudy and windy (see page 1, post 1) in the winter. Simply stated, 9000 BTU/hr is not going to cut it at 0F in that climate even with a 1000 sq. ft home and average R30. Twenty five years of work allows me to guess - and that's what I did.

24,000 - 30,000 BTU/hr to get a delta t of 70F in that climate and with those conditions.

It is physics.

Steve