Hi! - and- HVAC tutorial, Part 1 - Thermal Mass

[chuckspeed]

Hi guys!

I was invited over from another board; I'm an inveterate car guy who happens to make a living designing/engineering/refitting HVAC systems for industrial facilities. Worked on 20 million + square feet of facility to date and have experience with virtually every HVAC system you can imagine - from woodburners to forced air to radiant heat to passive solar systems - got a few patents in the area, to boot!

Anyway, I see a lot of discussion re: pros and cons of various systems types. While most of the stuff posted is true - I figured a thread on the 'why' of system selection would be interesting, and help in the selection process. Soo...here goes!

First off, sizing and selection of an HVAC system for a garage/shop is fundamentally different than selecting one for a residence. Residential systems are selected to satisfy design conditions, or more simply, the coldest and hottest days of a typical year in your neck of the woods. These design conditions are calculated in conjunction with the heat loss (and solar gain in southern climates) and the equipment is sized accordingly - normally. The system is sized to maintain a given space temperature on the design day - and therin lies the difference.

In the case of a garage/shop system, the process of heating and/or cooling is usually not continuous - it's a batch load. We don't normally live in the garage (except on the days we piss off our wives), so we only need to maintain an environmental condition when we're using the space. Problem is, however, there's literally several tons of material (shop equipment, cars, bikes, beer cooler, concrete) which has to be brought up (or down) to temp before the space temperature can be maintained. The amount of stuff in the space has something we call thermal mass, and the amount of heat required to bring it up (or down) to a specific temperature is call a pickup load.

If you attempt to heat a very cold 2 car garage space with a 30,000 BTU forced air unit, for example, you'll get to a decent space temp pretty quick - but the remaining space will 'seem' cold. That's because all the metal and concrete in the shop has not been brought up to temp, and is literally 'sucking' heat outta your body. an example of this in the real world can be found by walking thru the freezer section of the grocery store with a hand-held thermometer...the thermometer will show 72 deg F, but you'll feel cold! Your body is literally 'radiating' heat to the coolers - you're losing heat, so you feel cold!

Anyway, a system sized for a garage-sized heat loss will normally take a long time to heat up - on a cold day. Heat's gotta go into all the stuff to bring it up to temp; otherwise that 30,000 BTU forced air unit will click back on the instant it shuts off, 'cuz air has a very low specific heat, or the ability to 'store' heat energy. In most northern climates, this pick-up load is actually larger than the building's heat loss - especially now that we build efficiently insulated buildings!

So - If you really want to get 'trick' about equipment sizing, it's best to take into account the amount of material (machines, cars, bikes, concrete, etc.) which comprise the space and solve for the pickup load. Rather than let this post get all longish (it kinda is already - sorry) I'll follow up with some of the quick calcs for pickup load shortly.

Keepin' warm,

chuck.

[DanC]

sweet info! keep it coming! need to put heat in my garage and have lots to learn...

[Bruce T]

Sounds good. Whens part 2? I just priced out a radiant tube heater for my future garage and the guy mentioned some of your terms. He also did a heat loss summary based on the size, location (directions facing north), windows, doors, insulation being used ect. Thanks for your time.

[mike944]

So, that means we should under-size a garage heating system?

[Gregdoo]

Thanks for the great info...this is helpful. Looking for the rest of the story when you get to it.

[REFLEXX]

Super good stuff and welcome aboard!

A/C stuff coming up? I want to see if I was right in my criteria when I chose my window A/C units.

[bmwpower]

So, that means we should under-size a garage heating system?

Sounds llike we'll need an OVER-sized system in order to heat up the surrounds, too.

[W-Cummins]

I have noticed that the near 100 yards of concrete in my floor works nicely as a "thermal flywheel"

[Crosley]

Here in the metro Phoenix area of Arizona we use a bit more a/c than some places.

I have a 5 ton York heat pump ducted into my shop of 2800 sq feet. It works well. Added about 95 - 105 bucks to my electric bill when used in the summer . I do not try to keep the place at 72*F..... usually around 80*F with a programable thermostat.



http://bigbikeriders.com/photopost/data/500/760full_house-06-med.jpg

[Rowdy Rat]

Chuck,

Good information, thanks for taking the time to post it.

I'm in the planning stages of my house/garage and am looking for the best solution for heating and cooling both the house and garage as efficiently and economically as possible.

I'm hoping that I'll be pointed in the right direction after your series of posts is completed.

Regards,

Stan Falenski

[chuckspeed]

Guys -

I've been traveling and in meetings (remember, I do this for a living) so's I haven't had a chance to pos up part II yet.

quickly -

northern climates may require a slightly larger heating system than the heat loss calls for; southern ones - the normal heat loss calcs will be pretty close.

Crosley's comment on 80 deg F. for cooling is dead-nuts on; we call that air tempering; I was gonna cover that inna 'nother post. Gotta go - will post up later.

[chuckspeed]

Okay gang -

Here's what the numbers look like on a pickup load:

Let's assume the garage is 24 X24, has a 4" thick concrete floor, and two 3500 lb cars plus 1000 lbs of tools and machinery. Temp inside the garage is 30 deg F, and you wanna bring it up to 60 deg F.

Here's what you need to know:

Heat load = lbs of material x specific heat x temp rise (deg F.)

Density of concrete - 1260 lbs/yd.
Specific heat of concrete - 0.20 BTU/lb. - deg. F
Specific heat of steel - 012. BTU/lb. - deg F

you've got about 7.2 yards of concrete in the floor, so solving for the mass of the floor yields 9000 lbs (prox.)

Heat required to raise floor to 60deg F.:

Q = 9000 lbs concrete x 0.20 BTU/ lb. - deg F x 30 deg. F.
Q = 54,000 BTU

Heat required to raise tool temp and car temp to 60 deg. F.:

Q = 8000 lbs steel x 0.12 BTU/lb. - deg. F x 30 deg. F.:
Q = 28,800 BTU

Pickup load of garage:
Q = 54,000 BTU + 28,800 BTU
Q = 82,800 BTU.

So...If you had a 30,000 BTU furnace and were tryin' to heat up the garage:

Pickup load / furnace capacity = time to heat.:

82,800 BTU/ 30,000 BTU/hr. = 2 hours, 45 minutes (prox.)

Truth is this number is a bit high-ish as the calc has been done with a uniform slab temp - the slab can have a temp gradient across it and the garage will still feel comfy. Still and all - it's a 2 hour pickup load from 30 degrees to shirtsleeve temps; youse guys that live in the polar Midwest (minnesota) would need 4+ hours for the same equipment to go from zero to comfy.

the reason for bringin' this up is there isn't a whole lotta difference in price between a 30,000 BTU heater and a 60,000 BTU unit - last time I checked. The 60,000 BTU unit would heat up the space in half the time - a little over an hour and a half. dunno about you guys, but there are times during the winter months when getting out to the garage quickly is IMPORTANT!!! - assuming you're married. Solving for the pickup load can get you into that garage sooner - and help maintain some semblance of peace in the household.

[Tom]

Is there a way to 'cheat' when it comes to heating? Like the opposite of the cooling thread where what you strive for is low humidity and air flow.

[chuckspeed]

Tom - That's a good question!

Yes - and no. Unfortunately, there's no way to 'cheat' a pickup load; it's the heating equivalent of watching a pot of water boil. You hafta put a specific amount of heat into it; the only real variable is time. While it's possible to downsize the equipment, the offset is longer time to heat. If you can live with that, then install the smaller stuff.

Actually, if you read the sample calc section, you'll notice there is a 'cheat' already built in - the calcs are all predicated on 60 deg F. Somewhere in my work archives I have a table on comfort heating and working - it shows that if you're moderately active, 60 deg. F is a nice temperature.

As a case in point, I've been working on a '55 Chrysler since Jan 1 in an uninsulated (save for the garage door) space with a small-ish heater. Since the garage is attached to the house and the mechanical room (closet with furnace and water heater) is in the garage, the space rarely goes below 50, and with a 'little' extra heat I can get it to 60 in about an hour. I've spent a fair number of hours in there this winter wearing little mor than jeans and a polarfleece pullover - and a hat - and I've not been cold.

The other cheat is a neat one - size the equipment normally, and add a woodburner for supplemental heat. There is NOTHING like radiant heat, period - more on that later - and a little wood stove from Tractor Supply is pretty cheap - and can heat to beat the band. If you live in an urban area, a good source of wood can be found around shipping centers...Wood pallets. Pop burned scrap pallets for years in Shop #2; the pallets steel wire and flat stock are shipped on are usually oak, which has a high heat content. The size cut pallet stock is perfect for a small woodburner; one pallet will provide enough heat to bring a typical 2 car garage up to temp...And if you play your cards right, they're free!

[bmwpower]

In theory, this formula looks fine. The problem I see with this is that what happens when the air temp reaches the furnace setpoint? The heater shuts down when the air temp = setpoint, so the surrounding never reach the desired temp. When the heater shuts down, the surroundings start to cool again until the heater kicks on again. Unless I'm reading it wrong, does this calculation account for this?

[chuckspeed]

the calculation is heater independent; it merely explains the behavior of the space.

Typical forced air heating systems have a 2 degree deadband, which is to say the temp drops 2 degrees B4 the cycle is re-engaged - in a simple system. Industrial heating systems use much more complex controls and PID loops to offset the tendency to shut off before the material in the space is brought to temp. If you time it, the short cycle is pretty short initially - the rate of heat flow (log mean temperature difference, or LMTD) is greater when the space is cols, so the flow is faster then. As the thermal mass approaches setpoint, the time between on cycles increases.

The reason the calc is heating system independent is a radiant system alleviates short cycling - the heat flows into the mass of the space and the air temp follows the temp of the material. That's one of the reasons why radiant is soo neat for garages; once the material in the space is brought up to temp, you can open the garage door, remove a vehicle, close the door, and the space temp will recover in a matter of minutes.