HVAC guys - need design help

[turbo6bar]

I recently bought a small fixer upper and am having trouble deciding how to heat and cool. It has 1500 sf downstairs and a 500 sf upstairs addition. The original plan was to use a standard ducted split air conditioner and furnace for downstairs and a simple ductless split heat pump (for cooling and some heat) for the upstairs master suite (with a ventilation fan bringing conditioned air in the master bath.

However, I'm thinking about putting the master down and put two bedrooms and a bath upstairs. I only need about 2 to 2.5 tons cooling for the entire house and about 35000 btus of heat. I'd go with a single unit and run zone dampers and controller, but I'm concerned of pushing a lot of cooling CFM to the upstairs zone. It just sounds like it could be a real problem if I don't get the zone design just right. A ductless split with multiple zones starts to get into bigger bucks.

A retired refrigeration pro suggested a chiller and thermostats in each room, but questions whether such small chillers exist. It also doesn't address heating.

I'm game for ideas. I'm trying to keep the budget under $3k for materials. My original choice of 90% efficient furnace and 14 seer condenser/coil is $1500, and an inverter ductless split heat pump, 9000 btu, is around $750. I'm doing the install.
Jurgen

[MarKoBrow]

I would look into one 3Ton Heat pump and design a duct system with dampers in each room, you might also want to design in multiple returns. It sounds like a very simple system and I would keep it that way.

[Zeke]

With the aid of an HVAC company, I eliminated damper controlled zones in a large single story home. I did not feel that this was good design and after talking to a couple of different installers, I decided to let one of them redo the system. We did put individual butterflies in ducts as close to the T connections as we could to balance the system. No A/C in this system.

On another job I did the design myself for a 2 level addition using the existing basics. I got my duct sizing by observing what had been done previously and recently to the home and online with the aid of many sites devoted to duct planning.

Both homes work very well and are nicely balanced.

Not seeing your house, I would recommend a large 10" or 12" duct to the upper floors depending on overall length. Split it down to a couple of 7 inchers and maybe a 5" to the bath. With dampers, you should be able to get a nice distribution. Of course, not pinching anything down is a key goal.

[MarKoBrow]

In HVAC we use something called a Manual D for residential duct sizing, this system helps with air velocity and volume. Before suggesting to someone use a 10" to 12" duct which is used for a 4 to 5 ton system, I would suggest that they perform a heat load on the house. I would also worry about air exchange and humidity levels before suggesting a duct size. I am also wondering what "pinching down" is. Are you referring to reducing duct size to keep velocity constant over the duct run?

I often have to redesign systems that are over designed and instead of cooling a house they over cool and cause the windows to sweat, when they are in heat mode they dry out everything in the house.

Keep it simple, design your system based on known facts and practices and you will be happy in the long run.

[Zeke]

Originally Posted by MarKoBrow

In HVAC we use something called a Manuel D for duct sizing, this system helps with air velocity and volume. Before suggesting to someone use a 10" to 12" duct which is used for a 4 to 5 ton system, I would suggest that they perform a heat load on the house. I would also worry about air exchange and humidity levels before suggesting a duct size. I am also wondering what "pinching down" is. Are you referring to reducing duct size to keep velocity constant over the duct run?

I often have to redesign systems that are over designed and instead of cooling a house they over cool and cause the windows to sweat, when they are in heat mode they dry out everything in the house.

Keep it simple, design your system based on known facts and practices and you will be happy in the long run.

Well, that's what is really needed here is an expert, which I'm not. Using an over sized duct doesn't seem to be the worst case to me, but undersizing might be.

I assumed that (again, not seeing the house) that the air handler would be downstairs and a long run would be needed to supply the upstairs. I also assumed that the upstairs would be supplied from the ceilings as floor boots are hard to retrofit and duct. Therefore, I again assumed a larger duct would help supply the upper floor.

As one can see, assumption is not going to get this done.

By pinching, I meant having to over correct bad design. One thing I did learn is that controlling air flow at the register is the wrong approach. You're creating a wasted dead or poorly flowing line. Am I correct on that?

[wdfifteen]

Originally Posted by MarKoBrow

In HVAC we use something called a Manuel D for duct sizing,

MANUEL!! Dude! I thought jou got deported! Glad to see jou are back in the HVAC beesnezz.

[MarKoBrow]

Originally Posted by wdfifteen

MANUEL!! Dude! I thought jou got deported! Glad to see jou are back in the HVAC beesnezz.

I probably should learn how to spell or use spell check, now that I look, it is pretty funny. I will go back and edit it.

[turbo6bar]

I plan to use HVAC-Calc for the Manual J. I used a simple spreadsheet to get the heat loss/gains in the house. Also, I will use Manual D to size the ductwork.

The issue is keeping the upstairs section cool in the summer. If I use one unit for the entire house, I'll be cooling the downstairs when it isn't needed. That was sort of the purpose of the ductless mini split upstairs, whereby I can control humidity and temperature separately from the downstairs area.

The house is a total gut and redo of a 1950s home that had been abused by previous owners and an improperly flashed roof. I'm trying to stay in budget with HVAC so more money can be thrown into architectural details (conversion to craftsman styling).

A system of dampers and zone controller might work, but it sounds like a plan that would be great if it works fine, but a nightmare if it doesn't work.

milt, I agree that you shouldn't control flow at the grills. Put manual dampers in at the wyes and balance the flow while still maintaining adequate throw and low noise at the register.

BTW, I've run HVAC-Calc on three systems and did the install. Every time, I added a fudge factor and oversized equipment. I should have stuck with the program's numbers. Not going to make that mistake again.

[MarKoBrow]

Sticking with the numbers is the way to go, when I did my first job I oversized it and ended up a disaster, Won't do that again! You can damper between upstairs and Down send the majority of cool air upstairs. Perhaps you might even consider placing the unit upstairs and damper air for the downstairs. I design single systems all the time that are very quiet and don't over cool or over heat just using proper duct sizing( it can be done without dampers).

Option two would be to use the two split units. With that option you would be able to control your operating costs and designing ducts would be a lot easier, provided you have room for the air handlers.

[turbo6bar]

Only thing I don't like about two conventional splits is the small sizing. Upstairs might need 1 ton. Downstairs could use no more than 1.5 tons. Same with the furnace sizing. The smallest natural gas furnaces I've seen are ~40k btus. I need less than 40k btus for the entire house.

The ductless split is ideal, but I need two zones upstairs. A dual zone ductless split is easily $1500. If I use a regular split unit, the smallest I can get is 1.5 tons, which is definitely oversized, and would never control humidity worth a damn. In TN, like many places, humidity is as important or more important than temperature control.


MarKo, do you install single units for two story homes?