2 Pipe Chilled Water/Hot Water Changeover Fan Coil System 1

[MorrillB]

In a situation where you have the system as stated in the thread subject, how is it possible to use the hot water changeover for each unit without having an outrageous LAT or delta T for when the system is in hot water mode. The selected cooling coil for the units and EWT for heating(180 to ~160) make for these numbers to be too large.

In order to get the LAT needed for heating, some units are going to have to pump a significantly low GPM and in turn have a delta T of near 100F.

In this case there is only 2 options I can think of:
1. Scrap the changeover if possible and go electric heat.
2. Select a 4pipe FCU and use control valves to flow from the original 2 pipe into the heating coil when needed.

For the 4 pipe there still is the issue of the changeover being manual, so how would the unit know that the water coming in is heating or cooling? (water temp sensor somewhere?) On top of that, even if it did know.. when there is a call for cooling, it simply wouldn't be there in heating season, and vice versa in cooling(though there is baseboard for the heating application when in cooling season)

Anyone have any insight into the issue?

Thanks

 

[Drazen]

i mostly don't understand your post at all, but for fan coils you have changeover controllers available at some manufacturers that will recognize working mode by sensing supply flow temperature.

 

[cdxx139]

If you dont end up scrapping the changeover syste, (4 pipe is way more money), the size your pipe and flow for cooling, then back engineer the heating coil with the same gpm. THEN, reduce the hot water system temeprature to meet your worst case fan coil (or whateverr else is on the system). This works great with a condensing boiler sysytem. I did it in a dormitory, and was able to get my return HW temps down to 100F, increasing my boiler efficiencies intop the mid 90's efficiency.

What your missing for the thread, is the changover period. My energy code requires a 4 hour no heating no cooling changeover period (according to OA), so the system temp can reduce naturally so you dont shock the chiller and dont waste energy.

knowledge is power

 

[willard3]

The strong drawback of two-pipe system is that it takes a lot of time to change over. It's a bad system choice for a building that is occupied continuously.

 

[NegativeZero]

where will this system be used in. will this system be the only set up to maintain temps?

I can see a two pipe working when OSA temps fall and you can have a setback controller, controlling water temp based on OSA temps. The FCU would need to have a danfoss valve or something sensing room temp controlling the valve.

But depending on the sun, some days you will require some heat and some cool. will piping be zoned? will there be interior supply fans?

electric heat could cost as much in the long run. should do cost analyses.

 

[engr50]

With today's technology, you may look for a temperature controller with automatic changeover feature for application in your 4-pipe FCU. The temperature controller shall select the right motorized valve, which is installed on hot or chill water pipe, to open.

 

[ChasBean1]

I understand the issue and your concern. A cooling coil can tend to have way more fins per inch than what is required for heating, so you size the coils based on the limiting factor, which is cooling. When you heat with this “cooling” coil you have a higher heat output than what is required. This could result in control difficulties. But if you look back at the history of heating (fire make heat; steam radiators make heat; heat happens by rotating knob; enter thermostat heating by go-no-go—too cold, heat, warms up, stop heat), heating is not so picky… My house has this heat/no heat philosophy and it’s okay here. Me likey when heat kicks on, but me ‘survivy’ when heat idle…

 

[EnergyProfessional]

using the same coil for heating/cooling also is a maintenace nightmare. The cooling coil has some stuff growing on it since it is wet. Then you switch over and heat and bake that stuff on the fins. After a few cooling (wetting) and baking cycles you have quite some cake baked on. With the closer fin spacing it is hard to clean. Assuming you care about IAQ, using harsh chemicals isn't really possible.

 

[DRWeig]

My notes on this topic that haven't been covered above:

1) Strap-on water temperature switches are found often on these units to reverse the action of the thermostat.

2) Use caution at changeover time from heating to cooling. If your building piping is full of 180°F water, you don't want to immediately dump that through chiller tubes or you'll be buying a new rupture disk and load of refrigerant.

3) Explain in writing to the building owner that there will definitely be uncomfortable spaces during mild weather. Make him or her sign off on it in the presence of a notary and keep a dozen copies in a dozen different places.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[ChasBean1]

HerrKaLeun, good point