Low temp fan coil
Last Post 25 Aug 2014 12:43 PM by joe.ami. 24 Replies.
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19 Aug 2014 12:17 AM
We are building a new house, 3000sf 2 story in Seattle area. 

Trying to get the remaining details of the heating system figured out.

W2W geo system, radiant slab main floor on an outdoor reset control. 

Separate hot and cold buffer tanks and a ducted fan coil for heat & a/c upstairs.

The slab tubing is ran at 9" o.c. so with the outdoor reset controller it should keep the hot buffer tank at a fairly low temp.

So my question is with regards to the fan coil-

Do I need to find a way to raise the temp of water to the coil?

Or get the biggest coil i can find and run it with low temp water?

Or is there a fan coil unit available that is designed for low temp water?
jonrUser is Offline
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19 Aug 2014 12:20 PM
You can check the ratings for btu output vs temp and compare to what is needed. I'd verify that it doesn't blow cool air on anyone (during mild weather).

You can also create a more complicated control system where the heat pump can produce higher temperatures for the fan coil/buffer tank and lower temps for the slab. Running at the same higher temp for both will reduce efficiency.
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19 Aug 2014 03:24 PM
If the buffer tank/radiant floor is running 90*-ish range would it make sense to just use an electric tankless water heater to raise that to 120-130 for the fan coil?

Looking at performance data on fan coils I can't find anything for EWT's below 120*.


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19 Aug 2014 05:46 PM
It's pretty wasteful to use electric heat when you have a HP available that can handle the load.
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19 Aug 2014 06:08 PM
http://www.ttherm.com/tthermstore/Terra-ThermVariableSpeedFanManual.pdf

This is only one example. Geo is such a small segment of the market most manufacturers don't publish performance figures at lower (non standard) temps. Phone calls or emails might get you capacities at lower temps.

Calculate load first then specify fan coil. Keeping your W2W temps low should be your first priority. You can always put electric strip in the air handler to make up any short falls just like W2A systems.
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20 Aug 2014 09:26 AM
You have seperate tanks, if need be employ the "cold tank" as the fan coil tank so you can select different temps from the floor. Why two seperate tanks though?
Joe Hardin
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20 Aug 2014 12:40 PM
The idea behind two tanks was that one zone could call for heat the same day as another calls for a/c.  I think with one tank it can take a while to change over from hot to cold.

I guess the warm and cold tanks could be used as warm and hot tanks during the majority of the heating season.

One thing I'm worried about is what the higher temp load will do to COP? 

With the 90* load I could be as high as 4.25 COP, but demanding 120* will lower that.

So is using the heat pump the most efficient way to get from 90 to 120?

What if a desuperheater tank or DHW tank had a coil inside that water could pass through on the way to the fan coil?

I may be splitting hairs here and unnecessarily over-complicating things.

BTW the desuperheater option is another aspect of the system I am still undecided on.
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20 Aug 2014 03:44 PM
Just as you can have a HP maintain two tanks at cold and hot, you can have a HP maintain 3 tanks at cold, warm and hot. Then realize that a slab has plenty of thermal mass, so you don't need the warm tank.
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20 Aug 2014 09:59 PM
What is the load that the radiant is serving and what is the load of the air handler is serving?
What size heat pump will be installed? What size buffer tank will you be installing? What do you think the run time of the radiant versus air handler zone would be? are they two separate spaces?

Might have a few ideas after those are answered...
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21 Aug 2014 04:35 AM
Posted By ACES-Energy on 20 Aug 2014 09:59 PM
What is the load that the radiant is serving and what is the load of the air handler is serving?
What size heat pump will be installed? What size buffer tank will you be installing? What do you think the run time of the radiant versus air handler zone would be? are they two separate spaces?

Might have a few ideas after those are answered...

Radiant load (main floor) is 23,096 btuh heating and air handler load (second floor) is 10,939 btuh heating.

Total load for the house is 34,035 heating and 24,540 cooling. 

Planning to go with a 3 ton heat pump.  120 gallon hot buffer tank with a coil for domestic water to pass through on the way to the water heater.

Main floor is pretty much one big room with tall cathedral ceilings, second floor is lofted, open to below and partially over garage.

I was planning to put the return air vent on the main floor to help draw radiant heat through the duct work to upstairs but I think the effect from this will be minimal.

As far as run times I am not sure.  Bin analysis for the geo design shows the heat pump running 100% when the outdoor temp is below 27F or above 92F.

I thought the radiant with an outdoor reset would run most of the time and adjust water temp as necessary to satisfy the load?

And run time on the air handler I really have no idea, that is part of what I am still figuring out.  But I can say that it will be interlocked with an HRV, if that makes a difference.
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21 Aug 2014 08:34 AM
Yikes. How did you arrive at the 120 gallon buffer tank?

Here's what you need, One tank, 40 gallons tops, 2-3 ton hydronic air handler with 5k electric auxiliary heat. DSH to help with hot water and you are done. If you want to go full on demand hot water, you'd probably want a larger heat pump.
Joe Hardin
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21 Aug 2014 09:23 AM
I agree with joe above. With such a low load on the second floor a 2 ton hydronic air handler will serve you well at radiant reset temps at lower fan speeds (300-400 CFM) in heating while covering total cooling load in cooling (800-900 CFM). You will have to decide if your climate will require separate heating and cooling tanks. We do heat and cool tanks when there is a specific requirement for simultaneous heating and cooling with 2 units (think equipments rooms, reheat dehumidification, etc). I am not sure it makes sense for you. Do temps change that much in a 24 hr period?
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21 Aug 2014 09:24 AM
I was planning to put the return air vent on the main floor to help draw radiant heat through the duct work to upstairs but I think the effect from this will be minimal.


Don't do this if there are closed door bedrooms involved.

If the air handler can absorb the full HP output, then you may not need buffer tanks on that loop; just use one for the DSH.
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21 Aug 2014 09:41 AM
I think we all suggest you not reinvent the wheel. Perhaps you should put a return air vent on the main floor for when you cool that floor. The reason I suggested 2-3 tons is I don't know what air handler you will end up with and you would have to look at their specs to make sure you get desired out put with entering water temperatures.

You absolutely do want a buffer tank.

Switch from heating to cooling will only take about 30 minutes with that size buffer.
Joe Hardin
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21 Aug 2014 01:27 PM
Posted By joe.ami on 21 Aug 2014 08:34 AM
Yikes. How did you arrive at the 120 gallon buffer tank?

Here's what you need, One tank, 40 gallons tops, 2-3 ton hydronic air handler with 5k electric auxiliary heat. DSH to help with hot water and you are done. If you want to go full on demand hot water, you'd probably want a larger heat pump.

A mechanical engineer who helped me with the system concept believes bigger is better when it comes to buffer tanks.  I would tend to agree unless there is an argument against this.  More storage should mean longer, less frequent run times.

A heat pump salesman told me DSH don't work because the heat pump never actually gets hot enough in our climate.  I would tend to disagree because this is coming from someone who believes in oversized geo systems.

So the 40 gallon buffer is for DSH?  No buffer at all for hydronic heating?
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21 Aug 2014 01:33 PM
Posted By G.O. Joe on 21 Aug 2014 09:23 AM
I agree with joe above. With such a low load on the second floor a 2 ton hydronic air handler will serve you well at radiant reset temps at lower fan speeds (300-400 CFM) in heating while covering total cooling load in cooling (800-900 CFM). You will have to decide if your climate will require separate heating and cooling tanks. We do heat and cool tanks when there is a specific requirement for simultaneous heating and cooling with 2 units (think equipments rooms, reheat dehumidification, etc). I am not sure it makes sense for you. Do temps change that much in a 24 hr period?

I guess during the cooling season the cold buffer would serve the air handler for a/c while the hot buffer would keep the radiant floor 3* above the dewpoint. 

So even if it's not for heat and a/c in the same day it allows multiple temps.
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21 Aug 2014 01:49 PM
I wouldn't heat/cool/heat a single tank on a frequent basis. Nor would I heat and cool simultaneously in an open floor plan. If the AC isn't enough to keep the floor above dewpoint, use a standalone dehumidifier.
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22 Aug 2014 05:21 AM
A packaged dehumidifier cools then heats the air in single box. I makes perfect sense to make the whole house the box and use the efficiencies of the geo system to do the simultaneous heating (radiant) and cooling (airhandler AC). In a humid climate the comfort is unsurpassed when controlled properly.
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22 Aug 2014 09:49 AM
An air handler sized for heating/cooling is so over-sized for dehumidification only that it would short cycle and be less efficient. But I'd be interested in seeing a plumbing and control diagram for such a setup (ie, no use of the ground loops when only dehumidification is needed but can still switch to heating or cooling).
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22 Aug 2014 10:54 AM
Buffer tanks are sized to prevent short cycling. Air speeds and load side hydronic flows are controlled to produce 70f-72f neutral and dehumidified air. Heating and cooling have priority over dehumidification. Most of the time we do it with 4 pipe air handlers but have applied with radiant and 2 pipe AH. If I told you how I would lose my job. It's all in the control logic.
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