water to air hx in downflow furnace

[Medman]

Is anyone out there using a water to air heat exchanger in a downflow furnace? I have some concern that the heat from the coil, rising through the furnace when the blower is not on, will cause the temp limit to trip and run the blower even when the thermostat is not calling for heat. This would lead to short cycling of the blower and potentially overheat the house.

Would running the blower on low speed continuously, ramping up as the thermostat calls for heat, eliminate this problem?

Your input is appreciated!

 

[Redox]

You probably don't want to be circulating the water continuously through the coil for a variety of reasons, control being one of them. Set up the thermostat to turn on the circulator or open the zone valve on a call for heat and you shouldn't have a problem.

Chris

 

[BrownianHeatingTech]

The thermostat should turn on the pump, which sends hot water to the coil. A temp sensor on the coil should turn the blower on (the control in the furnace will not work directly, as the coil is located after it).

That's the simplest way to do it...

Joe

 

[Medman]

I have been trying to get more specific info on the temp sensor that will turn the blower on. Is there manufacturer or model numbers you can suggest?

Can this sensor system be wired in parallel with the standard blower controls for the furnace, such that I can continue to use the electric element in the furnace as backup? Ideally the backup system and primary boiler heat system will both be available at all times so that the electric will automatically kick in should the boiler be too cold.

I am just trying to visualize the wiring for this system.

Suggestions?

 

[BrownianHeatingTech]

I have been trying to get more specific info on the temp sensor that will turn the blower on. Is there manufacturer or model numbers you can suggest?

Can this sensor system be wired in parallel with the standard blower controls for the furnace, such that I can continue to use the electric element in the furnace as backup? Ideally the backup system and primary boiler heat system will both be available at all times so that the electric will automatically kick in should the boiler be too cold.

I am just trying to visualize the wiring for this system.

Suggestions?

A standard "make on rise" Honeywell aquastat would work. The specific model will depend upon the specifics of your installation, but you want "make on rise" or SPDT contacts, and an adjustable differential. There are types with immersion wells that get installed into the pipe, and there are types that strap onto the outside of the pipe.

You wire it in parallel with the blower control. Sometimes that will be a line voltage system, and sometimes it will be on the low-voltage side (if your thermostat has a "fan" function). Just make sure that, if you have a multi-speed blower, it is triggering the correct speed.

Joe

 

[Medman]

Thanks, Joe. The Honeywell switches were already on my mind as the ones to use.
My existing furnace uses a line voltage fan with different speed options, and I had already installed a manual switch to run the fan on medium speed (fan only) just to circulate air in the house when heating with my woodstove. Using the limit switch in this installation is fairly simple.
However, I am looking at replacing the furnace with an air handler and split system AC at the same time as the boiler install and I am wondering about blower control with the variable speed fan. Most furnaces use a circuit board to control fan speed, tying speed to thermostat setting and presence of flame/heat. Have you installed or seen a way to add a rise in heat exchanger temp to the control board?
I am ok with having two separate thermostats, one for the conventional air/heat system and one to switch the circulator pump. My issue is being able to have the fan run on low all the time, run at medium speed when the heat exchanger reaches the high temp limit, ramp back down to low when the heat exchanger reaches the low limit, and energize the electric backup coil if the heat exchanger is not hot. Also, have the fan run on low all the time in summer and ramp up to med or high speed when cooling is required.

I am looking specifically at Goodman air handlers, model AEPF303616.

Is this possible, or will I have to manually switch between the two systems?

Also, in a downflow application, what order should the components be stacked? A-coil, water to air HX, then blower on bottom or water to air HX, A-coil, then blower?

 

[BrownianHeatingTech]

However, I am looking at replacing the furnace with an air handler and split system AC at the same time as the boiler install and I am wondering about blower control with the variable speed fan. Most furnaces use a circuit board to control fan speed, tying speed to thermostat setting and presence of flame/heat. Have you installed or seen a way to add a rise in heat exchanger temp to the control board?
I am ok with having two separate thermostats, one for the conventional air/heat system and one to switch the circulator pump. My issue is being able to have the fan run on low all the time, run at medium speed when the heat exchanger reaches the high temp limit, ramp back down to low when the heat exchanger reaches the low limit, and energize the electric backup coil if the heat exchanger is not hot. Also, have the fan run on low all the time in summer and ramp up to med or high speed when cooling is required.

I am looking specifically at Goodman air handlers, model AEPF303616.

Is this possible, or will I have to manually switch between the two systems?

It looks like that air handler is two-stage compatible. So you just need a two-stage thermostat. The first stage will call in the hot water. The second stage will turn on the electric backup element if the temp continues to fall (usually one or two degrees below the first stage setpoint).

The first-stage signal from the thermostat connects to the pump control, and the Honeywell aquastat connects to the first-stage terminals on the air handler's control board. Also, add a make-on-rise aquastat at the wood boiler, connected in series with the pump. That way, if the wood boiler is cold, the pump won't even try to run (you don't want to be circulating cold water through the coil, while the electric element is trying to heat the house).

Also, in a downflow application, what order should the components be stacked? A-coil, water to air HX, then blower on bottom or water to air HX, A-coil, then blower?

In downflow, it should be (from top): A-coil, blower, then water-air HX.

Blower in the middle, and the heating coil on the outlet side means that the blower is never subjected to heated air. Almost every piece of equipment out there puts the A-coil before the blower, regardless of whether it is upflow or downflow.

A recommendation on the thermostat would be the Honeywell IAQ thermostat. It's a bit more money than a standard two-stage, but it uses communicating technology, so you don't have to drag 8 wires all the way to the thermostat - the wall-mounted unit communicates with the equipment interface module using only three wires. Only at the EIM do you split out into the separate control wires. It's also highly programmable, allowing for adjusting all the stage differentials with ease, which helps you fine-tune the system. And it will control both humidification and dehumidification, all from one unit. Very slick. I use them on nearly all jobs, now.

Joe

 

[Medman]

Thanks, Joe. That's just the answer I was Looking for. I will look into that thermostat for my system.
Now if I could just find an insurance company willing to cover this install...

 

[Medman]

What size heat exchanger would you recommend in this application to maximize airflow? I have the opportunity now to resize the trunk where it comes off the air handler, into the distribution system. If I use the dimensions of the air handler, 22 x 24, what size hx will actually fit? I'm thinking 20 x 20 will be fine considering the output of the EKO 25 is 80,000 btus and my house load according to the slantfin calculations is 38,000 btu/h loss.

 

[heaterman]

What size heat exchanger would you recommend in this application to maximize airflow? I have the opportunity now to resize the trunk where it comes off the air handler, into the distribution system. If I use the dimensions of the air handler, 22 x 24, what size hx will actually fit? I'm thinking 20 x 20 will be fine considering the output of the EKO 25 is 80,000 btus and my house load according to the slantfin calculations is 38,000 btu/h loss.

Considering the size of the coil is very wise. Introducing a second coil to the air stream can, and often does lead to excessive static duct pressure. You will notice this first in the cooling season when the A/C that used to work adequately before the second coil was added, suddenly doesn't. The second way you will notice this is when you start replacing blower motors every 3 years for some mysterious reason. The reason is that the second coil has lowered the airflow through the system and if there's one thing an A/C system needs, it's airflow. I usually go a couple sizes larger on the heating coil and make an adapter for the existing plenum. On a job we're doing right now for example, the plenum is 16x19 and I'm installing a 20x20 coil. This gives me about 30% more surface area and that's the basic rule of thumb I use. There is no downside to this other than a few $$ more for the coil and the plenum adapter. A larger coil will also let you run lower water temps which can really help keep wood consumption to a minimum. It will also give you a faster warm up because it can transfer more BTU's into the air stream all other things being equal.