Can't figure it out

[crs7200]

Been a while since I've been on this forum. Right to the point...

I have been using a boiler for 7 years. Standard install with a forced air furnace system. Hot water is delivered to a water-to-air heat exchanger in the plenum of the oil furnace, using the blower from the forced air system to carry the heat thru out the house.

I have one issue that I have had for the entire time, and have never been able to figure it out. When the outside temp drops below zero ( give or take) , the system can't keep up with the heating of the house. I have to put the oil furnace on to maintain the heat. The water temp is fine, pump is fine, etc. Why is it not able to keep the house warm? As soon as I put the oil furnace on, the house heats up fine. Does the oil burner create more heat than the boiler or is it because the heat exchanger is farther from the blower? Or is it something else? This has bugged me every year, so any ideas would be appreciated, unless there is nothing I can do, and if so, I guess I'll have to just deal with it. Thanks.

 

[Bob Rohr]

Been a while since I've been on this forum. Right to the point...

I have been using a boiler for 7 years. Standard install with a forced air furnace system. Hot water is delivered to a water-to-air heat exchanger in the plenum of the oil furnace, using the blower from the forced air system to carry the heat thru out the house.

I have one issue that I have had for the entire time, and have never been able to figure it out. When the outside temp drops below zero ( give or take) , the system can't keep up with the heating of the house. I have to put the oil furnace on to maintain the heat. The water temp is fine, pump is fine, etc. Why is it not able to keep the house warm? As soon as I put the oil furnace on, the house heats up fine. Does the oil burner create more heat than the boiler or is it because the heat exchanger is farther from the blower? Or is it something else? This has bugged me every year, so any ideas would be appreciated, unless there is nothing I can do, and if so, I guess I'll have to just deal with it. Thanks.

Often times those retrofit water to air heat exchangers are undersized. It usually show up only at design or colder days. My experience shows the one that fits the ductwork is the one that gets installed most times, no actual sizing, or load calculation is performed.

To move heat energy you need two things, temperature and flow.

Use an accurate temperature gauge and determine the temperature going into the HX and returning out. If you know how many GPM is flowing through the coil, the math is simple. It is tougher to guesstimate the actual flow, pump size, piping lengths and size can narrow down the number.

If there is any information on the coil you can often go to the manufacturers website to see how much energy that HX can move based on temperature, flow, and air flow across the HX.

Also you need to move adequate air flow thru the HX coils. Sometimes adding that HX coil reduces the CFM the blower can move.

Are there clean furnace filters in the system? Can you see if the fins on the HX are clean? How warm is the air at the vents? Ductwork in cold spaces, crawlspace, basement, attic? Is the ductwork well insulated? Sometimes a roll of duct insulation is all that is needed to get you over the hump.

 

[stee6043]

I would likely agree with the above but my first question would be -- how big is your house?

I've always wanted a properly designed "coil" for my plenum so I can take my storage temps lower than I do currently but my "run of the mill" HX has never had a problem keeping up with my load even at peak/design load kind of days. And I did buy the HX that fit my plenum, as stated above.

Do you know the temp going in and coming out of your HX? I wonder if you're getting way too much temp drop in your HX. You should be shooting for 20-30ish. If you're getting 30+ you need to push more water through that HX. If you're getting sub 20 drop you're HX is likely not doing the job it was meant to do. That means you're not moving enough energy from the water to the air via the HX. Time to upgrade in that case. I think a "high end" custom coil can be had for $400-500 if I recall correctly.

 

[Sawyer]

As expressed above, I have an undersized HX in my air handler.

I have a 3 pass coil in my air handler. It will "maintain" at design temperatures with the LP boiler and supply temps at 180* minimum. When operating with my Garn it will "maintain" at design temperatures if my supply temps are 170* or above. When I tried to set back my thermostat at night to save fuel it took forever to satisfy the heat load and still raise the house temperature. I now leave the temperature set at 70*. At 182* supply to the coil my return temp is 166.6* and drop over the HX is 17.2*. If my supply to the coil is 152* my return is 141* and drop over the HX is 11*. I am considering a larger coil for Nationwide to extract more heat so as to get more of a 30* drop over the coil at high temps and 20* at lower temps. That and a couple of panel radiators in the living room should make life much easier.