Fan coil water temps with Condensing boiler?

wkotelko

Hi guys,

Looking for some insight to my project.
Background:
Purchased a home last year which has a fairly complex heating system and after all the tinkering I have done I think it is a poor design. Have also concluded that it was very poorly maintained and installed.
So its middle of February in Alberta -30C my Original boiler (NTI TI 200 installed in 2012) goes down on low system pressure after my glycol make up tank pumped dry. Luckily with gas fireplaces on upper and lower levels the house kept warm. After hours of troubleshooting (being new to hydronics) I concluded the heat exchanger had failed.
Working hastily since I had no DWH, radiant, or forced air heat I installed a new Weil McClain boiler WMB-155h that I got a great deal on. The replacement heat exchanger for the NTI ti 200 was going to cost almost 2x as much as the new boiler not to mention that model is discontinued.

The new boiler is installed and working and I feel that downsizing the boiler was a good move since it is keeping up in the cold.
The system is piped primary/secondary with the primary loop handling priority DHW through a 40G indirect tank. The secondary loop has a fan coil and 5 radiant zones with the fan coil first to pull from it. Primary and secondary pumps are 3 speed Grundfos 26s and the 5 zone circulators are a taco 006. The fan coil is a lennox AM-30 and has its own built in taco circulator as well.

Questions:
1) The fan coil needs high water temps to be effective heating the upper level of the house. I have been running water setpoint at 150 - 160 and its working but its not allowing the return temps to be low enough for good efficiency, especially when the radiant zones are satisfied. With the outdoor reset connected the fan coil runs for ever, never reaching temp upstairs. Is there anyway to make this work properly? I feel the only way to make this system most effective and efficient is to eliminate the fan coil and install a HE forced air gas furnace. We are frozen 6-7 months of the year and its nice to be comfortable without wasting energy. The old boiler ran its whole life at high temps being poorly configured and I believe it took years off its life and wasted lots of fuel.

2) Also, when DHW is called to the indirect tank, I needed to limit the max modulation to keep the boiler from short cycling as it was overshooting the setpoint. It doesn't cycle now but will run at 170 steady until its satisfied. Even know it takes a matter of minutes to satisfy the call wouldn't it make sense to run cooler temps like 120-130 and run a little longer to make the boiler more efficient?

Brewbeer

Can you swap out the fan coils for some that will work at a low enough temp?

pecmsg

Why are you running high temperatures? If you lower the supply temperature are the rooms comfortable?

EBEBRATT-Ed

@wkotelko

I would put a larger fan coil in rather than go through the furnace aggrivation. Fan coils are designed to run higher temps if you blow air at 100 degrees or less it will feel cold.

You need a supplier who knows fan coils and can select one that will heat the space at 130 deg water.

This will not be a stock fan coil . So you need a larger coil with the same airflow as you have now.

wkotelko

Running a lower temp works well for all the radiant zones that are in concrete(basement and garage). Two of the radiant zones are upstairs under tile floor. To make these rooms comfortable they need also seem to need hotter temps as the heat does not hold long in the floor. I have no idea how they insulated under there. The area we spend most of our time is in the kitchen and upper living room heated with the fan coil.

Jamie Hall

As @EBEBRATT-Ed suggested, you need to find someone who can provide you with a fan coil unit which will provide the heat you need for the space with a lower input temperature. It's a matter of finding someone who can actually do the math. Not that hard, but not plug and play, either.

From there it's more of a control problem than anything else. You may find that you need to mix down the two distinct radiant areas to different temperatures, using the fan coil and domestic hot water as the higher temperature circuit. With the proper controls and valves, it should be possible at least most of the time to keep the radiant zones running at their happy point all the time, with the DHW and fan coil running at a higher happy temperature when they are needed, and then the boiler condensing most of the time when it is firing (nothing like radiant floors to be able to keep the return temperatures down!).

If the radiant is run continuously, you may find that that upper floor doesn't need to be quite as hot.

wkotelko

Good info thanks guys. Higher room temps in the two basement zones definitely improved efficiency . I was keeping them at 18C but just moving them to 20C and stabilizing I can see the boiler is working more efficiently keeping return temps low enough to condense. I do feel that extra heat upstairs. I will need to do some homework and get someone to spec out a different fan coil...thinking about It, its quite possible that this fan coil is scaled up internally and the heat is not transferring well..It is almost 20 yrs old. Could be worth removing the coil this summer and having it deep cleaned in an ultrasonic bath or something.

Jamie Hall

A deep descaling after 20 years? Surely wouldn't hurt...