right-sized boiler: unexpected problem

Chuckles_3

Having learned that one should not oversize boilers, we got a right-sized boiler last fall. This has resulted in a problem that I never expected (and I didn't read about it anywhere).

Most of the house is 80 years old with lots of excess radiation (radiators). There is also a newer extension, with the minimum of cast-iron baseboard (Burnham Baseray) needed to heat this zone on a design day at 180F (there's no space for any more baseboard).

I was well aware that I would need to set the reset curve based on the baseboard zone, not the radiator zone. What I didn't know is that it doesn't matter how I set the curve: the supply water never reaches 180F when the radiator zone is calling for heat, never mind what the curve says.

The problem is the excess radiation in the radiator zone. When the supply reaches 150F, the radiators are already spewing out all the heat that the boiler produces, so the water can't get any hotter than that. The boiler is not undersized, so 150F is more than enough to heat the radiator zone. But 150F doesn't carry enough heat to the baseboard zone! It keeps getting colder and colder.

When the radiator zone stops calling for heat, the supply temp to the baseboard zone zooms up to 180. But this is not enough! the baseboard zone needs 180F all the time on a design day, not just occasionally.

I hope this long and involved explanation makes sense. I know the baseboard zone needs more radiation, but this is easier said than done. I alleviated the problem by shutting off as many radiators as I could; this caused the water temperature to rise to 165F. But I wish I had anticipated this problem before buying such a small boiler!

J.C.A._3

Why not....

Run the radiators only on reset,with a mixing device and put the baseboard on a second "always high temp" zone?

Rarely is it possible to run 2 different types of emitters on 1 temperature water. The fintube has no MASS !The radiators hold heat for hours vs. minutes for the fintube.

Best of luck, and please let us know what you decide to do. Chris

Mike T., Swampeast MO

It can be rough when you try to mix radiation of wildly varying temp requirement and emission characteristics with a modulating boiler.

It's not really a problem of the boiler being undersized, it's a problem with the standing iron being able to liberate heat faster than the boiler can produce it.

Here's an idea for a very simple fix--I'm assuming that the boiler is piped primary-secondary. Drastically close all of the hand valves on the standing iron--not quite "off" but very near. This should reduce flow considerably and allow the boiler to come up to the reset curve.

Boilerpro_3

A Simple solution

Add a manully adjustable 3 way valve on the supply to the radiation and a thermometer on the outlet. Mix some of the return water from the radiation back into the supply so that when radiators need 150F water, the boiler runs at 180F. I do this all the time on new installs in order to get multiple reset supply temps out of one reset curve.

Stick with your small boiler it was the right decision. It just needs to be piped properly.

Proper set up will allow you to use whatever emitters you want and at any temperature with a single source.

Boilerpro

Chuckles_3

I am leaning towards increasing the radiation in the baseboard zone. I can't add more baseboard, but I can replace one section of baseboard with a panel radiator. By my calculation I need to increase the radiation in the room by 50%, which will allow the same output at 150 that is currently achieved at 180. A finned Runtal 23" high panel rad will let me do that, and I can order the exact length so I can use the inlet/outlet of the baseboard section I'd be replacing...unless there is something I haven't thought about.

Side benefit: more condensing.

Of course it will look a little inconsistent visually. I can't find any way to match the looks of a Burnham Baseray in a panel rad!

STEVE N

I agree with boilerpro

Boilerpro has the right idea. Use a Taco 5000 series 3 way valve and knock down that water temp to the radiators. Its a super easy fix and once tuned it will work perfectly and allow your boiler to reach 180 if so desired.

Good luck

Steve

Steve Rockwell_2

TRV's

We had a similar problem, though we have baseboard and CI rads on the same zones. When we put the Ultra in last fall, I had thermostatic radiator valves installed on all the radiators, which, like yours, were oversized for the applications.

My wife, who runs hot, always hated the CI rad in our master bath because it was always way too hot when the rest of the house was too cold, loves the new system. The TRV's control the rads like they are on their own zone, so they never overheat. She likes it because she doesn't overheat, and I love it because she doesn't open the bathroom window on 5* days anymore!

.

Of course, TRVs don't help my problem in any way, but thanks.

Mike T., Swampeast MO

TRVs would help because they would greatly reduce flow through the standing rads. Down-side of TRVs, flow throttling and the three-way valve is some reduced efficiency from keeping the boiler loop at a high temp used only in one small space. The panel rad is likely the best solution efficiency wise.