condensing boiler w/set back thermostat

bob eck

installing a TT PE110 nat gas boiler on a cast iron radiator system with 2" mains and the home owner wants to use a set back thermostat.

Setting the thermostat at 72*F when home and going to 67*F when at work and when sleeping.

Will this save the home owner money or shoud they just keep it at 72*F ?

TonyS

Turn on the boost function

When the outdoor curve is set up properly it can take a while for the boiler to "catch up".

I personally think it doesn't save any money and try to stay away from setbacks with outdoor resets because the boost raises temperature and decreases efficiency.

This would be a good math problem for Siggy.

wheels

It Will Work Great

Outdoor reset with a set back thermostat will work fine. Leave the boost to ZERO. Set the out door reset curve so that min water temp is 90. The set back thermostat will take care of timing for you (assuming you are using one with anticipating technology) BTW - the German engineers figured that 7F is the optimal setback! Make today a great day! wheels

Greg Maxwell

Setback

It would also help to use a H/W tstat with "Intelligent Adaptive Recovery". That tstat will remember how long the past calls from setback took before satisfying, and will "adapt" accordingly. That said, I use setback with outdoor reset, and still get some lag time when there are big temp swings in the shoulder seasons, but its my house, so I accept it. Also, the homeowner should understand that no system will recover quickly from a drastic setback, and homeowners sometimes think that setbacks to 55 will save money. I advocate for no more than a 5* setback. That will still give fuel savings, but will offer comfort as well.

Jean-David Beyer

Setback and "Intelligent Adaptive Recovery"

I am responsable for heat in a 200 year old church-like building. Extremely high (over 2 stories) ceilings, walls timber with spaces filled in with mortared bricks, etc. One of the rooms is allowed to drop to 42F during the week, but we try to heat it to 65F on Sundays. Forced hot air. In very cold weather, it takes about 9 hours to recover. Since it runs only once a week, the intelligent adaptive recovery does not really work. I do not believe the thermostat will anticipate much more than an hour, and do not know if it would ever recover 10 hours ahead. Since there are 4 time slots per day, I have it aim for 55F at about 12:15AM, 60F at 2 AM, and 65F at 3AM. Except on the coldest days, this gets it up to 65F around 9AM.



Of course, a larger furnace would recover faster; the current one is 125,000 BTU/hr. When we put those in, we called in three heating contractors, and the one we selected was the only one that did a heat loss. It was a little larger than the former furnace (120,000 BTU/hr). Actually there are two identical furnaces, and each heats about half of the building.

Ironman

Comfort vs Savings

Sounds like this may be a converted gravity system. Regardless, those 2" mains can radiate a lot of heat and take a long time to bring up to temp. If the boost is disabled, the customer may be complaining about comfort and calling you back to the job. The construction of the house is also a factor. An old house with large pipes and c.i. rads. is usually a high mass structure that may not be that well insulated or sealed. Putting it on setback would mean it would take longer than an "average" or better house to get back up to temp and without any boost the condition would be exacerbated even more.



I've had experience with German engineering, and as good as it is, the one thing they seem to refuse to understand is that American hydronic systems are many times existing high temp applications that are going to remain that way and their product is going to have to adapt to our market if they want us to sell it. I sell and install a lot of Buderus mod/cons, but they have no boost feature and that has not only caused issues with some customers, it has also cost them some jobs that they could have had due to their stubbornness.



Bob, I think the issue here will come down to comfort vs. savings and what the customer's willing to trade off in either area.



Wheels, I think your point is viable with better insulated houses and customers who are willing to give up some comfort to save a little. But, take either one of those out of the equation, and you're gonna have complaints. BTW, welcome to "The Wall"

TonyS

Depends on how close

you set your heating curve. I just finished a tt175 today on a old gravity conversion with 3 inch mains and three floors of radiation. I did a heat loss on the building before I sold the boiler and then a radiator count to set the heating curve. The mass of water and cast iron is similar to a radiant heated slab. When you set the heating curve properly you will only be adding what is being lost and your heating system should be running almost 24/7. There is very little extra heat being added to the system to gain temperature. Setting the TT boost will keep increasing the temp of the water until the stat is satisfied then it will reset to the outdoor mode again. It works well.

I have a TT in my shop feeding 2400 s/ft of 6 inch concrete radiant. I installed a 8000 stat to test the algorithms for setback, it will chase its tale all day, not much better with boost.  But for the cost of a setback, put one in and try it. I think alot of guys don't really set their heating curve, they just leave the TT at 186 and leave. I like to keep lowering the curve until it cant keep the building at 70 and then bump it up. This keeps the boiler running at the lowest possible temp. The system I converted today says 150 on design day (9f) so that is where I will start until it gets cold.

Jean-David Beyer

I forgot all about "boost"

My boiler has that feature. I would never use it with my radiant slab, where I use no setback, but maybe I should try it with my baseboard zone. Currently I have the reset so close that it takes several hours up there to recover from a 1F setback. Maybe with boost, I could use more setback up there.



Right now I do not use boost, but I can turn it on and off separately for each zone, so I will try it. It has not gotten cold enough to try it yet. Boiler has run about 4 hours so far this season.