Hot water heating struggling when temps fall below Zero F

GaryJB

Hi,

I have a Viessman furnace and I am observing my heat is not able to keep up when the outdoor temperature is falling below Zero. I have hot water baseboard and the baseboard temps are around 110-120F during that time. Hot water seems to be fine all the time.
When temps are above 10F heating seems to be fine and holds 68F in my thermostat and baseboards temps are between 125-135F.

Struggling to understand the problem, any insights or reasoning behind this behavior?

Thanks

Alan (California Radiant) Forbes

What model Viessmann boiler do you have? A picture would help.

You either don't have enough baseboard or the water temperature is not hot enough. Are you saying that the water temperature to your baseboard convectors is 110-120F? That's too low and you can raise it.

GaryJB

Thanks Alan for your response. We have been in this house for last 5 years and started observing this year only. From thermal temp sensor on convector baseboard I see that temps.

Attaching pictures of furnace and temp setting.

JakeCK

Where are you located? I ask because the design temperature for your area might be higher then 0f and that would mean this is normal if the boiler is sized correctly. I know you say you have been in the house 5 years but this January, really just the past few weeks is the first time we have had really cold temperatures for an extended period of time in a long time. We were around 11 degrees below normal for the last half of January here in Ohio with many nights where we were well below design. 

GaryJB

We are in Boston, MA, yes it is very cold this year (touched -10F couple of times) but something I got changed last spring so I want to make sure that is not causing this
- Got some of the hanging copper pipes in the basement replaced with pex pipes (with oxygen barrier).

I removed some power adaptor in the basement, not sure if this was connected to outdoor temp sensor?

Zman

120 is way too cold for baseboards. How are you measuring it? What does the boiler temp read? The pipes going to the baseboard would typically be 150 degrees or more and way too hot to touch.

GaryJB

@Zman - I am using a thermal sensor to get temp reading from the baseboard. I do not have a easy way getting boiler temp without opening? I increased the knob setting from 118F a bit, after this the baseboard temps are reading 135F.

Zman

Are you shooting the pipes directly? To get an accurate reading on pipes you need to put something less shiny on them. Electrical tape will work in a pinch.
Your infared will work on the pipes in the boiler room as well.

JakeCK

Reading a temperature from a shiney surface will give a lower temp than what it really is.

JakeCK

The design temperature for Boston is 9f if the table I just looked at is correct. Which means a properly sized system will not be able to keep up at 0f. 

GaryJB

Happening only this year, we had similar temps last year too in patches.

JakeCK

In patches... The length of time it is below design makes all the difference. It has not only been cold but it has been persistently cold. But let's pin down more specifics how many btu's does the boiler output, can you easily measure how many feet of baseboard you have, describe the construction of your house? Is it old construction, new? If old has it been sealed and insulated, good storm windows or newer windows, doors? Ranch, colonial, bungalow? How many square feet and is it a slab, basement, crawl space?

GaryJB

house is 1962 ranch and later half-floor built on top by previous owner.
Main floor is where I am having heating problem, whole zone has around I would have around 120'-130' of baseboard, whole floor is about 48x24=1152 soft. Windows are good but there are some cold spots from fireplace and doors, not too bad, with the thermal gun they show ~50F during freezing cold days.

JakeCK

Boiler output? Should be a boiler plate some where with all the ratings. Snap a picture of it.

Alan (California Radiant) Forbes

I'd start by playing around with the heating curve dial on the far left. Increase it from 1.6 to 1.8. If still not heating properly, bump it to 2.0.

You will also have to adjust the maximum boiler water temperature from 75C to 90C.


Here are the operating instructions for your Vitotronic 100 control:
https://www.viessmann.ca/content/dam/public-brands/ca/pdfs/doc/v100/vitotronic_100-kw10oi.pdf/_jcr_content/renditions/original.media_file.inline.file/vitotronic_100-kw10oi.pdf

GaryJB

Thanks for the manual and details, i will adjust the curve and see if it improves the heat.
Another question: does the outdoor (faulty? or disconnected?) sensor play role here?
Last sprint I had some work done in the basement and some old hanging wires and a power adapter were removed, I don't know how the outdoor sensor is connected back to the system! Too much snow near that area now, do not have easy access to open and check.

Alan (California Radiant) Forbes

The outdoor sensor is an important part of the system. Read through the literature on the Vitotronic to find out more. I just don't have time right now.

ACE_in_NH

I agree with all of the great advice you have received. The outdoor air temperature sensor as previously mentioned is critical component for the type of system you have to perform properly. You mentioned that you can't get to the sensor because there is too much snow? I am asking because the sensor's location on your home also makes a big difference when the temperatures get low like they have been. Could you detail where on the house ie. stuck out a basement window and buried in snow, up in an eave in the sun, etc. also which outside wall...North, South, East, etc. Seems like a small detail but could make a big difference when the mercury shrinks up.

STEVEusaPA

Do you have enough water in the system, and properly purged of all air? What's the system's PSI cold

GaryJB

Sensor is around 3 ft above ground on north side of the house but reaching there with 2 feet of snow last week was difficult, but now i guess i can check that.

@STEVEusaPA - i do get some wave sound when the furnace turns on, looks like there are air bubbles, i need to purge the zone for sure but do not have confidence yet to do during winter if i mess up something!