After Mod-Con install, should I upgrade baseboard radiators?

amazer98

Hi All,

I'm new to this forum and have a question I was hoping you could answer.

Six years ago my wife and I moved into this 1600sf condo in Maine. The heating system was an oil-fired boiler that fed into 4 zones-- one zone for each of the two bedrooms, one zone for the entire common area downstairs (kitchen, dining room, living room, front hallway), and one zone for an indirect water heater.

The common area is at least six times the size of either bedroom, and opens up to a vaulted ceiling with an adjacent loft. In this zone, there are five runs of baseboard totaling perhaps 50-60 feet. The hot water runs from one baseboard to the next (i.e., daisy-chained), so that by the time it reaches the last baseboard it was cooled down considerably.

Even with the oil boiler, which heated up to 198-202 degrees, the house would get a bit chilly on the very coldest days when the outside temps dropped below 10 degrees or so. The system couldn't warm the place more than approximately 68 degrees. Our heating contractor told us there was not enough baseboard in this central zone.

Two years ago natural gas was brought into our town, and we converted to a Veissmann mod con boiler. This unit heats the water to only 171 degrees and can't be adjusted to go any higher. With my Nest thermostat I can see how many hours the boiler runs each day. Even this time of year, when the temps drop only to the low 30s at night (and typically mid 40s in the day), the unit is running 14 or 15 hours to keep the house at 70. On the coldest days, the house gets to only 65 or so. We can increase the temp by turning on our gas fireplace.

Our new heating company says the best fix is to install high-output baseboard and connect it differently. Rather than daisy-chaining the baseboard radiators, he suggested installing a plenum, which would be looped to each of the five baseboards in the zone. That way, each baseboard would get 171 degree water rather than progressively cooler water, as they do now. This upgrade would cost approximately $5K and I am a bit reluctant to spend the money after already 'upgrading' the boiler.

So... my question is: would we significantly cut our gas consumption if we upgraded the baseboards? Maybe the new system would emit heat so much more efficiently that the boiler would need to run only a fraction of the time... I don't know.

Based on what I described, what would you recommend?

Leon82

There is high output baseboard like smith but also available are panel radiators which offer more mass than fintube allowing you to use cooler water to get the same heat and condense more thus extracting more energy from the flame.

The home runs from a manifold is what he's talking about. But a heatloss survey should be done before buying anything.this will allow you to size them to run lower starting water temp.

Jamie Hall

Two things here: first, the hours a mod/con runs is a very poor guide to its output, unless you also know what it has modulated to during those hours. Which the Nest won't tell you.

Second, for a given temperature inside and outside, given reasonably comparable boiler efficiencies, changing from one style of baseboard to another, or even changing the plumbing of the baseboards, won't change your fuel consumption.

Now. What will make a difference, and possibly make all the difference, is repiping the baseboards in that space so that, as your plumber suggested, they all get high temperature water.

Before you decide to go with new baseboard, though, you really do need to determine the heat loss of that space -- Slant/Fin has a nice online calculator to do that -- and see whether you really need to do that, or whether simply repiping will do the trick.

PerryHolzman

Ideal if your Viessmann mod-con is set up perfectly it should run 24 hours per day during deep winter heating season - at just enough firing rate to heat the house (that's why mod-cons are so efficient). It would run lower hours in the fall and spring where you only need heat part of the day. Note: I have a Vitodens 200 installed in my house with cast iron baseboards.

So, please ignore how many hours the Nest thermostat says it running as its not telling you anything meaningful.

Re-piping the baseboards will improve the ability of them to heat the house in the coldest weather. You may not need to split all 5 individually; it may be enough to leave 2 daisy chained in a segment.

Most baseboard heating is designed for 140 F water supply to heat the house on the "Design" coldest day of the winter. Now it does occasionally get colder than that and I have found in my own house that I need 155 F water to heat it when it gets to -30F (temperature, not windchill). as the "Design" temperature in this area is -15F (I've always wondered why its not -30F as we normally see -30 for a few days each year).

The fact that your system is needing about 200 F water to function reasonably well is in fact an indication that there is not enough baseboard related to the insulation value of your house according to modern standards for hot water heat. It actually makes me wonder if perhaps this was originally a steam system?

It may be a much better investment to increase the insulation and seal leaky windows/doors in the house than to change the heating system or equipment (increasing insulation & reducing air leakage = lower water temps needed to heat the house = lower energy bills every year).

I also recommend the Slant Fin calculator. But, also recommend a likely "whole house" energy evaluation (which includes a pre and post update pressure door test for house leakage). You do need a certain amount of air leakage for normal ventilation purposes; much more than that is often just an energy waste.

I likely accomplished nearly as much energy savings with a case of caulk and a few cans of expandable insulating foam and 2 days of work to seal up major air leakage after the energy audit identified significant air leaks as I did by upgrading my boiler efficiency.

If you are not sure of how your outside walls are insulated. You might wish to find an out of the way place to cut a hole and find out (I found out that I have 2" Rockwool in my 4" airspace. A lot better than nothing; but, also prevents blowing in insulation to better insulate the walls - and I would need to strip and replace the inside walls to really improve things).

I wish you well with this,

Perry

hot_rod

What size boiler(s) do you have in a 1600 sq ft condo? Even at a very large load of 30 BTU/ sq ft, you should only need 48,000 BTU/hr on design day. How many feet of fin tube currently?
As @PerryHolzman suggested, do a load calc, find out if the load is high, and why.
A blower door test may be available cheap or free from a local utility. I always suspect infiltration issues with high loads on small structures. Unless all walls are glass?

amazer98

Thanks for all the replies and comments, guys!

Leon82: Yeah, I had asked our heating company about those sleek European radiators, but he kept steering me toward high-output baseboards, saying they were easier to install and would work great. And, you're right-- the term he used was manifold (not plenum).

Jamie: Thanks for pointing out that changing baseboard styles won't reduce gas consumption. That's really what I wanted to know. On the coldest days, we can get by reasonably comfortably by running our gas fireplace as an extra heat source.

Perry: Thanks for pointing out that boiler running hours is not a good metric to use. I know that our house is 'under-baseboarded', but it is also decently insulated. We had a blower test and infrared heat loss analysis done a few years ago, and the house passed. We have six-inch walls filled with fiberglass and 1-inch or 1.5 inch foam board with an aluminum foil Reflextive layer. I also had our sill foamed three years ago.

HotRod: We have a Vitodens 100-W boiler with a rated input of 21 to 125 MBH - 6.2 to 36.6 kW. I think it has 94,000 BTU output. Our entire house has about 95 feet fo baseboard in total.

Robert_25

High output baseboards may be a very good solution. A manifold setup would work, but splitting the loop and using a larger pipe to feed it may work fine as well.

You mention the system as 4 zones - does each zone have its own circulator?

Have you ever cleaned the baseboards? If they are plugged with dust/debris, it will noticeably reduce the output.

amazer98

Robert25,
Yes, each zone has its own circulator. And I do periodically vacuum the fins... but probably don't do it often enough

amazer98

At this point, based on all your comments, I'm thinking I won't invest in the upgrade. If I thought I could appreciably cut back our natural gas consumption, that would be a convincing reason to move ahead. But we can keep reasonably comfortable by using the gas fireplace on the coldest days.

I can think of many more enjoyable ways to spend $5K!

Thanks for chiming in, everyone!

Robert_25

It would not hurt to call another heating guy.  A few other thoughts:

Splitting that long loop in half would provide a higher average temperature through your existing baseboards, and should cost considerably less than a total repipe.

This is what you have now:  https://heatinghelp.com/assets/Uploads/baseboar-loop-heating-temperatures.gif

This is what I am suggesting:

https://heatinghelp.com/assets/Uploads/baseboard-split-loop.gif

Lastly, make sure you don't have a bunch of furniture pushed tight up against the baseboards.  Doing so blocks the airflow. Pull everything away about 6" or so and see if you notice a difference.  

amazer98

Thanks Robert,

That’s good advice. For sure, I should pull my sofa out from the wall a few inches. Also, I like your suggestion about doing a simpler piping modification. I can totally see how that would give some of our key baseboards hotter water.

bburd

Standard 3/4” copper fin tube baseboard puts out about 600 Btuh per foot at an average supply water temperature of 180°F (Reference: Slant/Fin’s fine line 30). You are probably running closer to 160° average or less, so you would be lucky to get 480 Btuh per foot per S/F’s derating chart:

https://www.slantfin.com/images/stories/Technical-Literature/ratings_fineline30_r.pdf

95 ft x 480 Btu/ft/hr= 45,600 Btuh. You don’t have enough radiator surface to deliver nearly your boiler’s full output to the space. You may not need the full output… But clearly you are not getting enough heat from those radiators.

Any improvement to the situation by adding radiator surface and/or repiping to split that long main zone will result in heating your home to a more comfortable temperature on those cold days—by burning more fuel. 

amazer98

Thanks bburd! This is good info to know. I think the heavy duty baseboard we have may emit more btus per foot, so our whole house capacity may be 20% more... but still well under the boiler’s ability.

hot_rod

If in fact you 95' at 480/ft output at 160 AWT= 45,600
divided by 1600 sq ft of condo= 28 BTU/sq. ft.
It seems like that should be adequate, possibly shy on a design or below day.

If the oil boiler supplied 202° as you mentioned, maybe assume 180 AWT x 95' @ 600 BTU/ft = 57,000 BTU/ hr÷ 1600 sq ft=
35 BTU/ sq ft available.

You also mentioned water temperature cools considerably from first to last baseboard? What type of circulator? Could be a bump in gpm may get you the extra output.

You have plenty of boiler BTU, but that limited to 171° SWT, is gonna hurt you with fin tube.

If in fact the load is that high, look at upgrading that great room to a high output fin tube. The SlantFin H3 should get you around 600 BTU/ ft

amazer98

Thanks, hotrod! I will pay close attention to the system as winter settles in and monitor how often it struggles to keep up.

JUGHNE

With the 171 supply what is your return water temp?

amazer98

Actually, it turn out that the peak boiler temp is 174 degrees. And I have no idea what the return temp actually is or how one could measure it. But the last radiator in the loop doesn’t get very hot... and of course even the first radiator in the loop isn’t as hot as it used to be.

Big Ed_4

Condensing boilers are designed to be connected to low temperature radiation . Radiant heat or Radiators , with baseboard which runs on high temperature it would be not a match . Look into ripping out the baseboard and installing panel radiators or radiant heat ... A condensing boiler efficiency tanks supplying high temperature to (sorry) inexpensive fin tube baseboard . If you wanted not to change the radiation you would be better off with a good cast iron boiler rather then a condensing boiler..

amazer98

I know that now. When the gas company made natural gas available three years ago, our state was offering $3200 credits toward the high efficiency boilers, which is why I and many others ended up with these modcons... even though a more traditional boiler would have worked better.

Leon82

Did you find out your pump model? 

amazer98

When you say ‘pump model,’ do you mean which circulator do I have?

Leon82

Yes, if it's a 3 speed you can try the next speed up

JUGHNE

Most mod cons have a fairly high tech screen that will tell you more parameters than most people want to know.
Return temp and differential are usually usually easy to read.
If you have primary/secondary piping then it may not give you the actual system return temp.

amazer98

It turns out the circulators on our system have 3 speeds. They're currently set to #2, the medium setting.  Assuming that #3, the Fast setting, will deliver hotter water to our large baseboard loop, why wouldn't everybody select this setting?  Are there any advantages to setting the circulators to Slow or Medium speed?

In other words, what are the guidelines for determine which circulator setting to use?

Leon82

So they can make one pump for a few applications or adjustability.

I would try it at 3 for a day or 2. It's free and may help get the last emitters warmer water

amazer98

But why not make all circulators go at top speed? What kind of situation would warrant installing a circulator at a slower speed?

hot_rod

Increasing the pump speed increases the fluid velocity and it may become a noise issue, try it and see.

Leon82

It depends on the piping combination. Mine are all set to low because the deltaT would be too narrow pumping faster.

Some of the pumps like the Taco 1816 have a dial to adjust speed and others have temp sensors that will speed up or slow down to maintain delta t 

hot_rod

Leon82 said:

It depends on the piping combination. Mine are all set to low because the deltaT would be too narrow pumping faster.

Some of the pumps like the Taco 1816 have a dial to adjust speed and others have temp sensors that will speed up or slow down to maintain delta t 

While we design hydronic systems around a specific condition or delta T, fact is that delta is always moving.
On a cold start the delta will be wide, as the system approaches setpoint expect to see the delta close up.

This is true in an indirect tank, fun tube, radiant loops, air coil, etc.

Forcing the distribution circulator to run at a constrained delta will in some, most cases limit the emitter output.
It's basically a heat injection system. It can be used to lessen boiler short cycles, just know it does that by reducing or limiting the output to the loop.