Increase BTU output from hydronic baseboards to allow heating with lower temperature water

MikeMcNally

My home heating system consists of a gas fired boiler that provides heat and about 130 feet of fin on copper tube baseboards to distribute it. The system works well with water output from the boiler at 140F.

According to the baseboard manufacturer the baseboards put out 340 BTU/H with water at 140F

I'd like to replace the gas boiler with an air to water heat pump. However, I find that there is considerable doubt about a heat pump putting out 140F water. It looks like 115F would be more achievable. So, the existing distribution system will not work.

Among the options I see is replacing the baseboards with ones that will put out 340 BTU/H from 115 F water. But, doing that would include plumbing and redecorating, quite a big job.

Now, I see a brand of baseboard whose output at a fixed water temperature varies with different overall heights. It looks the additional height is just an empty space which provides a chimney effect to increase air movement through the coil.

If that is the case it suggests an alternative to replacing my existing rads with all the plumbing that would entail.

The alternative would be removing the room side covers and louvres on the existing rads to leave only the coils and their supports. Then, install custom made and painted sheet metal covers with a significant vertical dimension to create a warmed air space over the coil and thereby create a chimney effect to draw additional air over the old coil.

Has this been tried?

pecmsg

as you lower the water temperature you need to increase the output of the emitters.

There are several ways of doing it. Easiest is extend the length.

Another is to extend the height.

hot_rod

Did you do a room by room heat load? Maybe not all need to be increases? Also any structure upgrades will lower the heat loads in the rooms.

I would design around a 120SWT with a HP.

How often are you at or below design at your location?

Jamie Hall

And how much does the output vary with differing heights? You might be able, with a little math, to determine just how high the proposed "chimney effect" output increase would have to be to maintain the output of your baseboards at the low temperature.

For reference, the baseboard's output at that low temperature, without modification, will not be more than about 170 BTUh at 115 F , and likely a good deal less.

LRCCBJ

https://forum.heatinghelp.com/discussion/197653/increase-btu-output-from-hydronic-baseboards-to-allow-heating-with-lower-temperature-water

According to the baseboard manufacturer the baseboards put out 340 BTU/H with water at 140F

They only make 340 at 4 GPM.

And, there is a bit of a scam in the ratings. They are all inflated by 15% if you read the fine print. So, the best you have now is 289 at an elevated flow rate.

You'll never be successful with this system at 115F.

And a good reason to replace the gas boiler with a heat pump that is inadequate is………………………….????

hot_rod

A blower door test could help pinpoint infiltration losses

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HomerJSmith

Mike McNelly, It's more complicated than just adding more baseboard. Too long a baseboard, cumulatively, per zone, you will run out of heat energy before entering the last baseboard. As I recall, altho, I can't remember what I had for breakfast, is at 180 deg the cumulative length is 65 feet. You can increase the length by increasing the flow, however, there is a limit to flow depending on the dia of the pipe.

You can replace the baseboard with low temp wall panels that put out the same output of baseboard at a lower temp. But, remember, the water only carries so many BTUs at a given temp so it would require shorter runs.

The sys is like a commuter train. The train stops at the station (boiler) and the people (BTUs) get on and the train stops at the next station (heat emitter) and some people (BTUs) get off , soon all the people get off as the train stops at each station. Pretty soon, there aren't any more people on the train. You run out of heat energy (people).

You may need a buffer tank.

I should add environmental delta T has a lot to do with heat transfer.

fentonc

@MikeMcNally How big is your current boiler? You said the water temp is 140F, but is that just the aquastat high limit on your boiler? Mine continuously bounces between whatever high limit I set, and 20F below that, so the average supply temp is probably 10F below whatever I set it to. Does your system run non-stop on the coldest days of the year, or is it still cycling on and off all day? My CI boiler has a minimum aquastat high limit of 140F to protect it from flue gas condensation, but based on actual run times, the different zones in my house would probably be fine with 110F to 120F SWTs at the design temp if they ran non-stop.

Hot_water_fan

you can use the gas boiler for the coldest days and the heat pump for everything else. The way you could do it is have the return from the baseboards go to the heat pump, then go to the boiler. So the heat pump wouldn’t heat to 140, it’d take 115F water to 120F for example then the boiler would take it from there. In this scenario the heat pump would operate all winter but not provide 100% of the heat.

Ironman

IDK why you wanna replace the boiler with a heat pump. You also didn’t state if it’s natural or LP gas.

If it’s natural, then you’re not going to save money on fuel; in fact, you may spend more when all the costs are factored in.

There’s a big push to get everything converted to electric by the government and the power companies which doesn’t make sense to me considering how strained the grid already is.

You may also want to consider that the life expectancy of a heat pump is half that of a cast iron boiler.

All that being said, if you’re set on doing a heat pump, why don’t you keep the boiler for supplemental/back up heat? A competent hydronic contractor will know how to integrate the two heat sources.

Doug_7

May not be in your Budget, but the best way to allow heating with lower temperature water, would be to ditch the radiators and install in-floor heating. Will easily achieve 115º or lower heating water temperature. That plus tightening up the building envelope will give you the lowest long-term heating cost. This is well proven and how you would want to do it if building new.

Doug

DCContrarian

https://forum.heatinghelp.com/discussion/comment/1808836#Comment_1808836

It's not that simple.

When the temperature drops, two things happen to a heat pump: its capacity drops, and the COP drops. When you have the option of gas backup, there are two decision points:

1. When the capacity of the heat pump falls to the point that it's no longer sufficient.
2. When the COP of the heat pump drops so low that the heat pump is no longer cost-effective compare to the boiler.

There is no guarantee which one is going to happen first. This matters, because the response is different. At point 1, you want to augment the capacity, but as little as possible. At point 2 you want to just shut the heat pump off and turn the boiler on.

Shutting the heat pump off and turning the boiler on is easy, you just have an outdoor temperature sensor and a relay that turns one off and the other on.

Having the boiler augment the heat pump is complicated, because the heat pump senses load and modulates its output accordingly. There's no way for the boiler to know whether the heat pump is modulating because it senses a low heat load, or is actually unable to keep up with demand. The heat pump has to tell it. Some heat pumps — like the Chiltrix — have the capacity to call for auxiliary heat when they can't keep up. The Chiltrix can only control a resistance heating element, but that's the idea, have the heat pump ask the boiler to come on.

Note that point 1 is controlled entirely by the choice of equipment, you could choose a heat pump that always has enough capacity in your climate, although it might be ridiculously oversized in moderate weather. Point 2 is controlled somewhat by the choice of equipment — specifically the COP curve of the heat pump — but also the relative cost of electricity and gas where you are.

If it were me, I'd look at the cost of fuel. If Point 2 wouldn't be reached in your climate, I'd size the equipment for the climate and not mess around with gas. If Point 2 is reached in your climate, I'd size the equipment so that Point 2 is hit before Point 1, and just have a simple temperature-based switchover.

In either case I wouldn't try to mess around with augmenting the heat pump while it's running, it's just not worth it.