How to lower heating system temperature?

[Piwoslaw]

Having read about how lower system temperature increases efficiency when using heat pumps, heat buffers, condensing boilers, etc., I'm thinking about increasing the radiant area of our heating system. We have old, cast iron radiators and the temperature of the water that exits the boiler is as high as 65°-75°C (149°-167°F) during the winter months. I'd like to lower that somewhat since I'm thinking about upgrading to a condensing boiler in a few yaers when this one gives up, but condensing boilers thrive at temperatures below 55°C (131°F). Insulating to limit heat loss is in the plans, of course, but increasing the radiator area would also help.

What are my options?

1. Hydronic floor heating. But we have hardwood floors downstairs which can't be touched, while the upstairs floors were replaced only 2 years ago. The only places where hydronic floors can be installed are the bathrooms, kitchen and downstairs hallway, plus MAYBE the radiator on the stairwell could also get replaced. Radiant floors in the bathrooms and kitchen wouldn't really heat the house anyway, since the 'dirty' air from these rooms usually gets sucked out of the house.
2. Radiant walls. The walls would need to be exposed to radiate heat, so all of our furniture would have to be in the middle of the room. Plus the cost/complexity. Not really an option.
3. Warm air ventilation. Somewhere in the future I'd like to re-do our ventilation system to recover heat, then the air going back to the house could be heated. But this will not happen any time soon, plus the size of the system won't allow the whole house to be heated during the winter.
4. So I'm back to radiators. Doubling their size is out of the question - the family won't allow anything that takes up more wall space than the present radiators do, maybe I could squeeze in an extra 5-10% here or there. But maybe there are new types of radiators which radiate more heat without taking up more space? I know that our system, with huge cast iron radiators and large diameter plumbing, is antique compared to the new standards with thin, aluminum and/or copper radiators and small diameter tubing, but other than much quicker reaction time (80-100 liters of water in the system compared to our 250-300 liters) do these actually transfer more heat? And will they allow the water temperature to be lowered?

Any other ideas on how to lower our systems temperature?

[Ryland]

If you install a fan under or behind the radiator you will get more heat transfer, also if your radiators are painted it is going to be best if you can strip the paint and pant them black or with a good heat transfer paint.
For locations like bathrooms and kitchens you can get little radiators that fit in the toe kick that have a blower on them that forces air over it based off the demand for heat, you can get ones like that as well that fit in the floor or wall but they tend to get pricey.

[Daox]

Yeah, the fan idea would probably really help. You could probably get away with a computer fan with a thermal sensor. It would automatically turn on once the radiator warmed up. Just power it with a wall wart.

[Piwoslaw]

Originally Posted by Ryland

If you install a fan under or behind the radiator you will get more heat transfer

That's a good idea! I'll play with a few fans to see how to set them up so that they stay unnoticed by the rest of the family (noise, drafts, etc.).

I'll also check which method of activation will work for me: via thermal sensor as Daox proposed (better), or by a relay on the boiler or circulation pump (simpler).

Another thing that came to mind are thermostatic valves - they close the radiator off proportionally to a set temperature, so that they don't heat a room which is already warm. If I get these installed before next winter then I'll see how they effect the water temperature of the whole system.

I'm also looking for a pair of cheap thermometers to strap onto the main pipes entering and exiting the boiler. These will make it easier to monitor temps without having to bring my IR thermometer each time.

[Xringer]

Our heat is from an oil-fired forced hotwater baseboard system (typical of this area).
I ran it hot for years, most over 180f in the winter.

But, over the last couple of years, I've started using at lower temp.
140f low, to 160f high seems to work pretty well with a very low water flow.
No pump running, just free-flow.
It's not so great on a really cold day, but on most days, it does a pretty good job.

When it gets down under 12f, I turn it on to back up the Sanyo ASHP.
Last winter, we hardly had to use the water pump at all.
Since the coldest times occur when we are in bed and don't need the house to be very warm.

[Ryland]

having the fan under or behind the radiator set on a thermostat that is out in the room seems like the best idea, that way it's pulling more heat off the radiator when you want the room warmer.
I haven't worked with radiators much, but why is cooler better? your pump has to run more if it's cooler because the pump is not moving as much heat in the hot water, but then I suppose more of the heat from the flame is going in to the water.

[Piwoslaw]

Originally Posted by Ryland

I haven't worked with radiators much, but why is cooler better? your pump has to run more if it's cooler because the pump is not moving as much heat in the hot water, but then I suppose more of the heat from the flame is going in to the water.

That seems to be the main reason. Also, condensing boilers only condense the exhaust gasses (boosting efficiency by ~10%) when the system's water is below a certain temperature.

Heat pumps have a higher COP when their output is at a lower temperature. As for heat buffers, lower temps allow more heat to be extracted from them.

[AC_Hacker]

Originally Posted by Xringer

...But, over the last couple of years, I've started using at lower temp. 140f low, to 160f high seems to work pretty well with a very low water flow. No pump running, just free-flow.

Why don't you run a pump?

-AC_Hacker

[Xringer]

Originally Posted by AC_Hacker

Why don't you run a pump?

-AC_Hacker

The HS Tarm has a limited storage, (76 gallons IIRC) and turning on the
pump will deplete it's heat fairly quickly.

What I want, is a long slow release of heat. Like cutting the air back on
a wood fire. You get less heat, but for a longer time.

During that slow release time, we might also be able to add heat from
the Sanyo, solar gain from southern windows or solar PV assist heat
to the boiler.

[Piwoslaw]

Originally Posted by Ryland

If you install a fan under or behind the radiator you will get more heat transfer

OK, so I've collected a few 120mm 12V computer fans and plan to have them turn on/off with the boiler's circulation pump. I'm still trying to decide on how to wire them:

1. Put a 230VAC->12VDC transformer next to the boiler and run the DC out to the radiators.
2. Run AC to the radiators and run each fan off of its own transformer.
3. Run AC part of the way and have 2-3 transformers handling 2-3 fans each.

Option 1) will have the greatest trnasmission losses, since the 12VDC will have to go ~8-12 meters, with each fan pulling 0.15-0.25A. On the other hand, option 2) has greater losses from from having 5-6 transformers instead of just 1. Option 3) is a compromise between the first two.

So will sending 12V at up to 1A really cause huge losses over 8-12 meters? A small voltage drop (say to 10V) wouldn't be that bad since the fans would just be slightly quieter.
Oh, and another question: What if I used my metal plumbing as mass ('-' of the DC) and just sent the '+' through the wiring? Sending it through both wires should halve the resistance, but is it safe to mass it the pipes? Probably safer than massing 230VAC...