Question for Brad

mel rowe

I try to never miss one of your messages, since I have learned so much from you the last few years. I just read with great interest your post related to sizing for a replacement boiler. The part that really caught my attention was the suggestion to adjust the pickup factor and EDR for the pipes, if they were well insulated.
My question is -- Why not do this for rad. EDR as well, to reflect the difference between an old house with basically no insulation, as compared to an updated one with thermopane windows, insulation in walls, and better attic insulation? If you stretch to consider the air space around the rads as "insulation", then like the pipes, it seems the effective rad EDR should also be adjusted to reflect the better building insulation surrounding the rads. Just as with the pipes, shouldn't the attached load go down with increased insulation. This makes some sense to me as an explanation for why a number of boilers sized for the attached load (including my own now) only need to run about half of the time to hold 70 degrees, even on design day. Doesn't this suggest that the old EDR tables may need to be factored to reflect the amount of insulation currently installed? This would be somewhat like using heat loss calculations for other type systems.

Brad White_191

So how are things at Mel Rowe's Place?

Hi Mel

This is the best answer I can come up with.

Radiators and insulated piping are two different beasts. If the piping were not insulated, it would "all be EDR". Alas, the piping is there as a transport path. Like a road, it wants to be efficient and not defeat the purpose of delivery- heat to the spaces in which you live.

So, we insulate the piping, to lessen the burden on the boiler and speed delivery.

The radiators are a volume to be filled, simple as that, to a given temperature more or less. They do not care that the walls are insulated for they are intended to heat the surrounding air. The air they see is the same temperature regardless.

The question then is, how fast can we heat that air or, how well will that temperature be held once warmed?

Yes, you should insulate the house- to do so means that the house will come to temperature, possibly with the radiators "less than full" -at least after the first cycle or two.

But on that first cycle, from a cold start, even insulated piping may well be at 60 degrees. The radiators too and so the air around them. We have to drive the piping first, get it to near steam temperature, then go on to fill the radiators, then keep filling them until the air (space) temperature rises to setpoint.

The second cycle has an easier time of it. The piping is about 200 degrees this time, and so it goes. But the house, now insulated, reaches setpoint in less time than it once did.

So, the house load is less, the time it takes to heat is less, but filling that system, warming all of that iron (unchanged from when the house was built), remains the solemn duty of the boiler.

Does that help?

mel rowe

Yes Brad, that helps a lot. I was erroneously thinking that on design day the boiler would be running 100 % of the time, if it was sized correctly. What you have clarified is that in reality the capacity of the boiler has to be enough to fill the pipes and then the rads from a cold start. Obviously there would then be less of a load on the following cycles with the pipes hot and therefore less than 100% run time. I would guess that this automatically says there will probably be at least 25% or more down time during subsequent cycling, even on design day and with a perfectly sized boiler. My wife will be especially glad to hear this since I was threatening to shut down our boiler and do some experimenting during this spell of zero degree weather. LOL Thanks again.