New Piping of Old Cast Iron Radiators

FredRads

I own a 1905 uninsulated brick home that needs 30 cast iron radiators to meet the heat loss.  I've done the heat loss calc and its big, 260k total.  We're very excited to get them here this week, but I'm going back and forth with my plumber about piping.  My biggest radiator is 13.5k btu/hr so I assume 1/2" will suffice (barely), but I need reassurance. 

The plan for now is mod con boiler to a copper, 2 pipe reverse return, with 5 zones (35-70k per zone) instead of a home run 1/2" pex-al-pex (Im just not 100% on pex and TRVs at high temps over the long long run).

Will 1/2" give us enough heat for these monster rads?  I assume a large enough trunk (1" - 1 1/4") that gets closer to the rads and shortens the 1/2" branches will help? When rising to the second floor, should my pipes be larger or just trust the btu capacity throughout (1/2" 13k, 3/4" 35k, 1" 75k, 1 1/4 120k).  To be safe and "boost" the system should I oversize my trunks and keep the branches 1/2" or just go to 3/4" branches?

I know this can get very complicated very quickly and I appreciate the math part of it, but my house is not mathmatical (tall, leaky, uneven rooms) and every heat loss I've had done comes out different.  I am very happy with my end heat loss number, but it definitely falls within a range and I also have forced air heat to supplement. So we are seeking some sound guidelines and also reassurances that these new systems and concepts function well in old homes.

Thank you in advance.  I have already learned so much from this site, you all rock!

Jamie Hall

Keep in mind that those BTUh numbers for pipe sizes aren't really that. They're guidelines based on velocity (you don't want too much velocity) and required gallons per minute and, related, head loss. Elevation difference doesn't factor in. Length, however, is a consideration, as that is directly related to head loss.

Reverse return is an excellent way to minimise length differences -- and thus flow -- to various radiators. That's why it's used.

The main trunk lines should be bigger -- but it's not rocket science. Figure the BTUh which the trunk will be called on to supply, and size it for that. Then size the individual runouts for their load.

pecmsg

250K BTU’s per hour?
Where is this strip mall?

FredRads

I know it a big number.  12' ceilings, 7' single pane windows, a lot of unisulated exterior walls, it adds up.

Thats good news Jamie.  My trunk layouts have plenty of total btu capacity as is and I didnt want to oversize them. 

Another question: My plumber wants PEX, if we go that route does 1/2" PEX still provide ~13k btu or do the interior fittings reduce that number?

Ironman

I would highly recommend that you put TRVs on the rads, that will help compensate for any imbalances.
Also: remember that water takes the path of least resistance. Even if a 1/2” line may be sufficient for a 13k btu rad, if a 4K btu rad is also connected, it’s gonna heat up quicker. In that scenario, I’d use 3/4” for the larger rad.
Some of this goes beyond sizing tables and math and is just using common sense

FredRads

Thank you ironman, that does make sense.  My smallest radiator is around 6k, but I have a good number of 9k rads.  Do you think 1/2" PEX homeruns will suffice for those or go larger diameter for the 9ks also?

mattmia2

You can also accomplish this by closing valves most way on the small radiators and opening them fully on the larger radiators to balance the system.

FredRads

With TRVs, how do you keep the boiler from short cycling every time a valve opens?

mattmia2

What kind of boiler is it? The water and cast iron contained in cast iron boilers act somewhat as a buffer. TRVs modulate so they aren't immediate full on or off.

Ironman

FredRads said:

Thank you ironman, that does make sense.  My smallest radiator is around 6k, but I have a good number of 9k rads.  Do you think 1/2" PEX homeruns will suffice for those or go larger diameter for the 9ks also?

I’m somewhat confused: you said that you didn’t want a homerun system or pex (which is exactly what I’d do); now you’re inquiring about sizing for that?

Please clarify what you’re referring to.

FredRads

I'm trying to understand and price both options, but the TRV question could apply to reverse return or homerun.  My boiler is a mod con boiler and it just seems to me that with 30 valves opening and closing that short cycling would be a concern.  I'm imagining maybe a large tank would be required to buffer the boiler?  If not, thats great, I've never done this before and definitely want to make sure I'm not setting this up for a short life span.

FredRads

And I am still concerned about my 9k rads on a 1/2" pex-al-pex, if that will be enough.  Does anyone have experience with that?

Ironman

Again, line sizing has to be proportional and some good judgment used. I would really have to be on site or have a detailed drawing to what’s needed for sizing. They do make pex-al-pex in 1/2”, 5/8”, and 3/4”.

As far as TRVs causing the boiler to short cycle, I think that your concern is unfounded with the mass of the CI rads and a modulating boiler - assuming it’s not over-sized. The TRVs modulate, they don’t just go from fully open to fully closed.

mattmia2

With a mod con you probably want to set it up for outdoor reset. You can likely run the radiators at a lowish temp all of the time, but can run them at a lower temp most of the time. That will mean they will heat more slowly and evenly, will overshoot less, and likely will need to be turned off by the TRV less of the time.

EBEBRATT-Ed

@FredRads

Your pipe sizing looks ok. I would check the Id of 1/2" pex you are using and see what it measures. If it's not 1/2" like copper is bump it up in size.

Your original post said 5 zones so overheating shouldn't be an issue 30 rads on 5 zones is 6/zone (Yeah I know its just an average)

If you do pex home runs and manifolds with control valves on them and skip the trvs which is what I would do.

woobagooba

A few of the reputable plumbers that quoted my re-piping (Eastern MA) discouraged the use of pex-al-pex for higher water temperatures. Evidently there is some history with de-lamination. We ended up using O2 barrier pex. Might want to confirm this detail. Cheers