Standing and Tachnun

In Minhag Ashkenaz, are we supposed to stand for any portion of tachnun? The Hirsch siddur gives no instruction for standing by tachnun but does give such instruction in other places (like the end of pizukei d'zimrah).

I received an answer to this question that the custom is to stand as each part of vehu rachum contains 18 names of Hashem. With permission, from Dovid Yechiel Cohen -

"I have never seen a minhag to sit for Vehu Rahum on Mondays and Thursdays as mentioned in the Shulchan Aruch and Rema orah haim 134. This was also the minhag in Franfurt A.M, as mentioned in Divrei Kehilos and this is also meduyak from the Maharil as he used to sit down after Vehu rachum and wait for the Chazan to start Tachanun.

"The origin of this Minhag is brought by the Kolbo Perek 18 When a boat of Jews exiled from Yerushalayim arrived at a city they were asked where they had come from, upon hearing that they are Jews the Ruler said to test them by throwing them into the furnace as Chananya Mishael VeAzrya. When one old man was thrown into the furnace the fire divided into three and there were three old men in the fires each said one part of Vehu Rahum. Each part contains 18 names of Hashem and is like Shmone Esrei to be said standing.

"If someone is not able to stand for so long he may say it seated."