I have two brood boxes in place down here in Ballarat where the weather is still fairly chilly, with a few mild days now and then. The top box is doing fine. Each frame has capped and uncapped brood, some honey, and the bees are very active. The bottom box is a different story. While each frame has a lot of bee activity nothing much is happening. I can see no capped or uncapped brood, no indication of eggs and only a little uncapped honey present. One new frame I inserted a few weeks back has new comb while another remains empty. All the old comb on the other frames is very dark brown.
Hi @Tony1,
What you are describing is quite normal for multiple boxes beekeeping. In the winter queen moves in the top box. It is warmer there. She will move down when the weather will become warmer. You have two options. Leave it as it is and wait, or swap the upper and the lower box to stimulate the queen to move up again and start to lay in the box that is now empty. It is also a convenient time to replace the old comb when it is needed. While the queen has not moved down yet. Replace the old comb with new frames with wax foundation and put it on top of the active box.
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Thanks ABB, that has cleared up a lot for me actually. Are you able to give any clues on when it is time to replace old comb? My filled out frames are 3 years old.
Standard advice in the US is 3 years, but most commercial beekeepers go up to 5 years with no major issues as long as the hives are healthy. 
The brown colour in the comb is just shellac from the bee pupae in brood frames. It is not harmful, but it does make the cells slightly smaller. Some people see this as an advantage in the fight against varroa, but that shouldn’t be an issue for you, yet… 
It is not about the years, but a number of generations hatched from this comb. In climate zones where locally adapted strains stop laying for the winter, brood comb may serve longer. In Australia, we use various mongrels based on Italian bee that does not really stop laying during the winter, just slows down. Combined with short periods of cold weather, we have comparatively high usage of the comb.
Now we need to understand how bees use and service the brood comb. Each generation leaves behind a cocoon. A layer inside of each layer. Plus some feses at the bottom of the cell. Each layer reduces the volume of the cell. Bees tolerate the reduction of the cell diameter to a certain degree and then start to chew cocoons from the cell walls to keep the diameter at the minimal acceptable size. That is why even the ancient comb does not turn into pinholes with microscopic bees hatching from it. While doing that, they are much less efficient in cleaning the bottom of the cell. They compensate for the depth by elongating the cell.
Now, let’s put those two together. Counting generations of hatched bees is not really practical, and we will leave it to egg-headed research beekeepers. There is a rule of thumb for practical beekeepers to know when a comb requires replacement. Look at the Sun through the frame. If the light does not penetrate the bottoms of cells, it is time to replace this comb. It is still possible to use it, but bees spend a lot of effort to keep it serviceable. And this effort could be used elsewhere with more return.
The comb used in supers for honey only is different. It is about keeping it undamaged, for example by the wax moth or contaminated by pesticides. Well stored super comb in areas unaffected by agricultural activities can be used for 10-15 years and more.
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My experience has shown that comb replacement is more of a luxury management technique than a necessity…I’ve seen the same comb in brood chambers used for decades in commercial operations. But I’m continually rotating brood combs with honey frames these days.
All of my brood/honey frames have the black plastic foundation…Pierco. Sometimes I’ll simply manually shave the dark comb down to the black plastic foundation base…the bees seem to love this as they rebuild it out perfectly.
In the photo below is a brood frame that has been re-conditioned…I left a strip of original dark comb for illustrations sake. There are so many advantages to using plastic foundation in your frames.
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thank you ABB for your useful explanation of this.
Not sure if I responded or not. If not many thanks for your detailed reply