I too was considering spiral balances, I figured that since my original rebated reveals have been bricked up I wouldn't have space to make weights work without it looking weird. But I was walking round Beamish the other day and noticed that a bunch of their windows are just installed in a flush reveal anyway and you wouldn't question it. I think they're probably salvaged windows that they've made fit. But the difference in frame size between weights and spirals might actually be less than you think, and there are some rectangular weight sections that you may be able to work with to squeeze them a bit thinner too.
Still early days, but the most confusing thing to me so far is the sealing details. There doesn't seem to be a clear "best practice" option, just a list of options as long as your arm. But it's quite an important thing to get right. Some have a seal on the parting bead, some have two seals on the inside and outside of each sash, there can be all sorts of different brush seals and compression seals depending on where it is on the window...
I can imagine a way of doing most things with basic hobbyist tools, but the one thing I haven't seen a good method for is making the stepped cills. All the youtube videos use some crazy industrial router or circular saw setup. I guess you could hand plane it, or is this something people tend to buy pre made?
I suggest seal the lower sash by routing in a pile carrier channel 8mm by 3mm along the bottom and one accross the meeting rail.
Then use parting bead and staff bead with built in pile carrier.
If the parting bead extends over the head then there is no need to apply pile carrier to the top sash, if it doesn`t then use it there as well.
I prefer the 2 part plastic parting bead as you can pop it out for maintanance easily but you can get wood ones if you prefer, the reason I suggest using the beads with carrier rather than built in pile is because then you can choose which pile or Schlegel type seal you need for the particular gap.
I would also advise fully painting the sashes before adding the self adhesive carrier to ensure a really good waterproof seal there, do the channel a little oversised to account for paint.
I do not believe there is any advantage to putting a seal on the edges of the sashes and have often seen these causing binding with seasonal movement and I always remove them if I refurbish a window with them on.
Mightons version of the plastic 2 part parting bead is better than the reddiseals one, also their beads are cheaper in general and good quality, the stick on pile carriers are the same.
Jewsons stock box sash cill but they call it door cill its Meranti and varies in quality a lot I always used to try and find the heaviest ones., I make my own now from Utile, don`t let them charge more than about £35 a linear metre because they will try it on !!
If making new boxes use the pulley covers that go on the inside of the box as well.
Do not use spiral balances, just no advantage to them. They are difficult to balance perfectly, the metal bracket is a nightmare with tiny screws that will eventually lead to rot in the corners and rust out. They will break at random eventually and they are inclined to smash you in the fingers with the winder when installing them.
A well balanced window with weights and cords will work effortlessly.
