New space heating and boiler options / advice

[Nickfromwales]

Excerpt from a previous thread. A member asked..... 

Hi,

I'm currently in progress with gutting, extending and refitting my 50's semi including all plumbing, electrics etc. Stud partition walls have been ripped out, upstairs floors are (almost) up, ceilings down, downstairs ceilings coming down and concrete ground floor being dug up.. I pretty much have a blank canvas on which to work having moved out a few weeks ago.

The idea is to have full UFH downstairs and radiators upstairs, with ideally an unvented cylinder to supply hot water to a bathroom and ensuite. The boiler and unvented cyclinder are to go in the loft. Due to the completely different location of the new boiler, and relocated mains water feed, all downstairs plumbing is to be re-done and the only worthwhile plumbing that could remain is the loop that feeds three radiators on the first floor.

I have a couple of questions - 
Would it be best to re-do all plumbing for heating and dhw / cold water in plastic pipe or to reinstate in copper? What are the relative merits of doing either? It would be nice to have the visible pipework for the rads in copper and I'm guessing that with plastic I'd save quite a bit of time and cost not requiring earthing of all pipework.

The other question is around the spec of a boiler. If I wish to have UFH and rad circuits along with an unvented cycinder (the latter is not critical but I think would be better due to wanting a power shower and such like) what kind of spec boiler am I going to be requiring?

Any pointers on this lot would be very much appreciated.

[Nickfromwales]

The first reply was.....

I'm not really a plumbing expert, but found plastic a lot easier to work with wherever it was going to be out of sight, You need far fewer joints and that alone is a good thing. The downside is that exposed plastic pipework looks dreadful, it's pretty-near impossible to get tidy surface runs with the stuff, so what I've done is plumb with plastic for all the inaccessible pipe runs (no hidden joints) and convert to copper for all the exposed runs.

Some words of caution about UFH. To work efficiently it needs a lot of good insulation underneath it to prevent heat loss to the ground. Secondly, the maximum heat output is restricted by the heated floor area and can be a lot lower than with radiators. If your house is being insulated and sealed to a reasonably high level then UFH should be fine, but otherwise you may well find that with an older house you just won't be able to keep it warm with UFH. As a guide, work out your heating requirement for the coldest weather you're likely to get and then divide that by the area of floor that actually has UFH pipes in. If the answer comes out at over about 50 to 60 W per square metre than you either need to reduce the heating requirement or switch to radiators.

UFH is very good, but is always less efficient than radiators and has this potential limit on output, because you can't realistically run the heated floor area at much more than about 28 to 30 deg C (you can run radiators at 45 to 50 deg C, so get a great deal more heat out of them).