Converting 8mm microbore to 10mm

[Stuart]

Four of my 7 rads are done in 8mm microbore from a manifold in my hall and the other 3 rads are fed by 10mm tee'd in to the 22mm before the manifold . As I am getting a replacement condensing combi boiler and the fitter said 10mm was preferable to 8 I wondered if cutting the 8mm just after it leaves the manifold and then replacing it with 10mm would work better or would the couple or three inches of 8mm still create problems ( if at all) . I had thought all were done in 10mm and it was only after he left ( thinking they were all 10mm as I was) that I had a closer look and noticed the difference so lifted a couple of boards over the manifold and checked what was what .

I don't know why it was done that way ...can you not get 10mm manifolds ?

Stuart

[Doctor Drivel]

With a condensing boiler, balancing the rads and boiler to have a wide flow/return temperature differential means smaller bore pipes can be used. If 8mm was fine before it will be more than fine now.

Your fitter is probably a plumber who are good at drains.

[Peter Andrews]

Do your 8mm rads have difficulty maintaining the required temperature? If not then don't do anything.

Also make sure that your system is properly balanced and the rads are actually large enough for the room, however if you do decide to change to

10mm then all the connection needs to be enlarged to 10mm.

Peter

[Stuart]

Not sure what you mean by that last remark..disregarding the bad grammar...lol.......the system was fitted years ago and has nothing to do with the guy who was out today .

[Stuart]

The present rads work fine other than they are old and really need replacing with modern ones It's a very old Kingfisher CF60 and a Primatic cylinder setup ...I'll leave it as it is and see how it works ...if necessary the 8mm can be changed to 10mm later.

[Ian Stirling]

There is a small, but significant energy gain on a condenser, by keeping the temperature difference small.

[meow2222]

they work fine but need replacing?

I doubt a corgi would fit your new boiler without bringing the rest of the system up to modern spec

I'd look at replacing the manifolds before mucking about with bodges, but its likely you wont need to change this aspect anyway.

NT

[Dave Plowman (News)]

Assuming they are decent quality ones and you've had inhibitor in the system there's no reason rads should ever need replacing - unless you are referring to looks.

[Jim Alexander]

That doesn't make sense. What temperature difference exactly? Not "wide flow/return temperature differential" as correctly stated by DD. Perhaps you mean keeping a low average flow temperature, in which case 8mm will suffice perfectly but the radiator output will disappoint.

Jim A

[Stuart]

Why is that causing you a problem . ?.... The rads are about 20+ years old so are not as modern looking and a bit greyish and have been painted .

[Stuart]

It was looks I was meaning .

[Stuart]

Can you explain what you mean having regard to how I described the pesent setup what I said in my OP.

I'll leave it as it is and see how it works out .

[Ian Stirling]

For a given balanced system, with a given pump, there will be a certain flow.

The higher the flow, the lower the temperature difference between flow and return.

Increasing the flow will therefore decrease the temperature of the flow pipe, marginally, or perhaps not marginally increasing the efficiency of the condenser, by allowing it to condense more water from the gas exhaust.

[meow2222]

Its not, theyre your rads not mine. I was just guessing it might not be true, since such statements usually arent. And you seem to have confirmed that.

Ah well, if theyve been painted you do need new ones.

NT

[Jim Alexander]

From the Mohammed el Fayed book of central heating that. For your low temperature difference system you would need to set the boiler flow temperature at a temperature others would achieve on the return with a wide differential temperature setup (the norm). As I said, listen to DD on this one.

Jim A

[Andy Hall]

That would assume that the boiler is not measuring flow and return temperatures and adjusting burn rate accordingly (modulating).

Condensing boiler efficiency is dependent much more on the return temperature - not so much the flow. In a design for a new system, the normal choices are 70 degree flow and

50 degree return and sufficient radiator capacity, using those temperatures to meet the heat loss under conditions of -3 degrees outside. At higher outside temperatures, both temperatures can be lower than this.

This is an area where people become confused. On the face of it, a larger temperature drop implies greater release of heat coupled with the appeal of a lower return temperature and hence greater efficiency. The flaw is in missing the point that if this is achieved by reducing flow rate, there is less heat transfer anyway.

This is based on the simple formula of heat transfer being proportional to temperature rise or fall multiplied by the mass of water and by a constant.

Since the objective is to deliver an adequate amount of heat to compensate for heat losses, a boiler should ideally control the pump speed and the burn rate. However, most just control the burn rate by a simple measure of temperatures and the pump runs on a constant setting.

The main limitation is the maximum temperature drop that the heat exchanger can sustain. This is a physical limit and is generally around 12-15 degrees for a conventional boiler, 20-25 for a condensing one.

[Stuart]

I wasn't meaning they need replacing because there is something wrong with them working .I was meaning they need ( in the widest use of that word) because they don't look particularly good compared to a brand new shiny one .

[Matt]

But to do that means removing him from my kill file :)

[Doctor Drivel]

Perfect grammar.

read again.

[Doctor Drivel]

Lord Hall, please keep me in your kill file.