Cable for hot water cylinder and UFH

[SJRSJR]

Hello

I’m looking to run cable for underfloor heating to a bathroom and an en suite directly back to the consumer unit as I’ve read that’s what’s recommended (although possibly not mandatory). It’s a total of 12.5 sq m and 100w mat so I make that 1.2 kW.

However at some point in the future I’d also like to add a hot water cylinder to the current combi boiler and in that case I think a dedicated circuit back to the consumer unit would be necessary. (There is space at the CU.)

The upstairs is on two separate rings.

Looking at the existing cables I think I could just about route one new cable (2.5mm max) into the CU cupboard, but two would be difficult because they crowd through a hole into the wall cavity.

Do I have any options as regards combining the hot water cylinder and U/F heating on one circuit? Or adding the UFH together,or separately, to the rings?

I know new circuits are notifiable and it will all be completed by an electrician. I’m just looking at cable runs. Thanks.

 

[EFLImpudence]

Yes, you could have both items on the same circuit; dedicated does not mean exclusive - it means designed for such items.

If there will be just one element in the cylinder, that would be 3kW plus 1.2kW so 17.5A.
Therefore a 20A circuit with 2.5mm² T&E cable (assuming no derating factors because of the cable route) would be adequate.

Why would you want a cylinder when you have a combi-boiler?
Also electric under-floor-heating is not exactly wonderful for a bathroom.

 

[SJRSJR]

Thank you.

In my previous place the combi boiler was a pain if you wanted to run two showers at the same time or flush the loo without the flow dropping. I think that’s usual with combis so I’m thinking ahead.

The UFH is only to take the edge off the tiles, there will be radiators. But yes, I am still on the fence but want to run the cable ‘just in case’.

 

[EFLImpudence]

In my previous place the combi boiler was a pain if you wanted to run two showers at the same time

Don't then.
Have you thought about the plumbing?

or flush the loo without the flow dropping. I think that’s usual with combis so I’m thinking ahead.

Mine doesn't.

The UFH is only to take the edge off the tiles, there will be radiators. But yes, I am still on the fence but want to run the cable ‘just in case’.

Ok.

 

[SJRSJR]

Have you thought about the plumbing?

Yes, I’m putting in 22mm pipes to boost the flow rate. I’d like to avoid the expense if possible so I’ll see how it goes. But if I do need a cylinder I want to put the pipes and cable in place so the place doesn’t need to be torn apart later on.

Mine doesn't.

That intrigues me - for me it’s always been the downside of combi boilers. It was fine living on my own but even then I had to wait for the cistern to fill before the shower got to full flow.

 

[endecotp]

The UFH is only to take the edge off the tiles,

If you want it to feel warm, put down something that feels warm (e.g. vinyl or cork). Don’t put down something that feels cold (i.e. tiles) and then spend electricity to warm it up.

 

[Harry Bloomfield]

The UFH is only to take the edge off the tiles, there will be radiators. But yes, I am still on the fence but want to run the cable ‘just in case’.

All the bathroom hot water pipes, and heating pipes, in my house, run directly under the bathroom floor, which means the floor during heating season, is always warm.

 

[ericmark]

Why would you want a cylinder when you have a combi-boiler?
Also electric under-floor-heating is not exactly wonderful for a bathroom.

That was in a way my thoughts, but also I have to admit, instant water heating has some problems.

The UFH is only to take the edge off the tiles, there will be radiators. But yes, I am still on the fence but want to run the cable ‘just in case’.

I have done that, and it was just a waste of cable.

I ripped up the floor in mother's house, both to lay pipes, and insulation for the electric under floor heating. Once insulation fitted, the floor was not that cold, even with tiles. And the heating was in real terms a waste of time, materials and money.

I moved to instant water heating around 1980, the idea was to release the space used by the cylinder, and it did, first it was a bedroom, then an office, at that point not combined, but two independent boilers, there were problems, the pipes were too big, so took long time to get hot water to kitchen, and the boiler had to warm up, which further increased the delay, and taps had to be full on, or the boiler would cut out. And of course took an age to fill a bath, and the shower had to have a large rose so it passed enough water, and we were having problems with mould as the shower was over the bath.

Parents had their tank removed around 2006, by this point the combi had hit the marketplace, to get rid of the problem of having to turn taps on full, it had a small reservoir of hot water, great for the taps, useless for the shower, as the water started cold, went hot, then cold again, and then hot and stayed hot. One could turn the reservoir option off, called "eco-mode", but then back to having to turn sink taps full on.

This house back to storage tanks, main advantage is free hot water from solar panels in the summer. I can't stop central heating when running heating the DHW, but don't need to run boiler to get hot water in the summer. When we had a burst down the road, we still had water. Some cold taps stopped working. And we have an airing cupboard again.

I wish our showers were direct from hot water tank, but surprised how good they are, smaller rose of course, today I note when wife had a shower, it showed 1.5 kW coming from the grid, so it seems around 0.5 kWh from battery and solar, and 0.5 kWh from the grid, so shower cost around 21p in cost for import plus loss of export. At that cost, not worth changing shower type.

Yes if daughter and wife took a shower at the same time, at night at around 5 am, when likely no battery power left, we could be using 70 amps, and the house is on a 60 amp fuse. But that is an unlikely scenario.

But a combi boiler (oil) is really just a smaller reservoir, so what would be the point?

 

[SJRSJR]

So you’re saying that insulation below the tiles will make a noticeable difference? Was it PIR board or rockwool?

I’d like to save on the expense of UFH where possible but I had it in a previous place. The room was tiny and had no radiator so in winter it was freezing The UFH took the edge of it especially in bare feet. I don’t known if it had any insulation.

This is my one and only chance to do it so I don’t want to regret my decision!

 

[ericmark]

We had around 9 inches of polystyrene, then the plywood on top of that, the floor had to come up for drains so may as well put a load of insulation, this bit was not done by me, and the builder made an error, so finished floor level about ½ inch too high.

It was used a bit when it first went in, but heat uptimes were too long, it was ½ hour before you could feel any warmth with the hand, and an hour before warm to walk on, and it simply would not heat the room, the extractor fan removed more heat than the floor could give. Lucky, we had a towel rail.

When the sensor failed, it got stuck in the pocket, so could not be replaced, but even left on 24/7 it did not get that warm, so did without it. It was, all in all, a waste of time and money.