House replumb, new combi, lose the cold water storage

[RussTaylor]

Thanks Chris, yeah just a figure of speech really, having had it explained on this thread I'll certainly be an expert on my water mains before I choose the boiler.

My pressure gauage with outside tap fitting just arrived. Does the pipe bore affect the static pressure, e.g. my outside tap is on about an 8mm extension, should I disconnect my water mains into the house and connect it direct? The whole house is being replumbed anyway. Same question for pressure testing with a tap or two on...

I just did some googling, it seems the pipe bore will make a difference, although I don't understand that (thinking about car braking systems). It's just the size of the surface the water is acting upon that reads the pressure and that's governed by my pressure tester?

Thanks again guys, much appreciated, almost want to become a plumber all this new stuff I'm learning.

 

[mysteryman]

The bore size doesn't matter when you are checking the static pressure; it is important for maintaining a good dynamic pressure and flowrate under conditions of actual use. There's no point in fitting a storage combi if your main can deliver only 15 litres/minute.

 

[Agile]

He means an open pipe flow rate of 15 li/min.

If it was 15 li/min @ 1.0 Bar dynamic pressure then that would be quite good.

However, anyone who specifies a system without FIRST measuring the dynamic flow rate is an idiot heading for a possible disaster!

Tony

 

[RussTaylor]

Hello,

Ok I had a quick test on the weekend of the pressure, haven't had chance to rig up something to test it with some better bore pipe yet though.

Static pressure 36psi - 2.5bar
Dynamic pressure 13psi - 0.9bar

To get the dynamic pressure I turned the shower and downstairs taps on flat out, so everything was running that could be. Also the tap I measured from is teed into the same line that tap I used is on, I don't know if this is a bit drastic and I should have only put the shower on?

I'll try to measure flow rate properly asap.

 

[ChrisR]

It's only your cold mains flow you want to measure. The tank in the loft will be refilling, at some rate slower than the water can come out of the tank, but it could be a high or tiny rate. The current shower will be off the tank?

So you'll probably be left with the cold kitchen tap and outside taps, maybe a washing machine tap.
If you measure at a tap you won't be able to measure its flow as well, so just turn it on and off to see if it affects the others' flows, ....etc.
If you have a meter you can measure flow at that, depending...

Without being there - if your 0.9 bar is with over a dozen or so litres/minute it'll work. If the flows a bit low the temp can be a bit higher, etc.
There can be large variations in the mains supply pressure through the day/year - always an unknown. Check at 8am.
Check the boiler flow versus pressure figures if they publish anything. Check everything really...

 

[RussTaylor]

Hi,

Yeah, didn't turn on any taps from the tank, just ones run direct from mains (kitchen tap and shower) to get as much pressure drop as I could for dynamic pressure reading. But my static pressure of 3.5 bar is quite good right?

I need to rig something up direct off the mains to measure flow properly. I'll make something that goes from 25mm mains to two 15mm pipes on a T piece. I'll leave one open and put a tap on the other to measure the dynamic pressure from.

If I measure the litres/m of the 25mm mains, and halve it, would that be a realistic underload flow rate reading?

I have all the plumbing exposed, and I did everything so far myself, so I can chop things about as needed and know my system inside out now. Anything I can do to ascertain if I have enough water for this 837/937 boiler I'll do, just let me know what

Thanks very much

 

[ChrisR]

3.5 bar is quite good right?

Right.
Depends then on the resistance from the road to your house, and them IN your house.
Sounds promising but there's no yes/no answer until you connect everything in, and wait for all the pressure variations.

If you've used a couple of cheap 1/4 turn isolators and the pressure drops to 1 bar in the street when you want a shower, it will be a sub-optimal shower. But it'll still get you wet.

 

[RussTaylor]

Thanks Chris.

I'll avoid cheap isolators then, never considered that, thanks!

My plan was to run 22mm from the mains to the boiler, and T off that in 15mm for the rest of the house. So from that run one 15mm through the house and T off it for each tap.

Does that sound reasonable? Or should I run 22mm across the house and T off that in 15mm?

 

[Agile]

Hi,

But my static pressure of 3.5 bar is quite good right?

Thanks very much

That would be better but you only said it was 2.5 Bar. You must get these things right!

Best to measure pressure at garden tap or WMC valve.

Use 22 mm to boiler input an d15 mm on the outlets. Best of all to run single 15 mm plastic to each HW outlet from boiler outlet.

 

[DP]

DH, what is the relevance of following statement

You mention Vaillant 937 and 838. There's no point installing one of these in a three bed house as the central heating output will be much too high.

The boiler will modulate down to 12kw. 37 or 38 designation is DELIVERY OF POWER to HOT WATER. CH is capped at max 30kw or thereabouts.

 

[ALEC1]

actually the new ones go lower and the heating can be rated to avoid ramping up to fast...

 

[RussTaylor]

So now my gf is considering UFH for the bathroom, so that's added some grief. Could go electric, but will try to figure out how to do it with the 937, although I believe that makes it complicated.

Here is my plan, so mostly 15mm copper, seperate feeds to different areas of the house (upstairs/downstairs).

Any schoolboy errors please?

Thanks

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[RussTaylor]

Anyone please?

Still waiting on Vaillant about the UFH, electric might be better or not at all

Thanks

 

[ALEC1]

UFH is nice but costly in wooden floors...excellent in screed with tiles on top and but in wood still requires a high flow temperature to overcome the thermal resistivity of wood so no energy efficiency gain there. In small areas electric heating is easier probably to install...(ie less expensive)...

if it is house comfort you are looking for then go for the compensation controllers VR470 or 392...they avoid over and undershoot of normal heating controls and people comment that a compensated controller gives a nice even temperature throughout the house just like underfloor heating...so its a big saving to adopt these controls as opposed to installing UFH...

 

[RussTaylor]

Thanks Alec.

Yes was going for a 470 with weather compensator.

The UFH is for a first floor and just to help the floor dry/remove the cold tile feeling. The UFH would be sandwiched above the floorboards and levelling board and the tile, so hopefully the heat would be ok. I'd still need a towel radiator.

I think I should just get a bath mat but I think my gf would disagree ...

Do my pipe sizes look right please?

Thank you