Valliant 937 suitable? No one knows!...

[jetsetwilly2000]

You are heading for a problem!

A flow rate of 20 li/min from an open tap is NOT suitable for getting 20 li/min out o a shower.

You need a DYNAMIC flow rate of 20 li/min @ say 1.0 Bar.

Tony

OK, so as instructed I went away and tried to find a good definition of dynamic flow rate - I didn't get very far! But I can see how it could be defined as 'flow rate at a given pressure, that pressure to be achieved by opening other outlets' - is that right? How do you measure it?

I also found another thread which described the vagaries of boiler specs, basically saying that mfrs quote for 35deg rises but actually the boiler is usually set up for a larger rise (42deg) at a lower flow. So we might measure flow at 16l/min and think that's too little for a 937, when in fact it's more than the boiler can produce...

 

[Agile]

I measure the dynamic pressure as thats the only way to get a meaningful idea of the pressure/flow rate.

Typically I measure the flow rate obtained whilst leaving a pressure of say 1.0 Bar in the supply pipes.

Just measuring the open pipe flow rate leaves you unable to accurately guess the actual flow rate that you will obtain through a combi into a shower.

Very typically the dynamic flow rate might be HALF of the open pipe flow rate but that can vary according to the mains pressure and the supply pipe resistances.

Tony

 

[jetsetwilly2000]

I measure the dynamic pressure as thats the only way to get a meaningful idea of the pressure/flow rate.

Typically I measure the flow rate obtained whilst leaving a pressure of say 1.0 Bar in the supply pipes.

Just measuring the open pipe flow rate leaves you unable to accurately guess the actual flow rate that you will obtain through a combi into a shower.

Very typically the dynamic flow rate might be HALF of the open pipe flow rate but that can vary according to the mains pressure and the supply pipe resistances.

Tony

So how do you do that? Stick a pressure gauge on a tap and then open other taps till it reads 1.0 bar?

And why 1.0 bar? What are you allowing for, other water use or losses in the boiler and pipes?

 

[Agile]

Pretty much what you do.

I suggest 1.0 bar because thats often the minimum pressure that shower makers specify.

When you take hot water you lose pressure through the resistance of the supply pipes and through the boiler.

Then with a shower you need 1 bar pressure to provide that exciting tingling feeling as the high velocity water jet impinges on your back or other body parts.

Even measuring the dynamic flow rate you are not ensuring that everything will be perfect but its far better than wrongly measuring an open pipe flow rate and then wondering why it does not perform as expected. But thats what 90% of plumbers seem to do.

Tony Glazier

 

[jetsetwilly2000]

Pretty much what you do.

I suggest 1.0 bar because thats often the minimum pressure that shower makers specify.

When you take hot water you lose pressure through the resistance of the supply pipes and through the boiler.

Then with a shower you need 1 bar pressure to provide that exciting tingling feeling as the high velocity water jet impinges on your back or other body parts.

Even measuring the dynamic flow rate you are not ensuring that everything will be perfect but its far better than wrongly measuring an open pipe flow rate and then wondering why it does not perform as expected. But thats what 90% of plumbers seem to do.

Tony Glazier

So given that we're replumbing the whole house, how do you suggest I maximise our chances of a good flow at the shower(s)?

We have a funny squashed-looking main that might be 1/2" (1956 house), and 15mm after the indoor stopcock, and lots of funky runs over doors because of our solid downstairs floors.
We are going to site the new combi in the downstairs utility room which is right next to the rising main, and from there it can run straight up to the shower(s).
I was thinking of asking for 22mm to the boiler and for the hot water, and ultimately replacing the main from the outdoor stoptap with 25mm. How much of a gain would you expect form that lot, or am I just being naive?

 

[Agile]

It all depends on the flow available from the street.

You might as well fit all 22 mm up to the boiler input.

All HW output from the boiler should be in 15mm.

Ideally a seperate 15mm from the boiler just to the shower as thats the critical flow rate.

But I have only once seen a spur system where a seperate 15mm was taken from to boiler directly to each outlet. It uses a lot of tube.

Also all hot water pipes should be lagged!

Tony

 

[jetsetwilly2000]

well we have 3.0 bar static, and about 16l/min at the kitchen tap

 

[Agile]

That is not very good! But its the dynamic measurement which would count.

Tony

 

[jetsetwilly2000]

That is not very good! But its the dynamic measurement which would count.

Tony

Which isn't good? Specifically, is 16l/min what you'd expect from 3.0bar?

If it's lower, then surely opening up the pipework would help? How is the mains pressure regulated? Is the street supply usually so abundant that you can't create a drop in pressure no matter how big your house's main?

 

[jetsetwilly2000]

Back to the OP's point, I've been looking at boilers and considering overspeccing our combi, but nobody's mentioned central heating so far. I've looked at a couple of online tools and used a 50W per m3 calculation, and I get to about 10-13kW for our 4bed, 160m2 house. Does that sound right? Seems low!

The big combis that I'm looking at seem to go down to about 12kW, which if my calcs are right seems quite close, and I don't want a cycling boiler.
The WB combis go lower, and the Highflow 440 goes down to 7.4 kW, but then we're back to whether it'll be a problem that our water flow may not match up to the boiler - which was the OP's question!

ho hum...

 

[ianniann]

Which isn't good? Specifically, is 16l/min what you'd expect from 3.0bar?

If it's lower, then surely opening up the pipework would help? How is the mains pressure regulated? Is the street supply usually so abundant that you can't create a drop in pressure no matter how big your house's main?

Admittedly, there has been a lot of discussion but you haven't really taken it in. The flow rate is not particularly closely related to the pressure. A simple way to see this is to empty a bucket over you head. Very little pressure but a spectacular flow rate. Similarly, the flow rate from a tank in the loft could be two or three times what you quote at a much lower pressure.

In your case the pressure is quite good but the flow rate is mediocre. I'm not sure how or where you measured it, but 16l/min from a fully open tap is nothing special and you obviously wouldn't get so much through a shower. Opening up the pipework will almost certainly improve the flow rate but depends on the pipe run you need. Hot water flow through a combi may be limited internally anyway.

 

[ianniann]

Back to the OP's point, I've been looking at boilers and considering overspeccing our combi, but nobody's mentioned central heating so far. I've looked at a couple of online tools and used a 50W per m3 calculation, and I get to about 10-13kW for our 4bed, 160m2 house. Does that sound right? Seems low!

The big combis that I'm looking at seem to go down to about 12kW, which if my calcs are right seems quite close, and I don't want a cycling boiler.
The WB combis go lower, and the Highflow 440 goes down to 7.4 kW, but then we're back to whether it'll be a problem that our water flow may not match up to the boiler - which was the OP's question!

ho hum...

Sounds believable. It could be anything from half that to double, depending on your house design and quality (or quantity!) of insulation.

You will have a cycling boiler. Get used to it. The (say) 10kW is what you would need on a very cold day (or more likely night) to reach and maintain a comfortable temperature. In practice, most of the time you will need less. You can easily see this by taking your total gas usage in a winter month and dividing it up into hours. You will probably end up with something like 5kW as a 24/7 average. Obviously you don't run the heating 24/7, but still there will be plenty of times not in the middle of winter when you need nowhere near 12kW once the radiators have heated up. Then your boiler will cycle, probably several times an hour, to avoid overheating the water. Most likely your thermostat would cycle the boiler every hour or so anyway even if it could modulate down low enough, unless perhaps your system modulated down the flow temperature, but then the boiler would be forced to cycle to keep the flow temperature low/ Just have to accept that.

A 40kW boiler does heat up the radiators spectacularly fast when you turn the heating on

 

[kevindgas]

Back to the OP's point, I've been looking at boilers and considering overspeccing our combi, but nobody's mentioned central heating so far.
ho hum...

just work on a ballpark figure of 1.2 - 1.5Kw per rad /rail this will give you a rough guide to heat required

so 7 rads + 1 towel rail @ 1.2kw + pipework say 3kw = 12.6kw

i just use 8 rads @ 1.5kw (I don't bother with pipework) = 12kw

these figures have done me ok for 30 years!! but i'm sure some pedant on here will argue!!

 

[The831Bunny]

Back to the OP's point, I've been looking at boilers and considering overspeccing our combi, but nobody's mentioned central heating so far.
ho hum...

just work on a ballpark figure of 1.2 - 1.5Kw per rad /rail this will give you a rough guide to heat required

so 7 rads + 1 towel rail @ 1.2kw + pipework say 3kw = 12.6kw

i just use 8 rads @ 1.5kw (I don't bother with pipework) = 12kw

these figures have done me ok for 30 years!! but i'm sure some pedant on here will argue!!

Spot on I always use 1.5Kw for terraced or solid brick properties, 1.2Kw for newer spec insulation & 1Kw for WC or systems that are continuously on (OAP etc). This works fine when range rating condensers