New Combi Boiler - Flow and Return Pipes

[doitmyself]

15 mm tube normal design 1 m/s will carry 0.141 ls or heat rads up to 5.9kw the pressure drop pm run will be 1000pa
15 mm tube pushed to 2 m/s will carry 0.205 ls or heat rads up to 8.7kw the pressure drop pm run will be 1600pa

If the velocity doubles then the volume doubles.

You have assumed a temperature differential of 10C. Modern condensing boilers are designed for a differential of 20C. This required a flow rate half that of a 10C differential, so a 15mm pipe can then carry twice as much heat at 20C than it can at 10C. Of course the rads have to be sized correctly taking the reduced output due to a higher differential.

A max velocity of 1 m/s is the norm for domestic systems.

 

[happyflyer]

If the velocity doubles then the volume doubles.

You have assumed a temperature differential of 10C. Modern condensing boilers are designed for a differential of 20C. This required a flow rate half that of a 10C differential, so a 15mm pipe can then carry twice as much heat at 20C than it can at 10C. Of course the rads have to be sized correctly taking the reduced output due to a higher differential.

A max velocity of 1 m/s is the norm for domestic systems.

I have not assumed anything, I have worked those figures out on DegC drop as you spotted and you are right that with 20 DegC drop the flow rates would be different, I have not assumed anything with regard to temp drop across rads just in case they have been sized for 10, the worst scenario. You need to start somewhere with novices, until now I didn't even have the rad outputs, we need to move slowly we these people too much info and they will go into overload.

Doitmyself, perhaps you could show this guy the way forward on his pipe sizing, I am sure he would appreciate it.

I can't find nothing wrong with you statement at all.

Good luck with him.

 

[doitmyself]

I have not assumed anything, I have worked those figures out on DegC drop as you spotted

Then you have assumed, as the basis for you calculation, that the differential is 10C.

But that's a side issue. You haven't answered my, implied, question about flow rates.

You said:

15 mm tube normal design 1 m/s will carry 0.141 ls or heat rads up to 5.9kw the pressure drop pm run will be 1000pa
15 mm tube pushed to 2 m/s will carry 0.205 ls or heat rads up to 8.7kw the pressure drop pm run will be 1600pa

I was querying why the flow rate had only increased from 0.141 l/s to 0.205 l/s even though the velocity had doubled from 1 m/s to 2 m/s.

 

[happyflyer]

Then you have assumed, as the basis for you calculation, that the differential is 10C.

But that's a side issue. You haven't answered my, implied, question about flow rates.

You said:



I was querying why the flow rate had only increased from 0.141 l/s to 0.205 l/s even though the velocity had doubled from 1 m/s to 2 m/s.

take at Look:

 

[happyflyer]

take at Look:

View attachment 15746View attachment 15747

These are CIBSE water flow rates and you can work on the basis of constant velocity, constant pressure drop or just ignore them both and work on mass flow rate
and read off PD and still look at limiting velocity in the stepped bands. You have the best of both Words all based on water in pipe at 75 DegC, there are correction factors for different temps
but you are going to be cracking very small nut with a very large sledge hammer, doubt if that is needed on UKP

 

[happyflyer]

Doitmyslf,

Do it your way if you can help the lad, work out is PD and flow rates, he might be OK with what pipework he has installed looks to me they are all very small loads,
may have something totally wrong, and balancing by a novice is only one step down from balancing by an expert.

 

[Ermintrude]

Disable the weather comp. there is a setting in the controllers installers section. Failing that put wirless compensator in freezer. This will kick boiler up the rse. Your heating should run its nuts off!
If rads still poor I would suggest that the bit of 15 needs to come out. Are the flow and returns fully open on boiler? Is your filter open and clean ( assuming you have one).

Pressure and flow rate are not proportional !

I wouldn't worry too much about the design it's an 831 31kw to hot and about 28 to heating? Pump is a good one as with boiler. Are the flue screws in the collars on boiler?

 

[happyflyer]

Disable the weather comp. there is a setting in the controllers installers section. Failing that put wirless compensator in freezer. This will kick boiler up the rse. Your heating should run its nuts off!
If rads still poor I would suggest that the bit of 15 needs to come out. Are the flow and returns fully open on boiler? Is your filter open and clean ( assuming you have one).

Pressure and flow rate are not proportional !

I wouldn't worry too much about the design it's an 831 31kw to hot and about 28 to heating? Pump is a good one as with boiler. Are the flue screws in the collars on boiler?

There you are SORTED!!! In short its just a case of tweaking it. Buxton you should be happy with that analysis, I will leave you and the experts to tweak away. Not certain which bits of 15 mm you think need to come out Ermi.

 

[Ermintrude]

Seems a bit off hand. It's a small domestic installation which appears to be more DIY than design. Getting all confused with flow rates and pumping head. Custard seems a bit of a techie. (Drawings and valliant wireless controls)

There isn't enough information on that drawing as previously stated, but it's a small domestic system. The problem lies with the boilers controls and setup.

The first item on his agenda should be getting the heat unit to produce heat; as everything else is conjecture and misleading.

Once the boiler is returning hot water then look at system design. The boiler would reach max temp with two 600 450 p+ rads fed on 8mm and returned on 28mm if the comp was set up properly!

The bit of 15 I was referring to was the concentric bit 22 - 15 - 22 .

The installer ???? Is out of the picture by sounds of it , so better focus on the op. ( installer??)

 

[cbuxton1984]

The bickering is amusing and rather familiar, I see the same thing on a Tech forum that's in my area of expertise.

Thanks all for the help. I believe I have something to work with now.

The guy that installed the boiler and controller didn't really know much about the WC so the defaults were left in place. The heat curve was left at 1.2 so I will increase that to push the flow temp up and see what that does.

I still think that while I have easy access to under the floorboards it may be worth changing the main flow and return pipes to 22mm but I will leave that to see the impact of increasing the flow temp.

After I get the flow temp up I'll check that the rads are getting warm enough and whether the flow and return from drop 2 are both warm. I was concerned that because the drop uses 22mm pipes the 15mm pipes upstairs wouldn't have enough flow to push the returning water back up.

The original installer isn't out of the picture he just doesn't really know anything about the WC. I thought it would be a good idea to get some opinions before changing the pipes which I can do myself. No point getting someone in to replace two lengths of 15mm pipe with 22mm when I could do. I'm sure everyone on here does things themselves that they could get someone else in to do, does that mean it's done any worse than if you paid a professional?

 

[happyflyer]

Buxton,

You are more observant than most, but bickering it isn't, its not even a difference of opinion, but you are smart enough to work out what it really is, this last line of your post is 100% spot on.

"I'm sure everyone on here does things themselves that they could get someone else in to do, does that mean it's done any worse than if you paid a professional?"

I wish you well with your endeavours, I would not blame you if you did not come back and tell us how you went on.

Tony

 

[doitmyself]

I suggest you take a look at the same table. The flow rate of 0.205 l/s is for a velocity of 1.5 m/s not 1.0 m/s. The table does not show the flow rate through a 15mm pipe at 2 m/s.

 

[happyflyer]

I suggest you take a look at the same table. The flow rate of 0.205 l/s is for a velocity of 1.5 m/s not 1.0 m/s. The table does not show the flow rate through a 15mm pipe at 2 m/s.

View attachment 15751

How right you are, me at 5 am must stop posting at that time, nice one, nice colouring in too, colour scanner, woow, and like I said you won't find me designing down the bottom of the page at those high velocities, 0.5 to 0.75 is fast enough

 

[doitmyself]

I replaced all the radiators with bigger doubles and installed some new ones in rooms that didn't have them.

According to the data you provided later, you now have 9.6kW of rads (BTU are no longer used).

Do you know what your actual requirements are? If not, use the Whole House Boiler Size Calculator, but set the Domestic Hot Water Allowance to 0. This will give you the central heating requirement, which I would like you to post.

While the CH warms the house we haven't managed to get it to the desired temperature yet. I had it set to 21 degrees for 6 hours today and it only reached 14. The flow temperature on the boiler never exceeded 53 even though it's set to up to 75 on the boiler. I have the weather sensor 470f which I know overrides the flow temp on the boiler but I would have expected the boiler to put more effort into warming the property. If the desired temp is 21 and temp on the thermostat says 14 then surely it should ramp up the flow temp to try increase it, that doesn't appear to happen. I'm concerned that the flow is causing the problem.

I suspect that the reason for the house not heating up properly is that the boiler is cycling due to it being oversized compared to the total rad output.

You have 9.6kW of rads fed from a boiler which can produce between 5.5 and 25kW. It is starting off at max output, but is unable to modulate down fast enough, so the water temperature rises very quickly and the boiler has to cut out and cool down. The average temperature is therefore much less than expected, so the rad output is less.

The solution is to restrict the max output of the boiler to say 12kW

As for pipe sizes, it all depends on what temperature differential you use. The boiler expects a 20C differential, so you should really be balancing the rads at 20C. The result from the boiler calculator (link above), when compared to the total rad output, will tell you if this is possible. The rad total need to be at least 20% greater than the heating requirement to allow a 20C differential.

If this is possible, there is no real need to change any of your pipes.

 

[cbuxton1984]

Hi all. Thanks for the responses.

I've managed to get it sorted by implementing a number of things people have mentioned.

1. I did a chimney sweep and got the flow up to 75 degrees, did it twice so it ran for 30 minutes. I still had the same issue with the 22mm drop 2 pipes. Flow pipe was too hot to touch, return was just better than cold.

2. Increase heating curve to 4 which sorted the flow temp, reduced back to 2.4 after I did the below things as 4 was too excessive. It was set to 1.2 originally.

3. Fully closed all lockshields for upstairs radiators then gave them a 1/4 turn on. Someone I had read about else where. Still had the same problem with 22mm drop 2 pipes although the return was now hotter and the rads got warmer than before.

4. Drained the system, replaced the main flow and return with 22mm pipe. Feeding each individual rad with 15mm pipes. Added an additional rad on the landing upstairs and a second rad in the dining room which comes off drop 2. Added inhibitor through my towel rad and refilled the system.

The house now gets up to temp despite having poor insulation due to the other refurbishment work I'm performing. The flow and return to drop 2 seems to be much closer in temp, no longer have a cold return pipe.

I have the night temp set to 12 degrees which it maintains with the flow temp of around 25-28. This is probably a little higher than it will be when the house is finished because I still have 14mm underlay and carpets to lay, external door frames to seal and insulate, two 4 inch extractor fan holes to insert extractors and mortar, open chimney from where the back boiler was etc...

All in cost me an additional £54 for 22mm pipe, £40 for fittings and £400 for two additional rads plus 6 hours of my time yesterday. The missus insisted on having a designer horizontal rad for the landing hence the expense on the rads.

Thanks for the help

Chris

 

[happyflyer]

Chris,

I had an idea that you would not let go and you would fix it yourself, well done and well thought out, I never thought you would do it by just tweaking or the boiler was oversize

Nice you came back after all!!!

Now get the house airtight