New Combi Boiler - Flow and Return Pipes

[cbuxton1984]

Evening all. Thanks in advance.

I've had a new Valliant 831 installed at my property but I'm not convinced the upstairs flow and return pipes are sufficient for the number of drops and radiators. I've attached two diagrams, one showing the current layout and another showing my proposed layout. In both diagrams the red lines are flow and blue return, bolder lines are 22mm pipes and the thinner are 15mm. Hopefully the diagrams are useful.

The property used to have a back boiler in the living room which I've marked as drop 2 hence the 22mm pipes that go upstairs from those radiators. Drop 1 now has 3 radiators, it used to have a single radiator.

Before the new boiler was installed I replaced all the radiators with bigger doubles and installed some new ones in rooms that didn't have them. Due to installing bigger radiators I replaced the flow and return pipes to each of them with 15mm speedfit barrier pipe from the main 15mm flow and return pipes.

The new Valliant has been installed in the utility room immediately under bedroom 3. The gas man fitted 22mm copper pipe for the boiler up into bedroom 3 then connected to the 15mm copper pipe using 15mm PEX. Since it was installed I've noticed that while the flow down to drop 2 is hot the return is considerably colder, colder than I would expect.

My concern is that because the 22mm pipes go into 15mm pipes then back to 22mm for the drop to the living room that the pressure may not be sufficient to push the water back up the 22mm pipes for the return to the boiler.

Since we're renovating the property and it has no carpets or other flooring I wonder if it's worth me replacing the main flow and return pipes upstairs with 22mm PEX so there isn't a bottleneck. Would this be worth doing and would it be beneficial to my CH system?

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.

The distances are not reflected in the diagrams. The pipes upstairs are longer than they look on the diagram.

Hopefully this makes sense to everyone. I've tried not to waffle.

Existing Pipework



Proposed Pipework

 

[lame plumber]

looks like your running 6 rads via 15mm pipe and that is wrong as 15mm pipe cant carry enough heat for that sort of output. you need to have the 22mm as a complete run and T off the 15 mm to the rads with a max of 3 rads of med size in 15mm. trying to keep things simple here so no heat losses etc being included

just seen 2nd pic, thats the ticket, may be better to run 22mm to the utility kitchen etc to be sure of enough heat reaching all three

 

[Ch4 plumbing]

Your better off getting someone round to have a look who knows what they are doing, I could tell you the pipes are undersized but really that's probably only the start of things!

 

[Ermintrude]

Agreed. I would say bottleneck is a bit rough. As suggested 22mm is a good upgrade, I would as a trial set partial load to its max 28kw? Push the chimneysweep buttons see how the house heats up?

 

[cbuxton1984]

Thanks for the responses.

The problem is that I had someone round, multiple people when I got the new boiler priced up. Nobody mentioned an issue with the existing pipework.

I have a 7 year warranty with the boiler so if it turns out that there is an issue with that I'll be contacting the installing engineer or Valliant. I don't live in the property yet and have access to under the floorboard so I thought it may be worth replacing the existing 15mm copper before contacting anyone.

 

[stevetheplumber]

you show pipes running in 22 and reducing down to 15 its normal practice to reduce only at tees so 22 to a tee and 2 x15 carrying on

 

[tamz]

If the boiler temp isn't getting above 53º the pipes are not your problem.
Look to the weather comp settings.

 

[Ermintrude]

Thought comp had been overridden?

 

[lame plumber]

If the boiler temp isn't getting above 53º the pipes are not your problem.
Look to the weather comp settings.

ha ha pays to read the whole thing tamz, good point, got bored after para 1.4.2.5

 

[tamz]

If it was a flow problem that wouldn't stop the boiler getting up to temp.
The problem with weather comp is it is not as smart as it thinks it is and neither are most of them who push it.

 

[cbuxton1984]

So how do I override the weather comp to test getting the system up to full temp? The weather comp doesn't have very many settings to check. The boiler doesn't have any problems getting up to temp for the hot water it just seems like it doesn't want to warm up the house even though it's way off the desired temp.

 

[Ermintrude]

DHW isn't controlled by comp. it's probably set to wrong curve. Firstly is the sensor on a north wall about 1.4 to 2m from floor? Is it wired or wireless?

Read the instructions it's a little confusing going frame to frame but u will get it. Installer code should still be 17. If you push two lower buttons on the boiler control panel below LCD boiler will fire into chimneys weep mode as said before? This will blast the daylight about of system. Are the rads the correct sizes for rooms? Is the house insulated? Were windows and doors closed? Did any rooms heat?

 

[Ermintrude]

Has there been a bypass fitted is it too low? I.e shorting system out

 

[cbuxton1984]

Yes all the radiators are sufficiently sized for each room. The BTU output exceeds the recommended for each room and all rooms have a radiator. The house has roof insulation but no wall cavity. 220mm solid brick. There are no carpets or underlay if that's likely to make a difference, also currently no internal doors.

I'll have a read through the installers manual because the operators manual didn't contain anything useful. Code on controller is still 000.

Sensor is 2m high on a north facing wall and is wireless.

 

[tamz]

Vaillant controls are notoriously difficult to set even for pro's who know and work these type things day in day out.
I haven't a clue how a 470 works and i have no interest in learning it. Get your installer back but if it was your idea to fit this i wouldn't hold out too much hope that he is any better educated than you on how to go through the settings.
You can probably override it short term by pressing the 2 bottom buttons. This will put it onto high fire for 15 minutes. When it cuts out press the again.
Also the bypass comes set quite far open on these. Get your installer to set it properly.

 

[tamz]

What is the total output (roughly) of all your rads. If you replaced them you can look it up.

 

[tamz]

YThe house has roof insulation but no wall cavity. 220mm solid brick. There are no carpets or underlay if that's likely to make a difference, also currently no internal doors.

Big big difference. Weather comps come set for well insulated houses ad the curve has to be shifted to suit. Your 53º flow temp is about right for an out the box setting for an outside temp of around 10º.

 

[happyflyer]

At first glance it doesn't look too good but you have only told us half a tale, go back to the drawing and fill in all the missing bits so we don't need to take up 40 post guessing because that is what's happening, 15 mm will feed 6 radiators and I use the number 6 on the fly because it would depend on the output of each of the six, BTW you would never put 6 rads on a circuit anyway in real life, your drawing is useless without, radiator outputs and pipe lengths and diameters. I don't know the boiler set-up and I am not going to go searching, if it has and internal pump and you simply need more pump power then you may well be shot. I have repeated said this until I am blue in the face, boilers with internal pumps give you no get out of jail, the maximum pump power is fixed, you need to size the system pie work to the size of the pump, which to me as an engineer is not right.

Lets go back to that 15 mm tube, domestic system pipe work usually has water going thorough them at around 1 to 1.5 mtres per second, more than this and you will hear the water going around the system the data below might explain:

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


22 mm tube normal design 1 m/s will carry 0.311 ls or heat rads up to 13 kw the pressure drop pm run will be 520pa
22 mm tube pushed to 2 m/s will carry 0.457 ls or heat rads up to 19kw the pressure drop pm run will be 1000pa

So you can see if you are prepared to push the envelop and you are a little deaf or have tinnitus like me you won't notice the water flow
noise. 15 mm will feed 6 rads each with a output of 8.7 divided 6 = 1.45 kw each, as I said in real life we all like an easy life if you did this it would be a pig
to balance near impossible

If you fill the as fitted drawing in properly we may be able to help, be aware of what Tamz said you need to have the boiler flow temp correct, however taking the compensation
slope out of the equation is not the answer you want.

BTW I would never design a heating system with pressure drop in excess of 500 pa per mtre run, I would go even lower to reduce pumping power and energy used in the life cycle.

PS is the pump inside the boiler case, I hate even looking for the manual, brain dead design inside the case and if it fails it will cost you twice that of one outside the case.

 

[happyflyer]

I can see through their spiel you don't need the data sheet you can see it's got and internal pump
and made in Europe by our EU friends, wonder when China will start churning them out.



Vaillant ecoTEC plus - YouTube

 

[cbuxton1984]

I've added the BTU values and pipe lengths.

I can see two problems, they may or may not be related but that's why I'm asking for advice.

The flow temp doesn't increase up to its maximum or come close. The outside temp was 1.5, room stat said 14 and desired was 21. The outside temp decreased gradually from 6 to 1.5 over 6 hours yesterday afternoon. The room temp increased from 9 to 14 over the 6 hours.

The second issue I see is that the 22mm flow pipe to drop 2 is hot but the return pipe is cold. All the other return pipes that are 15mm are not as warm as the flow pipes but are still hot.

Either these issues are both related to the boiler not getting up to temp or I have two issues and one is the flow which is why I was considering replacing the 15mm pipe with 22mm upstairs.

Thanks for the replies so far.

 

[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