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

 

[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