Combi boiler conversion

[HarveyDIY]

I currently have a conventional boiler and open vent cylinder with 2 water tanks up in the loft. I have a 1930s 4 bed detached house. i want to get rid of the cylinder and tanks and so convert to a combi boiler. currently considering the Vaillant ecotec plus 937. It will be very rare for two people to have a shower at the same time but I would like to have a shower without the flow decreasing when someone opens the kitchen tap. We have 14 radiators in the house.

Had two plumbers to give quotes. Neither really tested water flow or pressure. One just ran the tap and guessed by eye I had 9 litres/min. I have since measured myself. tested flow into a bucket and it was 17 litres/min and pressure was 3.2 bar at washing machine tap. Upstairs from bath tap the flow was 14 litres/min. The cold supply is 15mm from stopcock. The plumber who guessed the pressure said I should run 22mm pipe from stopcock in kitchen to the boiler. This would be about 1.5m run from stopcock to boiler. Then he suggests running 22mm hot and cold to upstairs as currently 15mm pipes run to the upper floor for hot and cold.

So based on my pressure and flow readings of 17 litres/min at 3.2 bar do you think it is necessary to change the 15mm pipe from the stopcock to 22mm to feed the boiler and then run 22mm hot and cold even though the vaillant 937 only has 15mm outlets?

Thanks

 

[Butchybutch]

Personally Id leave the pipework alone, your pressure and flow rate is really good, I cant see any benefit in upsizing, like you say the vaillant takes 15mm inlets anyway!

You will need 22mm gas to the boiler, and if it was me Id fit an 837, the 937 is a big boy, dont forget you will need a blow off for the storage vessels which is another pipe you have to get down to the ground!

Good luck, make sure you get to see the installers gas safe card, take the registaration number and punch it into the gas safe website.

 

[Agile]

Only the dynamic flow rate is meaningful!

Worth feeding the boiler in 22 mm but outlet can be in 15 mm. Althoug best to take shower feeds in 15 mm directly from the boiler in each case if you want to try to use both together.

Tony

 

[swbjackson]

17l/min is not that great, especially if you want to run two outlets at once. I work on needing twice the required hot water flow rate on the incoming main as a rule of thumb. So if you have a 24Kw combi giving 9 l/min I'd like to see 18 l/min into the property. This gives leeway for a cold outlet to be opened without affecting the hot water.

I would be looking at testing the flow rate direct from your main stop tap before decided on boilers or systems. There's no point in sticking a large combi in if there's not enough water to use it to it's full capacity. As this is more work than just opening a tap you may be looking at paying someone to do it. If I was doing this sort of thing I would charge for it but knock the cost off the price of the boiler install if I got the job. This means that I can be fully independent in my assessment as I'm not wasting my time if someone comes in with a lower price.

 

[HarveyDIY]

Looks like conflicting views from different people. What swbjackson said is what the first plumber said. He would change the stop tap to a 22mm and measure the flow from there and then assess what to do after this. It would be a charge of £150 to do this. This guy does mainly commercial plumbing so I wasn't sure if he was going over the top.

When I fully opened both the kitchen tap and the tap in laundry sink the flow rate dropped to 9 litres/min and pressure read from 0.5-1 bar.

Will be calling in a third plumber for a quote and see what he has to say.

 

[HarveyDIY]

Just thinking if I went for a vaillant unistor unvented cylinder and system boiler instead of combi would I have the same problems regarding flow rate. So if my mains is 17 litres/min at 3 bar static then would I get 17 litres/min at all taps and showers? Or would it be the same problem. Someone opens a tap then the flow rate drops to 9 litres/min?

 

[swbjackson]

The incoming flow will be shared between all of the open outlets with those closer to the stop tap taking more.

 

[swbjackson]

It doesn't matter what appliances you choose you will only get 17 l/min shared between all outlets unless you fit a booster set or an accumulator to increase the flow rate.

 

[muggles]

If you've got 17 litres per minute out of one outlet, and you open a second, it stands to reason that both will drop to 9 litres a minute(ish), providing of course the first outlet isn't restricting the full flow of water in any way. Twice the open outlets = half the flow from each outlet.

A 22mm pipe from the stopcock will only help if the upstream side of the stopcock (incoming main) is of a similar bore to the 22mm copper. It's no good running 22mm copper off a stopcock if the incoming mains only has a bore equivalent to 15mm pipe, because you won't gain anything.

A combi in a house of your size is inadvisable

 

[CBF]

Looks like conflicting views from different people. What swbjackson said is what the first plumber said. He would change the stop tap to a 22mm and measure the flow from there and then assess what to do after this. It would be a charge of £150 to do this. This guy does mainly commercial plumbing so I wasn't sure if he was going over the top.

When I fully opened both the kitchen tap and the tap in laundry sink the flow rate dropped to 9 litres/min and pressure read from 0.5-1 bar.

Will be calling in a third plumber for a quote and see what he has to say.

if i was quoting on your job i wouldn't be recommending a combi or an unvented cylinder, sometimes in this country with our old water supplies, flow & pressure, a good ole cylinder & roof tank is the best way & this is what i would be recommending unless you want to go to the expense of replacing the incoming supply pipe & even then you need the pressure & flow in the pipe in the road.

 

[HarveyDIY]

If 17 litres/min mains flow is not enough for the vaillant 937 then maybe I can feed it off an accumulator which I believe can increase my flow rate. Anyone any experience of this?

 

[Butchybutch]

You can ask questions on here all day long, everyone will have a different opinion and you will be none the wiser!

I have fitted a few 937's not over impressed with them the boiler is huge, and like i said earlier you need a blow off pipe which can be a pain if your fitting it upstairs or in a loft.

I have fitted a lot of 837's , this is a good boiler and will do what you need, 3 bar is plenty of water pressure.

Like I said you will need a 22mm gas supply to the boiler, up to you wether you upsize the hot and cold, but it wont make much difference.

Nuff said!!!!

 

[mickyg]

i would not fit a 937 with poor flow rate, which is what you've got. 17l/min is not great. 937s are very restrictive, by the time its been through the boiler you will be lucky to get 10l/min out the other side. Im guessing the 3.2bar is static pressure, id be surprised to see 3.2bar working and 17l/min. Unvented out the question also same reasons.
An accumulator can help but you would ideally need an accumulator twice the size of the unvented cylinder, or with a 937 the equivalent of twice the volume of water your likely to use in one single use.