Plumbing ,conversion to combination boiler advice .

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W1ghty

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Hi Folks ,
I am thinking about changing the boiler from a pressurised central heating system with roof tank and hot water cylinder .
There is a chance I might have a roof extension in the next couple of years and the stairway to the roof will go through the airing cupboard and because it would be handy not to have the cold water roof tank in the roof , so I’m looking at Combi boilers . (I will be getting a corgi guy in to fit the boiler )
My current 20 year old Vaillant boiler is 24kw and a lot of the new Combi’s seem to be in the 24-30kw range so there isn’t any issues there .
My question is regarding the supply of hot water from the new Combi to the existing system . When I strip out the copper tank and roof tank myself (I’m fairly competent and want to save what money i can ) , I’ll alter the pipe work to the bath to 15mm down from 22mm and do what I can to alter existing hot water pipes so there aren’t any long dead legs or very long routes for the HW to travel from the new Combi .
The boiler will be sited in the downstairs toilet , there is a hand sink next to it with 15mm hot supply to the tap , so finally the question to the experts is can the new boiler supply hot water to this pipe as it’s only a couple of feet away ?? Or am I missing something because it sounds to easy and we all know there’s no such thing with DIY projects .
Thanks
 
Yep. That’ll be fine. You can also branch off the cold feed. 👍

I wouldn’t stress too much going to the bother of stripping out 22mm feeds either. It’s a lot of work for very little gain.
 
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Yep. That’ll be fine. You can also branch off the cold feed. 👍

I wouldn’t stress too much going to the bother of stripping out 22mm feeds either. It’s a lot of work for very little gain.
Thanks a lot , I was hoping it was as easy as that . The cold mains shut off is just below the boiler as well.
Thanks for the 22mm advice , I’ll give it some thought and probably won’t change it if there is little benefit . I could always do it later if I thought the flow wasn’t enough .
 
Thanks a lot , I was hoping it was as easy as that . The cold mains shut off is just below the boiler as well.
Thanks for the 22mm advice , I’ll give it some thought and probably won’t change it if there is little benefit . I could always do it later if I thought the flow wasn’t enough .
They’ll be no difference whatsoever in flo. Only marginal benefit to changing to 15mm would be less volume of wasted/unused hot water. 😊
 
Check your incoming water flow rate - can be done with a jug at the kitchen sink. Then look at boiler output.
At a previous house the previous owner (elderly lady) had been conned into a new combi boiler. Flow rate into the house was around 10 litres/minute and the boiler she was sold could only output 9 litres/minute. Narrow bath took 20+ minutes to fill.
I installed a new 32mm alkathene supply to replace the 15mm copper supply pipe. I disconnected the hot water output form the combi boiler and connected a new pressurised hot water cylinder on a motorised valve on the radiator circuit to achieve an adequate supply of mains pressure hot water.
Check the output from the new combi boiler will meet your needs.
 
If you leave the hotwater feeds in 22mm you will draw off a lot more cold water through a half inch sink or basin tap before it gets hot enough to wash, than if it were a 15mm feed from boiler. Bit of a waste of water, but ok if you don't mind the wait.
 
My question is regarding the supply of hot water from the new Combi to the existing system . When I strip out the copper tank and roof tank myself (I’m fairly competent and want to save what money i can ) , I’ll alter the pipe work to the bath to 15mm down from 22mm and do what I can to alter existing hot water pipes so there aren’t any long dead legs or very long routes for the HW to travel from the new Combi .

I've only recently looked at the same issue and I did the sums to try and get a handle on it. A given length of 22mm copper contains 2.36 times more water than 15mm. Those figures are based on actual bore sizes of 20mm and 13mm respectively. When you translate that in KWh of heat lost it's not a very big deal, in my case 22mm pipe would lose 0.178 Kwh more heat vs using 15mm at a cost of approx. 0.8p. The cost of wasted water was higher at 1.54p. Even so It would be a long time before the cost of installing 15mm pipe paid back.

In the end the problem went away because I decided I would keep the hot water tank as my bathrooms are very close to the tank but 28M of pipe work away from the prospective combi boiler ( it's a very long bungalow) and I simply couldn't put up with the time delay in hot water coming through to the taps. With 22m pipe it was going to take more than a full minute for hot water to reach a bath tap. and 2 minutes for a basin tap. Doesn't sound a long time until you try standing there with a tap running and time it.

Bottom line is all hot water systems are relatively wasteful of energy due to loses and I wonder if instantaneous under sink heaters are the answer for basins in the future. We already do it for showers but Baths couldn't realistically be done as you would need a 25Kw electrical heater.
 
I solved the run off time by putting in a pumped loop on a time clock. But badly advised by plumber as I could really do with reversing the circulation & plumber advised me to reduce the last leg from 22mm to 15mm to save energy.
Now to reverse the flow I would have a 1st leg of 15mm followed by 22mm for the rest of the circuit which I'm not sure would work well
 
We had this done recently, and things we never thought of:

1. We get a lot more noise from the pipes when using the water now

2. some taps are not suitable for mains pressure

3. when we use certain taps it stops the water flow completely to other taps !

I’m sure these things can be rectified.

otherwise, we are very happy with it. The central heating is now silent. We also changed to smart rad valves and they are great.

We replaced a 40 year old boiler and was surprised not to notice a saving in gas bills.

good luck with it
 
When you do your loft, will it have another bathroom? how many in the household?
 
When you do your loft, will it have another bathroom? how many in the household?
It will have another bathroom but I may remove an en-suite bathroom which has a macerator in it because it’s on the wrong side of the house for easy access to the mains drain so that will be a straight swap as far as boiler capacity is concerned . A roof extension will also add another couple of rads taking the total to 12 .
The house was a 3 bedroom which had a small extension added (bedroom and dining room) , so I’m not going to need a huge boiler . With the two of us I’m not really going to have to worry about multiple hot taps being used at the same time .
 
Check your incoming water flow rate - can be done with a jug at the kitchen sink. Then look at boiler output.
At a previous house the previous owner (elderly lady) had been conned into a new combi boiler. Flow rate into the house was around 10 litres/minute and the boiler she was sold could only output 9 litres/minute. Narrow bath took 20+ minutes to fill.
I installed a new 32mm alkathene supply to replace the 15mm copper supply pipe. I disconnected the hot water output form the combi boiler and connected a new pressurised hot water cylinder on a motorised valve on the radiator circuit to achieve an adequate supply of mains pressure hot water.
Check the output from the new combi boiler will meet your needs.
Just checked on the mains outside tap , managed 19L a min :eek: . Serious pressure !
 
Just checked on the mains outside tap , managed 19L a min :eek: . Serious pressure !
Just out of curiosity I checked our flowrate from the kitchen tap 20L/min....

Oh, and a wet floor and trousers!! :rolleyes:
 
Flow rate is the key, but also worth checking pressure. We did this change to combi some years back - the flow rate was magnificent, at over 24 l/min, but we do have an issue with one shower valve (also not very old) because the pressure is also very high - so the valve squeals when in use. Better than the alternative problem of low pressure, but I will have to fit a couple of pressure reducing valves to shut it up (current workaround is to turn on a couple of taps at the same time as the shower!).
 
W I hope you get it fixed ..But for insurance purposes you will need to get it serviced through a gas company cover plan . They will come and inspect the system and check out the flues and over flow out lets, and make sure the gas supply as fitted per regulations ...So if your going to do it yourself i would look in to the intallation rules first .You could do it to a plan they are easy to find on the web . Then ask a corgi engeneer to inspect it first so you know you have the installation right .
 
W I hope you get it fixed ..But for insurance purposes you will need to get it serviced through a gas company cover plan . They will come and inspect the system and check out the flues and over flow out lets, and make sure the gas supply as fitted per regulations ...So if your going to do it yourself i would look in to the intallation rules first .You could do it to a plan they are easy to find on the web . Then ask a corgi engeneer to inspect it first so you know you have the installation right .
Thanks , I’m only doing the monkey work of preparing the pipes for the Corgi Gas fitter to do the boiler .
 
Combi boiler 🤮
I think that’s more the old ones buddy . I’m in a catch 22 , my 21 year old Vaillant is limping along so far but if I just get a new system boiler I’m screwed if I want a roof extension :( .
The 10 year warranty on the new Worcester Bosch ones seem attractive .
My parents had a Combi installed a few weeks ago and that seems fairly faultless over the old system boiler . Only issue so far is it’s making a 25year old mixer shower drip 💧.
 

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