House replumb, new combi, lose the cold water storage

[RussTaylor]

Hiya,

I'm replumbing my house at the moment, 3 bed detached, 1930s. It has old steel pipe pretty bunged up, cold water and heating storage tanks in the loft, a massive old boiler and a hot water tank.

I'm going to go for a combi to get some space back by losing the hot water tank, probably a Vaillant 937 as it has a built in mini hot water store with it. Failing that a 838 (no store).

My water pressure is OK far as I can tell, currently though (mainly due to internal pipework) my electric shower struggles for pressure (direct from mains).

When I put my new boiler in I can junk the cold water storage and run it all direct to the taps/boiler/toilets. Or should I run a new storage tank and put a pump on it (to get decent pressure again)?

My main priority is having a really good mixer shower no matter what's happening in the house. I don't want pressure to drop because someone flushed a toilet or is watering the garden (which currently happens).

To achieve this will I need a cold water tank still? I will run direct to the boiler/kitchen taps, and feed everything else from the tank then I suppose? Ideally I'd save fitting a new tank/replumbing, but I don't want to go to all this work to find out my new system isn't that good.

Thank you very much for your advice

 

[swbjackson]

Run everything off the cold main if you have a high enough flow rate. If not you need to seriously think if a combi is suitable for your needs.

 

[RussTaylor]

Ok, that makes life easier and cheaper thanks. Also a bit more space in the loft and no worries about stuff freezing up there anymore.

Good point about the combi, I'm sure the pressure is ok, and I just had a new water main run in from the road.

Thanks again

 

[dcawkwell]

If you have a good flow and pressure to your bathroom taps then the problem is with your shower rather than the mains.

 

[RussTaylor]

The bathroom taps are slow, but they're from the old tank/old plumbing. The shower comes in on the new main downstairs, through some ancient galvanised pipe to the loft, then into new copper to the shower. The shower is ok, but on highest setting in the summer when the water is warmer to start off with it complains of low pressure.

My girlfriends parents shower is immense, I'd quite like something like that. They have a 40yr old boiler and a pump though and I'd like something more efficient and to get some space back by removing the tank in the bathroom.

The Vaillant 937 has a tank on the back of it, hopefully that'd run the shower well, just worried about cold water feed to it.

Cheers

 

[mysteryman]

Consider getting the water main replaced in new 25mm or bigger plastic. This should provide plenty of flow. A good combi-compatible mixer shower should then match a power shower for performance. As stated, the existing basin and bath taps may have their cold supply from the tank, so it is totally meaningless to measure the flowrate there.

 

[RussTaylor]

Consider getting the water main replaced in new 25mm or bigger plastic. This should provide plenty of flow. A good combi-compatible mixer shower should then match a power shower for performance. As stated, the existing basin and bath taps may have their cold supply from the tank, so it is totally meaningless to measure the flowrate there.

Hi,

Yeah the water pipe has been replaced with 25mm blue EPDM.

Ok that's interesting, I'll make sure the shower I aim for is combi compatible, thanks.

All but one point in the house comes from the tank and has poor flow.

I was just worried that if I turn the cold tap on in the kitchen or my pressure washer outside, is it going to interrupt my shower? In which case I was going to fit a tank for it.

I'll plumb it all without and at worst I'll have to put one back in

Thanks

 

[D_Hailsham]

My water pressure is OK far as I can tell

You really need to get the water pressure and flow rate checked before thinking about a combi boiler. If you get rid of your cold water tank, every tap (hot and cold), toilet, washing machine, garden hose etc etc will be supplied from the one source. So if the flow rate is not fast enough and the pressure high enough, someone having a shower will feel the effect of someone else washing the car.

If you have a garden tap (usually of the mains) you can check the flow rate by using a litre-marked bucket and a watch. Time how long it takes to fill to the top mark (usually 10 litres) and convert this to litres per minute. 10lpm will allow one tap on at a time. You really need 15-20 for multiple outlets running at the same time.

As for pressure, there is a simple test without going to the expense of buying a pressure gauge.

Repeat the test, but this time with the kitchen cold water tap full on (they always come of the cold mains). If the flow rate at the garden tap only drops a little, you have good pressure. But if it drops a lot, say 25-50%, you will have problems running two outlets at the same time.

You mention Vaillant 937 and 838. There's no point installing one of these in a three bed house as the central heating output will be much too high.

Use Whole House Boiler Size Calculator to find out your heating requirement.

 

[RussTaylor]

Thank you very much, very helpful reply!

I suppose I wanted to go big on the boiler in case I build an extension, and thought a plumber had recommended that one due to the hot water store. It's physically massive though, I'd like a smaller one that fits in my downstairs loo / utility better.


Using the calculator you linked to it says I need 10kw-14KW (depending on my roof insulation situation) only for CH so say 20KW with HW.

The dwelling type is detached with single glazed windows with wood/plastic frames.
The external walls are cavity, filled with insulation.
The roof is pitched with 50 to 75mm of insulation.
The Dwelling is in the south west.

The following three major contributors to the heat requirement
are all affected by the levels of insulation.

The WINDOW heat loss is 1.94 kW
The WALL heat loss is 0.91 kW
The ROOF heat loss is 1.98 kW


Using http://www.vaillantsystemfinder.co.uk/ it says 34kw and recommends the 838!

Number of residents: 4 persons
Year of construction: 1930 - 1959
Type of house: Detached house
Surface area: 200m²
Type of boiler : Combination boiler
Hot-water use in bathroom: Bath and shower
Your CH power requirement: 34 kW
Your hot-water comfort requirement: CW 6
Type of shower: Power shower


Who is right? The smallest Vaillant is 23KW combined, and my house/arrangement isn't the smallest. This one seems very detailed, I'll do it tonight room by room. http://www.heatlineradiators.co.uk/btuscalculator.html



I hadn't considered I can check the flow myself. My garden tap is tapped into the same pipe as the kitchen tap, my shower is direct though so I'll switch that and the tap on an see what happens tonight. Thank you very much, saving me a lot of trouble and mistakes

 

[D_Hailsham]

I suppose I wanted to go big on the boiler in case I build an extension, and thought a plumber had recommended that one due to the hot water store. It's physically massive though, I'd like a smaller one that fits in my downstairs loo / utility better.

The likely life of modern boilers seems to be only 10 years or so. You have to think how long it will be before you build the extension.

Using the calculator you linked to it says I need 10kw-14KW (depending on my roof insulation situation) only for CH so say 20KW with HW.

Heating and HW requirement are two different things if you have a combi boiler. The calculator only gives you the heating requirement, say 15kW. However this is the maximum you need, when it is very cold. For most of the year your requirement is much less. So the maximum CH output should be about 15kW (say 18 including the extension) and the minimum as low as possible.

The HW requirement for a combi boiler depends on the incoming cold water flow rate. You need 2.5kW for each litre per minute, with a maximum of about 35kW.

Using http://www.vaillantsystemfinder.co.uk/ it says 34kw and recommends the 838!

Who is right?

Vaillant are trying to sell boilers, the calculator I suggested is produced by the Building Research Establishment, so completely independent. It is also the one recommended by Energy Saving Trust etc.

This one seems very detailed, I'll do it tonight room by room. http://www.heatlineradiators.co.uk/btuscalculator.html[/QUOTE]
I recently looked at a number of online calculators and most of them leave much to be desired. On some the method of calculation could not be found out - so they were discarded. Others used very crude methods, e.g room volume X 5!!. Of the rest, the only one which seemed to give sensible results was the Home Supply Rad Calculator

 

[RussTaylor]

The likely life of modern boilers seems to be only 10 years or so. You have to think how long it will be before you build the extension.

A good point, well made. I could probably reduce my boiler cost by 25% if I stop worrying about 'forever'.

Heating and HW requirement are two different things if you have a combi boiler. The calculator only gives you the heating requirement, say 15kW. However this is the maximum you need, when it is very cold. For most of the year your requirement is much less. So the maximum CH output should be about 15kW (say 18 including the extension) and the minimum as low as possible.

The HW requirement for a combi boiler depends on the incoming cold water flow rate. You need 2.5kW for each litre per minute, with a maximum of about 35kW.

Great, thanks. I have purchased a pressure gauge and when it arrives I'll check my pressure and flow rates. I'm (not really) surprised that none of the plumbers that quoted me for a boiler bothered to check that.

Vaillant are trying to sell boilers, the calculator I suggested is produced by the Building Research Establishment, so completely independent. It is also the one recommended by Energy Saving Trust etc.

I recently looked at a number of online calculators and most of them leave much to be desired. On some the method of calculation could not be found out - so they were discarded. Others used very crude methods, e.g room volume X 5!!. Of the rest, the only one which seemed to give sensible results was the Home Supply Rad Calculator

I'll stick to that one, thank you.

This is all good food for thought, I was certainly aiming a bit high with the boiler and don't really need instant hot water / use the bath so I can avoid the 937.

Now your words have sunk in I realise that the max output of 37KW for the HW output on a 837 maybe overkill, and I'll only know when I know how much water I can put through it.

So from the Vaillant ecotec plus range there is

824 - 23kw HW 9.4l/min / 3.8-19 kw CH
831 - 31kw HW 12.7l/min / 5.2-24 kw CH
837 - 37kw HW 15.2l/min / 6.4-28 kw CH

So if I can get enough pressure/flow from the mains and have the space / cash to buy the boiler I might aswell get the 837 for the flow rate, or the 937 if I want 20l/min for 10 mins reducing to 15.

If I don't have the pressure/flow from the mains to run the big one I need to aim lower in the range.

Thank you very much indeed!

 

[RussTaylor]

Just did the bucket in the garden ... 11.5 litres / minute. They're doing the water pipes in the road at the moment so maybe it'll improve but looks like my super shower with a combi idea might be out the window?

 

[xr4x4]

You might wanna think about running 22mm from the stopcock.

 

[RussTaylor]

Ok good thinking.

I'll disconnect the blue water pipe into the house tonight and see what the flow rate is then, currently it goes from 25mm blue water pipe down to 15mm, then from that one of those smaller bore tap into the pipe kits (about 10mm id) for my outside tap.

So would I run 22mm to the boiler, and tee off that in 15mm to run the rest of the house, spuring off that 15mm as I go, including the cold feed for the shower? Or 22mm for most of the hous and tee off in 15mm each time?

All this 22mm is going to get expensive lol

 

[ALEC1]

if you want a vaillant boiler to last then install it as they do in germany with the outdoor weather sensor, and in the case of a 937 use a blending valve..

Decent condensing boilers will last a lot longer than 10 years I know of several well over 12 years old and going strong...

the real issue is that the manufacturers fail to support them as there technical staff can't keep up with the changes..but thats a management issue not a boiler issue!