Loft conversion with regular/traditional heating system

[Harry_W]

I'm hoping for some help from anyone that's gone through the same process as I'm about to start. I'm looking to convert our loft and currently have a fully functional regular heating system with f&e tank, cold water tank and Worcester Bosch boiler that looks under ten years old all in the loft and a hot water cyclinder in airing cupbaord on the 1st floor. I have had British Gas out to quote and they have said that the property is unsuitable for a combi as they were getting 2 bar of pressure but a flow rate of 11 litres per minute. I have the water company coming out as there is water pooling in the hole where the stopcock was changed about 6 months ago.

It's currently a 4 bed detached house with one main bathroom and one en suite. The loft will be master bedroom with en suite and I think I am leaning towards and electric shower for redundancy purposes. If a combi boilder is out of the question, should I just replace the cold water tank with coffin-style tanks and locate them under the eaves to be boxes in and replace the f & e tank with an expansion vessel? Can I replace the hot water cylinder with an unvented cylinder, keep the regular boiler and replace it with a system boiler when it packs up?

 

[terryplumb]

With that pressure and flow rate you can forget an unvented cylinder and combi ,both of which require every outlet ,all taps,shower etc to be fed from the mains.

 

[oldbuffer]

1. Its a surprisingly low flow rate given the pressure, and suggests there might be an obstruction between the meter / water board stopcock and the point where it was measured.
2. Do you know where the flow rate was measured?
2.1 If at the kitchen tap, flow could be restricted by isolation valve and / or narrow bore flexible hoses.
2.2 If at an outside tap or a washing machine cold feed valve, then there SHOULD be an isolation valve, and it is likely to be narrow bore.
2.3 Is your main stopcock fully open?
3. Was the pressure measured with a mains connected tap open, or just at one point with all other mains cold outlets closed?
4. I'd consider measuring the flow rate myself by finding the time to fill a 10 litre bucket from a source immediately after a fully opened stopcock and with no restrictions.
5. If that gives you 20litres per minute or more, it would be worth measuring the static (no other outlets running) and dynamic (another outlet running) pressures.
6. To justify a combi in a three storey (ground, first, loft) you'd need an absolute minimum of 1.5 dynamic pressure and a flow rate of 15 litres per minute. You'd lose approximately 0.5 bar in pressure given the roughly 5 metres in height between ground floor and floor of loft.

 

[Harry_W]

Thanks for your replies.

The pressure and flow rate were done using the outside tap but, based on your replies, I've just checked the stopcock under the sink and it wasn't fully opened. I'm going to use a measuring jug tomorrow in the daylight to retest the flow rate....fingers crossed!

 

[MNW67]

Probably a stupid question! Will the CH feed tank under the eaves be high enough? Are you having radiators in the loft?

 

[Harry_W]

I was under the impression the feed and expansion tank can be replaced with an expansion vessel that can installed next to the boiler. I'd only need the cold water coffin tanks in the eaves.

 

[MNW67]

I'd misread what you intended. Thought I might have! So you're going to essentially convert your regular boiler to a system boiler and feed it from the mains.

When you said "tanks"under the eaves I thought you meant one for the DHW and one to feed the CH.

 

[oldbuffer]

Look up the exact model of Worcester Bosch boiler you have, check the installation instructions (usually available on-line) just to make sure it can be used in a sealed system. Most modern boilers can.

 

[Harry_W]

I've had the flow rate and pressure checked again....14 litres per minute and 4 bar of pressure. Is this good enough for a combi in the loft?

I've checked the regular boiler in the loft and it's a 2009 Worcester Bosch Greenstar 15ri so only 15kw and is only rated for 10 radiators. I've got 13 rads already!

 

[Madrab]

only 15kw and is only rated for 10 radiators

Who told you that? All depends on the size of your rads - If all 10 Rads were 600mm x 900mm K2's (T22) then yes that would max out that boiler but you need to understand what the total demand of your system is 1st.

14L/min @ 4 bar - is that a dynamic figure? If so then I'd suggest that your mains is being restricted as 4 bar dynamic should be able to deliver a lot more than 14L/Min. That being said if it is dynamic then yes, that should serve a combi.

Do note though, if you have more than one bathroom (showers/baths/etc) where more that one hot outlet may be used at the same time, then a combi may not be the best option.

 

[Harry_W]

Thanks for all your help and info. I went for a Viessmann Vitodens 100-W 35kw and I'm over the moon with it. I managed to negotiate a Nest thermostat so can control it using Alexa. No problems with the pressure and shower is far more powerful than before now it's on mains pressure!