Combi or standard boiler

[Camilla]

Hi, we are looking to change boiler from standard to combi but from what we read we need to have good water pressure for combi boiler, we have checked water pressure in the whole house and it's different downstairs to upstairs so in the kitchen cold tap has water pressure if 13l/per minute but hot tap only 4,5 , bath upstairs 8 for hot and cold tap and shower over the bath 1,8. Which pressure we go by choosing the boiler and should we change to combi ?the boiler is located upstairs and I would like it to stay there.

 

[Nwalky]

There is a lot of information that you would need to consider if changing your system. I take it that currently you have a hot water cylinder in your property? And you want to change this to a combi. A combi boiler saves space and does give hot water on demand but you will finf tgat it is more the flow rate that you need to take into cinsideration ratger thanbpressure. If you have 3 bar pressure on the cold water then you will have no problems with a combi boiler but you will fibd that ut will take a long time to fill a bath because the boiler needs to run at a slower flow rate so as to adequately heat the cold water on demand.

My advice would be to have someobe out to quote for you but i would also consider putting in an unvented hot water cylinder and a heat only boiler, this will give you the oppurtunity to store your hot water, give great flow rate and will supply it at mains pressure, essentialy keeping the same set up you have but bringing it ibto the modern age, this would probably cause less upheavel than converting to a combi.

Hope this information helps. If you have someone quote, ask them for a few options and see what they say, some people will just jump on the easiest solution for them which may not necessarily be in the best interests of the customer

 

[steadyon]

1. The figures you have quoted are flow rates, not pressures.
2. To consider a combi or unvented cylinder from a "will it work" point of you you need to have satisfactory:
2a. Static pressure. This is the pressure of the incoming cold water main with no taps running. Often most easily measured at an outside tap, a washing machine / dishwasher tap or the kitchen tap.
2b. Dynamic pressure. This is the pressure of the incoming cold water main with another mains connected tap running. Most easily measured at and outside tap with the kitchen tap running.
2c. The flow rate. Should be measured at a point on the incoming main with no restrictions such as isolation valves. stopcock must be fully open.
3. Suitable pressures and flow rates are:
3a. Combi - Static pressure 1.5 bar, dynamic pressure about 1.2 bar, flow rate 15 litres / minute.
3b. Unvented - Static pressure 2.0 bar, dynamic pressure about 1.7 bar, flow rate 20 litres / minute as a minimum.
4. Pressure can only be measured with a gauge, such as Screwfix item 82412. This is designed to attach to a 3/4" BSP outlet connector such as a washing machine / dishwasher tap or outside tap. It would nee some (cheap) fittings to attach in other ways.
5. Flow rate can be measured with a weir cup such as Screwfix 5784K, or by timing how long it takes to fill a 10 litre bucket and calculating the litres / minute flow rate from that. The weir cup is a bit expensive for one off use.
6. The points Nwalky makes are good ones, but in addition:
6a. You won't get two simultaneous showers from a combi unless it is a very powerful one and you make sure both showers are limited to a a maximum hot flow rate of around 6 or 7 litres per minute.
6b. If the boiler breaks down you will have no heating or hot water until it is fixed. With a hot water cylinder system (whether vented or unvented) you have the option of an immersion heater, so that you can at least have hot water.
6c. The unvented system is a good option, but if there is a break in the water supply, you will have neither cold not hot water. With an unvented system you could have both until the cold water storage system runs dry, albeit the cold water would need boiling before drinking.
6d. An unvented hot water cylinder must only be installed by a "G3" registered installer, and should be serviced every year.
7. My personal preference is a heat only boiler running a sealed central heating system and a vented hot water system. Electric pump to boost hot, and if necessary cold, supplies. Simple, reliable, cheap although the pump(s) if necessary can be a bit noisy.