Advice on optimal system for large house?

[JoeMac]

I am looking for advice on the best boiler system for my home.

It is currently:

• a 4-bedroom mid-terrace period house of stone construction
• 1 toilet on ground floor
• 1 bathroom with bath and electric shower on 1st floor
• 28kw combi boiler fitted in kitchen
• 2000 sq. ft. with fairly high ceilings (not included attic i.e. approx 1,000 sq. ft. ground + 1,000 sq. ft. 1st floor)
• 10 large radiators and 1 heated towel rail

Currently in the process of renovating and converting the attic space to add 2 more bedrooms and a shower room so will become:

• 6 bedroom
• 12 large radiators and 2 heated towel rails
• 1 toilet on ground floor
• 1 bathroom with bath and electric shower mixer shower on 1st floor
• 1 shower room with mixer shower on 2nd (attic) floor

The existing 28KW combi is not going to cut it for this upgrade and the 2 mixer showers.

I am led to believe that a good mixer shower will require 15-20 litres/minute.

I could upgrade the combi to a 40KW combi, but that is still only likely to give about 17 litres/minute at best. Not ideal as I want the two showers to be able to run at the same time.

From some reading, I believe I should be looking at a System Boiler with an Unvented Cylinder. Something like the Worcester Bosch Greenstar Life System Boiler plus a Greenstore Unvented Cylinder. I'm not sure whether I would want a 40KW, 45KW or 50KW boiler, though, nor I am sure on the optimum size of cylinder. They come in 93L, 123L, 161L, 191L, 216L, 246L and 292L.

2 x 10 minute shower at up to 20L/minute x 60:40 hot:cold ratio = 240L.

Any comments to help point me in the right direction would be appreciated - as I am trying to have a rough of an idea of what I want before I start asking for quotes from installers.

 

[steadyon]

1. You need to be certain your incoming water supply provides, or can be upgraded to provide, a dynamic pressure of 1.5 bar, and preferably more, and a flow rate of at least 20 litres per minute. Given the size of the property, and if you want to run two showers simultaneously, minima of 2.0 bar and 30 lpm would be better. Pipework to the cylinder(s) should be 22 mm, not 15 mm.
2. You need to calculate the space heating requirements, taking into account the correct heat loss calculations for a stone built property. You will have to fit a condensing boiler, so work out the heating requirements based on a return at boiler temperature of 55 degrees Celsius. Allow an additional 10% for heating from cold.
3. Add the water heating requirements. Worcester Bosch are not the only people who make unvented cylinders. Look at OSO and Joule and possibly others. You can fit more than one cylinder if necessary, and might wish to go for fast recovery types. Decide on the cylinder(s) then calculate the heating required to keep them hot.
4. If you install a system which doesn't allow water and space heating at the same time you can use a boiler which satisfies the greater heat requirement of 1. and 2. above, otherwise you will need one which satisfies the sum of 1. and 2. above.
5. You might well find that the existing boiler is big enough. The fact that it is a combi doesn't prevent it being used to heat a hot water cylinder(s) - just another zone off the central heating circuit. If its in the kitchen, you could keep the combi hot water output for the kitchen tap.
6. It might be better in the long run to pay someone suitably qualified to design the system and specify what is wanted to give you a basis to obtain quotations.

 

[gmartine]

As it's a period property chances are you'll need to upgrade the incoming water supply, if you want at least say 25l/m you'll need that supply pressure and flow rate coming in. Don't guess the size of the boiler or worry about particular brands atm, any competent engineer should be able to calculate the correct size and as outlined above you can use any time of boiler to charge the cylinder and even possibly your existing one.

 

[JoeMac]

Thanks guys. Appreciate you both taking the time to respond and provide advice.

Very useful info about the mains flow rate and pressure. I will have a go at measuring the flow manually. I don't have the equipment for measuring the pressure but I will make sure that the engineer confirms that.

The 28KW boiler itself is about 8 years old and the whole central heating system was re-done at that time - so it's all pretty much brand new large double radiators and brand new copper piping throughout. A lot of the radiators are 2000x600mm K2 radiators - so probably about 10,000 BTU each. The system is working well for that purpose. The house heats up quickly and is warm and cozy for an old house. It is also working well for general hot water at the tap. There are no mixer showers at present - just 1 electric shower. I am unclear if the mains were upgraded when the central heating upgrade was done but will check.

I think the main thing that I am looking to future proof for is the removal of the electric shower and addition of the 2 mixer showers, so it sounds like the key next steps for me are:

1. Calculate pressure and flow rate of incoming water supply
2. If the above is less than 30L/min, look at upgrading the incoming pipework
3. Perform calculations to work out full heating and hot water demand
4. Work out the correct size of boiler and cylinder

If 1KW is 3214 BTU then I suspect that my 28KW combi (~90,000 BTU) is probably just about enough for all of my radiators, but the 28KW combi only has 11.5L/min of hot water flow.

I will have a look around and see if I can work out the calculations for working out the requirements for hot water demand and for working out the power required to heat the necessary cylinder. Brand new to this, so still reading and learning!

 

[steadyon]

If you go for an unvented hot water system the only hot water flow rate which depends on the water heating capacity of the boiler (your 11.5 lpm) are the outlets run directly off the hot water output of the combi. You could use the central heating side of the combi to heat the water in the unvented cylinder, just treating it as another heating zone. If you were prepared to have hot water and space heating running at different times (via a programmer) then the whole 28 kW would be available to heat the hot water cylinder OR run the central heating, but not both at the same time.