Which Boiler for Underfloor Heating

[tomcio01]

Hi,

Could someone offer advice on which boiler to get, please.

I am building an extension to the rear of my 3 bed semi, and puting in a downstairs loo too

I want to heat the extension (40m2 floor area) with underfloor heating only and keep the radiators in the rest of the house.

which boiler would do the job?

I was looking at the Veissmann Vitodens 100-W (35kW)

Any suggestions would be greatly appreciated.

Tom.

 

[D_Hailsham]

Make and model/size of existing boiler?

How many rads at present?

 

[tomcio01]

Currently I have a Glowworm SpaceSaver Mk2 (I think) It's about 30 years old.

there will be 7 rads, currently there are 8.

Thanks.
Tom.

 

[D_Hailsham]

Currently I have a Glowworm SpaceSaver Mk2 (I think) It's about 30 years old.

there will be 7 rads, currently there are 8.

So the boiler certainly needs replacing.

Presumably you have a hot water cylinder. Are you thinking of ditching this and installing the Combi version of the boiler? Or will you keep/replace the cylinder and install the System version of the boiler?

 

[tomcio01]

Was thinking of going combi but open to sugestions as we've not started on the plumbing yet.

 

[D_Hailsham]

Was thinking of going combi but open to sugestions as we've not started on the plumbing yet.

Has the incoming cold water flow rate and pressure been measured?
How many baths and showers in the house?
How many could be in use at the same time?

 

[tomcio01]

I've not measured the incoming cold water.

there is one bath and one shower but both in same bathroom so can't use both at once. I'm not puting in a shower downstairs so realistically only one bath or shower will be used at the same time.

to measure the flow do I just take a 10litle bucket and time how long it takes to fill it up?

Tom.

 

[D_Hailsham]

to measure the flow do I just take a 10 litre bucket and time how long it takes to fill it up?

Yes, but you need to use the correct tap. If there is an outside tap, use that at it will be an unrestricted flow. If not, use the kitchen sink tap, but be aware that some taps severely restrict the flow.

If you have a garden tap, measure the flow with only that tap open. Then repeat, but this time with the kitchen cold tap full on. The smaller the difference in filling time, the better the pressure.

Post the results.

 

[tomcio01]

Hi,

So took a 10L bucket to the outside tap and it filled it up in about 49 seconds.

The kitchen tap filled the same bucket in the same time.

Mind you the outside tap does use the sam pipes as the kitchen tap. The 15mm pipe branches out under the sink and goes through the wall to the outside. So not surprised at the same result.

Tom.

 

[D_Hailsham]

So took a 10L bucket to the outside tap and it filled it up in about 49 seconds.

That's just over 12 litres/min, which will be OK for a shower.

The kitchen tap filled the same bucket in the same time.

Mind you the outside tap does use the same pipes as the kitchen tap. The 15mm pipe branches out under the sink and goes through the wall to the outside. So not surprised at the same result.

You misunderstood my instructions, which is my fault as, on re-reading them, they are confusing. I wanted you to retest at the garden tap while both garden and kitchen taps are full on. This will give an idea of the water pressure and what will happen if the toilet is flushed or the washing machine turned on, etc while someone is in the shower.

Assuming 12 litres/ min, you would need a 30kW boiler to raise the water temp by 35C, ie. from 20C to 55C. This is more than adequate for a shower, which is normally about 40C.

 

[tomcio01]

Hello,

Well did the test as instructed...
Kept the kitchen tap on full and timed the outside tap.
To fill the 10L bucket it took 1min 14sec.

Hope this helps.

Tom.

 

[D_Hailsham]

To fill the 10L bucket it took 1min 14sec.

That gives 8 litres/min, which is on the low side for a combi boiler. What is this the size of the incoming main from the street? If it's only 15mm or 1/2 inch, it might be worth having it replaced by a 22mm. This will give you a better flow rate, but could be expensive.

A HW cylinder may be the better way to go. You would not need anything like a 35kW boiler to heat the house and cylinder. Use Whole House Boiler Calculator to find out what size boiler you would need.

 

[tomcio01]

thanks for the advice.

I'm pretty sure the incoming main is more than 15mm. As part of the extension work I uncovered the water main pipe and it looked like a 22mm. but it was lead.

I'll have to check the main tap in the house and probably replace it as it's quite old anyway and may be restricting the flow.

Tom.

 

[Agile]

Blind leading the blind!

This is going to result in a seriously oversized boiler if the cylinder is retained which would be the best ( and cheaper ) solution.

A mains water flow rate measured at an open pipe rate of 12 litres per minute is not the same as the dynamic flow rate which should always be used to assess the prospective performance of a combi boiler.

Depending on the mains supply pressure and the supply pipe resistance, the dynamic flow rate is often about half of the open pipe rate!

Tony

 

[tomcio01]

Tony,

Thanks for your comment. So going back to my original question.
With the flow rates I have provided what would be a good boiler I could go for, bearing in mind I'll have about 30m2 of Underfloor heating as well as the central heating rads?

What are my options.

Thanks.
Tom.