Combi DHW flow rate - sufficient?

ey_up

Hi guys.

We have had a couple of plumbers around to quote for a new central heating system and they have both have said combi route is a no brainer.

My only concern is the shower, we both like them and are wondering what would be sufficient. There are only two of us currently but this no doubt go up when kiddles arrive sometime in the next five years.

We have a larger than average 3 bed semi with one bathroom (bath, sink and boiler driven shower to be put in), two normal size sinks in kitchen and utility and two small sinks in two seperate toilet rooms.

The boilers suggest are from two different makes and have DHW flow rates of 12.4 and 15.2 at 35C. I wont go into which is best make but the 15.2 is £400+ more than the 12.4 (some of this is down the the make and some down to the specification).

In respect of the lower rated boiler, for the sake of not much more money I could upgrade to the next model up with a flow rate of 14.5.

Do the flow rates be sufficient?

I've also noted the other recent thread on shower pressures and rain showers. I dont think we would have such a shower head but would the flow rates be sufficient for one of these also?

Many thanks
eyup

macman

You need an GSR RGI, not a plumber, to advise on and do this work.
If you don't have a combi at present, is your pressure and flow rate suitable for one?
A combi is not a 'no-brainer'-it' depends entirely on your requirements and lifestyle, and not on the whim of the installer. If they didn't ask you about these factors then they are simply steering you towards the most profitable install for themselves.

ziggyman99

I haven't read the other thread on showers but I know that 12 l/min is a very good shower. How were the boilers specc'ed? Did the installers measure your flow rate at the kitchen tap? If you've only got a flow rate of 12l/min, a boiler with 15l/min is wasted. Also remember that 12l/min coming into your house will be usually shared between the cold and hot taps. If their both on then you won't have 12l of hot water. Also, due to reductions in flow rate caused by pipe work, it is unlikely that you will get 12l/min at the hot tap from 12l/min of cold supply. I always spec close to the maximum flow rate - better safe than sorry - but it's rare that the hot and cold flow rates match.
I always tell my customers to show an installer the door if he quotes without measuring flow rates and pressure.

ey_up

Hi, thanks for the comments.

We've had two of the three guys who visited quote the boilers above. All have been Gas Safe Registered, one is a gold star accreditted worcester installer. All have been recommended by either family tradesmen or friends who have used each guy rather than selected from the paper/yellow pages etc.

Not one of the three who have visited have tested that water pressure coming into the house. You have me worried now, is it possible that certain areas/ages of house etc wont give a problem? Or am I being duped?

All three have said Combi is the route without queston

macman

It's nothing to do with the age of the house or the area.
'Without question'? Did any of them actually bother to ask you what you need your system to do?

ey_up

Hi macman, thanks for quick response.

They all were given a thorough going round the house with all radiator locations, description of what was going in the bathroom and toilets (each would have to install the pipe work for the shower and additional sink in the toilet). We also explained our plans for the future in terms of extension etc including new second bathroom. The only item none of them checked was the water pressure which has now got me concerned.

cheers

HappyMJ

Without question? No Brainer. That really annoys me.... They must ask questions to determine what you want.

35 degrees is rather cool for a shower or is that the temperature rise? How hot do you like your showers? 45 degrees? That'll be OK in summer when the cold water coming in to the house is about 10 degrees but what about in winter when the incoming water temperature is close to zero. The maximum flow rate will be much lower.

Having unlimited hot water available can also increase your bills. Having only so much hot water available in a cylinder can self limit usage.

Personally I like my open vented traditional system. I can fill a bath in under a minute. Other people I know with combi's in winter turn the bath taps on then come back in 10 minutes but if you don't have baths then it doesn't matter.

The biggest advantage for me of having a cylinder is the unreliable boiler. When the boiler doesn't work I just turn the immersion on and I still get hot water then I can call out the engineer on a cheaper day rather than pay an emergency call out fee. As combi's only have a life of 10-15 years this really puts me off as I would have to have the money ready immediately to replace it when it becomes uneconomical to repair and have no hot water for several days.

Second bathroom you say....as long as you don't want to have 2 showers at exactly the same time then it's OK.

Canucklehead

Hi.

Given your plans I would be suggesting an unvented cylinder as long as you have the mains water supply to support one.

You might also consider (if you are south -ish facing) a dual coil cylinder with a view to some solar thermal sometime in the future.


GSR.

ormus

the only downside to a combi with low flow rates is filling the bath. it takes ages.
if baths are important to you, then get the highest flowrate boiler you can afford.

ey_up

Hi all, thanks for the many responses.

I think my comment may have been unclear/misunderstood. We explained our plans and needs to all the guys before they pointed out a Combi was the route to go. It wasnt as if they all just walked in and said combi. I do understand the point made though but it appeared that all thought combi was the best route.

HappyMJ - the water temperature was the comparable rating across all boiler spec sheets. I dont actually know if this is an industry standard temperature measure but all boilers looked at quoted the flow rate at 35C. Some did include rates for differing temps but wasnt consistently quoted across all makers.

I thinks its difficult to call really, I understand when its two of us our water needs are going to be somewhat limited but when the family arrives it will no doubt steadily increase. It needs planning for in that respect.

I've looked down the solar panel option but the costs are restrictive for us at the moment. I did speak to a installer before the tariff was cut and was looking at the extension roof being solar panels, its south facing and has direct sunlight for the majority of the day. May have to wait until systems become more cost effective.

Anyway, I digress. Have two more plumbers to arrange to come in next week.

Canucklehead

ey_up wrote: »

Hi all, thanks for the many responses.

I think my comment may have been unclear/misunderstood. We explained our plans and needs to all the guys before they pointed out a Combi was the route to go. It wasnt as if they all just walked in and said combi. I do understand the point made though but it appeared that all thought combi was the best route.

HappyMJ - the water temperature was the comparable rating across all boiler spec sheets. I dont actually know if this is an industry standard temperature measure but all boilers looked at quoted the flow rate at 35C. Some did include rates for differing temps but wasnt consistently quoted across all makers.

I thinks its difficult to call really, I understand when its two of us our water needs are going to be somewhat limited but when the family arrives it will no doubt steadily increase. It needs planning for in that respect.

I've looked down the solar panel option but the costs are restrictive for us at the moment. I did speak to a installer before the tariff was cut and was looking at the extension roof being solar panels, its south facing and has direct sunlight for the majority of the day. May have to wait until systems become more cost effective.

Anyway, I digress. Have two more plumbers to arrange to come in next week.

Hi: you are confusing solar PV (electricity) with solar thermal (hot water).

GSR