Mains pressure DHW what's best on limited flow?

[Buzzark]

I think I'm near a conclusion now!

It looks like an unvented is my most recommended option and we've gone through all the pro's and cons of the different approaches of combi/storage combi/heatbank/unvented.

We still seem to have borderline flow though in terms of an unvented.
Static pressure - 2.2 bar (goes up a bit from that with time of day)
Dynamic pressure - 1.3 bar (when above was measured)
Flowrate at 1st floor bath tap - 20l/m

I need to know if there's much difference between makes/models for unvented at that sort of flow. I've been unable to work out how much loss of head to expect when pushing water through an unvented?
I'm assuming it's similar to what happens with pipes for example going from 15mm to 22mm to 22mm looses a lot of pressure, but worse - this might be completely unfounded though.

There also seems to be a large variance in coil performance and I'd have thought the higher the coil rating, the cooler the return temp and therefore the more efficient in reheat times and in condensing?
e.g. for 150l size models Megaflow has 12.5kW coil, Ariston Prima has 26.7kW, Range Tribune is 18.9kW.

If this is true and we'd lose more than a bit, would a thermal store with a plated heat exchanger (e.g. DPS pandora, range flowmax) or a coiled heat exchanger (e.g. Albion Mainsflow) lose us less flow?
I realised the plated versions are more complicated and come with anti K.I.S.S moving bits to go wrong and the Albion wouldn't deliver anything like an unvented in terms of maximum flow, but has less to go wrong with it.

I just want to maximise what we have in available flow and pressure.

 

[Agile]

I would recommend a plain unvented.

The plate HE types get scaled up because people operate thermal stores at about 80°C to increase capacity and that will cause lime scale.

I also recommend a 300 li although you have never said how many people live there and you bath shower usage. If only two then a 200 li would do. But the increased size in less than proportional.

Reheat times are pretty irrelevant in a normal house!

Most installers will be quite happy with a flow of 20 li/min open pipe and yours is way better than that!

Tony

 

[Buzzark]

I also recommend a 300 li although you have never said how many people live there and you bath shower usage. If only two then a 200 li would do. But the increased size in less than proportional.

....

Most installers will be quite happy with a flow of 20 li/min open pipe and yours is way better than that!

Thanks Tony

It's 2 adults, 2 children. 2 showers in the mornings and 1/3rd baths in the evening for the children is our typical usage. Washing up etc.

300l seems larger than most of the manufacturer's estimates, but I see the extra capacity really removes the need for rapid recovery.

Yes, I guess we must have excess of 20l/m if I pulled the pipe off the stopcock and measured there.

I am favoring an unvented and we've started putting feelers out for recommended installers (ideally for new boiler + DHW) in the Stockport area to do the work.

 

[Dan Robinson]

Wise move - the larger tank will cost a little more, but the installation won't - if you see what I mean?

Ask neighbours for recommendations; and get references.

 

[Buzzark]

We know, of know of a really good plumber. Unfortunately he's not got a gap until next year.
We've a few other avenues yet though.

I know I've sounded picky so far (thanks for all the advice along the way) but it's a big investment. We're likely to be here a long time and if £100 extra spent on a larger UV cylinder suits us better well you might as well get it right the first time... I don't mind spending money but there's nothing worse than spending it and not getting what you thought you wanted.

I do have a habit of over researching sometimes though. But I do wonder how some people get through life by saying "That one looks nice, I'll have that!"

 

[Agile]

Dont get overinflated about the reheat times.

What you and many overlook is that its reheating as its being used. So for every 10 litres being used its heated up five have been heated up.

The 300 li costs little more and has that added extra storage in case of unexpected rapid requirements.

If you really are short of money then you can fit a new boiler later.

Why is it a national pastime to say they cannot afford a new boiler. This fellow in a £3M house told me he coupd not afford the £1600 to replace his dangerous and inefficient boiler yet he was telling me how he was going to replace his 1 y.o. jag with a top of the range new rangerover at £27k.

The Rangerover would cost £15k in depreciation in the first year whereas the boiler would save its cost in three years from the savings.

Tony

 

[Buzzark]

Why is it a national pastime to say they cannot afford a new boiler. This fellow in a £3M house told me he coupd not afford the £1600 to replace his dangerous and inefficient boiler yet he was telling me how he was going to replace his 1 y.o. jag with a top of the range new rangerover at £27k.

The Rangerover would cost £15k in depreciation in the first year whereas the boiler would save its cost in three years from the savings.

Tony

Some people have their priorities way out of whack!
I'm pretty practical, I'm pushing to get the house more energy efficient and the bigger things done sooner so we get more benefit from them.

Like you say, the boiler will pay for itself in just a few years so the sooner it's done the better.

We did learn that lesson in our old house though, for years it was pea soup green on the outside which both of us disliked. In the end we had it painted a nice colour and wondered why we hadn't just scraped the money together long ago. then of course we sold it and moved here losing all the benefit of it.

 

[Doctor Drivel]

From about 6 weeks ago

I would recommend a plain unvented.

The plate HE types get scaled up because people operate thermal stores at about 80°C to increase capacity and that will cause lime scale.

Larger plate heat Xs do not scale up. The plates flex preventing scale build up. Smaller plates in combis scale up as the plates do not flex enough.