Switch to a combi boiler

12 Feb 2024
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United Kingdom
Hi , hoping someone can advise me , we currently have had our old vented boiler removed from the loft as it was old and are planning on doing a loft extension , so wanted to move downstairs to the airing cupboard in 1 of the bedrooms where the old hot water tank was , I had 3 plumbers come round and my main concern was that I wanted a decent shower as we had a pumped aqualisa.
All plumbers said combis are brilliant these days you should be fine and have a decent shower anyway so we went ahead and had a Glowworm Ultimate 35 fitted . Anyway the heating is great but the shower is terrible , my current specs are 2.5bar of pressure and around 14l /min measured with a bucket , but the shower merely delivers 10l which in my eyes is terrible it’s even worse if one of the kids comes in and brushes there teeth in the morning . Like all family’s do .
After reading some suggestions I fitted a salamander home boost pump to help supply the boiler although this is better the flow rate is still very poor from the shower .
My questions are , is that just a combi for you ? Should I have had a non vented system fitted ?
Could I fit an accumulator to help with the shower ?
Also is it acceptable that 14l /min going in and only 10l/min coming out ?
I spoke to Glowworm and they told me all there boilers whatever the spec are limited to 12l intake , so I removed the limiter to see if it improved which wasn’t that noticeable.
I’m hoping someone else can enlighten me before I rip out the new system and kiss good bye to 3k.
My house is fed with 15mm copper pipe but has ok flow rate .
I’ve checked all filters going in and removed limiter , I’m contemplating buying an accumulator tank to help feed the boiler but is it worth wasting my money or shall I just put that towards a different set up?
Please someone give me an honest answer and no sales crap.
Thanks in advance
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First thing- your post is in the wrong forum, really needs to be in Plumbing and Heating.
Second, has anyone checked dynamic supply pressure (ie with cold tap wide open, what pressure is pushing water out).
Third, there's not a lot of point fitting boost pumps to the input of a combi.
Fourth (if you have the space) if your dynamic pressure is (as I suspect, pants) look at adding an open vent cylinder & header tank for the bathroom hot water supply
Thanks for the reply . Oh sorry first time in posting can I move the post.
When you say supply pressure ? Is that when you use the mug style thing , that was at about 12 goes up slightly with pump on .
I haven’t got much space but was considering fitting a big accumulator tank next to the boiler to supply the cold water feed for boiler and the feed for shower , having a pump fitted does mate somewhat of difference apart from drives us made going on and off all the time
Glow worm technical are correct, as 14 litres per minute would be limited coming out. What do you have at the taps?
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Hi I have a but 14l going in a 10l coming out of the taps , I understand there will be a drop off but that seems excessive
Hi I have a but 14l going in a 10l coming out of the taps , I understand there will be a drop off but that seems excessive
flow rates are limited by pipe diam and the number of draw points. Also a combi has a "theoretical" output as its based upon temperature rise but will always have an "actual flow" rate at whatever temp it happens to be (restrictions on the internal path through the boiler heat exchanger.
You also need to create "priority flow" of the mains so that the combi always has the maximum available flow rate (the cold water for the house is taken just before the boiler inlet)

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