Combi or Megaflow (or equiv)?


8 Feb 2008
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United Kingdom
I am currently in the process of getting a few quotes for a new boiler to replace a floor standing potterton diplomat 70/80. The hot water system is currently fed via a vented hot water tank in the bathroom. We want to remove this tank and the tanks in the loft, and put the new tank (if we don't go the combi route) downstairs where the floor standing boiler is currently sited (this will then eventually then be in a new utility room we are creating). The new boiler be positioned on the adjacent wall - within easy reach to the existing pipework, with the flue in the same position as the old boiler (through a flat roof). We will be keeping the existing radiators (the syetem will be flushed).

The question is if to go for a combi or pressureised hot water system. Advice so far has been mixed! The house is detached with three bedrooms (will eventually become four with thre creation of a small box room) , two reception rooms, kitchen and utility room. There will be a shower room, and a bathroom upstairs, and another shower room downstairs (this will probably be an electric shower though).

The pressureised hot water system will obviously cost more, but would a combi be OK?

One of the combi quotes for is a Potterton Promax combi HE 33. The quote to fit and remove the old boiler, and tanks was £2660 inc VAT. How does this sound? This includes a five year parts labour garentee.

I am still waiting for the quotes for the pressureised systems.

Thanks in advance....
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Easy option is to chuck a combi in. preferred option is to go the unvented route. Make sure that you specify a stainless steel unit. I personally prefer the models with external expansion vessel but others prefer those with a built in air gap.
Promax HE33 won't cope with more than one hot tap a a time.

An un-vented cylinder of about 170 - 210 litres and a system boiler would be my recommendation, but if you really want a combi, then the Vaillant 937 or one of the ATAG Q series would be your best option.
I can see into the future and can see you wishing you never asked this question.!


OK Dr, do your worst. :LOL:
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I can see into the future and can see you wishing you never asked this question.!


OK Dr, do your worst. :LOL:

Something I should know about? - not been on here for long :)

Regarding the comment about the Promax HE33 - we were assured that this could easily cope with more that one hot tap on at a time. (this is what we were worried about with a combi system) Have to say though the guy was more of a sales type than heating engineer...........

We really need to get rid of the header tank for the unvented system (removing supporting wall underneath ) so thats why we were thinking about a mains pressure hot tank system. Thinking about it though, we could maybe relocate the header tank to above a different supporting wall...hmmm

It's not just the Promax that wouldn't cope, it's any combi of this power or even a 37Kw.

For 2 hot outlets together you would need the Vaillant 937 to be able to deliver enough flow to both to be worthwhile.

Before making any decision you need to find out your cold mains inlet flow to your property.

For an un-vented or large combi you will need a minimum of 20 l/min and hopefully 3 Bar or more pressure.
One of the guys quoting measured the flow rate at the kitchen tap at 18l/min....

For a more accurate rate is it best to measure at a couple of taps at the same time and add the results together?

No one has yet measured the pressure, although I may do this tomorrow myself (I have a pressure gauge somewhere.....)

The incomming supply is lead, connected to a 15mm copper pipe, and then a gate valve.
You really need at least a 25mm incoming main from the street and 22mm to the unvented cylinder, or 937.

Without this you will be struggling to get the full advantage of any system.

If you are getting 18 l/min on 15mm then I would assume you must have good pressure.
Thanks Dave, we have another plumber coming tomorrow - I'll see what he suggests, but I am thinking now that maybe keeping a vented system is the way to go maybe with a bigger hot water tank, moved to the ground floor with a pump for the bathroom / shower room. That is assuming a vented system is OK with the tank on the gound floor? Should be OK shouldn't it as the pressure is from the header tank in the loft?

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