Unless your property has three or more bathrooms, there is no advantage in using 22mm piping for the hot water supply with a pressurised system. In fact, all that happens is the larger pipe will mean the hot water takes longer before it reaches the shower.
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I have a very similar situation but have a gravity fed system.
The Bathroom has been replaced recently. It was decided to swap bath ends so we could fit a shower on the house wall side. The plumber has connected the existing copper 22mm pipes for the bath taps to a 15mm speedfit around to the other end of the bath. He's then split the pipes feed from the bath taps up to a shower mixer.
The bath is about 40% less than before and the shower is barely acceptable. The shower and bath taps are Grohe 'Get' range.
I understand there is a difference between pressure and flow rate but there seemed to be more than enough pressure and flow rate before the change was made. Also, the plumber did not know what a full bore isolation valve was. That's not to say he's not fitted one but I can't check now that he's tiled the bath panel.
Houston, do I have a problem?
Yes, the solution would be to fit a combi or a pump but if he changed the 15mm pipes to 22mm pipes to the bath taps would that make a difference.
You've really answered your own question about the smaller bore and bath flow rate; it's 40% less, and caused by the change in pipework.
More to the point, any plumber worth their salt would be well aware of what a full bore serice valve is and where it should be used. They would also be aware that you cannot tee off low pressure bath supplies to feed mixer shower. Firstly, for safety and correct operation, they need dedicated feeds. By that I mean, cold direct from the header tank, and hot from the top of the hot water cylinder. Look at any mixer shower installation instructions for details. Secondly, it's likely that flow rates will be unbalanced anyway ie, low pressure hot and high pressure cold.
Has anyone checked the header tank size and height I wonder??
Question 1 Can I just clarify, the reason for the poor flow from the shower is because the bath pipes have low pressure and the combination of reducing the pipes to 15mm has reduced the flow to the shower?
Question 2 OK, so my only sensible options are a combi or pump. If a Combi/pump produces 16 litres a minute f(or argument sake) will shower perform the same whether I used a combination of 15mm and / or 22mm pipes?
Question 3 Does a combi/pump eliminate the issues of pressure as in a gravity fed system?
Q1 sort of misses the point really. Yes, the flow will be poor because of the reduction in pipe size, but most importantly, a gravity fed shower MUST have dedicated hot and cold supplies; those feeds should be 22mm their entire length and finally the header tank in the loft must be high enough to give an adequate head of water. Generally, unpumped gravity showers are mostly considered as poor by modern standards.
Q2. Showers supplied by a combi need only be 15mm and can be teed from bath supplies. Pumped supplies also need only be 15mm, but as mentioned before, for safe and reliable operation, MUST be dedicated and not teed from anywhere else.
Q3. In short yes, but the header tank still has to be suitable ie, big enough (you often need two tanks) and high enough to trigger the pump. Combi boilers on the other hand will, in almost all cases, outperform pumped showers. Combis only have to downsides. A) will only supply one shower at a time, and B) no backup hot water in the event of boiler failure.
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