Few questions, boilers and showers

[awmech]

Thinking of changing from my 28 y/o regular boiler system which I believe is combined vent / fill, and which is gravity fed with hot water cylinder but no immersion heater, to a combi boiler.
There is only me here, and my son at weekends, 3 bed semi detached with one bath / shower and 8 radiators total.

I’ve read the FAQ section which has dead / bad links on this subject.
Brief history, since autumn I’ve had issues with the boiler constantly tripping, I’ve had the system power flushed by BG but the boiler still trips, I’m told it’s overheating due to lack of flow, it only runs for about 20 seconds at a time, and starts again every 5-6 minutes.
I guess a possibility is the boiler heat exchanger, I’ve had a trusted plumber round since then, we drained water out from a downstairs radiator and he reckons the flow is ok.
I have no heat at all in any of the downstairs radiators, a little heat in the upstairs radiators, and can take about 12 hours or so to heat up the hot water cylinder after a shower or two.
The plumber thinks the boiler is probably the issue.
Should the heat exchanger not be cleaned out as part of the power flush?

1. Which are the best and worst combi boilers in your experience, ones to avoid, primarily looking for a cupboard fit (as the current regular boiler is) but that may change due to the pipe work involved with a combi.
I know I will have to change from 15mm to 22mm gas supply too.

2. What’s a good flow rate for DHW, I currently have a Mira Magna shower which I really like, but Mira have told me this can’t be used with a “high pressure / combi boiler” system, so I’m looking for a good flow rate from the boiler for a replacement shower, something like the Mira Platinum, or another similar non pumped digital mixer.
I’ve yet to check my mains flow rate and pressure.

3. I’ve seen folks saying that a bath takes an age to fill, whereas I was under the impression that combi boilers provide hot water @ mains pressure, but I do believe the flow rate of the boiler would be the factor there?

4. I have 10mm pipes to the radiators and it would be majorly difficult to replace with 15mm so I’m not planning replacing the pipework unless I absolutely have to, will this be ok with a combi? I’m planning to replace the radiators at some point.

TIA

 

[awmech]

Forgot to ask if I can control the new boiler via Hive, assuming just drop out the hot water from the Hive receiver?

 

[muggles]

Yes, a powerflush should clean the boiler's heat exchanger as well.

"Cupboard fit" rather depends on the dimensions of your cupboard. If you can tell us that, we can suggest good boilers which will fit in your cupboard. Generally though, I personally fit Vaillant Ecotec Plus for premium installs (very good but very expensive) or Vokera Pinnacle for the more budget-conscious. I use Navien NCB700 if the customer wants a particularly high flow rate but you'll need a big cupboard to get that in.

As you've identified yourself, the first thing to do is establish your mains flow rate & pressure. A higher powered boiler will give you a better hot water flow rate but only up to the limit of your incoming. Higher powered combi boilers tend to be physically larger so not really cupboard fit.

10mm pipes are fine.

I suppose you could use Hive if you must, but it's a nasty thing with no real intelligence that won't do your new boiler's efficiency much good compared to alternatives.

 

[Johntheo5]

Was the circ pump changed, whether or not, what make/model/mode setting?.

 

[awmech]

Many thanks for your reply, I’m willing to look at bigger boilers with the option of doing away with the wall cupboard it currently resides in.
Another option might be to install in the wiring cupboard, but running a condi drain and PRV drain from there would be very tricky as it’s in the centre of upstairs. Would also need to run a 22mm gas pipe up the side of the house and into the loft, then down from there, we will need to run a 22mm gas pipe regardless, as it drops down to 15mm somewhere under ground.
The internal dimensions of the cupboard are 680x570x290, so the ones I’ve seen would stick out the bottom of I wanted to keep the flue where it is.
My mains static pressure is around 3.2 Barg, dynamic pressure is around 2.9 Barg - 3 Barg, this was with the cold kitchen tap running, not a particularly fast tap, it maybe does around 10 LPM, whereas the outside tap produces more, still to measure that properly.

 

[awmech]

Was the circ pump changed, whether or not, what make/model/mode setting?.

Yeh, the pump was one of the first things I looked at after seeing online suggestions. It was charged by BG @ my request, but they found nothing wrong with the original pump. The new pump is a Grundfos and I have it on speed 3.

 

[awmech]

Yes, a powerflush should clean the boiler's heat exchanger as well.

"Cupboard fit" rather depends on the dimensions of your cupboard. If you can tell us that, we can suggest good boilers which will fit in your cupboard. Generally though, I personally fit Vaillant Ecotec Plus for premium installs (very good but very expensive) or Vokera Pinnacle for the more budget-conscious. I use Navien NCB700 if the customer wants a particularly high flow rate but you'll need a big cupboard to get that in.

As you've identified yourself, the first thing to do is establish your mains flow rate & pressure. A higher powered boiler will give you a better hot water flow rate but only up to the limit of your incoming. Higher powered combi boilers tend to be physically larger so not really cupboard fit.

10mm pipes are fine.

I suppose you could use Hive if you must, but it's a nasty thing with no real intelligence that won't do your new boiler's efficiency much good compared to alternatives.

Thanks, what’s a good alternative control instead of Hive?

 

[Johntheo5]

Yeh, the pump was one of the first things I looked at after seeing online suggestions. It was charged by BG @ my request, but they found nothing wrong with the original pump. The new pump is a Grundfos and I have it on speed 3.

I

Yeh, the pump was one of the first things I looked at after seeing online suggestions. It was charged by BG @ my request, but they found nothing wrong with the original pump. The new pump is a Grundfos and I have it on speed 3.

What model Grundfos, is it a UPS3?., if not does it display any data like the pwer in watts W and the flowrate in m3/hr?. (Grundfos Alpha models do display at least some of this data).
You said "boiler system which I believe is combined vent / fill", this means you have a OV system with a feed & expansion (small) cistern?, can you post a photo of where the the combined vent&fill is, does the vent carry on up over the F&E cistern and then bends back down above the cistern?

 

[awmech]

I

What model Grundfos, is it a UPS3?., if not does it display any data like the pwer in watts W and the flowrate in m3/hr?. (Grundfos Alpha models do display at least some of this data).
You said "boiler system which I believe is combined vent / fill", this means you have a OV system with a feed & expansion (small) cistern?, can you post a photo of where the the combined vent&fill is, does the vent carry on up over the F&E cistern and then bends back down above the cistern?

It’s a UPMO 15-70.
There is no up and over vent at the F&E tank at all.

 

[muggles]

Thanks, what’s a good alternative control instead of Hive?

It depends on the boiler you get really. If you get a Vaillant or a Navien then their own controls are best. If you get a Vokera then any decent OpenTherm control will do. I like the EPH CP4i for pure funtionality although I realise it's not as "pretty" as some of the others.