Change from open vented to system boiler?

[phil1235]

btw here is another heatloss cal:

http://www.idhee.org.uk/calculator.html

Thanks - just tried that, came to 16.53kw, incl 4kw for DHW as I have fast recovery cylinder.

I actually have 15 rads of various sizes, mostly K2, but only use 12. Separate 2-way valves for downstairs, upstairs and DHW, with room stats in hall and landing and TRVs on nearly all rads. Bathroom (with TRV) is on DHW circuit, which is nice. Run upstairs at 18C and downstairs at 19C, bit more when wussy relations staying.

Thanks everyone for a lot of useful stuff. Think I'll stay with open vented, but still have to decide whether 18 or 24kw.

Ref. the Range Rover or Mini, I chose second-hand 2.7 V6 car over 2.0 (diesel of course), but the subjective driving pleasure doesn't quite read across to a boiler!

 

[dumbrill]

I am not a plumber but I would consider a 4-pipe system boiler with weather compensation. This easily supports different boiler flow temperatures for CH and DHW. You won't get the best economy if the boiler can't drop the flow temperature below 60°C for CH, which is too cold for legal DHW.

Most boilers get over this by either heating HW or CH, but not both at the same time. Not really a problem if the recovery time of the cylinder is typically around 20 mins. Therefore when sizing the system the HW load shouldn't be added to the CH. Although for the best recovery times the boiler needs to be able to satisfy the full load from the cylinder.

 

[mehran]

though I really wish manufacturers would make it clear how many kw is need to achieve the re-heat time.

about 14kw will re-heat a 120L tank from 20 degrees to 60 degrees in about 20 minutes

 

[ajrobb]

Most boilers get over this by either heating HW or CH, but not both at the same time.

Yes, 4-pipe system boilers like the Viessman Vitodens 200-W are either on DHW or CH but not both at the same time. 2-pipe boilers tend to get stuck with a flow temperature of 70°C to satisfy DHW legal requirements and very little condensation savings. I don't think the Veissman is suitable for vented DHW.

 

[dumbrill]

Most boilers get over this by either heating HW or CH, but not both at the same time.

Yes, 4-pipe system boilers like the Viessman Vitodens 200-W are either on DHW or CH but not both at the same time. 2-pipe boilers tend to get stuck with a flow temperature of 70°C to satisfy DHW legal requirements and very little condensation savings. I don't think the Veissman is suitable for vented DHW.

The number of pipes coming out of the boiler is irrelevant. A boiler with two pipes can work exactly the same using external valves to direct the flow as required. I think that you'll find it a challenge to find a boiler that can't heat a HW tank to 60 degC and also modulate the CH water radiator temp down as required. They just won't be able to to do both at the same time (obviously).

 

[bookahandyman]

btw here is another heatloss cal:

http://www.idhee.org.uk/calculator.html

Thanks - just tried that, came to 16.53kw, incl 4kw for DHW as I have fast recovery cylinder.

I actually have 15 rads of various sizes, mostly K2, but only use 12. Separate 2-way valves for downstairs, upstairs and DHW, with room stats in hall and landing and TRVs on nearly all rads. Bathroom (with TRV) is on DHW circuit, which is nice. Run upstairs at 18C and downstairs at 19C, bit more when wussy relations staying.

Thanks everyone for a lot of useful stuff. Think I'll stay with open vented, but still have to decide whether 18 or 24kw.

Funny that "mr Gas man " My estimate of 11 rads was conservative and also stated only phil new at that minute how many rads he had. Yet as i know "nowt" he came up with more than eleven rads and 18kw required poss more. Maybe i had some knowledge after all, as i said he may get away with an 18kw originally. how about you wind your neck in once in a while and let others have a say, rather than have a ****ing contest.
wouldnt normally be offensive but sometimes you old boys need telling. and also put off others from trying to join in and help , and sometimes 'learn' from others

 

[transam]

Well the way I see it OP , its all very well talking about modulation ect ect but there is one important aspect that needs to be considered ? & that is reliabity !!

what one saves in running costs ? could very easily (Will ?) be wiped out in repair & spare parts costs ? down the line ! IMHO reliability in the condensing boiler market is pretty dire today in fact it might be a disgrace ?

So based upon this & the logic that less is best , there is a very compelling argument to go with Mehrans advice fit an Intergas boiler , less being best is that there are only 4 moving parts on the boiler ? not that I have fitted any , but he has (Mehran ) !

 

[Gasman77]

btw here is another heatloss cal:

http://www.idhee.org.uk/calculator.html

Thanks - just tried that, came to 16.53kw, incl 4kw for DHW as I have fast recovery cylinder.

I actually have 15 rads of various sizes, mostly K2, but only use 12. Separate 2-way valves for downstairs, upstairs and DHW, with room stats in hall and landing and TRVs on nearly all rads. Bathroom (with TRV) is on DHW circuit, which is nice. Run upstairs at 18C and downstairs at 19C, bit more when wussy relations staying.

Thanks everyone for a lot of useful stuff. Think I'll stay with open vented, but still have to decide whether 18 or 24kw.

Funny that "mr Gas man " My estimate of 11 rads was conservative and also stated only phil new at that minute how many rads he had. Yet as i know "nowt" he came up with more than eleven rads and 18kw required poss more. Maybe i had some knowledge after all, as i said he may get away with an 18kw originally. how about you wind your neck in once in a while and let others have a say, rather than have a p******g contest.
wouldnt normally be offensive but sometimes you old boys need telling. and also put off others from trying to join in and help , and sometimes 'learn' from others

No, he came up with 16.5 kw which included a 4 kw allowance for water heating, which most RGI's don't do anymore as its advisable to time your ch/hw so they don't clash, as most modern cylinder coils can absorb upwards of 10kw of heat, some over 20kw. so the boiler could be a 15kw, and also if the boiler is sized right you don't need to take up your floorboards to renew a gas supply which will become undersized if the boiler is oversized.

I do this every day fella, its my job, so wind YOUR neck in, unless you can tell me you are an RGI installing boilers evey week?

 

[phil1235]

Thanks again guys for all your thoughts - certainly didn't want to prompt a little spat! Since I'll be having an OV boiler I guess I can't prioritise DHW unless I have 2 separate programmers, which seems a complication too many. I also calculated the capacity of my rads, which came to 20.6kw for all 15 but 16.9kw for the 12 we use. House insulation has been improved since they were installed in late 80s.

Totally agree that reliability is the No 1 priority - otherwise I'd keep the old boiler for a few more years.

The old OV condensing boiler is 24kw but gas supply pipe (emerging from solid floor) is only 15mm - and about 4.5m and 6 elbows from the tee. I've read other threads on pipe size, and the Intergas and Remeha installation instructions. Neither mandate pipe size or forbid reduction from the 22mm inlet but both specify min inlet pressure with boiler and other appliances (gas cooker) full on. I guess if pressure requirement is met when tested on the old boiler it'll be OK for the new one.

Hovering between Intergas and Remeha Avanta, and 18 or 24kw. Spoke to UK companies: Remeha say 18kw fine, Intergas suggest 24kw adjusted back (to say 20 or 21kw max). Only other factor is Intergas say auto bypass must be at least 20ft from appliance, while Remeha say 1.5m. Removing one TRV could solve this I guess.

 

[DP]

A system boiler will give you a good flow rate along with hot water to the taps much quicker.

Can the poster elaborate?

OP, stay with open vented as system boiler will exert higher pressure on your system and any weaknesses will mean constant topping up will be needed.

 

[phil1235]

Thanks, I intend to stay with OV for just that reason, plus less to go wrong. Flow rate not a problem (from taps, that is ). With small grandchildren visiting and ceramic disc taps, the cold has to be throttled back a lot at the service valve anyway.

 

[bookahandyman]

Gasman you said, I do this every day fella, its my job, so wind YOUR neck in, unless you can tell me you are an RGI installing boilers evey week?

Sorry, but yes i am an RGI. OP has now confirmed my "average, rough guide" was right. so lets call it a draw.

 

[universal]

OK, after all that jazz - look at a Viessmann Vitodens 100-W Compact Heat-only Open Vent which can be equipped very easily with weather compensation. If fitting all new controls, get a W Plan valve [2 position 3 port] and use a Viessmann plug-in timer.

Good sound advice.

 

[Gasman77]

A system boiler will give you a good flow rate along with hot water to the taps much quicker.
Hot water back up via immersion heater (in case of boiler breakdown) and a presurised system to boot.
So quite a few advantages there!

I do take it that you're comparing a system boiler as opposed to going down the combi' root.

Servotech,
(A "technical" grade plumber at your service)

Can't see how we all missed this little pearl of wisdom....Servotech, can you tell us how going to a system boiler will give you a better flow to your hot taps? I have been grossly overcharging people by installing a unvented cylinder which I thought was essential to achieve this....

Do us and you a favour fella, get your head in the books before you come on here, give daft advice and the boast about being ' a technical grade plumber' what ever that means? of is 'Technical' another way of saying 'clueless'?

 

[phil1235]

Servotech, can you tell us how going to a system boiler will give you a better flow to your hot taps?

Gasman77, I'm the OP and a mere end user (a Chartered Engineer as it happens but with no pretensions to plumbing expertise), but my understanding is that a system boiler would run the HW at mains pressure, with an unvented cylinder as you say (and as Servotech recognised in mentioning the immersion heater back-up); so, compared with an OV (or heat-only) boiler, I guess it would push the HW through the pipes quicker - especially upstairs where the gravity head is less. I can't see what he said wrong or different to what you're saying. And it's a month-old post now!

After helpful advice from several posts, I'm now firm on staying with OV. (Live at bottom of a hill, mains pressure fierce, 20+ yr old pipes & rads, HW fast enough for our needs). Have now found out about 2-channel timers (eg. Honeywell 9400C), so can downsize to 18kw (and get lower modulation minimum) by staggering HW and CH start times. My short list of 2 makes (Remeha and Intergas) now up to 3 with Viessmann - all sound good.