controling vented cylinder and a combi?

[AnotherTom]

Hey folks,

Me mum wants a new boiler, the old oil firebird seems to be going through parts and fuel-oil a bit too quickly and she has the oporunity to connect to the mains gas. She is retired and it looks like she can get a voucher for the gas connection, as she spends more than 10% of her income on fuel.

At the moment she has a vented cylinder for her shower, but the rest of the hot water demand is fed from the combi. I think she would be reluctant to dump the cylinder as the mains flow is not fantastic, and it also gives hot water from the electric if the boiler has a fault.

going non-combi might work, but it seems a waste to keep a cylinder hot all day just for the washing up and she wants to avoid the disruption of major alterations to the plumbing.

The heating is mirobore and only has TRVs on some of the radiators. I have suggested fitting them to all or almost all.

The current controls don't allow separate hot water and heating schedules.

She will probably install a worchster-bosh on the basis of word-of-mouth.

on to the questions:
1. If a new combi is fitted, how would the system be controlled?
I take it the heating flow would want to be colder than the flow to the cylinder, so the controller needs to tell the boiler to adjust the flow temperature - does this mean it has to be a bosh controller, or will a third party one do this?
2. any suggestions of specific contollers? would an FW100 control a cylinder if fitted to a combi, or only when fitted to a system boiler?

3. Does the overall plan seem reasonable?

The old system was not what it could have been, so we want to keep a close eye on what gets installed this time round.

Tom

 

[Dan Robinson]

You won't be able to have two flow temperatures using a Bosch.

You could use an Intergas HRE and a set of relays to disable a weather compensator to give two flow temperatures using an S-Plan.

Then you simply set the weather compensation to do what you want for the heating flow temp.

They're better boilers too.


Other than that having a cylinder off a combi along with heating is simple to do.

 

[SimonH2]

1) As it is now - except you'd need to upgrade the controls. Dual channel programmer for CH and DHW, with valves to suit. Fit TRVs on all rads except where the room stat is located.

Unless the rads are well oversized, you'd need a similar temp for the rads as for the cylinder. If you just drop the temperature (other than by weather compensation) then you'll find the rads don't put out enough heat to keep the place hot in cold weather.

If she's got a cylinder now, then why the combi ? Other than showers or stupid taps with naff-all orifice in them, there's generally little that needs mains pressure. I'd just use the hot water cylinder for everything.

The hot water cylinder won't lose a lot of heat during the day as long as it's well lagged. Also, unless it's in an outhouse (not seen one like that myself) then any heat it loses will be part of the heating supply for the house so isn't wasted (except in summer if it's hot enough to have no heating on at all). There's a good chance that a competent heating engineer might suggest changing the hot water cylinder - if it's old then it might not be well matched to a decent new boiler.

 

[AnotherTom]

Thanks for you input folks, sorry if I seem to have wandered off, busy day.

I don't think the current tank is big enough for when someone comes to visit. The place used to have two separate small vented cylinders with separate headers, but one was scrapped when the first combi went in, as even the two together were a bit small for the four of us that lived there at that time.

I think she wants to keep the modifications/costs to a minimum but could probably be convinced to spend more if there was a good chance of saving the money back in the next few years.

I live hundreds of miles away and can't really remember the sizes of the radiators and tank, but I think the rads are a good size, and there are 12 of them in a three bed house.

I was hoping for weather compensation. She will use the heating a lot so I feel that any advantage that we can get is reasonably likely to pay for itself. I worry that the setting the flow temp to suit the cylinder could result in the heating return being too hot to condense quite a bit of the time - perhaps I worry too much?

anyone else have an opinion in intergas? I looked at the website and the twin heat exchanger idea they seem to be pushing seems logical, but looks expensive if it fails. Didn't see much info about control so might need to look again.