New Boiler Requirement

[tech749]

Hi everyone,
I have had an independent survey carried out to replace my current 30+ year old boiler. It is a Y plan system, with 10mm radiator piping and 6 radiators. It is controlled by a Hive system. The only hot water usage is for a bathroom and kitchen sink, we have an electric shower - no bath, with two of us in a 3 bed terrace property.

They have recommended a system boiler with weather compensation controls, still using the existing hot water cylinder. At present we only have the hot water heating on for one hour in the morning, and half hour in the afternoon to top it up, which is more than adequate for our needs.

The engineer I am using has recommended that the heating system will require changing to a sealed system this will require removal of heating expansion tanks. And changing of relevant pipework and incorporating a filling loop for self filling the new system.
The vented header tanks for hot water will stay in and will not be touched. However the coil from the cylinder will be re-routed to work directly from the boiler. Which will require pipes to run from current boiler position to cylinder. Provided a suitable route is achievable. A new 11kw system boiler will be fitted in the current boilers position.

Once this is completed the system will then be an X plan set up (priority hot water) a hot water sensor will be run from boiler position to cylinder. Meaning the hot water can be controlled via boiler user interface.
Weather compensation shall be introduced for better efficiency.

This plan may require quite a bit of upheaval inside the house to run the new pipes to the cylinder, including maybe lifting the bathroom floorboards, which are obviously under the bathroom fittings. I'm not sure I want to go through that sort of upheaval which will involve new bathroom flooring, and ceiling where the boiler is located. I am going to ask the engineer about this on his next visit, but would just like to have some knowledge about priority hot water, and whether I really need it before my discussion with him, so I can understand when he talks about it. I am a retired engineer, not heating, but can grasp most technical things, qualified in electric/electronics, and have a basic understanding of domestic heating system, I fitted the Hive myself.

The cost is not an issue, although it is expensive, just want to make sure I'm getting what I need. I probably won't recoup the cost in my lifetime, but I'd still like to pay less for gas than I am now, my current boiler is inefficient (original efficiency when new 60%) and quite heavy on gas ! Thanks for any insights into this.

 

[cross thread]

If going priority hot water the cylinder really wants to be a well insulated fast recovery type. Is the 3 port valve not the airing cupboard at the moment .

 

[ianmcd]

Hi everyone,
I have had an independent survey carried out to replace my current 30+ year old boiler. It is a Y plan system, with 10mm radiator piping and 6 radiators. It is controlled by a Hive system. The only hot water usage is for a bathroom and kitchen sink, we have an electric shower - no bath, with two of us in a 3 bed terrace property.

Far be it for me to argue with someone who has actually seen the job, if the only HW usage is only a bathroom sink and a kitchen sink, you would be far better to have a Combi fitted and all the tanks and cylinders made redundant/ removed

 

[tech749]

The 3 port valve is located in the airing cupboard, next to the hot water cylinder, is that what you were asking?
It is the old copper type with brand new insulation that I fitted on there. Not sure what fast recovery is, guessing its not.
I suppose what I'm really asking is, do I need hot water priority, if we have low consumption and its only on briefly. I'm probably wrong, but my understanding is that on hot water priority, the boiler wants to put all its effort into heating just the hot water, and not both central heating and hot water. If so, then I can always time the hot water to come on when heating is off, or am I totally barking up the wrong tree?

 

[tech749]

Far be it for me to argue with someone who has actually seen the job, if the only HW usage is only a bathroom sink and a kitchen sink, you would be far better to have a Combi fitted and all the tanks and cylinders made redundant/ removed

Hi thanks for the reply, he said he couldn't fit a combi, because when he measured the mains water pressure, it wasn't high enough?

 

[ianmcd]

Hi thanks for the reply, he said he couldn't fit a combi, because when he measured the mains water pressure, it wasn't high enough?

fair point

 

[fixitflav]

my understanding is that on hot water priority, the boiler wants to put all its effort into heating just the hot water, and not both central heating and hot water

It's because of the weather compensation. That automatically lowers the boiler flow temperature with rising outdoor temperature, to give maximum condensation hence greater efficiency. That could be below your cylinder stat setting, so it wouldn't work. To avoid that, the weather compensation is ignored during a HW cycle, the boiler reverting to its normal control-stat.
It's possible to have hot water priority without weather compensation, that's how W-plan, with either/or diverter valve works. It's what I have.

 

[DP]

Get an Intergas Eco or HRE combi. Instal as open vented.
Get rid of storage cistern and cylinder.

For controls, instal Honeywell Lyric and connect to Intergas as OT

Get installer to fit Megnaclean filter.

Simple. No need to do double flips in go through the loops.

Main issue can be worn rad valves and or fittings that are ok with open vented.
Seal the system and pressurise- higher pressure could find these weaknesses

 

[DP]

Hi thanks for the reply, he said he couldn't fit a combi, because when he measured the mains water pressure, it wasn't high enough?

If your electric showers work off the mains, then combi will work too.
Remember, the cold cistern is filling up to existing mains pressure. Boiler will be at a lower point.
Look at the existing mains cold water run- see if the run can be improved to reduce flow resistance ( full bore valves perhaps?)

 

[fixitflav]

However the coil from the cylinder will be re-routed to work directly from the boiler. Which will require pipes to run from current boiler position to cylinder. Provided a suitable route is achievable.

What's wrong with the existing pipe from the boiler to the HW cylinder? If the new system boiler is in the same place as the existing, can't you just remove the pump and plumb in the boiler flow?
But why go for a system boiler, or a combi? If you just replace the boiler with another heat-only that's likely to the cheapest option, without the risk of higher pressure. The controls will need to be modded to allow HW priority for weather comp in either case.

 

[tech749]

If your electric showers work off the mains, then combi will work too.
Remember, the cold cistern is filling up to existing mains pressure. Boiler will be at a lower point.
Look at the existing mains cold water run- see if the run can be improved to reduce flow resistance ( full bore valves perhaps?)

Yeah, the pressure was measured at my outdoor garden tap, which is basically a B and Q hose which is clamped to the water pipe in the kitchen, one of those garden tap kits. The mains comes in there under the sink. When I run the kitchen tap, the pressure seems much better than the garden tap anyway, garden tap has never been great, even though its close to the mains. Maybe ask him to measure at kitchen tap, shower upstairs is fine.

 

[tech749]

What's wrong with the existing pipe from the boiler to the HW cylinder? If the new system boiler is in the same place as the existing, can't you just remove the pump and plumb in the boiler flow?
But why go for a system boiler, or a combi? If you just replace the boiler with another heat-only that's likely to the cheapest option, without the risk of higher pressure. The controls will need to be modded to allow HW priority for weather comp in either case.

Thanks for the reply. He said the hot water needed to be monitored separately, to get the correct flow rate monitoring the outside temperature, so it could heat up the tank independently of the heating, at least that was my understanding of it, but I'm not a heating engineer, so not sure.

This is what he quoted :

" However the coil from the cylinder will be re-routed to work directly from the boiler. Which will require pipes to run from current boiler position to cylinder. Provided a suitable route is achievable. A new 11kw system boiler will be fitted in the current boilers position.
Once this is completed the system will then be an X plan set up (priority hot water) a hot water sensor will be run from boiler position to cylinder. Meaning the hot water can be controlled via boiler user interface.
Weather compensation shall be introduced for better efficiency ".

As I said earlier, I had an independant survey carried out to determine the best system for my house, and a system boiler was recommended, I'm just going on the recommendations I was given really. I had done some research previously, and there were so many different options, I wasn't sure what to go for, hence the survey. I didn't think it would involve replacing pipes from boiler to cylinder, causing some upheaval.

 

[ianmcd]

If you get a decent flow rate out of your existing cold water mains taps then you can have a combi and as already said if your electric shower is fine then a combi will be fine just heating a basin and a sink

 

[fixitflav]

However the coil from the cylinder will be re-routed to work directly from the boiler. Which will require pipes to run from current boiler position to cylinder.

Though it's hard to be too sure without seeing the place, as existing is Y-plan, it has a mid-position vale somewhere. One outlet port goes to the HW cylinder, the other to the rads. Where is the valve at present?

Maybe system boilers are available with separate flow connections to the HW cylinder and the rads, and internal diverter valve. I've not heard of it but I'm just a DIY-er, maybe pros on here can comment? If that's what he has in mind I still don't see why the pipe has to be rerouted, as the new boiler will be fitted in the current boiler's position. If not it still needs the valve, is he intending to move it?

He said the hot water needed to be monitored separately, to get the correct flow rate monitoring the outside temperature, so it could heat up the tank independently of the heating

I suppose that's his way of saying what I was trying to say in #7. The controls should take care of that.

 

[tech749]

It's because of the weather compensation. That automatically lowers the boiler flow temperature with rising outdoor temperature, to give maximum condensation hence greater efficiency. That could be below your cylinder stat setting, so it wouldn't work. To avoid that, the weather compensation is ignored during a HW cycle, the boiler reverting to its normal control-stat.
It's possible to have hot water priority without weather compensation, that's how W-plan, with either/or diverter valve works. It's what I have.

Hi fixitflav
I found this information in another forum, I'm presuming this system is what you are referring to, or something similar ?

I looked at the installation instructions, and it just requires some wiring changes, and another cable from the connector block in airing cupboard to boiler (which is quite easy via the loft and outside wall to boiler position). Looks like a simpler solution to me, but I'm not an installer, so I could be wrong.

Weather Compensating Kit - DHW.​

Vitodens 100-W Type B1GA Heat Only Weather Compensating with domestic hot water cylinder.
This is a simple accessory kit to upgrade an open vented boiler that is connected to a traditional S- or Y-plan system to a more efficient “4-pipe” system.

Guidance:​

• Y-Plan mid-position valves (eg Honeywell V4073A) must be wired to operate as a W-Plan.
• The boiler then needs to be re-commissioned and set to weather compensated operating mode.