Boiler Replacement Question on Current Gravity Fed and Pumped Heating System


Postby Dave From Leeds » Thu Jul 25, 2013 12:19 am

We have a conventional boiler with gravity fed hot water and pumped central heating. We are planning to have a new kitchen installed shortly. Someone from a kitchen company called today and mentioned to my wife that it would probably be in our interests to have a new boiler as our current one is (a) old and (b) floor standing in a corner of the kitchen. His comments were that installing a new boiler would free up some space in the corner and that, if he were to build around the existing boiler and it then packed in, access to it would be very difficult.

I understand that all new boilers now have to be condensing boilers and that these do not work with gravity hot water systems. I guess this would entail converting our system to a fully pumped system with motorised valves on both the hot water and central heating circuits. I also believe that we would need to install a cylinder thermostat and TRVs on all radiators except one, neither of which we currently have.

I'm tempted to turn down the idea because I fully understand how my current system works and where everything is. Upgrading to fully pumped will simply add more things to go wrong in addition to the fact that a condensing boiler would be far more complicated than my existing one. Also if we do need to install a cylinder thermostat this will involve running a cable from the central heating programmer all the way up to the loft where the cylinder is located.

Another issue is that, after we had a loft conversion and I moved my F & E tank further up into the roof space, the central heating system has airlocked on refilling every time I've had to partially or fully drain it. I've cleared the airlocks by fitting a hose from a mains cold tap to the expansion pipe above the F & E tank then pushing the air up through the bottom of the tank. To do that after upgrading would also involve manually opening the CH motorised valve (I think).

Could someone please advise whether I would in fact need to install a cylinder stat & TRVs? If not I might consider upgrading the system to fully pumped and getting a new boiler.
Dave From Leeds
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Joined: Mon Nov 19, 2007 9:02 pm

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Postby Atticus Finch » Thu Jul 25, 2013 2:41 pm

There is no requirement to fit a cylinder thermostat on your current installation. If you decide to install a new boiler then you must convert it to fully pumped, to meet current Building Regulations.

The Regulations also specify that you must fit an 'interlock' to turn the boiler off when heating is no longer required, e.g when the HW cylinder has reached the required temperature (normally 60C). The interlock is normally the cylinder thermostat.

Installing TRVs is not a requirement when just changing the boiler, but is recommended as it's easy to do when the complete system is drained down.

As for running a cable from the cylinder stat to the boiler, you can now get wireless cylinder stats which will overcome this.

An upgrade of all your controls would probably be beneficial, e.g Honeywell Sundial RF2 Pack 4.
Atticus Finch
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Joined: Wed Mar 07, 2012 4:13 pm


Postby Dave From Leeds » Sun Jul 28, 2013 1:48 am

Thank you Atticus Finch.

Sorry for the delay in coming back to you - been busy. Thanks in particular for the link re. a wireless cylinder stat. If I do go ahead with the boiler change, I'll probably go with a hard wired stat though because my hot water cylinder is two floors above where the controller will be. One further question I have about this is whether it is acceptable to run the cable for a cylinder stat in the same boxed section that the pipework runs in, since it will be carrying mains voltage. I'm thinking it might have to kept away from the pipework in case of leaks.

What I'm likely to do initially is to temporarily re-configure the pipework and controls in the kitchen so I can run the hot water system as pumped then, if this works, add the CH system back in to ensure that both will run together as a fully pumped system using my existing boiler. The reason for this is that I'm not sure if my current pipe runs for the HW & CH systems will work on a fully pumped system, partly because of where I might have to route the expansion pipe.

If all this works I'll almost certainly go the whole hog and get someone in to install a new boiler, modify the layout to probably a Y plan and relocate the pump and controller. I'd originally been thinking S plan as I think it might give better flow through the two systems but using Y plan with a mid-position 3 port valve would eliminate the need for an automatic bypass if the new boiler has pump overrun. At some point along the way I'll almost certainly fit TRVs to all radiators except the bathroom whilst the system is empty because, as you say, it makes sense to do so.
Dave From Leeds
Posts: 179
Joined: Mon Nov 19, 2007 9:02 pm


Postby Dave From Leeds » Mon Jul 29, 2013 7:18 pm

Since my last post I've been thinking of possibly installing a System Boiler to eliminate the F & E tank & therefore it's vent pipe. I may also consider going over to a combi-boiler since there is a 22 mm pipe feeding all my hot water taps and a cold mains water feed pipe right next to where I would want to site the new boiler. Also it would then be an easy task to link the combi boiler into my existing CH system.

I've sent an email to a local heating engineer asking him to call round to discuss the various options with me. Thanks again for your help Atticus Finch.
Dave From Leeds
Posts: 179
Joined: Mon Nov 19, 2007 9:02 pm


Postby Atticus Finch » Tue Jul 30, 2013 1:46 pm

Dave From Leeds wrote:Since my last post I've been thinking of possibly installing a System Boiler to eliminate the F & E tank & therefore it's vent pipe. I may also consider going over to a combi-boiler since there is a 22 mm pipe feeding all my hot water taps and a cold mains water feed pipe right next to where I would want to site the new boiler.

If the pipework round the house is old, a system boiler may not be advisable as it works at a higher pressure (1 to 2 bar) compared to a system with a feed tank, which will be only 0.5 bar or so. There is therefore a possibility of joints leaking or even pipes pinholing.

A combi boiler is OK provided you are not using too many outlets (taps, toilets, washing machines, hose pipes etc) at the same time. You need to know your flow-rate and dynamic pressure (when water is flowing). The engineer who you have invited along should measure these; be wary if he doesn't.

Here's a quick way of checking; it assumes you have a garden tap fed straight from the mains.

Turn the garden tap full on and time how long it takes to fill a marked bucket. Then convert this to litres/minute. Now turn the kitchen cold tap full on and measure again at the garden tap. The less the drop in flow, the better the pressure.
Atticus Finch
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Joined: Wed Mar 07, 2012 4:13 pm


Postby Dave From Leeds » Thu Sep 26, 2013 3:31 am

Final Update.

Thank you again for all your helpful input Atticus Finch.

The engineer who came took measurements of our mains water flow and pressure. He said he was confident that we wouldn't have any flow problems were we to go for a combi boiler, especially since there are only the two of us left here now that the kids have moved out. Since this would do away with all the tanks and pipework in the loft and avoid having stored hot water constantly losing heat, I decided to take this option, capping off the redundant hot & cold water pipes in the kitchen beyond the take-off points for all outlets.

We had a new Worcester Greenstar 25Si compact installed three weeks ago, along with a radio frequency programmable room thermostat to control the heating system. I'm extremely impressed with it. No more advance planning of hot water requirements is one obvious benefit. Another bonus is that, since the engineer is a Worcester acredited installer, we've got a 7 year warranty with the new boiler.

It turned out, when I checked the junction box for the old system controls, located under the kitchen floor, prior to the installation that we already had a cylinder thermostat that I'd never been aware of! In any case, after finally shutting off the old boiler, I took out the old two channel programmer and connected the boiler feed cable directly from the main double pole fused switch to a new junction box under the floor for the power supply to the new boiler to be connected to. I wanted the system to have it's own dedicated supply rather than linking it into the ring main as the engineer had mentioned as a possibility.

I've decide to post this because I know just how frustrating it is when people are offered advice on this forum and then don't come back to report the outcome. So, thanks again AF.
Dave From Leeds
Posts: 179
Joined: Mon Nov 19, 2007 9:02 pm


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