heating regulations

[Peter Smithson]

Hi,

I really need a new gas boiler and I just need to check something I've been told.

One chap came round and quoted to replace the existing gravity system with another gravity system (hot water is on gravity, radiators are pumped). This is a conventional boiler.

Another chap came and said that it's against some regulations to fit gravity systems and also wanted to sell me a combi for various reasons. (easier to fit, more loft space, less corrosion due to sealed system and might save me a few quid in gas bills)

Is it true that gravity systems are not allowed?

Also, the 2nd chap said he has to fit a room thermostat even though he can't see the point in them as he fits those things on the radiators. Is that right? You have to have a room thermostat now? The first guy wasn't going to fit one.

Thanks.

Peter

[Christian McArdle]

Basically, yes. Part L Approved Document states that you must make provision to reduce primary circuit losses. The suggested method is to pump the circuit (to make heat transfer quicker) and install a cylinder thermostat (so the primary circuit is only heated when required). This keeps the primary circuit cold, and the boiler off, most of the time, whilst still keeping the cylinder at the correct temperature.

You need to have a boiler interlock, which completely turns off the boiler's burners when there is no call for heat. If you had TRVs on all radiators (and a bypass, but no room stat), then when all the radiators turned off via their TRVs, the boiler would still be pumping heated water round the bypass. This is not good. It can be overcome either by installing a room thermostat in a room without a TRV on the radiator, or by installing a flow switch on the circuit (but after the bypass) so that the boiler knows when to stop.

Christian.

[James]

Is it unrealistic to have a combi in a hard water area, unless you also have a water softener installed?

James

[Set Square]

You could achieve much of this with a C-Plan system, and *still* have a gravity HW circuit - which I assume would meet the regs?

However, a fully pumped system would still be better - preferably an S-Plan system.

[Tony Bryer]

This is not the way to go, regulations or no regulations. You need either a fully pumped system or a combi and do need a thermostat interlock as explained by Christian. You can realistically expect to see your gas bills fall by 20-30% by spending an extra £200-300.

Whether a combi is right for you or not depends on whether you bath or shower and how many people in your family - the merits and demerits of combi systems can be found in many past threads here.

[Christian McArdle]

Except that the poor gravity circulation might take ages to heat up the cylinder. If you could prove that you had efficienct gravity circulation that provided rapid Part L recovery, then it would be fine. This is rarely achievable in practice, however.

Christian.

[John Stumbles]

You only need a scale inhibitor, not a water softener. There are various =

types available: magnetic, electromagentic and phosphor-dosing. The=20 latter are cheapest but require replacement of the phosphor cartridge=20 approximately every year, at a cost of about =A315. They do however have =

the advantage over the other types that there is evidence that they do=20 actually work :-)

[Peter Smithson]

Trouble is, he's quoting about £700 more than the other guy. I'm not sure if this is down to him charging more or because he has to do a lot more work. He'll be taking out the hot water cylinder and the loft tanks and making all the pipework tidy. He sounded a lot more interested in his work and I'd expect a more professional job. He's also recommending an expensive boiler as he says they pay for themseleves in terms of maintenence.

I've read some of those threads in the past. I think I'm an ideal combi in that it's a small house with 2 people. The only thing against the combi which is unusual to me is that our road often has power cuts due to poor cableing in our street. The last one went on all day but I could still have a shower and go out for the evening with my hot water tank.

I think the combi is going to be better overall but money is tight. I just wanted to check that the 2nd guy (expensive combi man) knew what he was talking about and it sounds like he does.

Cheers

Peter

[Ed Sirett]

If you keep the temperature control down (people put it to maximum in the vain hope of makeing the boiler produce a faster flow rate). Then the scaling is not an issue. If the exchanger does become scaled then on many modern combis you can fairly easily remove and descale the unit or replace it. Phosphate dosing of the input water can also be used.

Some people believe in electromagnetically bewitching the water make sod all diffeence AFAICT.

[Owain]

"Peter Smithson" wrote | I've read some of those threads in the past. I think I'm an ideal | combi in that it's a small house with 2 people. The only thing | against the combi which is unusual to me is that our road often | has power cuts due to poor cableing in our street. The last one | went on all day but I could still have a shower and go out for | the evening with my hot water tank.

If power is really that bad, then a small inverter will easily power the combi (it only needs its electronics, gas valve and pump to be powered, same as an ordinary boiler system) from a charged-up car battery.

Owain

[MBQ]

Surely the boiler senses the return water temp and turn itself off when only the bypass is open? That's how our system works where all rads have TRVs (including in the room with the thermostat) and the bypass is a length of pipe in the airing cupboard with a gate valve. It works fine and the boiler shuts down when all the TRVs close. Or is that what you mean by a boiler interlock - just a temp sensor in the boiler? Are they not standard?

MBQ

[Christian McArdle]

The problem is that without a room thermostat or flow switch, the boiler won't know when any TRVs are opened. What will actually happen is that the boiler will start burning again once the circuit has cooled down a little. Your system, if left on at the height of summer, would actually use gas. This is not acceptable under the law.

Christian.

[Set Square]

Not quite the same thing! The boiler certainly has a thermostat, and stops firing when its temperature is reached. So the boiler will *initially* turn itself off when all the TRVs close. BUT, the boiler will then cool down to below its stat temperature - and will promptly fire again - even if there is no demand from space or water heating. That's why a boiler interlock is needed.

[Mike Clarke]

Is retrofitting a flow switch a practical option without modifying the internal boiler wiring?

The house we're moving into needs to have the heating controls improved. Currently there is no room stat and only the bedrooms have TRVs. The "standard" approach of a room stat in the living room and TRVs everywhere else doesn't appeal since there will be times when we will want to heat parts of the house without heating the living room. Separate zones would obviously be the best solution but I don't fancy the task of ripping up floors and modifying the pipework layout to achieve this.

The boiler has an integral bypass and (I think) claims to have anti cycle logic so TRVs on all radiators would be the simplest approach. A flow switch would undoubtedly improve efficiency but as far as I can see the only extra electrical connections shown in the manual for the combi boiler are for an external thermostat and for a frost stat. I imagine the flow switch needs to cut out the burners but leave the pump running so it can detect a TRV opening. I assume that connecting a flow switch to the external thermostat terminals would just permanently shut down the entire system when all the TRVs closed since it would turn off the pump as well as the burners.

[Christian McArdle]

Correct. You may find it works with an additional external pump set to minimum, if the boiler doesn't mind the extra help! The flow switch output would connect to the thermostat input on the combi.

Whether this would work or not depends on the internal design on the combi.

Christian.

[Andy Hall]

It can be done with an outside temperature sensor as well.....

.andy

To email, substitute .nospam with .gl

[MBQ]

But this doesn't happen. I guess what must be happening is that the room stat is shutting off the boiler before all the TRVs have closed. Failing that, how do I tell if I have a boiler interlock or not?

MBQ

[Christian McArdle]

If you have a room stat, then you almost certainly have a boiler interlock.

To test if you have a boiler interlock, with the system cold, set all TRVs and room thermostats very low. Now turn the central heating on. If the boiler fires, then you have no interlock.

Christian.