EvoHome: do I need to leave a single radiator in my house without HR92 or TRV?

[johnny]

Hello everyone, I am new here. I am about to have Evohome installed when the plumber replaces my radiators at home. I have a combi boiler and was therefore thinking of ordering the Evohome controller, a BDR91 to switch the boiler on/off, and an HR92 for each radiator in the house.

My plumber has not installed an Evohome system before and told me that it is good practice to leave a single radiator in the house (usually the one closest to the boiler) without a TRV, ie. in my case without an HR92. He indicated this was because if the combi boiler produces heat but all the radiators are turned off, the heat has nowhere to go, the boiler overheats and the system can get damaged.

That's all well and good but I have to say that I thought with an Evohome system, I'd be able to have an HR92 on each radiator, and that in case no heat is required by any of the radiators, the BDR91 in the system would switch off the heat production of the boiler and the problem described by my plumber would not happen?

Any thoughts you may have on this would be most appreciated.

Thanks
Johnny

[HenGus]

Has your chosen plumber done any Evohome installer training? I ask because you may be paying for him to learn on the job, and you may not get the results that you are looking for. For example, there are MINIMUM prescribed limits for the siting of boiler demand relays (BDRs) etc. Incorrect binding can also cause a raft of problems. The whole point of zoning is to have the ability to have heat where and when you want it. Leaving a radiator 'online' was/is a method of ensuring that there is always an open return path to the boiler when the TRVs/HR92s close. The preferred way of protecting your CH system is to get your installer to fit an automatic bypass valve: this will allow you to have HR92s on all your radiators. It is worth pointing out that, unlike conventional TRVs, HR92s do actually shut off heat to the radiator.

[G4RHL]

I have one small radiator in a downstairs toilet deliberately left with no TRV. If the boiler is meant to be off but still thinks heat needs to be dissipated and there is a pump overrun there is then somewhere for the water to be pumped. My boiler is coming up to 18 years old and no doubt when it goes the journey the replacement will have a bypass as has been mentioned. Am sure am on borrowed time with my boiler but it works!

[emmeesse68]

My boiler has an embedded automatic bypass valve but when all radiators are shut and there's some overrun (with TPI this happens some 10 times per hour), my boiler gets LOUD... I mean, the bypass is noisy indeed!

I'll try taking a TRV head off to check what happens, but IMHO it should be the smaller and the farther from the boiler the better...

[johnny]

Thank you all for your feedback, it is super helpful. My plumber is indeed not experienced installing Evohome systems which is not ideal, but I don't have much choice in the matter since it was hard enough for me to find a competent / responsive / available plumber to install my new combi boiler, and my new radiators before winter broke off for good.

I spoke with the Honeywell helpline and with my plumber today. My plumber confirmed that my new Vaillant combi boiler will have an embedded automatic bypass. The person I spoke with at Honeywell also indicated that for a combi boiler, there was no need to leave a radiator without TRVs, as the HR92s would 'tell' the BDR91 relay box whether any heating was required from the boiler. If none of the HR92s requests any heating from the boiler, then the BDR91 would know to switch off the heating production of the boiler, kind of putting it in 'summer' mode (i.e. hot water only, no heating). Anyway, this is what the total plumbing layman that I am understood but if anyone thinks I got this wrong, I am of course all ears.

Cheers
Johnny

[erik]

It is worth pointing out that, unlike conventional TRVs, HR92s do actually shut off heat to the radiator.

Conventional TRV's do this too when they're set to 0.

And you DO need a bypass or open radiator. I've seen very often that the HR92's were closed BEFORE the BDR91 would stop demanding heat from the boiler. Also, a boiler often keeps the pump running for a while after heat demand being gone. So if all HR92s close before the pump stops running, the water has nowhere to go and your boiler won't like it.

[Fursty Ferret]

Honeywell recommend an automatic bypass valve in these circumstances.

[johnny]

And you DO need a bypass or open radiator. I've seen very often that the HR92's were closed BEFORE the BDR91 would stop demanding heat from the boiler. Also, a boiler often keeps the pump running for a while after heat demand being gone. So if all HR92s close before the pump stops running, the water has nowhere to go and your boiler won't like it.

Thanks, very helpful information. The boiler to be installed is a Vaillant 832 ecotec plus, which does have an embedded diverter valve with bypass. Hopefully this means I don't need to buy a separate bypass valve but if you think differently, I would be delighted to know.

[erik]

I don't know if a built-in valve causes enough flow to not upset the boiler. Also, I wonder if it will be able to dissipate enough heat to prevent the boiler from overheating when the generated heat has nowhere to go.

[top brake]

I don't know if a built-in valve causes enough flow to not upset the boiler. Also, I wonder if it will be able to dissipate enough heat to prevent the boiler from overheating when the generated heat has nowhere to go.

I think maybe you're confusing pump overrun with bypass. The internal bypass on a Combi is there to ensure a minimum flow rate through the boiler to prevent overheating. Pump overrun is there to circulate the heating water after the combustion has stopped.

An external bypass is always recommended to ensure differential pressure in a system is not too high as this can cause system noise.

Of course all of these things are inter-related to an extent.

To answer the OP no you don't need a 'bypass radiator' but if you don't have one the need for an external bypass is even greater.

[erik]

I'm not confusing them. I'm wondering if an internal bypass can cause enough heat dissipation, because it's... internal. So where does the heat go?

[johnny]

To answer the OP no you don't need a 'bypass radiator' but if you don't have one the need for an external bypass is even greater.

Great thanks for the help, it looks like I am slowly getting to the bottom of think thanks to you all.

Final questions:

(i) if I do install an external bypass in a combi boiler system without any water pumps, should I go for the Honeywell DU144 (or DU145), or should I go for the Honeywell V4044C? All these models are listed on the Evohome shop website.

(ii) if I use a 'radiator bypass' with no TRV, say the radiator in the kitchen near where the boiler is located, does this mean that when I turn on a bedroom TRV at night, the kitchen radiator will heat up needlessly? Also, in this set up, how do I ensure the kitchen radiator turns itself on when I want it to? Could I use the internal thermostat of the EvoHome controller to do this?

[erik]

yes, it will heat up needlessly. Best to choose a small radiator, or one that needs to be on anyway (towel radiator in bathroom?). Maybe restrict the water flow going through it, to make sure it doesn't 'steal' too much heat from other radiators.

And turning it on specifically when you want it to.... hmmmm... Getting creative... what about a normal HR92 but screw it on loosely, so it can never close the valve completely? There might better solutions though

[top brake]

DU144 is a straight valve, DU145 is an angled valve

You don't need V4044


Bypass radiator typically in bathroom or loo

It will heat whenever any zone calls

Great thanks for the help, it looks like I am slowly getting to the bottom of think thanks to you all.

Final questions:

(i) if I do install an external bypass in a combi boiler system without any water pumps, should I go for the Honeywell DU144 (or DU145), or should I go for the Honeywell V4044C? All these models are listed on the Evohome shop website.

(ii) if I use a 'radiator bypass' with no TRV, say the radiator in the kitchen near where the boiler is located, does this mean that when I turn on a bedroom TRV at night, the kitchen radiator will heat up needlessly? Also, in this set up, how do I ensure the kitchen radiator turns itself on when I want it to? Could I use the internal thermostat of the EvoHome controller to do this?

[johnny]

Thank you so much, really helpful advice.