Central Heating Bypass on Fully TRV'd SYstem (Incl Honeywell CM37z Zone System)

[t]

We are in the process of having central heating installed, replacing an old partial system. We have done this before in other properties so know something about it. This property has 11 radiators. We are using a Honeywell CM37z system with radio-linked TRVs on every radiator except for the bathroom where the steam may corrode the electics of the radiator head.

Each radiator head in this system radios back to the relay unit that controls the boiler. If any one radiator head calls for heat the boiler will be active. If no radiator heads call for heat the boiler is off. There is no separate room thermostat as this would be entirely without a purpose.

The CORGI registered man fitting the boiler for us is saying that we must have a bypass. I know what this is. He says it is due to 'new' regulations. He does not say whether these are building regulations or Corgi regulations. He says "safety" and that he cannot sign it off (presumably Corgi-wise) unless it has a bypass. He is saying that in theory if every radiator valve failed and shut off, then the bypass radiator, which would be uncontrolled, would allow the passage of water for safety reasons. He also says it will save gas. The problem to us is he is insisting it must be one radiator that remains uncontrolled. This is anathema to me! An uncontrolled radiator chugging away every time the system is on for any one or more other radiators.

Clearly this cannot save gas. I thought that a bypass was to protect the circulating pump from pumping against nothing in the wildly unlikely event that all TRVs were closed or faulty. Is there another function of a bypass? Could we use a pipe or something automatic (I know there are various so-called automatic bypasses) instead, rather than a radiator? As we do not have a radiator in a room with a system thermostat. Our system will only fire the boiler if one or more stats calls for heat. I take this to mean it is fully interlocked?

Does any regulatory body, e.g. Building Regs or CORGI specify a bypass as mandatory? If they ever do, does a system that would not call for heat in the situation where all TRVs are -shut- require a bypass? Is it true to say the bypass is a safety item or is it to protect the pump? Or in some way supposed to save energy?

What should I read? Has anyone any references? Any input would be most appreciated

Nick

[Doctor Drivel]

None. Corgi is a registration authority only. You indicate that the bathroom rad has no thermostat valve on it. If so the by-pass is there. If it does, say a mechanical one, then a mechanical by-pass between the flow and return at the boiler may be fitted.

If you have electric thermostat valves on all rads then no by-pass is needed if the boiler is control "interlocked". That is when all are off the pump and boiler is off. You may want to put a by-pass in to maintain a minimum flow through the boiler if only one rad is partially open.

Another way is to have a Grundfoss Alpha auto speed pump and a flow switch on the reads return. When all rads are closed the flow switch cuts out the boiler but not pump. No flow and the pumps automatically winds down. when one opens it winds back up.

He is talking balls. The Baxi 133 HE Plus and Potterton Promax have an integral flow switch and rad valves can be fitted on all rads. What if that flow switch fails?

A Grundfoss Alpha solves that.

Flow switches: A selection at Hawco

but have a £75 min order. Try RS: switches from Farnell: FS-05 (3A contacts) 3/4" BSP - £16 FS-01 (3A contacts) 22mm compression £31.92. Twice the price for compression joints.

Data sheets:

Is there another function of a bypass?

It is there to maintain the minimum flow through the boiler. Some flow switches have a minimum flow switch-on. Find out min' boiler flow and match switch to suit. Fit an Alpha pump. If the flow switch is too low a flow rate for the boiler, then have a tee before and after the flow switch and by-pass the switch with 15mm pipe, in this pipe have an in-line isolator valve. This then will raise the minimum flow through the boiler before switching in the burner. A fix speed pump can be used as it will pump around the flow switch by-pass, only when flow get to the min boiler level will the flow switch, switch in and the burner come in.

The problem is the minimum flow through the boiler. If one rad is calling and just about open it may not be enough flow. The flow switch combined with a Grundfoss Alpha pump will maintain the min flow

No. Just a control interlcok, which you have, and makers specs of course.

The minimum flow problem again. You could have one thermo rad valve permanently partially open, to give the min flow if another rad is open. Do the rad valves have a min/max switching adjustment? If so you are sorted, and no Alpha pump needed.

Just ask me questions.

[Ed Sirett]

There are two things here:

1) Do you need a bypass. Yes. 2) Do you need one rad without a TRV. Not in this case.

Essentially you have a very extended S-plan system with each radiator effective become a zone. Any one radiator will make for a boiler demand (although in practice the likelihood is that several will be near calling for heat if they are not already doing so).

Since the closing of all TRVs shuts off the boiler there IS an interlock between the house being warm enough and the boiler demand. So in this case the usual advice that you need a wall thermostat and a nearby radiator without a TRV can be specifically ignored.

The bypass is almost certainly needed to provide a path for primary water to continue after the demand has ceased. The boiler will need this (some have this device internally so no need to add an extra one on the pipework).

The valve must be 'smart' not simply a gate valve on a piece of pipe with the handle removed. Using a radiator (often the bathroom towel rail) for a bypass is also no longer acceptable.

This is to comply with Part-L. CORGI are involved with this is as much as if the fitter is discovered to have been self-certifying non-compliant installations then he will put his registration in jeopardy. CORGI have an inspection process - he may get caught out.

HTH

[Doctor Drivel]

Open to question. It is NOT a definite yes at all.

depends on what the boiler manufacturers say.

Open to question.

It is. The idea is to:

1. Provide a minimum flow through the boiler.
2. Pump overrun to dissipate heat from the boiler.

A bathroom rad could do this. An auto by-pass valve is not desirable. as it raises the return temperature lowering efficiency. A flow switch, switching the burner, with a parallel by-pass is probably the answer.

A pity the CORGI man doesn't know that much. All too typical. They arm wave a lot.

[Phil Addison]

If I can elaborate on Ed's advice ...

Under the new Building Regs, Part L, this is no longer sufficient. You must have a separate timer for the hot water (perhaps you have done this) if the whole system is being replaced.

See "Heating controls" in

> What should I read? Has anyone any references? Any input would be most

The Honeywell guide summarises things reasonably well

[snip]

Indeed. For example, from my daughter's Ariston EuroCombi A/23 - A/27 MFFI Installation Manual ....

"By-pass: The appliance includes an automatic by-pass valve, which protects the main heat exchanger in case of reduced or interrupted water circulation through the heating system, due to the closing of thermostatic valves or c*ck-type valves within the system."

This requirement to have a by-pass valve (not necessarily automatic) stems from the boiler manufacturers installation requirements rather than from 'regulations'. However, the new "Basic and Best Practice Specifications" calls for an automatic valve to be used, rather than the old way of a fixed gate valve.

Central Heating System Specifications (CHeSS) - Year 2005

"An automatic bypass valve controls water flow in accordance with the water pressure across it, and is used to maintain a minimum flow rate through the boiler and to limit circulation pressure when alternative water paths are closed. A bypass circuit must be installed if the boiler manufacturer requires one, or specifies that a minimum flow rate has to be maintained while the boiler is firing.The installed bypass circuit must then include an automatic bypass valve (not a fixed-position valve). Care must be taken to set up the automatic bypass valve correctly, so as to achieve the minimum flow rate required (but not more) when alternative water paths are closed."

Only needs to be smart in the sense that it opens according to the water pressure across it - doesn't need any fancy electronics. e.g.

The uk.d-i-y FAQ is at Google uk.d-i-y archive is at NOSPAM from address to email me

[Ed Sirett]

I had assumed that there would a master time clock for the heating.

Yes, by smart I simply meant that it must be sensitive to the differential flow-return pressure,not just a semi-open valve. The latter has the double disadvantage that it i) Effectively reduces the boiler output thus requiring a bigger unit than needed to be installed. ii) Raises the temperature of the return water at all times thus reducing the boiler efficiency.

[Doctor Drivel]

Best practice is a "recommendation" only. There are better ways of doing it rather than with a auto by-pass valve. An auto by-pass valve lowers the efficiency of condensing boilers.

That explains what a by-pass does. It does not say you "have" to have one. Read my post on this.

[robert]

Bypasses, Room thermostat or flow switches all of interest as I am sorting out the spec for my replacement system.

The TRVs + flowswitch plus Grundfoss Alpha does seem the logical approach - I hate the single room thermostat approach and find that in the type of houses I have had they are useless at providing sensible control of when the boiler is required to be providing heat.

but re Grundfoss Alpha pump and bypass

Can the pump in condensing combi boilers such as the Worcester and Valliant range be replaced with a Grundfoss Alpha or are you stuck with the "manufacturers" specified type ?

Robert

[Phil Addison]

No criticism of you Ed. I was just adding some info for the general readership.

Phil The uk.d-i-y FAQ is at

Google uk.d-i-y archive is at NOSPAM from address to email me

[Doctor Drivel]

Some makers say you can fit them as a direct replacement. Others say no, as they don't want any deviation from the norm. In most cases you can replace like for like if it fits in. There is a larger control box on the Alpha.

Using an Alpha with an auto by-pass sometimes may not open the auto by-pass. See this page of auto by-pass setup:

A flow switch with a settable by-pass around it is the best option when using an Alpha pump. Or if you can get a flow switch which switches off the burner when flow is below the minimum the boilers makers stipulate.

[Christian McArdle]

Only the boiler manufacturer can specify if the boiler needs a bypass. Some do, some don't. Contact their technical department. Don't bother trying to explain your system. Just say it is "S-Plan" if they ask. Don't mention the radio TRVs it will only confuse them.

Christian.

[Doctor Drivel]

The makers specify a minimum flow. How you do that is your business. An auto by-pass is only one way, but can be problematic it fails and lowers efficiency raising the return temperature. Most fitted, like 90% of them, are incorrectly set anyway. Best avoided.