Add-on bypass circuit for combi boiler?

[Al N]

djc wrote in news:k7s9e8$6ou$ [e-mail]:

Yes; I just wish I could have TRVs on ALL of my rads, yet not provoke the boiler into firing up all the time and thus becoming inefficient.

I have just looked at the valves on the big uncontrolled rad in the lounge. They have plastic meddle-proof caps held on with a pozi-drive screw. Underneath each is a standard non-theromstatic rad valve. So I guess I can put a turnable knob on one side of the rad, thus making it closeable. I can then remove the thermostat head from one of the other rads in the house and have the the wireless stat in that room, as was suggested. But will my messing with the lounge's rad upset the balance of the system, I wonder? I guess one of the valves on that rad will have been set to a specific degree of openness when the installer balanced the system. (That's IF he balanced the system!)

A

[John Rumm]

Yup, since you have a wireless room stat that sounds easy. If the rad in the study already has a TRV, either set it to maximum of just take the head off it.

The lounge one you can either manually turn off, or add a TRV.

[John Rumm]

Careful use of timing via programmer (or better a programmable stat) can partially solve that one.

An easy test would be to close a valve (counting the turns required), and then reopen to the same setting, and do the same on the other side. If its been balanced at all then you would expect one to be in effect fully open, and the other only partially.

[Al N]

John Rumm wrote in news:- [e-mail]:

That makes sense. And if one of them was set fully open by the installer, I can use that as the manual control without even messing with the other side, I guess.

Thanks...

A

[harry]

d-

If a water flow is reduced through a labrinth (eg a boiler), it does not reduce uniformly throughout the boiler in practice. This means there is more reduced flow in some part(s) than others which can lead to local overheating which in turn can cause "tea- kettling" and boiler inefficiency.

Also the pump can cavitate which will damage it. (Most "wear" in pumps is caused by cavitation.)

Also as system pressure rises, it tends to defeat TV valves. This, if neglected, is one of the main reasons for poor functioning.

In an ideal system, the boiler has constant flow and the heating circuit has constant pressure.

The setting up of the bypass valve is critical if the system is to work well.

[harry]

[e-mail]:

Your installer is what is commonly known as a f******g tosser. Did he do a calculation to size the boiler? Did he even look in the loft for insulation or ask if you had cavity insulation? If not, the diagnosis is confirmed.

Oversizing a boiler is one of the best ways to get inefficient running. They do it because they don't know how to size a boiler. They go on the size of the previous/existing one which may itself be oversized.

[harry]

You are.

[harry]

That is thinking from years ago. Every time a door is opened the thermostat will be unduly influenced. The heating requirements of a house have many factors. Outside air temp is only one. Solar gain (ie sun shining through windows) and wind strength and direction are far more important.

Number if people is a factor but only a small one in an average house. Boiler size =3Dsay 20,000watts. One person=3D80watts. So neither here nor there.

[harry]

The valves with no handles are "lockshield" valves. There should be one of these on each radiator plus a TV. Their purpose is to balance the system, ie to ensure the heat is distributed around the house to suit requirements. Once set up, there should be no need to touch them in normal operation.

[harry]

As you (may )have no bypass valve, yes the system will be upset by altering TV/other valve settings Every time you close a valve, water will be diverted to other radiators because system pressure has risen.

The purpose of a "programmer" is to turn the whole heating system off in your absence. It can't do anything fancy.

You can get devices that start and stop the system at optimum times that vary with outside temperature. Also there are ones that can "setback" the temperaturs at pre-determined times. These will save a lot of money

But first you need to get your system working correctly.

Have a look at the boiler instructions and see if there is a bypass valve inside the cabinet. I'll be surprised if there isn't.

[fred]

In article , Al N writes

At an elderly relative's house they had the old traditional setup of having the thermostat in the hall (no TRV on rad) and TRV's elsewhere. As stated elsewhere this usually satisfies the requirement on a TRV controlled installation that the stat controlled location is the last to reach temperature (shutting down all heat - boiler demand off) and first to cool down (to give boiler demand before other rooms get too cool) but doesn't give great temperature control.

I added a wireless stat and moved it to the lounge (where most time is spent) and where tightest temperature control is most desirable. To overcome the TRVs messing up the stat control of that room they are set to max.

You could do the same for your setup, take the wireless stat to your office, max the TRV in that room and keep the other rooms cool by turning down their TRVs. In the evenings, if you want tight temp control in the lounge you could return the stat to there but you would need to re-adjust the TRV for the office (to control temp) and lounge (to max) every time you do this.

This will get very tiresome after a very quickly but to achieve better control than this you will require multiple zones and it sounds like you're not quite ready to go there yet ;-).

[Al N]

harry wrote in news:ccf0c2db-7240-460f-ba31- [e-mail]:

I'm not sue. They did take measurements of the room before selecting the boiler.

Yes, they did ask about both.

I will check on that. Thanks for your comments!

A

[Al N]

harry wrote in news:0dad452e-a2c6-40df-8397- [e-mail]:

Helpful comments; thank you! A

[Al N]

harry wrote in news:1291c556-b671-44a4-ad63- [e-mail]:

Thanks for the additional help. I can confirm that the bouler does not have any bypass built in (according to the maker's manual).

A

[Al N]

fred wrote in news: snipped-for-privacy@y.z:

That's very useful to know (and it makes sense). Thanks!

Thanks for the helpful suggestions. The zoning option is somthing I'll probably want to do - probably immediately after the warranty runs out.

I can see that getting a hot water central heating system designed and configured optimally is incredibly complicated, and there are many influencing factors, many of which are not taken into account, or even understood, by many installers. I imagine that in the interest of minimising one's heating bills, it probably makes sense to pay for the most expert advice one can find, prior to having a system installes or making changes to it, afterwards. Everyone agree?

A

[John Rumm]

Boiler size may be 20kW but hopefully you don't need all that power to maintain steady state! A few kW even in a relatively poorly insulated place should be enough most of the time.

Also note that the rad in a room will often be less than 2kW, so half a dozen people in a room do add up to a significant extra input for the room.

[John Rumm]

Its traditional on the rad in the room with the stat to fit lockshields at both ends so that it does not get fiddled with.

[John Rumm]

No need to get too worried. Combi boilers need to sized based on the hot water demands rather than the heating load. So the smallest you could sensibly fit would be 24kW and even that will give fairly feeble bath filling. So bigger is normally better. They typically output less to the heating side anyway (a 35kW combi may only feed the CH at a max of

25kW), and some can be adjusted down further during installation to more accurately match the needs of the property.

Modern boilers are also modulating (i.e. they can reduce their power output to match the actual heating load rather than running at a fixed output like harry's idea of a boiler). That takes allot of the sting out of a boiler being oversized. (a typical fixed output heating only boiler may be in the range of 12 to 15kW - however most modern modulating boilers will be able to modulate lower than that).

So at switch on, the boiler will run flat out. As the system and the house starts to come up to temperature and some of the TRVs start to close, the return temperature will rise. The boiler will respond to this by reducing its power output. In an ideal world it would modulate to the point where it matched the heat loss rate of the house. In reality that will usually be lower than the lowest rate on the boiler and it will then be cycled by the room stat to maintain the required temperature.

If you want to get more clever, then you can use weather compensating controls. These adjust the flow temperature of the boiler to match the outside temperature, and the difference between the inside actual and demanded temperatures. So on warmer days the boiler runs cooler (and more efficiently) than when its really cold.

[Fredxx]

It still is the thinking, though as Harry says it can be influenced by outside cold air especially if a front door is left open. The other advantage of a hall or landing is that doors are often left open where the temperature becomes more a mean of the whole house. Hallways and landings tend to be more closed areas with less solar heating through windows.

It is possible to have more than one thermostat, or even have zones with a thermostat for each. I installed CH in a 3 storey house where I had a zone for each floor with a programmable thermostat for each to set appropriate temperatures at different times of the day.

[John Rumm]

That was certainly the trend, although technology has come to the rescue to a certain extent with better controls and more flexible boilers.

Not necessarily, but I would favour gaining adequate expertise yourself, to make sensible decisions. Then you will be able to spec what actually suits your requirements rather than rely on an installers pet solution.

Part of the difficulty here is that labour charges are high, and people don't like paying them. However finenessing a system takes time and hence costs lots[1]. So lots of installers will for example do very little balancing of the system, and leave it up to the TRVs to sort out (which they will after a fashion - just at the expense of the place heating at an uneven rate).

[1] Hence payback times can also be very long when the extra work is being paid for, and the resultant savings are only modest.