New install condensing combi system

[Mark Begbie]

I have just bought a 2 bed lower flat into which I am intending to install a condensing combi system. Based on the on-line radiator calculators I need about 8.5kW of output in total for the six radiators and hot water, so my questions to the group are a few:

1) I had though I could use 15mm (plastic) pipe for the feed and return to the radiators, but it has been suggested to me that this should be upped to 22mm - is this good advice or would 15mm suffice for this sort of installation?

2) Being a condensing boiler I understand it will run more efficiently with a lower circulation temperature - does this mean I have to derate the radiators below their quoted capacity?

3) Since all the boilers I have seen are substantially more powerful than I require for this building, I will have to put in an over powerful model. I am guessing that I should also over spec the radiators, to an extent, in order to provide a decent load for the boiler and use a room stat as an additional step to prevent short cycling (the other rads will all have TRVs on them). Is this sensible?

4) I have collated a chunk of info / FAQs on this, but if anyone can point me in the direction of substantial / useful resources, again this would be greatfully received.

Many thanks in advance for any help,

Mark

[Christian McArdle]

It depends on the configuration. If installing the radiators off a snaking "main", then I'd use 22mm. If you are running each radiator pipe to a central manifold, then you'd get away with as low as 10mm (or even 8mm) for most radiators.

You don't have to, but it will be more efficient if you do. You'd have to derate them a little anyway, as they are normally quoted at an unreasonably high delta T.

Modern boilers are modulating and simply turn down the heat before switching off and cycling. Just ensure that your chosen boiler model has a low minimum burner rate.

There's no need to overspec the radiators for the reason given. However, oversizing will be more efficient as it would enable ever lower flow temperatures to be used, as the expense of wall space.

Christian.

[John Rumm]

What is the current heating setup? How is hot water provided at the moment?

15mm is only good for 6kW or so (or less at lower temperatures), hence I would use 22mm for the backbone pipework and 15mm runs to the rads.

Yes.

Most condensors will moduleate down to much lower outputs (say 6 - 8kW, some will go lower still) so not as much of a problem. Obviously once the house is warm the heating load will fall to the a low level and the boiler will cycle to maintain it.

Your most important consideration will be hot water performance. If you mainly take showers then a smaller combi (24 - 30kW) will be fine. If you prefer baths you may want faster water delivery than you will get from a smaller boiler. If you already have a hot water cylinder heated from a boiler then you could retain that for the bath (heated from the radiator circuit) and use the direct hot just for the other taps. If there is no current hot water cylinder then look at a larger combi or one that includes a storage tank. How much space you have available for the boiler will also be a consideration.

[Mark Begbie]

Thanks for the info, I'm getting more mentally clear about this by the day. So to answer some of your questions.

The current setup is a cold tank in the hall cupboard with hot cylinders in the lounge cupboard heated by an immersion. There is no boiler at all in the house with the only heating provided by a gas fire in the lounge (heading for a skip). Space is very tight (I have a lot of stuff!) so I am keen to go down the combi route and get rid of all the tanks. Also I can't remember when I last had a bath (yes, obvious riposte duly noted ;-) )

The system I am planning to put in is a combi running a feed and return loop to the radiators, so it sounds like I should really be using 22mm for the loop. Plan is for barrier Hep2O with copper through the floor. The boiler I was looking at is a Glow-Worm 24cxi as a) it's a condenser b) it modulates down to only 4.95kW and c) it's available from energysmart. Everything except (probably) the lounge will be on TRVs with a roomstat for the final rad.

I am now leaning toward overrating the rads so that the total is closer to the boiler output (18kW) than the "required" value of 6kW that the online radiator calculators come out with. Probably put in about 12kW worth of radiator area.

My master plan is to do all the installation and then get a CORGI engineer in to pressure test and hook in to the gas supply.

Many thanks again,

Mark

[Andrew Gabriel]

Not clear if this includes downrating the rads for lower temperature rating. In my case, I had to downrate the rads to 60% of their data sheet ratings for condensing boiler operation down to -3C outside. Some manufacturers overrate even worse and their rads will need downrating by 50%. That gives you your 6kW output from 12kW of rads. On the other hand, there no necessity to have the boiler still in the condensing region of operation when outside temperature are down to -3C.

Something else this enables you to do is to avoid predictive heating using a timeswitch, where in many cases for me at least, the heating would end up coming on ages before I arrived home. By being able to dump far more heat into the house than it needs to compensate for losses, you can do a damn fast bringing up to temperature. I now turn my heating on when I get in, and the warmup time is so fast I just don't notice it. (If it's really cold, I can ring up the home automation system and have it switch on the heating and/or hot water before I get home, or I can do it as I leave the office via the Internet;-)

[Tony Bryer]

I switched to a Glow-worm 24Cxi earlier this year from a Potterton Envoy (!!!) condensing boiler/cylinder system. A key plus of the combi arrangement is that for most of the year you can keep the CH flow temp down at the condensing level with beneficial effects on the boiler efficiency and heating performance (the rads aren't glowing and sending the heat straight up), whilst with a boiler/cylinder setup you need to keep the flow temp up (assuming a single boiler stat) if you don't want extended cylinder reheat times.

As you imply, on a condensing combi system it's the loss in efficiency caused by turning the temp up when it's really cold will probably cost a lot less than oversizing the rads.

[Andy Hall]

Of course you could put a temperature sensor outside the office window and hook it up to a machine in the office. Taking the average temperature differences over the last century for the differences between the home and office locations from the Met Office database, you could control the home system remotely. Of course it might take several phone calls to control it properly.

I would think that this would be a good application for a SunFire E25K, and every office worker should want one.

.andy

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[Andrew Gabriel]

I have one outside at home and hooked up to the computer which controls the room temperatures, call for heat, etc. However, apart from logging the outside temperature, I have not found it useful for adjusting the behaviour of the heating control system. This is in an old house which takes many days to cool down if you switch the heating off -- it might be different in a new house with almost zero thermal inertia.

Well, an E25K computer system is around 30kW. You can modulate it down by using dynamic reconfiguration (DR) to switch off CPUs. I don't know what the lowest setting with just 1 CPU running would be. An interesting idea though - a heating system which is just the control computer;-)

I actually control the heating with a 120MHz Pentium I, and it's way over-powered for the task.

[Owain]

"Andrew Gabriel" wrote | (If it's really cold, I can ring up the home automation system | and have it switch on the heating and/or hot water before I get | home, or I can do it as I leave the office via the Internet;-)

Of course, in the 'good old days' wives did all this without being told :-)

Owain

[Ed Sirett]

Since you are a shower man a 24kW unit will be quite adequate. That is the water heating power. A condensing model will try to modulate down to the few kW you need, it will likely do a much better job than a non-condensing unit could do.

Make the radiators up to about double what's needed by the heat loss program. There is no need to be too strict about this, if say, there are places where you don't wish to have the extra width or depth or rad.

I'm interested to hear from anyone who has a condensing combi. I want to know what your experience of it is.

I would be happy to fit Vaillant Ecomax 8xx units but the (trade) price is really to high ATM.

-- Ed Sirett - Property maintainer and registered gas fitter. The FAQ for uk.diy is at

Gas fitting FAQ CH FAQ

[Pete C]

Hi,

It could be used to optimise the flow temperature.

cheers, Pete.

[John Rumm]

I fitted a Ideal Isar HE35 in March this year. It has performed very well since then. Boiler is physically very small and neat (hence the reason for choosing it - it had to fit a pre existing gap left by a Gloworm Fuelsaver 55F).

Operation is very quiet (better that the one it replaced). Heating performance seemed fine using a programmable thermostat. It seems to modulate to match the actual load very well. Seems to run in condensing mode 90%+ of the time (judging by the pluming) when running the heating. This may improve later since I have not balanced the radiators yet, since I knew I would be adding more radiators to it in the near future anyway.

Not had chance to asses the impact on fuel usage since the time is too short, and for a good proportion of the first couple of months there were various holes in the house as I was getting stuck into the structural bits of a loft conversion. These I presume would not make for a fair like for like comparison!

Instalation was pretty straight forward, no setting up procedures to perform as such. Pipework all runs to a mounting tray which the boiler is lifted onto afterwards (that was the hardest bit!). Standard concentric flue turret at the top.

DHW performance is OK. It does not attempt to modulate the flow of water, so if the incoming water is cold enough and the supply is up to it, then demanding water too fast will result in the temperature falling. In the winter with ground water at 5 degrees you could open a bath tap to approx 70% max flow before reducing the temperature to below bath temp. In the summer our mains feed (about 22 lpm) is not up to dropping it below bath temp. The thermostat works well at setting a maximum temperature for the water. It runs a heat exchanger preheat every few hours to ensure that there is always hot water available "instantly".

Anything else you want to know?

[Andrew Gabriel]

I might do that when I've hacked my Celcius25 so the computer can set the flow temperature. I finally found some dual linear taper motorised pots, which was the key part I was missing for the design I have in mind.

[Andy Hall]

A servo driven by /sbin/twiddled?

Another pretty simple approach would be a suitable optical coupler with the output side replacing the pot, then drive the input side from a DAC. Have a lookup table in software to linearise the conversion. No moving parts.

.andy

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[John]

SNIP

I've fitted a few of the Glow Worm condensing range both Combi and straight boilers Ed. Generally the response has been highly satisfied customers with one exception where the water flow detector (turbine type) impellor stuck on its shaft a couple of times. Glow Worm sent their own guy out as an in warranty repair. No further problems encountered. One of the straight boilers had a displaced rubber joint on the condensate trap which showed up inside an hour and was a trivial job to correct it.

[Ed Sirett]

Ta. Price.

[John Rumm]

Twas about 900 including standard flue package from the local village plumbers merchant when I got it (or about 1K at the on-line discount places!). It seems to have come down in price a little since then. Best price seems to be about 800 at the mo unless you can get a decent trade discount. The range of Isars also includes a 30 and 24kW version which are cheaper.

[Pete C]

Hi,

Is it possible to set the flow temperature to maxium and get the computer to switch it out to control the flow temperature?

cheers, Pete.

[John Stumbles]

Apart from these problems are you happy with the design & build quality? Are they easy or fiddly to set up? They seem a hell of a weight - definitely a 2-man lift, then?

[Andy Hall]

One could, but it's a very bad idea because the boiler, pump and fan will be going on and off.

Having an analogue control is a much better way.

.andy

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