Combi's (Accepting Pre heated Mains water)

[kevymoss]

Hi,
I have talked to all teh kmajor gas boiler manufactures. They tell me that there is no such thing as a gas combi boiler that can take pre heated mains (solar heated) water!! Surely this isnt so
My intention is to add a twin coil (solar cylinder) unvented to my system. Draw the hot water from this to teh mains inlet of the gas boiler. Thus when the water in summer ( and hopefully winter) is above a certain temp (determined by boielr) the boiler does not fire up , but when it needs tops up the boiler will fire in it combi mode.

Any one any ideas?
Thanks
Kevin

[Justme]

Will the pre heated water be at mains pressure?

If so then wont all boilers cope?

Turn off when plenty of solar & on when its just warm & needs a top up, poss even controlled via the solar controller.

[kevymoss]

Yes it will be mains pressure. No all teh manufacturers stated that they will not accept water over 25*C

[desperate]

The makers I spoken to claimed that the plastic parts on the inlet side are not designed to withstand heated water, when I asked if any plans were afoot to modify this "I dunno".was the answer     


These people couldn,t run a bath let alone a boiler making company.

Desperate

[tony.]

I spoke to ferolli a few years ago they saif my boiler a modena 102 would accept pre heated water however some modification to plastic parts were required.

its not a condensing boiler so probably not worth it anyway.

regards

tony

[kennyt]

You could try combi made by Alpha, they take preheated water up to 60c. You would also need a
diverter valve on cold feed into combi, to change flow at 60c. Alpha will not sell you their solar
diverter valve unless you have bought their solar kit.

Kenny

[martin W]

I have a halstead Ace combi boiler (I think the wicks 102 is the same model). I contacted Halstead and they told me it will accept pre-heated water to 50°C.

Its not a condensing boiler, but you can turn the hot water temp down to use about 33% - 100% gass for heating the hot water. Ideal for solar, as long as water in Thermal store is say 30°C I can run the boiler on 33% gas consumption and still shower in nice hot water. PLus when thermal store is over say 55°C we can both have free hot showers, I just turn the boiler off.

[solar_cambridge]

Hi,
I have talked to all teh kmajor gas boiler manufactures. They tell me that there is no such thing as a gas combi boiler that can take pre heated mains (solar heated) water!! Surely this isnt so

Talk to Atag heating down in Sussex. Their combi boilers easily take preheated water - I have done it before on such a setup. I don't think they even specify a max limit, but I presume you will have a TMV3 set to something like 60c? Additionally they have two modes of operation, flow switch or temperature probe. You can set in the software which one is uses. Personally I use the temp probe as it allows the PHE to be prewarmed, however for solar you will probably want the flowswitch arrangement. The boiler will still fire up when the flow switch is triggered, but will immediately modulate down/off as soon as the solar heated water passes the temperature probe.

martyn

[Bargeman]

Hello,

The ATMOS Intercombi HE 32 is designed for solar preheat. No practical limit on input temperature but a good idea to use a TMV to regulate the temperature delivered from the store. I have recently spec'd one of these systems so feel free to query further.

regards

[wookey]

Search then many previous threads on this subject, which will list a few more boilers and also explain where the 'no more than 25C' answer comes from. It is not always technically true, but 25C is the definition of 'cold' water in this country and that matters for legionella regs so it's much easier for them to just tell you that warm water is not allowed (i.e max 25C) than to actually find out what might melt.

Viessman combis will accept hot water. Atmos are designed for it, so that's 5 to choose from - there are probably a few more.

I would read this thread before picking the Alpha setup: http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=2309&page=1 (someone who installed it before realising that an Atmos would have suited her much better, and a non-combi system possibly better still).

[noelsquibb]

Ive tested three different types of TMV to see if they would work in reverse. They all seem quite happy to do this once the non return valves are removed. the temp adjustment also works quite happily.

I think its probably reasonable to suspect that the idea for the TMV actually came from some smart cookie working out that you could reverse a thermostatic valve. So all I am suggesting is that reversing the flow is no more than making the valve do what it was originally made to do.

So if you only want water up to a certain temp to go through your boiler you can use the config shown in this diagram  http://www.atmos.uk.com/core_files/productDoc(114).pdf

The first TMV valve works in reverse and diverts a selected temp flow to the boiler or sends it past the boiler if its hot enough. The second works as a regular TMV to blend the flow back if its too hot for safe use.

How would we know that our boiler was not happy with incoming water at the legionella temp threshold ?
Would it have some kind of shut down response and would it stop sulking if the temp dropped a bit more ?

I might sound confident but I have yet to actually do this. It is part of the plan though  

Noel

[Juliancanoe]

Hi,
I have talked to all teh kmajor gas boiler manufactures. They tell me that there is no such thing as a gas combi boiler that can take pre heated mains (solar heated) water!! Surely this isnt so
My intention is to add a twin coil (solar cylinder) unvented to my system. Draw the hot water from this to teh mains inlet of the gas boiler. Thus when the water in summer ( and hopefully winter) is above a certain temp (determined by boielr) the boiler does not fire up , but when it needs tops up the boiler will fire in it combi mode.

Any one any ideas?
Thanks
Kevin

I have now seen at close quarters a half dozen instances where pre-warmed water has been fed into combis. All the combis were different and none were models deemed to be suitable for that purpose. All worked perfectly. Water was fed into each from a cylinder heated by a woodburner, pumped to mimic mains pressure. Water temp was kept down using a mixing valve at approx. 40 degrees.

I went to a show and spoke to various boiler manufacturers to ask why they say it should not be done (after not understanding why not for myself). It was very hard to get a meaningful answer. The two main reasons were a) boiler fittings on incoming water side not designed for heat and b) boiler will overheat the water. In situation A I believe that water would have to be a lot hotter than 40 degrees to ever damage a fitting. In situation B I would imagine that a boiler might only be upset if the incoming water temperature was a lot higher than 40 degrees. In effect water in summer is preheated anyway as mains water is warmer than it is in winter - so preheating is okay, just to what extent.

Most meaningful conversation was with somebody from one of the largest boiler manufacturers who said "we know it works we just don't say it because we are covering ourselves". She suggested that only at very high incoming water temperatures would there "maybe a few issues".

All over the Internet it says it cannot be done - but possible problem with the Internet is things get copy pasted like a virus. Same thing with "wood burners cannot be connected to combis".

I am not saying anybody should or should not pre heat water to a combi - just that I have just seen it working quite happily (with or without the combi boiler switched on).

[wookey]

All agreed, excet that incoming water could easily be a lot hotter than 40C.

Tim Small on this forum has had a pre-heat combi system for several years now. Details are in some very old post.

[noelsquibb]

thanks Juliancanoe

Safety has to drive everybodys thinking when you are considering putting a giant naked flame in peoples houses.

So I cannot believe that a device designed to quickly heat large amounts of water would incorporate low melt fittings anywhere that involved its primary functions.

However I'm not surprised that manufacturers will not extend their warranty to cover a situation that the designer was not briefed to cover but if word got back to poshnewboiler HQ that they were loosing sales because of the need for integration with other sources of water heating, I'm confident they would modify their response.



And thank you for confirming that a reversed temperature mixing valve is a reliable thermostatic diverter.

Noel

[martin W]

hm.. just had an oddd thought on pre-heated water and combis - well actually on combis.

Forget about the pre-heating for a moment - how do combi manufacturers stand with the risk of scolding when using their products? My Halstead would quite happily scold you if you ran the hot tap just open a little. The combi is supposily capable of heating the incoming water by 35°C (I think at full flow). Whats the water regs say about anti-scolding devices? I thought you must now fit a device to ensure that the person using the water cannot be scolded i.e. TMV set at 50°C Max (or is it 40°C at point of use)?

How does an installer stand with regrd to pushing pre-heated water thougth the combi? Must the installer now ensure that the combi output is not hot enought tot scold, even if the manufacturer has not?

I feel this is another 'where there is blame there is a claim' waiting to happen to some installer (or maybe combi manufactuer).