Bypass circuit... solid fuel stove... thermal store...

[RWhites]

Hi all.
I am soon to install a Specflue / DPS heatbank thermal store heated by both a solid fuel stove and a gas boiler. I'm not a pro, just a DIYer with ideas bigger than his budget....!

One of the safety features pre-installed onto the thermal store is an overheat stat which fires the central heating pump to dissipate excess store heat if the temperature reaches 90 deg.

I am also intending to install the Honeywell evohome system with wireless programmable TRVs on all rads.

My problem is that I don't really want to have a rad without any TRV on it - which would be the standard approach to having a 'heat dump' from the store in order for this overheat circuit to work....

I'm presuming that under normal circumstances (ie no solid fuel stove) I wouldn't need to fit an auto bypass valve, as the evohome would tell my heating pump to stop pumping when all rads TRVs are up to temp (is that correct????).

As a solution to the overheat circuit / heat dump problem, I'm considering fitting an auto bypass valve.... I'm debating whether I should try to use it to effectively bypass one of the rad TRVs (using an extra pipe into one of my rads), or to just bypass directly onto the rad circuit return pipe...?? Has anyone got any thoughts on either of these options? - Bearing in mind that my objective is to dump heat.

On the '3rd pipe into a rad' idea, is there a fitting that exists for this? I'm picturing a long rad tail with a 'T' between the TRV and the rad for my bypass circuit - or alternatively just piping via an additional lockshield into the top above the TRV. Am I in cloud cookoo land, or are these viable options???

Any help, assistance, or comments would be welcomed - even if it just to tell me I'm an idiot and my plans are ridiculous...

Many thanks in advance!

 

[Dan Robinson]

I don't think you are in cloud cookoo land - I own a gas heated DPS Excel store as well as using the EvoHome control system - in fact I have even broke several rules and laws and use the Evo to control the store temperature with mixed success.

However, I think you are misinterpreting several things:

As a solution to the overheat circuit / heat dump problem, I'm considering fitting an auto bypass valve....

This has to be power independent.

Ignore the gas boiler and the Evo.

You need to dump heat in a situation where the store is up to temperature, the power has failed, and the burner is burning.

AFAIK the only way to do that is the "Power-Free Overheat Protection via Discharge" option that pushes mains water through a coil at the top of the store when it hits 95 degrees and dumps it to a drain.

 

[Biggles..]

Just like Dan I'm also an installer of DPS stores.
The boil to drain discharge is one safety feature, but depending on what you've asked if their is an over heat stat that fires the pump you could if room add anothe radiator hidden away near the store as a heat leak, take that off the stove primaries, control valve etc.... Over heat stat fires, rather than dump into the heating re wire so it circulates round the heat leak rad.

 

[RWhites]

I don't think you are in cloud cookoo land - I own a gas heated DPS Excel store as well as using the EvoHome control system - in fact I have even broke several rules and laws and use the Evo to control the store temperature with mixed success.

However, I think you are misinterpreting several things:

As a solution to the overheat circuit / heat dump problem, I'm considering fitting an auto bypass valve....

This has to be power independent.

Ignore the gas boiler and the Evo.

You need to dump heat in a situation where the store is up to temperature, the power has failed, and the burner is burning.

AFAIK the only way to do that is the "Power-Free Overheat Protection via Discharge" option that pushes mains water through a coil at the top of the store when it hits 95 degrees and dumps it to a drain.

Hi Dan
Sorry, I should have said. I do have the power-free overheat protection system fitted as well. I've been advised that I shouldn't rely on it alone, and should still have the facility for the system to dump heat in a more conventional way. I had considered (briefly) not having a heat dump facility, but I don't think it'd be very sensible to ignore a perfectly good safety feature - which is a 1st line of defence, and doesn't involve sending my perfectly good hot water down the drain!

I'd partly hoped that the evohome system might have some kind of TRV override option - to open the rad valves on receipt of a signal from the central unit, but they tell me this isn't a feature....

So, what I'm trying to achieve is a 1st line of defence (powered) heat dump circuit that doesn't involve having a permanent heat dump rad kicking out heat when I don't want / need it, but is a very effective heat dump when I do need it.

Lee - I've not really got my head around how I could achieve what you've suggested - I guess a couple of diverter valves?? - My main problem with it is a massive lack of space.... the location for the store has only about 4" clearance at the sides as it is.....

So, given that I do have the quencher coil as a 2nd line of defence... does my idea have any merit? Would it work? And do you guys know if the rad fittings exist??!?

 

[RWhites]

As a solution to the overheat circuit / heat dump problem, I'm considering fitting an auto bypass valve.... I'm debating whether I should try to use it to effectively bypass one of the rad TRVs (using an extra pipe into one of my rads), or to just bypass directly onto the rad circuit return pipe...?? Has anyone got any thoughts on either of these options? - Bearing in mind that my objective is to dump heat.

My problem is that I've never used an auto bypass valve before, so I've no idea if it will work as I would like it to work in this instance.

Can anyone tell me if it will do what I want it to do?

 

[Dan Robinson]

blimey....

right an ABV is just a sprung loaded isolation valve - effectively.

the evo system will be useless for your needs here.

you could plumb a couple of rads without trvs and then wire the heating pump up with an overheat stat in series, but it is a real bodge and not one i would consider for a customer.

if you can't fit the system to comply with the rules governing the safety of you and yours, do you think you should ignore those rules and plough on?


the evo is a clever bit of kit, but it cam't be relied upon in ANY way to provide a safety mechanism. it is a system based purely on wireless communication!

 

[mysteryman]

In these circumstances, you MUST HAVE a permanently open heat dump radiator. This is to dissipate heat if the pump fails or there is a power cut. Failure to make this provision is dangerous. The heat dump must be capable of dissipating whatever output the boiler is producing at any sudden pump or power failure.

 

[RWhites]

blimey....

right an ABV is just a sprung loaded isolation valve - effectively.

the evo system will be useless for your needs here.

you could plumb a couple of rads without trvs and then wire the heating pump up with an overheat stat in series, but it is a real bodge and not one i would consider for a customer.

if you can't fit the system to comply with the rules governing the safety of you and yours, do you think you should ignore those rules and plough on?


the evo is a clever bit of kit, but it cam't be relied upon in ANY way to provide a safety mechanism. it is a system based purely on wireless communication!

Dan.
I think (that is 'think') you've misunderstood my plans and intentions.

There are safety systems for use with uncontrolled heat sources; I should use them. I've got that.
Safety matters. I've got that - two kids, wife etc etc.
There are two safety systems fitted to the store. One kicks in at 90 deg, the other at 95 deg. I've got that.
The 90 deg safety system (the one I'm on about on here) needs the facility to dump heat - somewhere. I've got that.
Evo won't help me (sadly). I've got that (but I was hoping you were going to tell me otherwise!).
The easiest solution is to have a rad or two without a TRV fitted. I've got that too - but this is where the question arises.

Having rads without TRVs is surely a really wasteful solution under normal operating conditions? I'm trying to come up with a more efficient way of achieving the same (safe) end result.

If the ABV idea won't work (any no-one has actually told me it won't), I guess I'm back to a version of LeeC's idea (I think)...

The system I'm imagining is a fully TRV'd system under normal operating conditions - ie sub 90 deg. What I want to achieve is that my largest rad's TRV is bypassed if the store temp reaches 90 deg (hence triggering the store's 1st safety system and circulating the heating pump). If I can bypass this TRV, I have a heat dump rad. But only when I actually need it.

The way I see it, I need to feed an additional 'flow' pipe to the rad in question, with a 'normally closed' valve separating it from the standard rad circuit, but this valve opens when the store hits 90 deg, so flow through the rad in question is independent of the rad TRV. I guess I need to use a diverter valve(?) rather than a ABV, wired to the same safety circuit as the 90 deg pump trigger.

I was hoping someone would tell me there is an existing fitting that would take the 3rd pipe at the rad tail, but failing that, I could take this extra pipe into the top of the rad, at the opposite end to the lockshield. Not ideal, but should work(?).

As an added bonus, the rad in question is in the same room as the stove, so would almost certainly be 'off' at the TRV - meaning that cold(ish) water would be circulated round the system, helping to quench the store.

To me, it sounds like a reasonably neat solution. The reason I'm posting it on here is get expert opinion from yourself and others who have far more experience than I do at this kind of thing, and to see if I've made any glaring omissions in my assumptions about how it will work.

Rich.

 

[RWhites]

In these circumstances, you MUST HAVE a permanently open heat dump radiator. This is to dissipate heat if the pump fails or there is a power cut. Failure to make this provision is dangerous. The heat dump must be capable of dissipating whatever output the boiler is producing at any sudden pump or power failure.

The store has a power free safety system pre-installed that kicks in at 95 deg to circulate mains cold water round the store to quench it. this is the last line of defence, and is not the safety system I refer to in this thread - which by definition needs power to operate, so does not fall into the realms of 'power free safety features'.

 

[Dan Robinson]

To make use of the heat sensibly then would require a budget tank fed via a pump; wired as I suggested before, and then used to preheat the water feeding the plate.


But you said you didn't have much space...

The eco/abv? Well, assuming all the hr80's shut down when they sense the room temp suddenly rise it might work, but the ABV should be there anyway unless you have a modulating pump, but even so - towel rails are always nice to have warm?

A third pipe to a rad wouldn't work IMHO


Anyway posting from a mobile so rereading the whole thread is a bit of a pita.

 

[mysteryman]

In these circumstances, you MUST HAVE a permanently open heat dump radiator. This is to dissipate heat if the pump fails or there is a power cut. Failure to make this provision is dangerous. The heat dump must be capable of dissipating whatever output the boiler is producing at any sudden pump or power failure.

The store has a power free safety system pre-installed that kicks in at 95 deg to circulate mains cold water round the store to quench it. this is the last line of defence, and is not the safety system I refer to in this thread - which by definition needs power to operate, so does not fall into the realms of 'power free safety features'.

The OP stated that he did not want to rely on this alone. In this case he needs to follow my advice, or take the risks.

He has hit upon the fundamental disadvantage in that you can't achieve complete controllability from the oil or gas boiler with complete safety from an uncontrolled source on the same system. A thermal store - of itself - does not do everything required.

A good alternative is to fit a dry woodburner and leave the central heating alone.

 

[RWhites]

To make use of the heat sensibly then would require a budget tank fed via a pump; wired as I suggested before, and then used to preheat the water feeding the plate.


But you said you didn't have much space...

The eco/abv? Well, assuming all the hr80's shut down when they sense the room temp suddenly rise it might work, but the ABV should be there anyway unless you have a modulating pump, but even so - towel rails are always nice to have warm?

A third pipe to a rad wouldn't work IMHO


Anyway posting from a mobile so rereading the whole thread is a bit of a pita.

I did consider leaving the TRV off the towel rail; but putting aside some of the issues (tiny bathroom, can get too warm), I'm not convinced that rad is big enough to dissipate sufficient heat from a 350lt store.

Why won't the 3rd pipe to a rad work?

 

[Norcon]

Back in the ole days many solid fuel systems were fitted with no heat leak rad.
I worked on one recently where gravity circulation supplied the vented hot water cylinder and in a power cut the householder would just turn on a tap which cooled the coil.

That was the manual version of the SCWS auto release valve except its more efficient today.

It worked that way for over 30 years with no issue. And it will easily work for well over another 30 years with no issue.
Unless of course a box ticking official gets through the door.

 

[Dan Robinson]

Problem I think the OP wants to prevent is the O/H discharge and use the "wasted" heat.

All adds up to one big plumbing oxymoron to me.

This - if he hasn't got bored yet, and replies, we might find out.

 

[simond]

I thought DPS went t*ts up last year?

Have they risen again?