Oil boiler on an ASHP designed system

[plumbiz]

Hi. I have a potential client that has all the radiators fitted suitable for an ASHP (but pump not plumbed in) now they want an oil boiler plumbed back in instead but want to keep all the big radiators , just in case they decide to change their mind As they still have the ASHP but not fitted. ..
Some small rooms even have two large doubles.
What are the potential pit falls for this. My first thoughts were would the return temp now be too low (as rads sized for ASHP) and corrode the boiler. Expansion vessel will need to be a lot bigger.
There have been a few plumbers already !!! One installed a new oil boiler, Then a few months later ASHP crew came in and removed all pipework and fitted all new rads with a few extra ones . Now they have gone (don't know why) and client just wants the oil boiler plumbed in..
My gut is saying don't get involved as I may be the 3rd ..or 4th plumber on site..
But also they have had no heating for months and I would like to help but it not come back and bit me if the new boiler corrodes..?

 

[fixitflav]

Hi. I have a potential client that has all the radiators fitted suitable for an ASHP (but pump not plumbed in) now they want an oil boiler plumbed back in instead but want to keep all the big radiators , just in case they decide to change their mind As they still have the ASHP but not fitted. ..
Some small rooms even have two large doubles.
What are the potential pit falls for this. My first thoughts were would the return temp now be too low (as rads sized for ASHP) and corrode the boiler. Expansion vessel will need to be a lot bigger.
There have been a few plumbers already !!! One installed a new oil boiler, Then a few months later ASHP crew came in and removed all pipework and fitted all new rads with a few extra ones . Now they have gone (don't know why) and client just wants the oil boiler plumbed in..
My gut is saying don't get involved as I may be the 3rd ..or 4th plumber on site..
But also they have had no heating for months and I would like to help but it not come back and bit me if the new boiler corrodes..?

If it's a modern condensing boiler I don't see why corrosion would be a problem, it's designed for condensation, which is improved by generously sized rads allowing low water temperature. But worth checking with the boiler manufacturer.

 

[Madrab]

Do they know the system design temp? The latest ASHP's can run much hotter than the originals, who's design temps were around the 50Deg mark. The latest pumps using newer gases and can run >60degC.

Even then though, if the system has been designed around ~ 50DegC then just run the oil boiler at that temp to maintain the design heat loss output. Only other consideration would be the HW storage temps and the requirement to have the system temps raised to ensure the HW storage can reach >55DegC periodically.

 

[Johntheo5]

Hi. I have a potential client that has all the radiators fitted suitable for an ASHP (but pump not plumbed in) now they want an oil boiler plumbed back in instead but want to keep all the big radiators , just in case they decide to change their mind As they still have the ASHP but not fitted. ..
Some small rooms even have two large doubles.
What are the potential pit falls for this. My first thoughts were would the return temp now be too low (as rads sized for ASHP) and corrode the boiler. Expansion vessel will need to be a lot bigger.
There have been a few plumbers already !!! One installed a new oil boiler, Then a few months later ASHP crew came in and removed all pipework and fitted all new rads with a few extra ones . Now they have gone (don't know why) and client just wants the oil boiler plumbed in..
My gut is saying don't get involved as I may be the 3rd ..or 4th plumber on site..
But also they have had no heating for months and I would like to help but it not come back and bit me if the new boiler corrodes..?

Grant and I think Firebird specify a minimum return temperature of 40C to prevent corrosion of the primary HEX, the secondary HEX is stainless, a bit surprising as one would think that the gases leaving the primary would be well avove the due point and at ~ 150/200C with a flue gas exit temp of ~ 65/75C. Anyhow probably prudent to ensure the return temp isn't below 40C.
This can be achieved by keeping the target flowtemp sufficiently high and high(er) circulation rates, TRVd rads though have a mind of their own and can throttle the flowrate right down until the return is almost at room temperature, I would point all this out to your prospective customer. Again, probably prudent to ensure the return temp isn't below 40C., ideally install a temperature controlled bypass valve (Tapstat) between the flow & return at the boiler, or at the very least, a temperature gauge.

 

[Exedon]

First if your the third or fourth plumber in a short time I'd be very wary .
As above return temperatures on most oil boilers need to be above 40c.
We have achieved this in past using temperature blending valves to keep return temperature up.

 

[fixitflav]

We have achieved this in past using temperature blending valves to keep return temperature up.

On sludge digestion plants on sewage works, using biogas in the boilers, we used minimum return temperature control, 60°C IIRC. Because of sulphur compounds in the biogas, but hadn't heard of it on natural gas.

 

[Johntheo5]

On sludge digestion plants on sewage works, using biogas in the boilers, we used minimum return temperature control, 60°C IIRC. Because of sulphur compounds in the biogas, but hadn't heard of it on natural gas.

The above concern is a oil fired boiler, kerosene contains a very small % of sulphur I think, and acid dewpoint is well over 100C, so maybe this is why a minimum 40C return temperature is required.

I spent over 30 years running 50mW boilers on a mixture of nat gas with up to 15% bio gas from a waste water plant (Citric Acid production), these boilers didn't have economisers but had 10 ton Heat Wheels that slowly (2.1RPM) rotated between the fluegas out and the cold air in, the combined cold end temp of the flue gas exit temp + cold air in temp had to be kept at a certain level (can't remember it now) to avoid corrosion of the heat wheel elements, this was done by controlling/heating the cold air in with steam preheaters.